A THESIS Entitled GEOCHEMICAL INVESTIGATIONS of DEEP SEA

THESIS

entitled

GEOCHEMICAL INVESTIGATIONS OF DEEP SEA SEDIMENTS

FROM THE INDIAN. OCEAN

Submitted for the

Degree of Doctor of Philosophy of the

University of London

MICHAEL FRANCIS HORDER

Applied Geochemistry Research Group Royal School of Mines

Imperial College January 1979 ABSTRACT

Data collected from the analysis of two hundred and forty Indian Ocean deep sea sediments using bulk and partial chemical techniques for the elements Ca, Al, Ti, Fe, Mn, Ni, Co, Cu, Cr, Cd, Pb, Zn, Mg, Ba, Li and As by Atomic Absorption Spectrophotometry (AAS) have allowed conclusions to be drawn on the general geochemistry of these sediments since the Late Jurassic.

Comparisons between metal-rich sediments from the crest of the seismically active Central Indian Ocean Ridge and from the base of Deep Sea Drilling Project boreholes on the flanks of the ridges show that both groups are in general geochemically similar and confirm that the latter are ancient analogues of the former group, which have been moved to their present position by sea floor spreading. The analysis of the carbonate-rich sediments by AAS for certain elements was subject to spectral interference caused by the large amounts of Ca present. A correction procedure was developed and has been applied to all• the data. The analysis of the carbonate-rich sediments has further shown that proportions of such metals as Pb, Ni, Ba, etc. may be held in the lattice of the carbonate minerals in the tests of dead foraminifera.

Data from chemical partition studies and multivariate statistical analysis have shown that it is possible to account for metal enrichment in Indian Ocean sediments in terms of five major processes. These are the hydrothermal leaching of metals from hot basaltic rocks; direct input of possibly deep seated volcanic/hydrothermal fluids; the weathering of volcanic material; authigenic precipitation from seawater and biogenic concentration. The processes of metal-enrichment have been cyclic and some can be related to periods of increased sea floor spreading rates along segments of the Indian Ocean Ridge system since the Late Jurassic. ACKNOWLEDGEMENTS

The research described in this thesis was carried out in the Applied Geochemistry Research Group, Department of Geology, Imperial College of Science and Technology under the general direction of Professor J S Webb whilst the author was working as a NERC Research student. The project was initiated and supervised throughout by Dr 0 S Cronan.

The author would like to thank Dr D S Cronan for providing the samples collected by the West German research vessel "Valdivia". Furthermore, the author wishes to thank the Lamont-Doherty Geological Observatory, New York, and the Scripps Institution of Oceanography, California, for providing sets of samples. The Deep Sea Drilling Project samples were provided from the East Coast Repository under the auspices of the US National Science. Foundation.

The author would also like to thank all the staff and students of the Applied Geochemistry Research Group and numerous members of the Department of Geology, Imperial College, who helped in a variety of ways or who took part in many helpful discussions. Thanks are due to Mr A Thompson and other members of the technical staff for help with the analytical work and to Dr Ian Henderson, for assistance in identification of the micro-fauna. Thanks are also due to Mr R Boyling of the University of London, Library Photographic Depart- ment, for the preparation of the photographs and to Miss Meriel Beale and Mrs Rosamund Macartney for the typing of the manuscript.

Special thanks are due to Dr Richard Howarth for his critical reading of parts of the manuscript and to Mr S A M Earle, for both assistance and discussions concerning the data handling and statistical analysis; to Or E J W Jones, Department of Geology, University College, London, for helpful discussions and critical reading of parts of the manuscript; and to Or S A Moorby for helpful discussions during the course of the research.

The author gratefully acknowledges the receipt of a NERC research studentship for the period of three years for the preparation of this thesis.

Finally, I would like to thank my wife, Felicity, for her encourage ment, support and understanding during the research and writing of this thesis. TABLE OF CONTENTS

Title Page Abstract Acknowledgements Table of Contents Table of Tables Table of Figures Table of Plates

Section 1. GEOLOGY, OCEANOGRAPHY AND SEDIMENTATION IN THE INDIAN OCEAN 1

1.1 Introduction 2 1.2 Geology and Evolution of the Indian Ocean 8 1.2.1 Introduction 8 1.2.2 Structural Features of the Indian Ocean 8 1.2.2a Introduction and Geographical Setting 8 1.2.2b The Mid-Ocean Ridge System 10 1.2.2c Aseismic and Inactive Ridges 16 1.2.2d Continental Fragments 24 1.2.2e Continental Margins 25 1.2.2f The Ocean Basins 27 1.2.3 Evolution of the Indian Ocean 30 1.2.3a Introduction 30 1.2.3b Fit of the Southern Continents and the Timing of the Initial Breakup of Gondwanaland 30 1.2.3c Pre-Breakup Sea Floor Spreading (140 - 130 m.y.B.P.) 31 1.2.3d Sea floor spreading 130 - 80 m.y.B.P. 31 1.2.3e Sea floor spreading 80 - 53 m.y.B.P. 33 1.2.3f Sea floor Spreading 53 - 32 m.y.B.P. 37 1.2.3g Sea floor Spreading 32 - 0 m.y.B.P. 40 1.2.3h Concluding Remarks 42 1.3 Oceanography - Ocean Currents in the Indian Ocean 43 1.3.1 Introduction 43 1.3.2 Previous Work on Indian Ocean Currents 43 1.3.3 Antarctic Bottom Water (AABW) 44 iv

TABLE OF CONTENTS (cont'd)

1.3.4 Antarctic Circum Polar Current (or Deep Water) 48 1.3.5 Minor Ocean Currents 48 1.3.6 Concluding Remarks 49 1.4 Sedimentation in the Indian Ocean 50 1.4.1 Introduction 50 1.4.2 Sedimentation in the Indian Ocean since the Late Jurassic 51 1.4.2a Pre-Late Cretaceous Sediments 51 1.4.2b Late Cretaceous Sediments 51 1.4.2c Early Eocene Sediments 53 1.4.2d Early Oligocene Sediments 55 1.4.2e Neogene to Recent Sediments 57 1.4.3 Present-Day Sedimentation in the Indian Ocean 58 1.4.3a Present-Day Sediment Distributions 58 1.4.3b Present-Day Sedimentation Rates and Sediment Thicknesses in the Indian Ocean 61 1.4.3c Clay Mineral Provinces of Present-Day Sediments in the Indian Ocean 65 1.4.3d Calcareous Sedimentation in the Oceans and the Distribution of Calcium Carbonate in Present-Day Indian Ocean Surface Sediments 69 1.4.4 Concluding Remarks 73

Section 2 THE GEOCHEMISTRY OF INDIAN OCEAN OSDP BASAL SEDIMENTS 74 2.1 Introduction and Sample Description 75 2.1.1 Basal Metalliferous Sediments in General 75 2.1.2 Indian Ocean DSDP Site Location and Geological Setting, Sample Descriptions 81 2.1.2a Introduction 81 2.1.2b Leg 22 (Darwin - Colombo, 1972) 82 2.1.2c Leg 23 (Colombo - Djibouti, 1972) 87 2.1.2d Leg 24 (Djibouti - Mauritius, 1972) 89 2.1.2e Leg 25 (Mauritius - Durban, 1972) 95 2.1.2f Leg 26 (Durban - Fremantle, 1972) 97 V

TABLE OF CONTENTS (cont'd)

2.2 Bulk Geochemistry of Indian Ocean DSDP Basal Sediments 101 2.2.1 Introduction 101 2.2.2 Geochemical Variations within Indian Ocean DSDP Cores 102 2.2.2a Leg 22 (DSDP Sites 211, 212, 213, 214, 215 and 216) 102 2.2.2b Leg 23 (DSDP Sites 220, 221, 223 and 224) 106 2.2.2c Leg 24 (OSOP Sites 235, 236 and 238) 111 2.2.2d Leg 25 (DSDP Sites 239, 245, 248 and 249) 118 2.2.2e Leg 26 (OSOP Sites 250A, 251A, 254, 256 and 257) 125 2.2.2f General Trends 131 2.2.3 Metal Accumulation Rates 133 2.2.4 Data Handling - Correlation Coefficients and Factor Analysis 138 2.2.4a Introduction 138 2.2.4b Correlation Coefficients 138 2.2.4c Factor Analysis 146 2.2.5 Summary 152 2.3 Geochemical Partition of Indian Ocean DSDP Basal Sediments 156 2.3.1 Introduction 156 2.3.2 Results and Discussion 158 2.3.3 Summary 186 2.4 Discussion of the Geochemistry of Indian Ocean DSDP Basal Sediments 189 2.4.1 Geochemical Comparisons Between Groups of Sediments 189 2.4.1a Introduction 189 2.4.1b Comparison of Indian Ocean DSDP Metal-Rich Sediments with Indian Ocean DSDP Basal Carbonates, Clayey-Carbonates and Clays 189 2.4.1c Comparisons of DSDP Indian Ocean Metal-Rich Sediments with DSDP Metal-Rich Sediments from the Atlantic and Pacific Oceans 192 2.4.2 The Composition of Metal-Rich Sediments from Locations of Special Interest in the Indian Ocean 196 2.4.2a Introduction 196 vi

TABLE OF CONTENTS (cont'd)

2.4.2b Metal-Rich Sediments Associated with Intrusive Sills 196 2.4.2c Metal-Rich Sediments from the Ninety East Ridge 197 2.4.3 Metallogenesis and Regional Geochemical Variations in. the Indian Ocean 202 2.4.3a Introduction 202 2.4.3b Metallogenesis in the Western Indian Ocean 204 2.4.3c Metallogenesis in the Eastern Indian Ocean 208 2.4.3d Concluding Remarks 212 2.4.4 Sources of Metals in Indian Ocean DSOP Basal Sediments 212 2.4.5 Conclusions 218

Section 3. THE GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE 223 3.1 Introduction and Sample Description 224 3.1.1 Previous Geochemical Studies of Sediments from Indian Ocean 224 3.1.2 Geological Setting, Sample Location and Description of Recent Sediments from the Central Indian Ocean Ridge 227 3.1.2a Introduction 227 3.1.2b Geological Setting of the Central Indian Ocean Ridge 228 3.1.2c Valdivia Surface Sediments 238 3.1.2d Lamont-Doherty Geological Observatory Sediments 239 3.1.2e Scripps Institution of Oceanography Sediments 239 3.1.2f General Comments 244 3.1.2g Biogenic Components of Central Indian Ocean Ridge Sediments 245 3.2 Bulk Geochemistry of Recent Sediments from the Central Indian Ocean Ridge 274 3.2.1 Geochemical Variations Across the Central Indian Ocean Ridge 174 3.2.1a Introduction 274 3.2.1b Traverse 1. (230 16'S 66° 50'E to 19° 33'S 71° 48'E) 274

vii

TABLE OF CONTENTS (cont'd)

3.2.1c Traverse • (180 S 620E to 130 50'S to 720 3O'E) 276 3.2.1d Traverse 3. (14030'S 610 30'E to 130S 700 45'E) 276 3.2.1e Traverse 4. (110 30'S 640 30'E to 50 1O'S 700 4O'E) 279 3.2.1f Traverse 5. (100 30'S 6303O'E to SOS 700 30'E) 279 3.2.1g Traverse 6. (70S 630E to 50S 700 5O'E) 279 3.2.1h General Trends 279 3.2.2 Vertical Geochemical Variations 285 3.2.3 Goechemical Variations within the Sample Area 286 3.2.3a Introduction 286 3.2.3b Metal Distributions within the Sample Area 286 3.2.4 Metal Accumulation Rates 303 3.2.5 Data Handling - Correlation Coefficients and Factor Analysis 308 3.2.5a Introduction 308 3.2.5b Correlation Coefficients 308 3.2.5c Factor Analysis 314 3.2.6 Discussion and Summary 319 3.3 Partition Geochemistry of Recent Sediments from the Central Indian Ocean Ridge 323 3.3.1 Introduction 323 3.3.2 Results and Discussion 325 3.3.3 Summary 343 3.3.4 Geochemistry of the Carbonate Phase of Indian Ocean Recent Sediments 345 3.3.4a Introduction 345 3.3.4b Removal of Adsorbed Trace Metals 346 3.3.4c Geochemistry of the Carbonate Material 346 3.4 Discussion of the Geochemistry of Recent Sediments from the Central Indian Ocean Ridge 354 3.4.1 Geochemical Comparisons Between Groups of Sediments 354 3.4.1a Introduction 354 3.4.1b Comparisons of Fracture Zone, Crestal and Non-Crestal Sediments with other Indian Ocean Surface Sediments 354 viii

TABLE OF CONTENTS (cont'd)

3.4.1c Comparisons of Fracture Zone and Crestal Sediments from the Indian Ocean with Surface Sediments from the Pacific and Atlantic Oceans 358

3.4.2 Sources of Metals in Recent Sediments from the Central Indian Ocean Ridge 362 3.4.3 Conclusions 367

Section 4. GENERAL DISCUSSION : THE GEOCHEMISTRY OF

INDIAN OCEAN METAL-RICH SEDIMENTS 370

4.1 Introduction 371 4.2 Geochemical Comparison between Indian Ocean

Surface and Basal Sediments 371 4.2.1 Comparisons of Basal Metal-Rich with Surface Metal-Rich Sediments from the Indian Ocean 371 4.2.2 Comparisons of DSOP Indian Ocean Metal-Rich Sediments with Metal-Rich Sediments from the East Pacific Rise and the Mid-Atlantic Ridge 374 4.3 General Summary : Variations in Metal-Enrichment Processes with Time in Indian Ocean Sediments 378

Bibliography 384

Appendix Al. Sample Preparation and Analytical Procedures 404 A1.1 Sample Preparation 404 A1.2 Analytical Procedures for Analysis by Flame Atomic Absorption Spectrophotometry 404 A1.2.1 Introduction 404 A1.2.2 Bulk Chemical Analysis 405 A1.2.3 Chemical Partition Analysis 406 A1.2.3a Introduction 406 A1.2.3b Chemical Partition Procedures 407 A1.2.3c Analysis of the Leached Residues 408 A1.2.4 De-Ionized Water Washed Samples 409 A1.2.5 Atomic Absorption Spectrophotometry - Method of Operation 410 viv

TABLE OF CONTENTS (cont'd)

A1.2.5a Basic Principle 410 A1.2.5b Configuration 410 A1.2.5c Method of Operation 410 A1.3 Analytical Procedures for Analysis by Flameless Atomic Absorption Spectrophotometry 414 A1.3.1 Introduction 414 A1.3.2 Sample Preparation 414 A1.3.3 Volatile Hydride Method - Flameless Atomic Absorption Spectrophotometry 415 A1.4 Colorimetric Determination of Silica (SiO ) 415 2 A1.4.1 Introduction 415 A1.4.2 Reagents 416 A1.4.3 Sample Preparation 416 A1.4.4 Method of Determination 417 A1.5 Calcium Carbonate Determinations 418 A1.5.1 Introduction 418

A1.5.2 Method of Determination of CaCO3 418 A1.5.3 Results 420

Appendix A2. Precision and Accuracy 421 A2.1 Introduction 421 A2.2 Precision 422 A2.3 Accuracy 424 Appendix A3. Calcium Interference - Effects and Correction Procedure 426 A3.1 Nature of the Interference 426 A3.2 Effect of the Interference 427 A3.3 Method of Correction 427 A3.4 Deuterium Background Correction 430

Appendix A4. Data Handling 437 A4.1 Recalculation of the Element Concentrations 437

A4.2 Calculation of CaCO3 values 438 A4.3 Data Processing 440

TABLE OF TABLES

2.1.2a DSDP Indian Ocean Site Data 84 2.1.2b Indian Ocean DSDP Sample Data 90 2.2.2a DSDP Site Bulk Composition Averages 112 2.2.2b Average Composition of Carbonate-Free Fraction of Indian Ocean DSDP Basal Sediment Groups 132

2.2.3 Metal Accumulation Rates of Indian Ocean DSDP

Basal Sediments 134

2.2.4a Interelement Associations in Indian Ocean DSDP

Basal Sediments 139

2.2.4b Interelement Associations in Indian Ocean DSDP

Metal-Rich Sediments 140

2.2.4c Interelement Associations in Indian Ocean DSDP Basal Carbonates 142

2.2.4d Interelement Associations in Indian Ocean DSDP Basal Clayey-Carbonates 143

2.2.4e Interelement Associations in Indian Ocean DSDP Basal Clays 145

2.3.2 Summary Table of Partial Composition of Major Types of Indian Ocean Basal Sediments 172 2.4.1a Composition of DSDP Sediments from the Indian Ocean 191 2.4.1b Chemical Composition of OSOP Metal-Rich Sediments from the Indian, Pacific and Atlantic Oceans 194 2.4.3a Metal Contents of DSDP Basal Sediments from the Western Indian Ocean 206 2.4.3b Metal Contents of DSDP Basal Sediments from the Eastern Indian'Ocean 209 3.1.2a Station Data of Recent Surface Sediments from the Central Indian Ocean Ridge 230 3.1.2b Frequency of Occurrence of species of Planktonic Foraminifera in the Valdivia Sediments 240 3.1.2c Frequency of Occurrence of other Biogenic Components in the Valdivia Sediments 242 3.2.1a Bulk Chemical Composition of Sediment Groups from the Central Indian Ocean Ridge 284 3.2.4 Metal Accumulation Rates of Indian Ocean Surface Sediments 306 3.2.5a Interelement Associations in Surface Sediments from the Central Indian Ocean Ridge 309 3.2.5b Interelement Associations in Fracture Zone Sediments from the Central Indian Ocean Ridge 310 3.2.5c Interelement Associations in Crestal Sediments from the Central Indian Ocean Ridge 311 xi

TABLE OF TABLES (cont'd)

3.2.5d Interelement Associations in Non-Crestal Sediments from the Central Indian Ocean Ridge 312 3.3.2a Average Partial Compositions of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 333 3.3.4a Metal content of Foraminifera and other Biogenic Components (ppm) 347 3.3.4b Chemical Composition of Foraminifera Species in Central Indian Ocean Ridge Sediments 350 3.3.4c Interelement Associations in Foraminifera Species from Fracture Zone, Crestal and Non-Crestal Sediments from the Central Indian Ocean Ridge 352 3.4.1a Composition of Sediments from Locations in the Indian Ocean 3S5 3.4.1b Composition of Surface Sediments from the Indian, Pacific and Atlantic Oceans 359 4.2.1 Composition of Metal-Rich Sediments from the Indian Ocean 372 4.2.2 Chemical Composition of Indian Ocean DSDP Metal- Rich Sediments and Metal-Rich Surface Sediments from the East Pacific Rise and Mid-Atlantic Ridge 375 A1.2.5c Conditions of Operation of Atomic Absorption Spectrophotometers 412 A2.2 Inter-Batch and Intra-Batch (Analytical) Precision Values for Analyses carried out during this Study 423 A2.3 Concentrations of Major and Trace Elements in International Standards and Control Samples, together with values of Analytical Accuracy Determined for this Study 425 A3.2 Analysis of a Surface Sediment from the Central Indian Ocean Ridge by AAS, with and without the Application of the Ca Interference Correction Procedure 428 A3.4 Results of Use of Ca Interference Standard Correction Method and Deuterium Background Correction Method on Ca Spiked Standards and Sample Solution for the Determination of Cu

by AAS 435 XII

TABLES OF FIGURES

1.2.2a Outline Bathymetry of the Indian Ocean 9 1.2.2b General Structural Features of the Indian Ocean 13 1.2.3c Sea Floor Spreading History of the Indian Ocean, 140 - 130 m.y.B.P. 32 1.2.3d Sea Floor Spreading History of the Indian Ocean, 130 - 80 m.y.B.P. 32 1.2.3e Sea Floor Spreading History of the Indian Ocean, 80 - 53 m.y.B.P. 35 1.2.3f Sea Floor Spreading History of the Indian Ocean, 53 - 32 m.y.B.P. 38 1.2.3g Sea Floor Spreading History of the Indian Ocean, 32 - 0 m.y.B.P. 41 1.3.1 Deep Ocean Currents in the Indian Ocean 45 1.4.2b Sediment Distribution in the Indian Ocean, 75 - 70 m.y.B.P. (Campanian-Maastrichtrian) 52 1.4.2c Sediment Distribution in the Indian Ocean, 53 m.y.B.P. (Early Eocene) 54 1.4.2d Sediment Distribution in the Indian Ocean, 36 m.y.B.P. (Early Oligocene) 56 1.4.3a Present Day Sediment Distribution in the Indian Ocean 59 1.4.3b (i) Sedimentation Rate in mm/103 years in Indian Ocean sediments 62 1.4.3b (ii) Sediment Thickness in km of Indian Ocean Sediments 64 1.4.3c Clay Mineral Provinces in the Indian Ocean 67 1.4.3d Calcium Carbonate Content of Bottom Sediments in the Indian Ocean 72 2.1.2a Location of DSDP sites in the Indian Ocean 83 2.2.2a Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit 8 (Brown Zeolitic Calystone) of DSDP site 212, Wharton Basin, Indian Ocean 103 2.2.2b Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IIb (Intermixed volcanic clay and microcarbonate chalk with discrete beds of volcanic ash) of DSDP site 216, Ninety East Ridge, Indian Ocean 104

xIu

TABLE OF FIGURES (cont'd)

2.2.2c Vertical Distribution, in weight percent, of Element Concentrations in the carbonate-free fraction of basal sediments from Unit III (Brown clay) of DSDP site 221, Arabian Sea, Indian Ocean 107 2.2.2d Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit III (Grey microcarbonate-rich clay nannochalk) of DSDP site 224, Owen Ridge, Indian Ocean 109 2.2.2e Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit III (Nannochalk with zeolites) of DSDP site 238, from the North East end of the Argo Fracture Zone, Indian Ocean 116 2.2.2f Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit II (Brown silty clay and brown nanno clay) of DSDP site 239, Mascarene Basin, Indian Ocean 119 2.2.2g Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IV (Grey- black clay-rich, ferromanganese nannochalk) of DSDP site 245, southern Madagascar Basin, Indian Ocean 121 2.2.2h Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit III (Brown silt-bearing clay) of DSDP site 248, northwest Mozambique Basin, Indian Ocean 123 2.2.2i Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IIIB (Grey and olive black silty claystone) of DSDP site 249, Mozambique Ridge, Indian Ocean 124 2.2.2j Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IV (Brown detrital clay) and Unit V (olive-grey, greenish, olive-black detrital clay) from DSDP site 250A, Mozambique Basin, Indian Ocean 126 2.2.2k Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IV (Black, grey, brownish sandy and silty clay with pebble conglomerates) of DSDP site 254, Ninety East Ridge, Indian Ocean 127

xiv

TABLE OF FIGURES (cont'd)

2.2.21 Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit I (Brownish-grey detrital clay) of OSDP site 256, Wharton Basin, Indian Ocean 129 2.2.2m Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit I (Reddish-brown, laminated coccolith, detrital clay) of DSDP site 257, south east Wharton Basin, Indian Ocean 130 2.2.4a Rotated Factor Loadings Matrix resulting from R-Mode Factor Analysis on data from the carbonate-free fraction of Indian Ocean DSDP Basal Sediments 147 2.2.4b Distribution of Factor Scores for Factor 1 (Mn oxide), Factor 2 (volcanic) and Factor 3 (Biogenic/clay mineral) of Indian Ocean DSDP Sediments 149 2.2.4c Distribution of Factor Scores for Factor 1 (Mn oxide) and Factor 2 (Volcanic) of Indian Ocean DSDP Sediments 151 2.2.4d Distribution of Factor Scores for Factor 1 (Mn oxide) and Factor 3 (Biogenic/Clay Mineral) of Indian Ocean DSDP Sediments 151 2.2.4e Distribution of Factor Scores for Factor 2 (volcanic) and Factor 3 (Biogenic/Clay Mineral) of Indian Ocean DSDP Sediments 151 2.3.2a Distribution of Fe in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 160 2.3.2b Distribution of Mn in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 160 2.3.2c Distribution of Ni in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 164 2.3.2d Distribution of Co in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean OSDP Sediments 164 2.3.2e Distribution of Cr in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 167 2.3.2f Distribution of Cu in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 167

TABLE OF FIGURES (cont'd)

2.3.2g Distribution of Pb in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 171 2.3.2h Distribution of Zn in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 171 2.3.2i Distribution of Li in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 179 2.3.2j Distribution of Ba in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 179 2.3.2K Distribution of Ca in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSOP Sediments 182 2.3.21 Distribution of Al in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSOP Sediments 182 2.3.2m Distribution of Mg in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 185 2.3.2n Distribution of Ti in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments 185 2.4.1a Distribution of Ca, Fe and Al in the carbonate-free fraction of Indian Ocean DSDP Sediments 190 2.4.1b Distribution of Fe, Al and Mn in the carbonate-free fraction of Indian Ocean DSDP Sediments 190 2.4.1c Distribution of Ca, Fe and Mn in the carbonate- free fraction of Indian Ocean DSDP Sediments 190 2.4.1d Distribution of Ca, Fe and Al in the carbonate- free fraction of DSDP Metal-Rich Sediments from the Pacific, Indian and Atlantic Oceans 193 2.4.1e Distribution of Fe, Al and Mn in the carbonate- free fraction of DSDP Metal-Rich Sediments from the Pacific, Indian and Atlantic Oceans 193 2.4.1f Distribution of Ca, Fe and Mn in the carbonate- free fraction of DSDP Metal-Rich Sediments from the Pacific, Indian and Atlantic Oceans 193 2.4.3a Metal Concentration Variations with Time in DSDP Basal Sediments from the Western Indian Ocean 205 2.4.3b Metal Concentration Variations with Time in DSDP Basal Sediments from the Eastern Indian Ocean 210 3.1.2a Location of the sample area within the Indian Ocean 229

xvi

TABLE OF FIGURES (cont'd)

3.1.2b Physiographic Features of the Central Indian Ocean Ridge, 5° to 24°S 234 3.1.2c Location of Sample Stations on the Central Indian Ocean Ridge 235 3.1.2d Structural Features of the Central Indian Ocean Ridge, 5° to 24°S 237 3.2.1b Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 1 (23° 16'S 66° 50'E to 19° 33'S 71° 48'E) across the Central Indian Ocean Ridge 275 3.2.1c Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 2 (18°S 62°E to 13° 50'S 72° 30'E) across the Central Indian Ocean Ridge 277 3.2.1d Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 3 (14° 30'S 61° 30'E to 13°S 70° 45'E) across the Central Indian Ocean Ridge 278 3.2.1e Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 4 (11° 30'S 64° 30'E to 5° 10'S 70° 40'E) across the Central Indian Ocean Ridge 280 3.2.1f Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 5 (100 30'S 63° 30'E to 6°S 70° 30'E) across the Central Indian Ocean Ridge 281 3.2.1g Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 6 (7°S 63°E to 5°S 70° 50'E) across the Central Indian Ocean Ridge 282 3.2.3a Distribution of Calcium Carbonate in surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 287 3.2.3b Distribution of Cadmium in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 288 3.2.3c Distribution of Lead in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5o to 24°S 289 xvii

TABLES OF FIGURES (cont'd)

3.2.3d Distribution of Barium in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 291 3.2.3e Distribution of Cobalt in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge. 5° to 240S 292 3.2.3f Distribution of Lithium in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 293 3.2.3g Distribution of Manganese in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 240S 294 3.2.3h Distribution of Iron in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 295 3.2.3i Distribution of Arsenic in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 297 3.2.3j Distribution of Copper in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 240S 298 3.2.3k Distribution of Zinc in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 50 to 240S 299 3.2.31 Distribution of Magnesium in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 5° to 24°S 300 3.2.3m Distribution of Nickel in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge) 5° to 24°S 301 3.2.3n Distribution of Chromium in the carbonate-free fraction of surface sediments from the Central 0 Indian Ocean Ridge, 5 to 2405 302 3.2.3o Distribution of Aluminium in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 50 to 240S 304 3.2.3p Distribution of Titanium in the carbonate-free fraction of surface sediments from the Central Indian Ocean Ridge, 50 to 240S 305 3.2.5a Rotated Factor Loadings Matrix resulting from R-Mode Factor Analysis on data from the carbonate- free fraction of sediments from the Central Indian Ocean Ridge 315

Xviii

TABLES OF FIGURES (cont'd)

3.2.5b Distribution of Factor Scores for Factor 1 (Basaltic Detritus), Factor 2 (Biogenic/Clay Mineral) and Factor 3 (Volcanic/Hydrothermal) of Fracture Zone, Crestal and Non-Crestal Sediments from the Central Indian Ocean Ridge 317 3.2.5c Distribution of Factor Scores for Factor 1 (Basaltic Detritus) and Factor 2 (Biogenic / Clay Mineral) of Fracture Zone, Crestal and Non-Crestal Sediments from the Central Indian Ocean Ridge 318 3.2.5d Distribution of Factor Scores for Factor 1 (Basaltic Detritus) and Factor 3 (Volcanic/ Hydrothermal) of Fracture Zone, Crestal and Non-Crestal Sediments from the Central Indian Ocean Ridge 318 3.2.5e Distribution of Factor Scores for Factor 2 (Biogenic/Clay Mineral) and Factor 3 (Volcanic/ Hydrothermal) of Fracture Zone, Crestal and Non-Crestal Sediments from the Central Indian Ocean Ridge 318 3.3.2a Distribution of Fe in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 326 3.3.2b Distribution of Mn in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 326 3.3.2c Distribution of Ni in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 326 3.3.2d Distribution of Co in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 330 3.3.2e Distribution of Cr in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 330 3.3.2f Distribution of Cu in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 330 3.3.2g Distribution of Pb in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 337 3.3.2h Distribution of Zn in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 337 3.3.2i Distribution of Li in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 337 xix

TABLE OF FIGURES (cont'd)

3.3.2j Distribution of Ba in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 339 3.3.2k Distribution of Ca in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 339 3.3.21 Distribution of Al in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 339 3.3.2m Distribution of Mg in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 342 3.3.2n Distribution of Ti in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge 342 3.4.1a Distribution of Ca, Fe and Al in the carbonate-free fraction of Indian Ocean Sediments 357 3.4.1b Distribution of Fe, Al and Mn in the carbonate-free fraction of Indian Ocean Sediments 357 3.4.1c Distribution of Ca, Fe and Mn in the carbonate-free fraction of Indian Ocean Sediments 357 3.4.1d Distribution of Ca, Fe and Al in the carbonate-free fraction of Surface Sediments from the Indian, Pacific and Atlantic Oceans 360 3.4.1e Distribution of Fe, Al and Mn in the carbonate-free fraction of Surface Sediments from the Indian, Pacific and Atlantic Oceans 360 3.4.1f Distribution of Ca, Fe and Mn in the carbonate-free fraction of Surface Sediments from the Indian, Pacific and Atlantic Oceans 360 4.2.1a Distribution of Ca, Fe and Al in the carbonate-free fraction of Recent and Basal Metal-Rich Sediments from the Indian Ocean 373 4.2.1b Distribution of Fe, Al and Mn in the carbonate-free fraction of Recent and Basal Metal-Rich Sediments from the Indian Ocean 373 4.2.1c Distribution of Ca, Fe and Mn in the carbonate-free fraction of Recent and Basal Metal-Rich Sediments from the Indian Ocean 373 4.2.2a Distribution of Ca, Fe and Al in the carbonate-free fraction of Indian Ocean Metal-Rich Basal Sediments and Metal-Rich Surface Sediments from the East

Pacific Rise and Mid-Atlantic Ridge 376 XX

4.2.2b Distribution of Fe, Al and Mn in the carbonate-free fraction of Indian Ocean Metal-Rich Sediments and Metal-Rich Surface Sediments from the East Pacific Rise and Mid-Atlantic Ridge 376 4.2.2c Distribution of Ca, Fe and Mn in the carbonate-free fraction of Indian Ocean Metal-Rich Basal Sediments and Metal-Rich Surface Sediments from the East Pacific Rise and Mid-Atlantic Ridge 376 A1.2.5b Configuration of an Atomic Absorption Spectrophoto- meter 411 A1.5.2 Collins Calcimeter Apparatus and Method of Calculation of Wt% CaCO 419 3 A3.3 Example of Correction Graphs used in the Ca Interference Correction Procedure 429 A3.4.1 Results of Use of Ca Interference Standard Correction Method and Deuterium Background Correction Method on Ca spiked standards and sample solutions for Determination of Nickel (Ni) by AAS 432 A3.4.2 Results of Use of Ca Interference Standard Correction Method and Deuterium Background Correction Method on Ca spiked standards and sample solutions for Determination of Cadmium (Cd) by AAS 434 A3.4.3 Diagrammatic Representation of the Comparisons between the Spectra of Hollow Cathode and Deuterium Lamps 429 A4.3 Calcimeter CaCO versus Dymond et al's (1976) 3 CaCO3 Determinations 441 xxi

TABLE OF PLATES

3.1.2a (i) Globigerina bulloides bulloides (d'Orbigny) from station VA-45KH 247 3.1.2a (ii) Surface of G. bulloides bulloides (d'Orbigny) showing the pores and postulae (a), with coccoliths (b) and diatom fragments adhering to it 247 3.1.2b (i) Globigerina praecalida (Blow) from station VA-50GK 248 3.1.2b (ii) Blocky surface of G. praecalida (Blow) showing coccoliths adhering to it, from station VA-73P 248 3.1.2c (i) Globigerina eggeri eggeri (Rhumbler) from station VA-69P 249 3.1.2c (ii) Blocky surface of G. eggeri eggeri (Rhumbler) showing coccoliths (a) and diatom fragments (b) adhering to it 249 3.1.2d (i) Globigerina bradyi (Weitzler) from station VA-50GK 250 3.1.2d (ii) Coccoliths and small plate-like minerals (? clays) adhering to the surface of G. bradyi (Weitzler) 250 3.1.2e (i) Globigerina rubescens (Hoftier) from station VA-80GK 251 3.1.23 (ii) Hexagonal pores containing coccoliths and diatom debris in the test of G. rubescens (Hoftier). Small plate-like minerals present - ? clays 251 3.1.2f (i) Globigerinita gluttinata (Egger) from station VA-55GK 252 3.1.2f (ii) Helmet shaped radiolarian in the valve of G. gluttinata (Egger) 252 3.1.2f (iii) Interior of G. gluttinata (Egger) showing benthonic forams (?) 253 3.1.2f (iv) Coccoliths in the interior of G. gluttinata (Egger) 253 3.1.2g (i) Globigerinoides quadrilobatus immaturus (Le Roy) from station VA-80GK 254 3.1.2g (ii) Hexagonal pores in the test of G. quadrilobatus immaturus (Le Roy) 254 3.1.2h (i) G. quadrilobatus trilobus (Reuss) from station VA-69P 255 3.1.2h (ii) Hexagonal pore surface of G. quadrilobatus trilobus (Reuss) with coccoliths filling the pores 255 xx i

TABLE OF PLATES (cont'd)

3.1.21 (i) G. quadrilobatus sacculifer (Brady) from station VA-80 GK 256 3.1.2i (ii) Surface of G. quadrilobatus sacculifer (Brady) showing cocoliths, diatom fragments and diatom spicules (?) 256 3.1.2j (i) G. ruber (d'Orbigny) from station VA-S5GK 257 3.1.2j (ii) Pores of the tests of G. ruber (d'Orbigny) showing radiolaria, diatoms, coccoliths and plate-like minerals-clays (?) 257 3.1.2j (iii) Pores of the tests of G. ruber (d'Orbigny) showing radiolaria, diatoms, coccoliths and plate-like minerals-clays (?) 258 3.1.2k (i) Globorotalia pumilio (Parker) from station VA-69P 258 3.1.2k (ii) Postulae on the test of G. pumilio (Parker), showing coccoliths and diatom fragments 259 3.1.21 (i) G. cultrata menardii (Parker, Jones & Brady) from station VA-69P 259 3.1.21 (ii) Crystalline structure of the test of G. cultrata menardii (Parker, Jones & Brady) with coccoliths adhering to it 260 3.1.2m (i) G. fimbriata (Brady) from station VA-45KH 260 3.1.2m (ii) Fragment, interior of G. fimbriata (Brady) 261 3.1.2m (iii) Fragment, interior of G. fimbriata (Brady), with coccoliths and clay minerals 261 3.1.2n (i) G. tumida (Brady) from station VA-55GK 262 3.1.2n (ii) Surface of G. tumida (Brady), showing postulae and coccoliths 262 3.1.2o (i) Hastigerina siphonifera (d'Oribgny) from station VA-69P 263 3.1.2p (i) Orbulina universa (d'Orbigny) from station VA-45KH 263 3.1.2p (ii) Surface of 0. universa showing coccoliths, fragments of diatom frustules, diatom spines 264 3.1.2p (iii) Surface of 0. universa , station VA-80GK, showing solution marks 264 3.1.2p (iv) Orbulina universa (d'Orbigny) from station VA-80GK 265 3.1.2p (v) Surface of 0. universa, station VA-80GK, showing solution marks 265 TABLE OF PLATES (cont'd)

3.1.2q (i) Pullēniātina'ObliqūilOCOlata (Parker; Jones and Brady) from station VA-69P 266 3.1.2q (ii) Surface of P. obliquiloculata [Parker, Jones and Brady) showing dendritic solution marks, following crystal face edges, and coccoliths 266 3.1.2q (iii) Surface of P. obliquiloculata (Parker, Jones and Brady) showing dendritic solution marks, following crystal face edges, and coccoliths 267 3.1.2q (iv) Pulleniatina obliquiloculata (Parker, Jones and Brady) from station VA-46GK 267 3.1.2q (v) Surface of P. obliquiloculata (Parker, Jones and Brady) showing postulae, fragments of diatoms and coccoliths 268 3.1.2r (i) Sphaeroidinella dehiscens (Parker and Jones) from station VA-80GK 268 3.1.2r (ii) Surface of S. dehiscens (Parker and Jones) showing pores 269 3.1.2r (iii) Surface of S. dehiscens (Parker and Jones) showing diatom fragments, coccoliths and plate-like clay minerals 269 3.1.2r (iv) Surface of S. dehiscens (Parker and Jones) with coccoliths 270 3.1.2s (i) Radiolaria from station VA-45KH 270 3.1.2s (ii) Close up of diatom, Stephanodiscus stephanodiscus 271 3.1.2t (i) Radiolaria from station VA-69P 271 3.1.2t (ii) Surface of radiolaria from station VA-45KH showing diatom fragments 272 3.1.2u (i) Coccoliths on surface of iron oxide fragment from station VA-73P 272 3.1.2u (i) Coccoliths on surface of iron oxide fragment from station VA-73P with diatom spines and clayey material 273 3.1.2u (ii) Coccoliths and diatom fragments on interior surface of unknown foraminifera from station VA-45KH 273 9

Section 1

1. GEOLOGY,OCEANOGRAPHY AND SEDIMENTATION IN THE INDIAN OCEAN

1.1 INTRODUCTION

1.2 GEOLOGY AND EVOLUTION OF THE INDIAN OCEAN

1.3 OCEANOGRAPHY - OCEAN CURRENTS IN THE INDIAN OCEAN

1.4 SEDIMENTATION IN THE INDIAN OCEAN 2

1.1 INTRODUCTION

Studies of the geochemistry of pelagic sediments have indicated that they contain higher concentrations of certain elements than can be attributed solely to continental run off (Goldschmidt, 1938; Rubey, 1951; Wedepohl, 1960; Turekian and Wedepohl, 1961). The source of the additional amounts of these elements is still a matter of some controversy. However, it is now generally recognised that not all the enriched elements are supplied by the same process and furthermore that enrichment of a particular element may result from several different processes (Cronan, 1976; Heath and Dymond, 1977). The general view is that for ridge crest metal-rich sediments, such as those dealt with in this thesis, processes related to ridge crest volcanic activity, i.e. hydrothermal processes, are the main source of the majority of enriched elements in these sediments.

The view of a volcanic source for enriched elements in ocean floor sediments is not a new one. As early as 1878, von Gumbel postulated such a source for the Mn in manganese nodules. Similar explanations have been presented since that date to account for enrichments of particular elements in a variety of sediments (e.g. Arrhenius and Bonatti, 1965; Skyornyakova, 1964; Bostrhm, 1967). In 1966, Bostrhm and Peterson carried out the first really detailed study of metal-rich sediments from an active oceanic ridge. They attributed the enriched concentrations of ,Fe, Mn, Cu, Cr, Ni and Pb they found in East Pacific Rise sediments to mantle exhalations and deep seated magmatic processes.

Since 1966, numerous papers have appeared reporting occurrences of metal-rich surface sediments from the active mid-ocean ridge system throughout the world oceans (Bostrhm and Peterson, 1969; Bostrhm et al., 1969; Fisher and Bostrhm, 1969; Horowitz, 1970; Chester and Messiha- Hanna, 1970; Bender et al., 1971; Veeh and Bostr8m, 1971; Dasch et al., 1971; Cronan, 1972; Bruty et al., 1972; Aston et al., 1972a, b; Piper, 1973; Dymond et al., 1973; Horowitz, 1974a, b; Heath and Dymond, 1977). The majority of work on ridge-crest metal-rich sediments has been conducted on the East Pacific Rise and Mid-Atlantic Ridge. Until now, comparatively few data have existed on the occurrence and nature of metal- rich sediments from the active portions of the mid-ocean ridge system in the Indian Ocean. 3

Since the advent of the Deep Sea Drilling Project (DSDP) metal- rich sediments have been reported overlying basaltic basement that is no longer near the crest of an active mid-ocean ridge. They are fairly widespread in distribution, and have been reported from the Pacific Ocean by von der Borch and Rex (1970), von der Borch et al (1971), Cook (1971), Heath and Moberly (1971), Cronan et al (1972), Cronan (1973), Cronan and Ganett (1973), Dymond et al (1973, 1976), Swanson and Scott (1974), Bostr8m et al (1976), Cronan (1976), Dymond et al (1977); from the Atlantic Ocean by Peterson et al (1970), Bostrbm et al (1972), Horowitz (1974a), Horowitz and Cronan (1976) and Chester et al (1976); and from the Indian Ocean by Cronan et al (1974), Fleet and Kempe (1974), Marchig and Vallier (1974), Pimm (1974), Warner and Gieskes (1974), Gieskes et al (1974) and Fleet (1977)- Comparisons between the surface and basal metal-rich sediments have shown them to be chemically similar, and this, together with their widespread occurrence, has led to the now well supported theory that the basal metal-rich sediments represent the ancient analogues of the crestal metal-rich sediments which have been moved to their present locations by processes of sea floor spreading (Peterson et al, 1970; von der Borch and Rex, 1970; Cook, 1971; Bostrbm et al, 1972; Cronan et al., 1972; Cronan, 1974; 1976; Horowitz, 1974a; Horowitz and Cronan, 1976).

In general, basal and surface metal-rich sediments are enriched relative to normal pelagic sediments in Fe, Mn, Ba, Mg, Ni, As, Cr, Cu, V, Ag, Tl, Zn, Pb, Hg, Cd, U and B, whilst they are depleted, or show average concentrations of Si, AI, Ti and Mo (Bostr8m and Peterson, 1966; 1969; Bostr8m and Fisher, 1969; Bostrbm and Valdes, 1969; Cronan, 1969, 1972; Horowitz, 1970, 1974a). They differ from normal pelagic sediments only in the concentrations of the elements that they contain, Horowitz (1974a)having suggested that the method of entrainment of the metals in them may not necessarily be different fran that in normal pelagic sediments. The contribution of enriched elements from volcanic sources may be locally significant in these sediments. However, taken in the context of an entire ocean this contribution amounts to probably less than 10% of the total metal content of the sediments.

Compositional variations between metal-rich sediments from different oceans have been widely recorded (Bostr8m et al., 1969; Cronan, 1972; Horowitz, 1970; Bostr8m and Valdes, 1969) as have variations along a 4

single section of mid-ocean ridge (Horowitz, 1974a). Cronan (1974) has suggested that this reflects the localised nature of metal enrichment along sections of the mid-ocean ridges. Such compositional variations reflect the variation in amount of detrital input into the three major ocean basins, the Atlantic Ocean having the highest detrital sedimentation rate, the Pacific Ocean the lowest and the Indian Ocean an intermediate value (K u et al., 1968). The compositional variability of the sediments probably also reflects the variations in ridge activity, i.e. spreading rates; the East Pacific Rise being the most active, the Mid-Atlantic Ridge the least active and the Indian Ocean Ridges having intermediate spreading rates. Cronan (1974) has suggested that such compositional variations may also reflect variations in basalt chemistry between the ridges.

The occurrence of metal-rich sediments on areas of the ocean floor which are at present unassociated with high heat flow and volcanic activity, e.g. Ninety East Ridge (Fleet and Kempe, 1974; Fleet, 1977), Rio Grande Ridge (Bostrtim et al., 1972), the Lau Basin (Bertine, 1974), over sills in North Atlantic (DSDP site 138) (Horowitz, 1974a; Horowitz and Cronan, 1976) has led to the view that the originally suggested association of metal-rich sediments with mid-ocean ridges is not necessary but that their occurrence in close association with basaltic material is of greater importance (Horowitz, 1974a).

The first theory proposed to explain the metal enrichments in metal- rich mid-ocean ridge crest sediments was deep seated hydrothermal exhalations. Bostr3m and Peterson (1966, 1969) and Bostr8m et al (1969) suggested that such metals were derived from the lower crust or upper mantle. Although it is now recognised that a single element can be supplied to the sea floor by more than one process (Cronan, 1976; Heath and Dymond, 1977) the main process of metal enrichment is considered to be that which is associated with ridge crest volcanic activity, i.e. hydrothermal processes. Although metal enrichment has been suggested by such processes as extraterrestrial activity (Arrhenius, 1963), post. depositional re-mobilisation of elements such as Mn, Ni, V, etc. (Bostrtim, 1967; Bonatti et al., 1971), continental run off and ice rafting (Horowitz, 1974a; Aston et al, 1972c),the addition of metals from such sources may be of very minor importance in comparison to metal additions by hydrothermal processes along the mid-ocean ridges. 5

The exact nature of the hydrothermal processes of enrichment is still a matter of some debate. Bostr8m and his co-workers (Bostr8m and Peterson, 1966; 1969; Bostr8m et al., 1969; 1972; 1974; 1976) consider that the metals are derived from deep seated, lower crustal or upper mantle sources and are transferred to the surface in magmatic solutions which debouch along the ridge crest as a result of the same processes which lead to sea floor spreading. By contrast several authors have emphasised the importance of the reactions of seawater and the ocean floor rocks at elevated temperatures (Corliss, 1971; Elderfield et al., 1972; Piper, 1973; Hart, 1973a, b; Horowitz, 1974a; b; Cronan, 1976; Heath et al, 1977). This causes the leaching of the metals from the basalts, followed by their precipitation as Fe and Mn oxyhydroxides as the acidic, higher temperature hydrothermal fluids debouch on to the ocean floor and mix with the more alkaline, lower temperature sea water. The alteration of ocean floor basalts may also occur at lower or more normal temepratures, as a volcanic weathering process which may lead to metal enrichment in ocean floor sediments (Piper, 1973; Bertine, 1974). Such a lower temperature alteration process may be of significance in the absence of a relationship of the metal-rich sediments with high heat flow as occurs in the Lau Basin (Bertine, 1974) and above a sill as at DSDP Site 138 in the N. Atlantic Ocean (Horowitz, 1974a).

Bostr8m et al (1972) have shown that while it is possible to account for the Fe anomalies observed in metal-rich sediments by the leaching of tens of metres of ocean floor basalt; the amount of basalt required to account for the observed Mn anomalies is so much greater as to make it appear unlikely that the ocean floor basalts represent the only source of elevated Mn values in metal-rich sediments. The suggestiOn that elements may be supplied by different sources has been demonstrated for Fe and Mn in East Pacific Rise basal sediments by Cronan and Garrett (1973) using chemical partition techniques. Similar findings have been reported for other elements in the Pacific (Cronan, 1976; Dymond et al., 1973; 1977; Heath and Dymond, 1977) as well as for many elements in other areas (Chester et al., 1976; Horowitz and Cronan, 1976; Fleet, 1977). These studies have underlined the importance of chemical partition studies in the understanding of the sources and processes of enrichment of metals in metal-rich sediments as well as normal pelagic sediments. 6

It would seem probable, based on the evidence of chemical partition studies, that proportions of certain elements, e.g. Cu, Mn, Pb, Zn, are scavenged from sea water and co-precipitated with authigenic mineral phases such as FeMn oxides in metal-rich sediments and may also be adsorbed onto the surfaces of clay minerals (Chester and Messiha-Hanna, 1970; Cronan and Garrett, 1973; Sayles and Bischoff, 1973; Horowitz, 1974a, b; Cronan, 1976; Chester et al., 1976). Furthermore, a biological source of certain enriched elements e.g. Pb, Hg, Cd, in metal-rich sediments may also be probable due to the well-known association of the elements with calcium carbonate-rich shell material (Aston et al., 1972a; Cronan and Garrett, 1973; Horowitz, 1974a; Oldnall, 1975; Bostr8m et al, 1974). Certain trace elements are essential to the life processes of marine organisms (Bertine and Goldberg, 1972) which incorporate these elements into their hard (carbonate and/or siliceous) and soft parts. When the organisms die, their tests are incorporated into ocean floor sediments thus providing a pathway for enrichment of certain metals into the sediments. Although in mid-ocean ridge sediments the contribution from biological sources may be of minor importance in comparison to the contribution of metals from hydrothermal sources; for certain elements and in areas of high carbonate sedimentation rates, a biological source may provide significant proportions of a metal's overall concentration.

In order to study further the nature of active ridge derived metal enrichments in sediments and in order to aid in the clarification of some of the difficulties involved in processes of metal enrichment, two sets of samples, each related to different parts of the Mid-Indian Ocean Ridge System, have been investigated in detail. The first set is a group of carbonate-rich sediments from the crest of an active ocean ridge in the western Indian Ocean - the Central Indian Ocean Ridge; and the second set is a group of basal sediments collected by the DSDP during five separate cruises and related to sea floor spreading processes about several segments of the Mid-Indian Ocean Ridge system.

The majority of the work to date on basal metalliferous sediments recovered by the DSDP has been carried out on material collected from the Pacific and, to a lesser extent, the Atlantic oceans. The few studies which have been completed are in the nature of a general reconn- aissance, and have mostly only been concerned with the bulk geochemistry 7

of the deposits. On the present sediments, geochemical partition studies have been carried out in order to provide data on the methods of entrainment of the enriched metals contained in the sediments. The results of the bulk and partition geochemical analyses provide data on the variations in metal enrichment processes through time and show how such processes can be related to the overall geological evolution of the Indian Ocean since the Early Cretaceous.

Geochemical studies on present-day active ridge crest surface sediments, have in general been concerned with widely spaced samples, for a limited group of elements. Although investigations of active ridge crest metal-rich sediments have been carried out since 1966, in this thesis for the first time are described a set of carbonate-rich ridge crest sediments from the northern portion of the Central Indian Ocean Ridge, which is an actively spreading portion of the Mid-Indian Ocean Ridge System. The geochemistry of these sediments has been studied using both bulk and partition geochemical techniques. The geochemical partition data provide information on the methods of entrainment of the enriched metals in the sediments.

The surface sediments studied in this work were collected from stations above and below the lysocline. The bulk and partition geochemical data on these carbonate-rich sediments provide information on the effects of calcium carbonate dissolution in the oceans on the trace element composition of the sediments and upon the biological contribution to the metal concentrations of active ridge surface metal-rich sediments.

Comparisons between the basal and surface metal-rich sediments, in terms of their bulk and partition geochemistry have allowed some general comments to be made on the variations in processes of metal enrichments through time in the Indian Ocean.

1.2 GEOLOGY AND EVOLUTION OF THE INDIAN OCEAN

1.2.1 Introduction

The Indian Ocean is the smallest, tectonically the most complex, and still the least geologically understood of the world's three major oceans. The International Indian Ocean Expedition (110E), 1959 -- 1966, provided the first opportunity to study the geophysical and geological features of this ocean. This expedition was followed by the publication of several articles dealing with the structural features and evolution of the Indian Ocean (Heezen and Tharp, 1965; Laughton et al., 1971, 1972; McKenzie and Sclater, 1971, 1973), some of which used the results of the IIOE, as well as other geophysical and geological data. The results of the IIOE were published together in the 'Geological and Geophysical Atlas of the Indian Ocean' (Udinstev et al., 1975).

The results of the IIOE, although providing a careful overall. picture of the geology of the Indian Ocean have left the origin of many of the ocean's geological features little understood. The ability to drill into, and recover samples of, the basaltic rocks and overlying Sediments of the oceanic crust of the Indian Ocean during legs 22 to 27 of the DSDP has greatly improved our knowledge of the evolution of the Indian Ocean. However, much more evidence is still required if many ambiguous and contradictory data are to be understood. A broad structural outline of the Indian Ocean and its geological evolution is given below, with particular emphasis being placed upon those features which relate to the development of metal-rich sediments in the Indian Ocean.

1.2.2 Structural Features of the Indian Ocean.

1.2.2a. Introduction and Geographical Setting (see Fig. 1.2.2a, 1.2.2b),

The Indian Ocean is bounded to the south by the wide Southern Ocean, beyond which lies the passive continental margin of Antarctica. To the east and west the Indian Ocean is bounded by the stable and inactive continental margins of Australia and Africa respectively. On the latter side the continental margin is downwarped (Laughton et al., 1971, 1972) 9

• E: 20°1•: o°E 60°F 8o°1•: 100

Fig. 1 ._ %t Out 1 ine flathvmt•t t of the Indian 0ceal Map after l.aughton et al , 1971_ 1000 and 4000 met re contours shown

■

chelles

ascare a ~ r to n

Platea

0 Crozet Plateau Kerguel Platea 10

where considerable thicknesses of sediment exist off East Africa. To the north, the Indian Ocean is surrounded by the active plate boundaries of the Red Sea - Gulf of Aden Complex, the uplifted Himalayan Mountain Belt to the north of India and the Sumatra and Java trenches of the Indonesian island arc complex.

Many of the existing problems with respect to the evolution of the Indian Ocean relate to its unusual structural and morphological features. The Indian Ocean is unusual amongst the three major oceans in a number of respects. It has a highly fractured and segmented mid- ocean ridge system (Fisher and Sclater, 1971; Francis and Raitt, 1967; Weissel and Hayes, 1971; Bergh, 1971; Sclater and Harrison, 1971; McKenzie and Sclater, 1971) and it has a large number of aseismic areas, continental fragments and alike, such as Madagascar, the Seychelles Bank, the Broken Ridge, the Kerguelen - Heard Plateau, the Agulhas Plateau, the Ninety East Ridge, the Chagos-Laccadive Ridge, the Mozambique, Madagascar and Rodriguez Ridges, the Naturaliste and Wallaby Plateaux, etc. There is also a lack of active trenches, unlike in the Pacific Ocean which is almost completely surrounded by them (McKenzie and Sclater, 1971). The northern active margin of the Indian Ocean is also character- ised in part by downwarping of the crust and the accumulation of great thicknesses of sediments, as in the Arabian Sea and Bay of Bengal (Ewings et al., 1969; Curray and Moore, 1971). The nature and origin of these features is discussed below.

1.2.2b. The Mid-Ocean Ridge System.

The mid-ocean ridge system (MORS) of the Indian Ocean has been defined on the basis of shallow seismic activity (Laughton et al., 1971, 1972), and detailed bathymetric, magnetic and geological studies (Fisher et al., 1971; McKenzie and Sclater, 1971; Sclater and Fisher, 1974). It is highly segmented in contrast to the East Pacific Rise (Menard, 1967) and it is principally on the basis of fracture zones that the MORS is divided into five major segments. In the north, in the Gulf of Aden occurs the Sheba Ridge. This ridge is composed of a series of ridge segments of WNW/ESE or E/W trends, with a pronounced median valley, which are offset in a complicated pattern by a number of NE/SW trending fracture zones by as much as 18Okm. (Laughton et al., 1971). Earthquake epicentres cluster along the ridge segments and the fracture zones (Sykes, 11

1968). The magnetic anomaly associated with the median valley is most pronounced in the Gulf of Aden. High heat flow measurements in this region are also common (Laughton et al., 1971). The association in the Gulf of Aden of the Sheba Ridge and its continuation into the Afar Depression, with the East African Rift Valley and the Red Sea is clear (Laughton et al., 1971). However, the detailed mechanism which relates these features is not clear and consequently opinions differ as to their origin. However, seismic refraction data have shown that the whole of the Gulf of Aden is oceanic and probably formed as a result of the separation of Arabia and Africa along the Sheba Ridge (Laughton et al., 1971). The Sheba Ridge is joined to the second segment of the MORS, the Carlsberg Ridge, by the Owen Fracture Zone. The Owen Fracture Zone has a right lateral motion (Heezen and Tharp, 1965) and offsets the ridge crest by 300km (Laughton et al., 1971). This feature, trending NE/SW, is weakly seismically active and has a number of sea mounts distributed along its length (McKenzie and Sclater, 1971). The seismically inactive Chain Ridge is believed to form the southwesterly extension of the Owen Fracture Zone. The Carlsberg Ridge, is an actively spreading portion of the MORS, is seismically active and trends SE/NW (Matthews et al., 1965; McKenzie and Sclater, 1971). It has a pronounced median valley with a related central magnetic anomaly (Laughton et al, 1971; McKenzie and Sclater, 1971) and is offset by a few fracture zones which have a NE/SW trend, are seismically active and have a right lateral motion (Matthews et al, 1965; McKenzie and Sclater, 1971). At the south-eastern end the fracture zones and deeps, trending NE/SW, destroy the continuity of the median valley and its central magnetic anomaly (Laughton et al., 1971), and form the link with the third segment of the MORS.

The Central Indian Ocean Ridge, with its general N/S trend, extends 0 from about the equator to the triple junction at 22 S (Laughton et al, 1971). It is offset by a large number of fracture zones and has attracted much interest in recent years (Heezen and Tharp, 1965; Fisher et al.,

1967, 1971; McKenzie and Sclater, 1971; 1973; Laughton et al., 1972). The Central Indian Ocean Ridge was described on the basis of shallow seismic data as having a general N/S trend (Heezen and Tharp, 1965). However, the bathymetric and magnetic data indicate that it is composed of a series of NW/SE trending ridge segments offset by several NNE/SSW trending fracture zones (Fisher et al., 1971; McKenzie and Sclater, 1971). 12

The ridge segments, which are actively spreading at about 2.3cm/yr, have well developed magnetic anomalies associated with them, together with a central rift zone, although this is not so well-developed as in the deep transverse valleys associated with the offsetting fracture zones (Fisher et al, 1971; McKenzie and Sclater, 1971). Between the equator and the triple junction at 22°S, the ridge axis is offset by fifteen or sixteen fracture zones all of which have a roughly NNE/SSW trend. With the exception of two of these, L-L' and M-M' (Fig. 3.1.2d) which have left lateral offsets, all have right lateral offsets. Several of these features have been named by Fisher et al (1971) and McKenzie and Sclater (1971). The Vema Fracture Zone (10°S) in which the maximum depth in the Indian Ocean of 6402m was recorded (Laughton et al., 1971) offsets the ridge axis by 180 - 20Okm. The Argo Fracture Zone (15°S) offsets the ridge crest by 90km (Fisher et al, 1971), while the Marie Celeste Fracture Zone (17°S) offsets the ridge crest by 300km, and the Rodriguez Fracture Zone (20°S) offsets the ridge crest by 100km (McKenzie and Sclater, 1971). In the south of this portion of the ridge, the Rodriguez Ridge occurs as a rare E/W trending feature. Bathymetric and magnetic data suggest that this feature is later than, and probably not related to, the tectonics which formed the Central Indian Ocean Ridge (Fisher et al., 1971). Its origin is not clear although it may be related to the evolution of the triple junction in some complicated way, however Fisher et al (1967), have suggested that it is a volcanic feature, formed by eruptions which ended 1.5 m.y B.P. The age of the initial volcanism remains unknown (Fisher et al., 1967). The Central Indian Ocean Ridge is linked to the fourth and fifth segments of the MORS through the triple junction to the east of Mauritius and Reunion, at about 26°S. This feature is a typical R.R.R. triple junction (McKenzie and Morgan, 1969) and is probably the best example in the world of such a feature (McKenzie and Sclater, 1971). The development of the triple junction has been fully described by McKenzie and Sclater (1971) and probably was formed about 36 m.y. B.P. by an alteration in the direction of relative movement of the African and Indian plates. This was coincident with the formation of the Central Indian Ocean Ridge from the Chagos Fracture Zone (McKenzie and Sclater, 1971, 1971) (see below for more detailed description).

The fourth segment of the MORS - the South West Indian Ocean Ridge - is believed to be actively spreading (Luyendyk and Davies, 1974), from Fig. 1.2.2b. General Structural Features of the Indian Ocean. Map shows the approximate locations of actively spreading ridge segments, 30°E 50°E 70°E 90°E 110°E 130°E major fracture zones, some major magnetic anomalies and earthquake epicentres. Map after Luyendyk and 20°N Davies (1974) and Sclater and Fisher (1974)

10°N

LEGEND

Major Fracture Zones 20°S // Active Ridge Segments

30°s Major magnetic anomalies

0 Earthquake Epicentres

4000 metre contour shown

20°E 40°E 6o°E 8o°E 100°E 120°E

C.4 14

the evidence of earthquake epicentres (McKenzie and Sclater, 1971), sediment distribution (Ewings et al, 1969) and bathymetry (Heezen and Tharp, 1965). This is the least known segment of the MORS, it has no central magnetic anomaly associated with the median valley and it has a ragged topography (Laughton et al, 1971). The ridge crest probably trends ENE/WSW and is offset in a left lateral sense by NNE/SSW trending fracture zones, such as the Mozambique, Prince Edward and Malagassy fractures (Heezen and Tharp, 1965; Sykes, 1970; Bergh, 1971; Schlich and Patriat, 1971; Luyendyk and Davies, 1974). The fractures are probably sub-parallel to or continuous with such features as the eastern margins of the Mozambique and Madagascar Ridges (Heezen and Tharp, 1965; Laughton et al, 1971; Fleet, 1974). The spreading rate and age of the South West Indian Ocean Ridge are not accurately known (Luyendyk and Davies, 1974). Schlich and Patriat (1971), Bergh (1971), McKenzie and Sclater (1971) and Sclater and Harrison (1971) have all calculated spreading rates, from the identification of magnetic anomalies, of between 0.6 and 1.0 cm/yr in a northerly direction. The rates are however, difficult to determine accurately due to the discontinuous and fractured nature of the magnetic anomalies (Laughton et al, 1971; Luyendyk and Davies, 1974). McKenzie and Sclater (1971) suggest that the South West Indian Ocean Ridge originated about 35 m.y. B.P. at the same time as the last period of spreading on the Carlsberg and Central Indian Ocean Ridges began. However, it seems more probable that the South West Indian Ocean Ridge did not begin actively spreading until about 20 m.y.B.P.

To the east of the Triple Junction lies the fifth segment of the MORS - the South East Indian Ocean Ridge. The South East Indian Ocean Ridge is divisible into two portions. The first portion, to the west of the Amsterdam Fracture Zone, in the vicinity of the Amsterdam and St. Pauls Islands, resembles the Carlsberg and Mid-Atlantic Ridges (Laughton et al, 1971). The ridge crest is aligned NW/SE (Sclater et al, 1976). There is a central magnetic anomaly, high heat flow and the area is reported as being barren of sediments. Fresh basalts have been dredged from it (Laughton et al, 1971). On the basis of magnetic anomalies the ridge is reported to be spreading at a rate of 2.5cm/yr. (Sclater et al., 1976). Further to the souti-. and east in the vicinity of the Amsterdam and St. Pauls Islands the ridge is narrower, steepsided and more flat-topped (Francis and Raitt, 1967) than a typical mid-ocean ridge (Menard, 1967; Sclater and Harrison, 1971). This area is reported as being seismically active, in a NE/SW direction, perpendicular to the ridge crest (Schlich et al, 1974), rather than in a N/S 15

direction as reported by Laughton et al (1971). The Amsterdam Fracture Zone itself accommodates differential movement along the South East Indian Ocean Ridge (Le Pichon and Heirtzler, 1968) and may displace the crest by up to 1100km (Heezen and Tharp, 1965). The second portion of the South East Indian Ocean Ridge, to the east of the Amsterdam Fracture Zone, has a smoother and broader topography than the area to the west (Heezen and Tharp, 1965). In this respect it is similar to the East Pacific Rise (Laughton et al, 1971), and furthermore it would appear to be a fast spreading feature, probably at a rate of 4.5 - 6.4cm/yr. (Johnson et al, 1976). The ridge crest is aligned NW/SE and is offset by four major dislocations in this area. No evidence of a median valley is reported by Laughton et al (1971). Between 105°E and 140°E, to the south of Australia, the ridge crest is reported as spreading asymmetrically to the north (Weissel and Hayes, 1971). The ridge is linked to the Pacific- Antarctic Ridge to the south of Australia.

Analyses of rocks recovered from the MORS of the Indian Ocean (Laughton et al, 1971 and included references) have suggested that there are no essential differences between the ocean floor rocks of the Indian Ocean and other deep oceans. The basaltic lavas recovered from the MORS belong to the now widely recognised low K2O oceanic tholeiite group of basalts. Drilling on the flanks of the Carlsberg, Central, South West and South East Indian Ocean Ridges as part of the DSDP (von der Borch et al, 1974; Whitmarsh et al, 1974; Fisher et al, 1974; Simpson et al, 1974; Davies et al, 1974) has confirmed these observations and has provided additional evidence of the existence of such features as separate lava flows, pillow structures, inter- lava sediment layers; fractured lava surfaces, hydrothermally altered zones etc.

The MORS of the Indian Ocean, throughout its length, is offset by a large number of fracture zones (McKenzie and Sclater, 1971). Such fracture zones may expose layers 2 and 3 of the lower crust and upper mantle (Laughton et al, 1971; Engel and Fisher, 1975). Laughton et al (1971) report a wide variety of rocks dredged from the Indian Ocean fracture zones, and locations of ultrabasic rocks from the fracture zones of the MORS have been reviewed by Udintsev and Dimitriev (1975)- Engel and Fisher (1975) have reported on a wide range of both basaltic, mafic and ultramafic, intermediate and granitic rocks which they have dredged from fracture zones of the Central and South West Indian Ocean 16

Ridges. They report that differentiated rocks (e.g. lherzolite harzburgite, gabbros, Ti-ferro gabbros, anorthosites and Na-rich trondhjemites)occur below supra-crustal rocks which are similar in every respect to tholeiites from the crest of the MORS. They have suggested that most of the magma generated beneath the crest of the mid-ocean ridge may not reach the surface, but be trapped beneath a carapace of supra-crustal basalts and may develop into differentiate rocks such as those dredged from the fracture zones. It would appear therefore that such fracture zones, at depths of more than 4,500m act as 'windows' into layers 2 and 3 of the lower crust and mantle (Engel and Fisher, 1975). Furthermore, where such ultramafic rock types are associated in fracture zones with sulphide mineralisation, as in parts of the Central Indian Ocean Ridge (Rozanova and Baturin, 1971; Baturin and Rozanova, 1975; see also Section 3) it would appear that metallogenic processes may be concentrated along such lines of dislocation as these fracture zones (Bonatti, 1975). The occurrence of sulphide mineralisation in fracture zones has been previously reported in the equatorial Atlantic Ocean (Bonatti et al, 1976a, b, c). An understanding of such processes within fracture zones is of importance in any study of ridge crest metal- rich sediments.

1.2.2c. Aseismic and Inactive Ridges.

The Indian Ocean is notable for the presence of a large number of relatively shallow features whose common factor is that they are free from seismic activity. These features have been grouped together in the past as 'microcontinents' (Heezen and Tharp, 1965), whereas the geophysical and geological data suggest that they represent a variety of features whose only common factor is their lack of seismic activity. They can be grouped broadly into those which are elongated, generally in a N/S direction (Mozambique and Madagascar Ridges, the Chagos-Laccadive Ridge, the Mascarene Plateau and the Ninety East Ridge) and those which are more equidimensional or have different trends (Broken Ridge and Naturaliste Plateau, the Kerguelen Plateau and Heard Ridge, the Wallaby Plateaux and the Agulhas and Crozet Plateaux). MOZAMBIQUE AND MADAGASCAR RIDGES Little is known about Mozambique and Madagascar Ridges which run almost N/S and intersect the South West Indian Ocean Ridge (Langhton et al, 1.971). Both have scarps on their eastern sides and early work has suggested that they may relate to fracture zones which offset the South West Indian Ocean 17

Ridge (Heezen and Tharp, 1965), or that on the basis of their relationship to continental blocks they may be continental fragments (Laughton et al, 1971). Drilling as part of the DSDP at Sites 246/7 and 249 on the Madagascar and Mozambique Ridges respectively, coupled with other seismic reflection and geological data, led Schlich et al (1974) to consider the Madagascar Ridge as not having a continental structure and therefore not being an extension of Madagascar. It seems probable from bathymetric and magnetic data, that the Madagascar Ridge is the south western continuation of the Madagascar Basin, although the reason for its existence as a topographic high is not known (Schlich et al, 1974). Deitrick et al (1977) reported that the subsidence history of this feature has been complicated, and although it may have been in greater depths at one time, uplift has left it unusually shallow for its age. The origin of the Mozambique Ridge is still uncertain (Schlich et al, 1974). However, based on evidence from DSDP Site 249, the Mozambique Ridge may represent a, line along which localised volcanism, unrelated to volcanic events in E Africa and Madagascar, took place during Aptian - Cenomanian times (Schlich et al, 1974). Deitrick et al (1977) report that the subsidence history of this ridge is still very unclear. CHAGOS-LACCADIVE RIDGE. The Chagos-Laccadive Ridge is a steep-sided continuous feature, about 2200-2700km in length and is capped with coral atolls which form the Laccadive and Maldive Islands and the Chagos Archipelago. Subsidence along the Chagos-Laccadive Ridge appears to have occurred at a rate which is normal for the oceanic crust (Deitrick et al, 1977). The ridge is bounded on its southern and eastern sides by deep trenches which probably represent the line of transform faults (Avraham and Bunce, 1977). The Chagos- Laccadive Ridge has been interpreted in a number of ways, as the surface expression of a transform fault (Fisher et al, 1971; McKenzie and Sclater, 1971); as a subsided volcanic feature formed by the northward movement of the Indian Plate over a mantle hot spot (Dietz and Holden, 1970; Laughton et al, 1971; Morgan, 1972; Siddiquie et al, 1976); as a microcontinent (Krishnan, 1960); and as an extension of the Indian continental shelf (Narain et al, 1968). A recent study by Avraham and Bunce (1977) using evidence from DSDP Site 219 and seismic, magnetic and gravity data, proposed that the Chagos-Laccadive Ridge is composed of structural elements with different origins. Avraham and Bunce (1977) suggest that the portion north of 7°N is volcanic and was formed due to the northward movement of India along a leaky transform fault, as suggested by McKenzie and Sclater (1971). The middle segment, occupied by the Maldive 18

Islands from 7oN to 1oN, is probably composed of lower Cretaceous or Jurassic sediments which are probably a seaward extension of the Kutch Mesozoic basin on the Indian mainland (Avraham and Bunce, 1977). The separation of this portion of the Chagos-Laccadive Ridge, which is thus inter preted as a continental fragment (Avraham and Bunce, 1977), probably occurred prior to the Palaeocene extrusion of the Deccan trap lavas which although present on the mainland, are absent in the Maldive Islands (Avraham and Bunce, 1977). Sclater (1976, person. commun. in Avraham and Bunce, 1977) however, has suggested that, on the basis of evidence from DSDP Site 219 (see also Siddiquie et al, 1976), the entire Chagos-Laccadive Ridge is volcanic and has subsided, in a similar fashion to the Ninety East Ridge since the Early Eocene (Siddiquie et al, 1976; Deitrick et al, 1977). The northern part (1°N - 2°S) of the third portion (south of 1°N) of the Ridge is probably volcanic and formed in a similar way to that portion to the north of 8°N (Avraham and Bunce, 1977). The southern part, the Chagos Archipelago, of the third portion (south of 1°N) of the ridge is the only part which is seismically active. This seismicity is probably related to that of a fracture zone which offsets the Central Indian Ocean Ridge (McKenzie and Sclater, 1971). This probably reflects the separation of the Chagos-Laccadive Ridge from the Mascarene Plateau, about 36 m.y. B.P. due to spreading along the Central Indian Ocean Ridge (Fisher et al, 1971; McKenzie and Sclater, 1971). MASCARENE PLATEAU The Mascarene Plateau is a submarine, aseismic plateau lying to the east and north east of Madagascar and extending in a faulted composite arc for 2300km from the Seychelles Bank in the north southward through the coral reef capped volcanic structures of the Saya da Malha, the Nazareth and the Cargados-Carajos banks to the faulted Tertiary-Quaternary, oceanic, volcanic island of Mauritius (Fisher et al, 1967; Laughton et al, 1971). The feature is steep-sided and angular and other features associated with it are the Amirante Trench, the Rodriguez Ridge (see under Section 1.2.2b), the Mauritius Trench and the volcanic island of Reunion (Fisher et al, 1967). The Seychelles Bank at the northern end of the Mascarene Plateau. is probably a continental fragment (see Section 1.2.2d). The southern portion of the Mascarene Plateau probably has a volcanic strucutre (Fisher et al, 1967). Fisher et al. (1971) have suggested that this portion of the Mascarene Plateau, between Saya da Malha and Mauritius, 19

together with the volcanic foundations of the Chagos-Laccadive Ridge were formed by extrusion of volcanic material, with very slow widening along the Chagos Fracture Zone, between 50 and 30 m.y.B.P. Such volcanism may have caused the Seychelles Bank to become linked with the remainder of the Mascarene Plateau to the south (Fisher et al, 1967). The volcanic portions of the southern Mascarene Plateau and the Chagos- Laccadive Ridge were linked along this fracture zone during their formation in the middle Tertiary (Fisher et al, 1971). When spreading began in Miocene times in a NE/SW direction along the new Central Indian Ocean Ridge which formed from the Chagos Fracture Zone (see Section 1.2.3), the Mascarene Plateau and Chagos-Laccadive Ridge were torn apart. This probably gave rise to the steep scarp slopes on the eastern Mascarene Plateau and the western Chagos-Laccadive Ridge (Fisher et al, 1971). Drilling at DSDP Site 237 between the Seychelles and Saya da Malha Banks did not reach basement, but bottomed in Palaeocene sediments (Fisher et al, 1974). Evidence from this site suggests that it was near sea level during the Palaeocene and has subsided to its present depth, in a similar fashion to the subsidence observed for the Ninety East and Chagos-Laccadive Ridges (Deitrick et al, 1977). NINETY EAST RIDGE. The Ninety East Ridge is the most striking of the aseismic features of the Indian Ocean (McKenzie and Sclater, 1971) and was revealed by the results of the ĪIOE (La'.zghton et al, 1971). It extends for a distance of nearly 5000km, subparallel with the 90°E meridian from 32°S, northwards to about 9°N, where it is buried beneath sediments of the Bengal Fan (Sclater and Fisher, 1974). It can be traced in the subsurface to about 0 12 N, using seismic reflection data (Curray and Moore, 1971). It generally rises to heights of 2000m above the ocean floor and it is between 100 and 200km in width (Laughton et al, 1971). It appears to have three distinct physiographic provinces:- north of 7S° it has an en echelon, symmetrical topography; between 7°S and the Osborn Knoll (14 - 16°S) it is narrow, linear and asymmetric with steep, east facing slopes; and to the south of the Osborn Knoll the ridge becomes wider and has more relief (Luyendyk and Davies, 1974; Sclater and Fisher, 1974). A great deal of attention has been focused on the Ninety East Ridge and several models have been proposed to explain its origin. Francis and Raitt (1967) suggested on the basis of seismic evidence that it was a horst structure, in which oceanic crust was thrust up. This interpretation was supported by Le Pichon and Heirtzler (1968) who attribute its upthrusting since the Eocene to differential movements of crustal 20

plates. McKenzie and Sclater (1971) have suggested that it is the surface expression of a transform fault which allowed the northward movement of the Indian Plate in the Late Cretaceous and Early Tertiary. Morgan (1972) has suggested that the Ninety East Ridge has been formed by the movement of an oceanic plate over a fixed point mantle plume. More recent work has benefited from the data which was available as a result of the drilling of DSDP Sites 214, 216, 217, 253 and 254 on this feature. As a result of this and of gravity studies it has been shown that the ridge cannot be a horst structure (Bowin, 1973). Furthermore, the DSDP results also rule out the possibility of uplift (Pimm et al, 1974; Luyendyk and Davies, 1974) as suggested by McKenzie and Sclater (1971), since subsidence has accompanied the ridges northward movement since the Late Maastrichtian (Pimm et al, 1974). DSDP drilling results show that the Ninety East Ridge gets older to the north (Pimm et al, 1974; Luyendyk and Davies, 1974). Furthermore, magnetic anomaly data suggest that the ocean floor ages in a similar way on the west side of the ridge, while ageing in an opposite direction, i.e. getting older to the south, on the eastern side in the Wharton Basin (Sclater and Fisher, 1974; Luyendyk and Davies, 1974). This evidence suggests that the Ninety East Ridge was a line of offset between the Indian and Australian plates and was attached to the Indian Plate (Fisher and Sclater, 1974). The existence of such a fault to the east of the ridge is indicated from the geophysical and bathymetric evidence (Sclater and Fisher, 1974; Luyendyk and Rennick, 1977)- Recent studies of the Ninety East Ridge and other aseismic features of the eastern Indian Ocean (Sclater and Fisher, 1974; Luyendyk and Davies, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977), have indicated firstly that the three physiographic provinces of the Ninety East Ridge have different origins and secondly that these, together with the origin of the whole Eastern Indian Ocean may be related to movement of the Indian, Antarctic and Australian plates over two mantle hot spots, in a similar fashion to that originally suggested by Morgan (1972). Such an origin is supported by the Rare Earth Element (REE) and other geochemical data from pyroclastics recovered from DSDP Site 253 (McKelvey and Fleet, 1974) and basalts recovered from other DSDP sites along the Ninety East Ridge (Luyendyk 0 and Davies, 1974). The portion of the Ninety East Ridge north of 7 S was probably formed by the trace of a volcanic point source (Johnson et al, 1976; Luyendyk and Rennick, 1977). This portion is thus the same age as the Indian Plate to which it is attached. The portion of the Ninety East Ridge between 7°S and the Osborn Knoll (14-16°s) was probably formed by leaking along a transform fault to the east of the ridge between the Indian and Australian 21

plates, as India moved northwards at about 64 - 53 m.y. B.P. (Luyendyk and Davies, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977). This portion of the ridge is thus younger than the Indian plate on which it sits (Luyendyk and Davies, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977). The portion of the ridge south of 16°S was probably formed by a volcanic trace which the Antarctic plate moved over. This may also have been overprinted by later volcanic activity along the transform fault, associated with an 110 southern jump of the crest of the South East Indian Ocean Ridge (Luyendyk and Rennick, 1977; Johnson et al, 1976). The age of the southern portion of the ridge is thus about 53 m.y. B.P. (Luyendyk and Rennick, 1977). The extreme southern portion of the Ninety East Ridge was probably formed between 32 and 17 m.y. B.P. by movement over the Yerguelen hot spot (Luyendyk and Davies, 1974; Luyendyk and Rennick, 1977). BROKEN RIDGE AND NATURALISTE PLATEAU The Broken Ridge extends eastwards from the southern end of the Ninety East Ridge and it is about 500km wide. Its relationship to the Ninety East Ridge is unknown, and it is a shallow feature varying from 1 to 1.5km in depth. On its southern side it is bounded by a steep scarp falling away into the Ob Trench to a depth of 4 km. Its northern face dips gently into the Wharton Basin (Laughton et al, 1971; Luyendyk and Davies, 1974). Separated by a 600km gap to the east of the Broken Ridge lies the Naturaliste

Plateau. This forms the southwestern extension of the Australian continental. margin and lies at water depths of about 2.51on.. It has a gentle sloping northern edge, while its southern edge is a steep scarp, parallel to the Diamantina Fracture Zone. The fact that these two features lie on a similar structural trend has led to speculation that they may both be continental structures (Luyendyk and Davies, 1974). Francis and Raitt (1967) on the basis of seismic refraction work considered the Broken Ridge to be a continental fragment and with the Naturaliste Plateau to have been part of the Western Australian Shield. Deitrick et al (1977) have shown that although these features were in a shallow water environment at some time in their history, the subsidence is complicated and not clear, and in the case of the Broken Ridge has left it unusually shallow for its age. Evidence from the DSDP sites on those features (255 and 258) have led to the suggestion that the Broken Ridge and the Naturaliste Plateau are volcanic features formed during the Early and Late Cretaceous (100 - 80 m.y. B.P.) as Australia and Antarctica moved ESE at this period, over a mantle hot spot (Luyendyk and Rennick, 1977). Between 64 and 53 22

m.y. B.P. these two features were separated •from the Kerguelen Plateau as Antarctica and Australia rifted apart and were subsequently separated from each other by Eocene rifting (Luyendyk and Rennick, 1977). The Broken Ridge was above sea level before 38 m.y.B.P. as evidenced by the occurrence of beach deposits at DSDP Site 255, and it is probable that the Naturaliste Plateau was emergent or near sea level at this time (Luyendyk and Davies, 1974). Since this time both features have subsided in a complicated fashion (Deitrick et al, 1977) and since the Miocene have been receiving normal carbonate sediments as they lie above the lysocline (Luyendyk and Davies, 1974). KERGUELEN AND HEARD PLATEAUX The Kerguelen Plateau lies at the northern end of the Kerguelen Ridge that extends 2000km to the south east towards the Antarctic continental shelf. The Kerguelen Islands are composed of intrusive and extrusive volcanics of a basaltic and syenitic composition (Laughton et al, 1971; Watkins et al, 1974). Heard Island, farther south on the Kerguelen Plateau contains examples of basalts, trachytes and alkali lavas (Laughton et al, 1971). The similarity in geometry and magnetics between the Kerguelen Plateau and the Broken Ridge and Naturaliste Plateau led Le Pichon and Heirtzler (1968) to suggest that the Kerguelen Plateau was also a continental fragment. By contrast, Watkins et al (1974) having studied the extrusive and intrusive rocks of Kerguelen, concluded that there was no evidence to support this proposal. Evidence from DSDP Site 255 on the Broken Ridge further supports this conclusion (Johnson et al, 1976; Luyendyk and Davies, 1974). Luyendyk and Rennick (1977) suggest, on the basis of DSDP results and geophysical data from the eastern Indian Ocean, that the Heard Island is probably Late Cretaceous to Early Tertiary (100-80 m.y.B.P.), in age and was formed together with the Broken Ridge and Naturaliste Plateau as the Australian/Antarctic Plate moved ESE across a mantle hot spot. The Kerguelen Islands and the remainder of the ridge were probably formed later (Luyendyk and Rennick, 1977), at about 64 - 53 m.y. B.P. as the rifting began between Australia and Antarctica above a mantle hot spot (Luyendyk and Rennick, 1977). Volcanism has continued in the Kerguelen Islands up to the present, probably as a result of the presence of a mantle hot spot below the Kerguelen Islands (Luyendyk and Rennick, 1977). 23

WALLABY PLATEAUX Little is known about the Wallaby Plateaux which lie off the WNW continental margin of Australia at depths of 2100 to 2300 metres (Laughton et al, 1971). Heezen and Tharp (1965) have suggested that they may have a continental structure and are thus an extension of the Australian continental margin. This idea is supported by a recent reconstruction of the history of the Indian Ocean by Johnson et al (1976). AGULHAS PLATEAU The Agulhas Plateau lies off the southern coast of Africa at a depth of about 2.5km (Laughton et al, 1971). The southern portion of the plateau has a smooth topography covered by relatively undisturbed sediments about 0.5 to 1.0km in thickness. The northern portion shows rough topography with a thin, disturbed sedimentary cover (Barrett, 1977) which may be due to the influence of bottom currents (Luyendyk and Davies, 1974; Barrett, 1977). Gravity data have suggested that this feature may be a continental fragment rifted from South Africa (Laughton et al, 1971). However, magnetic anomalies suggest a structure more akin to normal oceanic crust (Le Pichon and Heirtzler, 1968). A recent study by Barrett (1977) supports the view that the Agulhas Plateau is thickened basaltic oceanic crust and has formed during the separation of the Falkland Plateau from South Africa in the Valanginian (125 - 130 m.y. B.P.). CROZET PLATEAU The Crozet Plateau lies on the south eastern side of the South West Indian Ocean Ridge in the Crozet Basin (Laughton et al, 1971). It has an E/W trend and at the eastern end is topped by the extinct, andesitic volcanoes of the Crozet Islands (Laughton et al, 1971). The plateau is smooth topped and covered by 0.5km of undisturbed sediments. A magnetic profile across the plateau (Le Pichon and Heirtzler, 1968) showed little difference from those taken across the Agulhas Plateau and the South West Indian Ocean Ridge. McKenzie and Sclater (1971) have suggested that it, together with the Afanary - Nikitin sea mounts was probably formed during a phase of reduced spreading along a ridge (South West Indian Ocean Ridge?) which caused the Indian Ocean to open in the Middle to Late Cretaceous (80 m.y.B.P., Anomaly 32) and it is thus an oceanic, volcanic feature. 24

1.2.2d. Continental Fragments. These features, while generally similar to the aseismic features in sharing a lack of seismic activity, could be justifiably given the name 'microcontinents', as suggested by Heezen and Tharp (1965) since they have been shown to be composed of continental, rather than oceanic material. SEYCHELLES BANK The Seychelles Bank lies at the northern end of the Mascarene Plateau and it is unusual amongst oceanic islands in being composed of Pre Cambrian granitic rocks, which are cut by later alkali granites, and basic dyke- swarms which give rise to large amplitude, narrow magnetic anomalies across the Bank (Baker, 1963; Fisher et al, 1967; Laughton et al, 1971). Seismic refraction and gravity data confirm that the crust underlying the Seychelles is of continental character and thickness (15km) (Fisher et al, 1967). This, and the presence of a linear fracture zone to the east of Madagascar has led Fisher et al (1967) to suggest that the Seychelles Bank separated from Madagascar in the Late Mesozoic over a distance of 1000 - 1200 km. Oceanic basaltic material has been emplaced between the intervening blocks and now floors the oceanic area between the Seychelles and Madagascar (Fisher et al, 1967). MADAGASCAR Madagascar, the largest island in the western Indian Ocean, lies off the east coast of E. Africa, on the same latitude as the Triple Junction and the Rodriguez Fracture Zone (McKenzie and Sclater, 1971). Laughton et al (1971) have suggested that Madagascar is a continental block which is separated from Africa by probable oceanic crust. An extensive geological study by Kutina (1975), using the patterns and trends of fracture zones in Madagascar, on the E. African mainland and off the eastern coast of Madagascar, has suggested that an area extending from the east coast of Africa, eastwards to the northern extension of the Malagassy Fracture Zore is continuous Pre Cambrian basement. This area has dis- integrated in the course of geological history and a great part has been subjected to 'oceanisation', by the emplacement of volcanics along the NNE trending fracture zones (Kutina , 1975). This interpretation does not conflict greatly with the study of McKenzie and Sclater (1971) which did not reveal the presence of any magnetic anomalies around Madagascar and the Seychelles. Such subsidence probably took place before 75 rn.y. B.P. prior to movements suggested by McKenzie and Sclater (1971) along the Chagos Fracture Zone. Drilling as part of Leg 25 of the DSDP has suggested 25

that the Mozambique Channel is composed of stretched and thinned continental crust which has subsided through time (Schlich et al, 1974) lending support to Kutina's (1975) model for the origin of Madagascar.

1.2.2e. Continental Margins. The nature of the continental margins of the Indian Ocean is fundamental to understanding the evolution of the Indian Ocean in terms of continental drift theory, since they form the pieces of the jigsaw puzzle which must be fitted together (Laughton et al, 1971).. The Indian Ocean continental margins can be divided into three main types: the inactive margins of south east Africa, Antarctica and Western Australia; the downwarped margins with thicknesses of accumulated sediments of E. Africa, the Arabian Sea and in the Bay of Bengal; and the active plate margins of the Indonesian Island Arc System. INACTIVE MARGINS In the west, the continental margin off S. Africa and Mozambique is very narrow, being less than 10 km wide, except in the vicinity of the Agulhas Plateau, where the normal sequence of shelf, slope and rise occurs, as in the classic Atlantic Ocean - type continental margin. In the north west, the western continental margin of India is about 200km wide, and has small sediment embayments which parallel the coast. On the southern Indian margin a normal slope and deep water separates the Margin from the Chagos-Laccadive Ridge. The eastern margin of India is very narrow and is incised by numerous submarine canyons (Laughton at al, 1971). In the east the Indian Ocean is bounded by the broad shelf of the western Australian continental margin. The continental slope is very gentle and shoals lie off the coast, such as the Wallaby Plateaux. Between the N.W. Cape and Perth the shelf is narrower and the slope steeper while the southern margin of Australia is parallel to the Diamantina fracture zone (Laughton et al, 1971). The southern boundary of the Indian Ocean is the continental margin of Antarctica. This has been studied as part of the IIOE and three zones have been recognised. These are an inner coastal hilly zone; a zone of deep faults en echelon to the coast, resulting from differential ice loading; and an outer zone of old shelf planes which have been isostatically uplifted in the Holocene. The transition from continental to oceanic crust occurs beneath the continental slope (Laughton et al, 1971). 26

INACTIVE MARGINS WITH SEDIMENTARY ACCUMULATIONS The Mozambique Channel contains 1.0 - 1.5km of sediments and has in the past been considered as a downwarped geosynclinal depression (Laughton et al, 1971). Kutina (1975) has suggested that this area may be a downfaulted trough, related to the development of Madagascar, in which sediments from the Zambesi River have accumulated. Off the coast of Kenya there is a 500km wide zone where magnetic anomalies are absent. This suggests that the shelf, slope and rise sequence is missing and that the area is covered by thick sediments. Seismic refraction work has shown that the sediment accumulations here are 4 to 12km thick, over a downwarped margin, which may have resulted from intermittent subsidence since the end of the Karroo period (Laughton et al, 1971). Further north, in the Arabian Sea, 0.5 to 2.5km of sediments occur to the north of the Carlsberg Ridge. These sediments form the Indus Cone, which results from the denudation of the Himalayas at a rate of 17cros/1000 yrs (Ewings et al, 1969). Off Karachi and Bombay on the Indian Coast and in the Oman Basin, 5 - 8km thick sequences of detrital sediments occur, which probably result from Himalayan erosion (Ewings et al, 1969; Laughton et al, 1971). To the east of India in the Bay of Bengal occurs the Bengal Fan. This is a massive, submarine deltaic fan, resulting from the erosion of the Himalayas at a rate of 70cros/1000 yrs. The sediments have been transported by, and deposited from, turbidity currents originating in the Ganges- Brahmaputra River delta, by flow through a submarine canyon and then through a series of meandering and braided valleys on the surface of the fan (Curray and Moore, 1971). The Bengal Fan measures 300km long by 1000km wide and extends from 20oN to 10oS. It is probably greater than 12km thick at its thickest and this thins to 4km thick at 4°S and then to less than 1km thick at 10°S, at a point furthest from the sediment source (Curray and Moore, 1971). The sediments of the fan are divided by unconformities of Miocene and early Pleistocene age which corresponds to periods of Himalayan orogeny (Curray and Moore, 1971). ACTIVE CONTINENTAL MARGINS The Indonesian Island Arc System extends from Burma to the northern coast of Australia through the Andaman and Nicobar Islands, the islands west of Sumatra, the ridge south of Java and through Timor. On the inner side of the arc lie the volcanic mountains of Sumatra and Java and on the outer side is the Sunda Trench. The whole system is seismically active and is coincident with a negative gravity anomaly of the Benioff zone of the Sunda Trench, down which the Indian/Australian plate is being 27

actively subducted (Laughton et al, 1971). Seismic refraction studies have indicated that normal oceanic crust lies to the south of the Java Trench and 3km of sediments occur in the vicinity of the Sunda Trench, while to the north side of the arc continental crust occurs which is downwarped in the sea areas (Laughton et al, 1971). Johnson et al (1976)have suggested that the Java Trench and E. Indies were formed after India had moved into its present northern position; they suggest this occurred later than 10 m.y. B.P. (i.e. in the Miocene).

1.2.2f. The Ocean Basins Seismic data, coupled with observations of magnetics and gravity show that the structure of the ocean basins in the Indian Ocean does not differ significantly from other ocean basins of the world (surveys sited in Laughton et al, 1971). Furthermore, the ocean basins exhibit lower than average heat flow than in the Indian Ocean as a whole (Langseth and Taylor, 1967) and generally show magnetic anomaly patterns which can be related to sea floor spreading about adjacent segments of the MORS. SOMALI BASIN The Somali Basin is mainly floored with detrital sediments which extend eastwards from the African Continental margin and obscure any pattern of magnetic anomalies. To the north of the Seychelles, McKenzie and Sclater (1971) have recognised WNW/ESE trending magnetic anomalies 23 - 29 of Late Cretaceous and Palaeocene age. These have probably been formed by sea floor spreading about the proto - Carlsberg Ridge (McKenzie and Sclater, 1971; Schlich et al, 1974). MOZAMBIQUE BASIN No clear magnetic anomaly pattern is observed in the Mozambique Basin (Schlich et al, 1974). However, dating of the basaltic basement at DSDP Sites 248 and 250 have given a Santonian age, which suggests that this basin was probably formed at the initial stages of Gondwanaland break up (Lugendyk and Davies, 1974; Schlich et al, 1974). MADAGASCAR AND MASCARENE BASINS These basins lie to the west of the Central Indian Ocean Ridge and are bounded in the south east by the South West Indian Ocean Ridge and in the west by the Madagascar Ridge. They are separated by a fracture zone, the Mahanora Ridge (Schlich et al, 1974) with the Mascarene Basin lying to the north of it. Magnetic anomalies, trending NW/SE from 33 (79 m.y. B.P.) to 19 (47 m.y. B.P.) are reocgnised and they are offset by four NE/SW 28

trending fracture zones, the Mohanora Ridge being one of these (Schlich., 1974). Spreading half rates calculated from the magnetics, suggest a considerable variation in ridge activity through time (Schlich, 1974) - An 33-31 (79-71 m.y. B.P.) 4.0 cm/yr; An 30 - 28 (71-65 m.y.B.P.) 12.2 cm/yr; An 27 - 23 (68-58 m.y.B.P.) 7.1 cm/yr; and An 22-19 (57- 47 m.y.B.P.) 4.0 cm/yr. Younger magnetic anomalies (1-5) are associated with and are parallel to the crests of the present Central and South West Indian Ocean Ridges (McKenzie and Sclater, 1971; Schlich, 1974). CROZET BASIN In the Crozet Basin it is possible to recognise a similar pattern of magnetic anomalies, and calculated spreading rates are similar to those for the Madagascar and Mascarene Basins. However, in the Crozet Basin the trend of anomalies 27 - 23 is WNW/ESE and they are offset by NNE/SSW trending fracture zones. Younger anomalies, 23 - 17, have a NW/SE trend and are offset by NE/SW trending fracture zones (Schlich and Patriat, 1971; Schlich, 1974). CENTRAL INDIAN BASIN The age of the ocean floor in this basin increases to the north (Sclater and Fisher, 1974) where the magnetic anomalies become obscured by the detrital sediments of the Bengal Fan (Sclater et al, 1974). Such sediments form the Miocene succession at DSDP Site 215 (Sclater et al, 1974). Magnetic anomalies 7 - 16 have a NW/SE trend and have been formed by spreading about the South East Indian Ocean Ridge at a half rate of 2.5 cm/yr. (Sclater and Fisher, 1974; Sclater et al, 1976). Older anomalies than this (back to No. 33) occur, but have an E/W trend and are offset by N/S trending fracture zones. The spreading half rates for these anomalies (McKenzie and Sclater, 1971; Sclater and Fisher, 1974) show wide variations through geological time - An 33-30 5.7cm/yr; An 30-27 12.Ocm/yr; 27-22 8.1cm/yr; 22-16 4.0cm/yr. The change in direction of spreading was probably brought about by the collision of India with Asia, causing a change in the relative motion of the Indian and Antarctic plates in the Oligocene. This alteration in relative motion caused a reorientation of spreading along the South East Indian Ocean Ridge between anomalies 18 - 15 (43-39 m.y.B.P.) from a N/S to a NE/SW direction (Sclater and Fisher, 1974; Sclater et al, 1976). The similarity in the pattern of magnetic anomaly lineation numbers 23-27 and the calculated spreading half rates for this period for the Madagascar, Crozet and Central Indian Basins, suggests that they were formed by spreading about the same ridge segment (Schlich et al, 1974) i.e. the South East Indian Ocean Ridge (McKenzie and Sclater, 1971). 29

WHARTON BASIN This is the most geologically complex of the ocean basins of the Indian Ocean, and as the results of drilling as part of the DSDP have shown (Luyendyk and Davies, 1974) contains the oldest ocean floor in the Indian Ocean. In the southern part of the basin, magnetic anomalies (1-19) occur which trend parallel with, and were formed by spreading along, the South East Indian Ocean Ridge since its formation by the separation of Australia and Antarctica 53 m.y.B.P. (Luyendyk and Davies, 1974; Johnson et al, 1976). The northern portion of the basin is floored by detrital sediments of the eastern limb of the Bengal Fan, which extend into the sedimentary sequence at DSDP Site 211 (Sclater et al, 1974). To the west of this site, magnetic anomalies, Nos. 17-33 have been identified and these are seen to get younger to the north (Luyendyk and Davies, 1974; Sciater et al, 1974). These E/W trending magnetic anomalies, are offset by a number of N/S trending fracture zones, among them the Investigator Fracture Zone (Sciater et al, 1974). Spreading half rates for anomalies 29-23 give a value of 6.0 cm/yr (Sciater et al, 1974). Such a magnetic anomaly pattern precludes a spreading history similar to that for the floor of the Central Indian Basin, to the west of the Ninety East Ridge (Luyendyk and Davies, 1974; McKenzie and Sclater,1971). Johnson et al (1976) have suggested that these E/W trending magnetic anomalies were formed by spreading, by the. N/S separation of India from Australia/Antarctica along a ridge in the northern Wharton Basin, which ceased spreading about 28 m.y. B.P. and is now obscured by the detrital deposits of the Bengal Fan. Johnson et al (1976) have further suggested that the separation of India from Antarctic/ Australia in the early stages of Gondwanaland break up (130-80 m.y. B.P.) has produced the oldest sea floor in the Wharton Basin off the western continental margin of Australia. Such an interpretation is supported by the Aptian (112 m.y. B.P.) age of the basement at DSDP Site 259 (Luyendyk and Davies, 1974), and the presence of magnetic anomalies, 127 m.y old in the vicinity of Perth (Markl, 1974). Spreading probably occurred at a half rate of 3.5 - 4.8 cm/yr (Johnson et al, 1976). Later spreading described above in this basin may have partially obscured such an early spreading history (Johnson et al, 1976). 30

1.2.3. Evolution of the Indian Ocean

1.2.3a. Introduction

The morphology of the Indian Ocean, described in some detail above, illustrates the complex evolution of this ocean. The lack of magnetic anomaly data older than 80 m.y.B.P. has meant that reconstructions of earlier events must largely be based on direct dating evidence. Such direct dating evidence has become available from the drilling results of the DSDP. These have made it possible to reconstruct the geological history of the Indian Ocean, since the break up of the southern continents..

1.2.3b. Fit of the Southern Continents and the Timing of the Initial Breakup of Gondwanaland.

Several authors have discussed the continental fit of Gondwanaland (Le Pichon and Heirtzler, 1968; Dietz and Holden, 1970, 1971; McElhinny, 1970; McElhinny and Luck, 1970; Smith and Hallam, 1970; Heirtzler et al, 1973; Laird et al, 1977; Norton and Molnar, 1977). The majority are agreed that Australia lay next to Antarctica, with India to the west and Africa to the west of that. Opinions differ, however, as to whether there was (McElhinny, 1970, McElhinny and Luck, 1970) or was not (Smith and Hallam, 1970; Dietz and Holden, 1971; Norton and Molnar, 1977) a gap between India and Australia. More recent reconstructions tend to the view that there was no gap due to the presence of 'Greater India' (Fisher and Sclater, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977). The majority of the reconstructions are in agreement as to the location of Madagascar, Ceylon and the Seychelles next to the east coast of India (McElhinny and Luck, 1970; McElhinny, 1970; Dietz and Holden, 1971). However, Smith and Hallam (1970) and Dietz and Holden (1970) positioned Madagascar and the Seychelles next to Africa, and such a position may be more in keeping with the origin of these features as suggested by Kutina (1975) and Fisher et al (1967).

The date proposed for the initial break up of the southern continents varies from 100 m.y. B.P. (Sclater and Fisher, 1974) to 180-200 m.y. B.P. (Dietz and Holden, 1970, 1971). The majority of the evidence from the geology of the southern continents and DSDP data suggests a date of 130- 140 m.y.B.P. for the initial rifting, which in the eastern basin may have been caused by the movement of Gondwanaland over a mantle hot spot 31

(Luyendyk and Rennick, 1977). This period is marked by the separation of Africa from India/Australia/Antarctica along the line of the present South West Indian Ocean Ridge (Le Pichon and Heirtzler, 1968; Johnson et al, 1976).

The sequence of events in the geological evolution of the Indian Ocean will be briefly summarised below with reference to events taking place simultaneously in the eastern (Luyendyk and Davies, 1974; Sclater and Fisher, 1974; Johnson et al, 1976; Sclater et al, 1976; Luyendyk and Rennick, 1977) and western basins (Fisher et al, 1971; McKenzie and Sclater, 1971, 1973; Schlich., 1974; Schlich et al, 1974).

1.2.3c. Pre-Breakup Sea Floor Spreading (140-130 m.y.B.P.) (see Fig. 1.2.3c).

Heirtzler et al (1973) have suggested that western Australia rifted off and separated from the remainder of Australia prior to the break up of Gondwanaland. In view of recently obtained geological evidence (Norton and Molnar, 1977) this seems to be extremely unlikely. However, it seems probable that sea floor spreading may have occurred prior to the breakup in the now N.E. Wharton Basin (Larson, 1975; Johnson et al, 1976), as is indicated by the basement ages at DSDP Sites 260 and 261 (Luyendyk and Davies, 1974).

1.2.3d. Seafloor Spreading 130-•80 m.y. B.P. (see Fig. 1.2.3d).

In the Eastern Basin during this period India separated from Australia/Antarctica, thus opening a landlocked sea about 130-100 m.y.B.P. This spreading probably occurred in a NW/SE direction at a rate of between 3.5 and 4.8 cm/yr (Johnson et al, 1976). This is supported by magnetic anomalies 127 m.y. old in the vicinity of Perth (Markl, 1974) and the Aptian (112 m.y. B.P.) age of the basement of DSDP Site 259 (Luyendyk and Davies, 1974). This movement eastwards of Australia/Antarctica, probably continued over a mantle hot spot (Luyendyk and Rennick, 1977) and may have given rise to the volcanic features of the Broken Ridge, Naturaliste Plateau and Heard Island at about 100-80 m.y.B.P. (Luyendyk and Rennick, 1977). The portion of the Ninety East Ridge north of 7°S was probably formed at this time, by the movement northwards of the Indian plate over a mantle plume as India separated from Australia/ LEGEND

254 DSDP Sites with (33) observed basement ages in brackets m.y.B.P.

• DSDP Sites shown in previous figure

Active Ridge Crest spreading segment

Lambert equal area projections Maps After Johnson et al, 1976 and Luyendyk and Davies, 1974.

Fig. 1.2.3c. Sea Floor Spreading History of Fig. 1.2.3d. Sea Floor Spreading History of the Indian Ocean, 140-130 m.y. B.P. the Indian Ocean, 130-80 m.y.B.P. 33

Antarctica (Luyendyk and Rennick, 1977). This portion of the ridge is therefore the same age as the Indian Plate on which it sits (Johnson et al, 1976; Luyendyk and Rennick, 1977).

In the western Indian Ocean, this period is marked by the separation of Africa from India/Australia/Antarctica along the line of the now South West Indian Ocean Ridge (Le Pichon and Heirtzler, 1968). Furthermore, it is possible that this separation was complete by the Mid. Albian (102 m.y.B.P.). No magnetic anomalies have been found which can be associated with this separation, and hence the spreading rate is not accurately known. Since this time, until more recent times (see below) the South West Indian Ocean Ridge appears to have acted as a series of fracture zones between the Madagascar Basin to the north and the Crozet Basin to the south (Le Pichon and Heirtzler, 1968). The Agulhas Plateau was probably formed during this period (Valanginian, 125 m.y.B.P.) as the Falklands Plateau separated from southern Africa with the opening of the South Atlantic Ocean (Barrett, 1977). It is possible. that local volcanism associated with, but post dating, this separation may have given rise to the Mozambique Ridge in Aptian / Cenomanian times (Schlich et al, 1974). Johnson et al (1976) have suggested that Ceylon probably reached its present position with respect to India about 100 m.y.B.P. This conclusion is based on the reconstruction of the southern continents by Smith and Hallam (1970), although Johnson et al (1976) do not suggest the exact mechanism for this positioning.

1.2.3e. Sea floor Spreading 80 - 53 m.y.B.P. (See Fig. 1.2.3e).

In the Eastern Basin the first part of this period is marked by the continued separation of India from Australia/Antarctica in the same direction and at a similar rate as was occurring prior to 80 m.y.B.P. (Johnson et al, 1976; Luyendyk and Rennick, 1977). At about 64 m.y.B.P. the pattern of spreading changed and became more rapid as India moved northwards (Johnson et al, 1976; McKenzie and Sclater, 1971, 1973). This spreading took place along two major ridge segments. To the south of India (i.e. on the Indian plate) spreading occurred along the South East Indian Ocean Ridge, between the Chagos Fracture zone in the west (the western boundary of the Eastern Basin in this discussion, McKenzie and Sclater, 1971, 1973) and a fracture zone immediately to the east of the Ninety East Ridge (Luyendyk and Rennick, 1977). Movement along these two 34

fracture zones permitted the northward separation of India from Australia (McKenzie and Sclater, 1971, 1973). Spreading along this segment of the MORS to the south of India produced magnetic anomalies 33 to 22 in the Central Indian Basin to the north of the spreading centre and anomalies 27 - 23 in the Crozet Basin to the south of it

(Schnell and Patriat, 1971; Sclater and Fisher, 1974; Sclater et al, 1976). This separation reached a peak rate of 12.2 cm/yr between anomalies 30 and 27 during the Maastrichtian and Early Palaeocene. The second spreading segment lay to the east of the Ninety East Ridge fracture zone, in the Wharton Basin, and allowed the separation of India from Australia and movement between the Indian and Australian plates (Johnson et al, 1976; Luyendyk and Rennick, 1977). The spreading centre was offset to the N.E. along a number of fracture zones, among them the Investigator Fracture Zone (Sclater and Fisher, 1974; Luyendyk and Davies, 1974). Spreading gave rise to the E/W trending magnetic anomalies 29 - 23 (Late Maastr —Early Palaeocene) at a rate of 6.0 cm/yr (Sclater et al, 1974). The anomalies young to the north, indicating a different history to the magnetic anomalies of the same age formed on the west side of the Ninety East Ridge. Such an interpretation is supported by the basement ages from DSDP sites drilled in the N.W. Wharton Basin (Sclater et al, 1974). During this rapid northward movement of the Indian plate it is probable that the central portion of the Ninety East Ridge, between 7°S and the Osborn Knoll (14 - 16°S), was formed by leaking along the transform fault immediately to the east of the ridge (Johnson et al, 1976; Luyendyk and Rennick, 1977). The end of this period in the eastern basin was marked by the initial rifting apart of Australia and Antarctica along the probable trace of a mantle hot spot (Luyendyk and Rennick, 1977). This caused the splitting apart of the volcanic plateau (see above under 1.2.3d) with the Broken Ridge - Naturaliste Plateau massif to the north and the Kerguelen Plateau to the south (Johnson et al, 1976; Luyendyk and Rennick, 1977).

In the western basin, the sequence of events is less clear (McKenzie and Sclater, 1971, 1973). It would appear that the volcanic feature of the Crozet plateau was formed at the beginning of this period, 80 m.y. B.P. although the exact nature of the mechanism of formation is not clear (McKenzie and Sclater, 1971). From evidence from DSDP Sites 248 and 250 35

Fig. 1.2.3e. Sea Floor Spreading History of the Indian Ocean 80 - 53 m.y. B.P.

Ninety East Ridge

LEGEND

As for Fig. 1.2.3d.

Non-active ridge crest spreading segment 36

the Mozambique Basin and the Madagascar Ridge (Schlich et al, 1974) were probably formed at the beginning of this period (Santonian, 82 - 76 m.y.B.P.) (Luyendyk and Davies, 1974; Schlich et al, 1974), and it is also probable that the Mozambique Channel and the island of Madagascar may have subsided at this time (Kutina,1975). The Late Cretaceous may also have marked the separation of the Seychelles Bank from Madagascar along the Amirante fracture zone (Fisher et al, 1967). Spreading in the western basin during this period occurred along the proto-Carlsberg Ridge which had an E/W trend and terminated against the Chagos and Owen fracture zones in the east and west respectively (McKenzie and Sclater, 1971, 1973). As the Indian plate moved northwards spreading along the proto-Carlsberg Ridge was very rapid (McKenzie and Sclater, 1971), and it reached a peak rate, between anomalies 30 and 28, of 12.2cm/yr. These anomalies are found in the Madagascar and Mascarene Basins. Magnetic anomalies (29 - 23) of Palaeocene age, found in the Somali Basin, to the north of the proto-Carlsberg Ridge, and in the Madagascar and Mascarene Basins to the south of the ridge were formed during this period at rates of 6.5 and 7.1 cm/yr, respectively (Schlich et al, 1974). It appears that during this period the South West Indian Ocean Ridge acted as a series of transform faults along which no spreading took place (Le Pichon and Iieirtzler, 1968). It seems possible that the northern portion of the Chagos-Laccadive Ridge was formed during this period, by the leaking of volcanic material along the Chagos fracture zone (McKenzie and Sclater, 1971; Avraham and Bunce, 1977). Towards the end of this period (mid-Palaeocene), with a decrease in the spreading rate and the northward movement of India (McKenzie and Sclater, 1971) it is probable that slow leaking of volcanic material along the Chagos fracture zone began the formation of the southern portions of the Chagos- Laccadive Ridge and the Mascarene Plateau (Fisher et al, 1967; McKenzie and Sclater, 1971; Avraham and Bunce, 1977). Such volcanism may have caused the Seychelles Bank to become linked to the Mascarene Plateau after its separation from Madagascar along the Amirante Trench in the late Mesozoic (Fisher et al, 1967). This period also marked the joining of the continental fragment of the Maldive Islands to the forming Chagos- Laccadive Ridge after its separation from the mainland of India prior to the Palaeocene (Avraham and Dunce, 1977). 37

1.2.3f. Sea Floor Spreading, 53 - 32 m.y. B.P. (See Fig. 1.2.3f).

In the eastern basin the beginning of this period is marked by the formation of the southern portion (south of 16°S) of the Ninety East Ridge by the leaking of volcanic material along the fracture zone immediately to the east of the ridge (Luyendyk and Rennick, 1977). The subsidence of this ridge from a shallow water environment, continued as it had done throughout its formation, as the ridge moved northwards with the Indian plate (Pimm et al, 1974). Little movement, at a rate of 3.0 - 3.2 cm/yr, occurred between India and Australia at the beginning of this period along the offset spreading centre in the N.W. Wharton Basin (Johnson et al, 1976). This period is marked by the collision of India with Asia (Johnson et al, 1976). Movement between India and Antarctica occurred along the segment of the South East Indian Ocean Ridge to the west of the Ninety East Ridge. This spreading was in a NNE/SSW direction, at a rate of 4.0 cm/yr (Sclater and Fisher, 1974; Sclater at al, 1976) and produced magnetic anomalies 22 - 16 in the Central Indian Basin and anomalies 22 - 17 in the Crozet Basin (Schlich and Patriat, 1971). Spreading to the east of the Ninety East Ridge was marked by a new pattern, due to the separation of Australia and Antarctica. This spreading was in a NE/SW direction and occurred at a rate of 2.9 - 7.0 cm/yr (Luyendyk and Davies, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977). Towards the end of this period at about anomaly 16 (Oligocene) the Australian and Indian plates became welded together, movement along the fracture zone to the east of the Ninety East Ridge ceased as a result, and spreading in the N.W. Wharton Basin came to an end (Luyendyk, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977). The collision of India with Asia caused a change in the relative motion of the Indian and Australian plates and a resultant change in the spreading direction from N/S to NE/SW at about this time (An 18 - 15) (Sclater and Fisher, 1974; Johnson at al, 1976; Sclater et a1,1976; Luyendyk and Rennick, 1977). This resulted in a jump of 110 to the south of spreading along the South East Indian Ocean Ridge to the west of the Ninety East Ridge (Sclater and Fisher, 1974; Sclater et al, 1976). Spreading along this segment at a rate of 2.5 cm/yr produced the NW/SE trending magnetic anomalies 16 - 7 in the Central Indian and Crozet Basins (Schlich and Patriat, 1971; Sclater and Fisher, 1974; Schlich et al, 1974; Sclater et al, 1976). The separation of Australia from Antarctica Fig. 1.2.3f. Sea Floor Spreading History of the Indian Ocean,

5 3 - 32 m.y.B.P.

LEGEND as for Fig. 1.2.3e. 39

along the South East Indian Ocean Ridge caused the continued separation of the Broken. Ridge and Naturaliste Plateau in the north from the Kerguelen Plateau in the south (Luyendyk and Davies, 1974; Johnson et al, 1976). It seems probable that Eocene rifting during this period caused the separation of the Broken Ridge from the Naturaliste Plateau (Luyendyk and Rennick, 1977).

In the western basin this period (An 22 - 6) is marked by slow spreading along the Carlsberg Ridge as India collided with Asia (McKenzie and Sclater, 1971). During this period of slow spreading it is probable that leaking of volcanic material continued along the Chagos fracture zone, producing the volcanic portions of the Mascarene Plateau (Fisher et a1,1967) and the Chagos-Laccadive Ridge (Avraham and Bunce, 1977). Spreading continued along the Carlsberg Ridge at a rate of 4.0 cm/yr or less and produced magnetic anomalies 22 - 19 in the Madagascar and Mascarene Basins (Schlich, 1974; Schlich et al, 1974). An absence of younger magnetic anomalies i'.dicates a period of slow or no spreading in this area (McKenzie and Sclater, 1971). The South West Indian Ocean Ridge appears to have continued to act as a series of fracture zones (Le Pichon and Heirtzler, 1968), although it has been suggested that it may have come into being as a ridge about 40 m.y. B.P., with only minor spreading occurring (McKenzie and Sclater, 1971; Schlich, 1974; Schlich et al, 1974). Towards the end of this period, 36 - 35 m.y. B.P. a major change took place in the western basin, coincident with that observed for the eastern basin, caused by the collision of India with Asia (McKenzie and Sclater, 1971). This caused a change in the relative motion of the Indian and African plates and hence a change in the spreading direction from N/S to NE/SW. As has been discussed by Atwater and Menard (1970) this meant that the Chagos fracture zone was no longer parallel to the direction of spreading, with the result that it split into a number of NE/SW trending fracture zones offsetting NW/SE trending ridge segments, thus forming the Central Indian Ocean Ridge (Fisher et al, 1971; McKenzie and Sclater, 1971, 1973). This alteration in spreading pattern also resulted in the formation of the Triple Junction at 26°S, by the mechanism described by McKenzie and Sclater.(1971). The formation of, and spreading along, the Central Indian Ocean Ridge caused the tearing apart of the Mascarene Plateau in the west from the Chagos-Laccadive Ridge in the east (Fisher et al, 1967; Avraham and Dunce, 1977). This reorientation of spreading direction from N/S to NE/SW caused a realignment of the Carlsbergc, 40

Ridge along this trend (McKenzie and Sclater, 1971). The separation of Africa from Arabia in the lower Eocene (Le Pichon and Heirtzler, 1968) which affected the Red Sea, probably gave rise to the Sheba Ridge (Le Pichon and Heirtzler, 1968; McKenzie and Sclater, 1971).

1.2.3g. Sea floor Spreading, 32 - 0 m.y. B.P. (See Fig. 1.2.3g).

In the eastern basin between 32 and 17 m.y. B.P. this period saw the formation of the extreme southern tip of the Ninety East Ridge, which was probably caused by the southerly movement of a mantle hot spot (Luyendyk and Rennick, 1977). Luyendyk and Rennick (1977) suggest that two mantle hot spots exist at present in the eastern basin - one below the crest of the South East Indian Ocean Ridge and one beneath the Kerguelen islands. The latter one could probably be the cause of post- glacial volcanic and intrusive rocks found on these islands (Watkins et al, 1974; Luyendyk and Rennick, 1977). This period saw the realignment of spreading along the segments of the South East Indian Ocean Ridge to the east and west of the Ninety East Ridge to a NE/SW direction (Johnson et al, 1976). Such spreading at a rate of 4.5 - 6.4 cm/yr has produced magnetic anomalies 5 to 1 in the Central Indian, south Wharton and Crozet Basins (Weissel and Hayes, 1971; Luyendyk and Davies, 1974; Sclater and Fisher, 1974; Johnson et al, 1976; Sclater et al, 1976). The Miocene was a period of uplift for the Himalayas (Curray and Moore, 1971). The erosion products of these mountains have been deposited as the sediments of the Bengal Fan in the northern parts of the Central Indian and Wharton Basins. These sediments have obscured the late Cretaceous - Palaeocene spreading centre in the N.W. Wharton Basin (Johnson et al, 1976) and are found in the Miocene sequence at DSDP Site 215 and also in the succession at DSDP Site 211 (Sclater et al, 1974). The Java Trench and the E. Indies probably formed after India had moved northwards to its present position at about 10 m.y. B.P. (Johnson et al, 1976).

In the Western Basin spreading has continued along the Carlsberg and Central Indian Ocean Ridges at rates of 1.2 - 1.3 and 2.3 cm/yr respectively (Fisher et al, 1971; McKenzie and Sclater, 1971) and produced magnetic anomalies 1 - 5 in the Arabian Sea and the Somali, Mascarene and Madagascar Basins (Schlich, 1974; Schlich et al, 1974). Fig. 1.2.3g. Sea Floor Spreading History of the Indian Ocean, 32 - 0 rn.y. B.P.

LEGEND as for Fig. 1.2.3e.

(Seafloor spreading about the South West Indian Ocean Ridge not shown) 42

Uplift and erosion of the Himalayas during this period caused the deposition of detrital sediments in the Arabian Sea (Ewings et al, 1969; Currey and Moore, 1971). Spreading along the South West Indian Ocean Ridge did not begin at the same time as on the Central Indian Ocean Ridge and the Carlsberg Ridge, i.e. 36 - 25 m.y. B.P. (McKenzie and Sclater, 1971). It is probable that on this highly fractured and complex ridge segment, spreading began in the Miocene about 20 m.y. B.P. (Schlich 1974; Luyendyk and Davies, 1974) at a slow rate of less than 1.0 cm/yr (Bergh, 1971; Schlich and Patriat, 1971; Sclater and Harrison, 1971; McKenzie and Sclater, 1971). The spreading has been occurring in a NNE/SSW direction, which is parallel to the fracture zones which offset the ridge crest (Bergh, 1971; Luyendyk and Davies, 1974).

1.2.3h Concluding Remarks.

Such an interpretation of the geological evolution of the Indian Ocean as that presented above, based primarily on magnetic anomaly evidence and geological reconstructions (Fisher et al, 1971; McKenzie and Sclater, 1971; Sclater and Fisher, 1974; Johnson et al, 1976), has been shown to be in a good agreement with direct geological evidence obtained from the drilling results of the DSDP (Luyendyk, 1974; Luyendyk and Davies, 1974; Schlich, 1974; Schlich et al, 1974; Sclater et al, 1974; Luyendyk and Rennick, 1977). It is hoped that further geological data will be forthcoming in the future in order to resolve many of the problems which still remain with regard to the geological features and evolution of•the Indian Ocean. 43

1.3 OCEANOGRAPHY - OCEAN CURRENTS IN THE INDIAN OCEAN

1.3.1 Introduction

The water column in the oceans is structured in such a way that it consists of layered water masses which move against one another and are different in terms of water temperature, salinity, turbidity, dissolved oxygen content, etc... (Warren, 1974). Such bodies of water, moving at different levels and speeds constitute the ocean currents. An understanding of the movement of ocean currents, particularly bottom currents, is important in a study of ocean floor sediments, since bottom currents can affect the ocean floor environment by transporting and eroding the sediments, transporting organisms and by enhancing or inhibiting the supply of dissolved metals to the bottom sediments (Ewing et al, 1968; Burkle et al, 1974; Kolla et al, 1976a; Watkins and Kennett, 1977).

Bottom current activity has the most marked effect on the ocean floor environment. In the Indian Ocean, the existence and form of bottom currents has been indicated from such lines of evidence as disconformities within the sediment column (Kennett and Watkins, 1977), the introduction to an area of fauna from a different latitude (Burkle et al, 1974), dissolution effects, in carbonate sediments (Kolla et al, 1976c), high content of suspended matter and redistribution of sediments (Kolla et al, 1976a, b), differences in temperature and salinity (Warren, 1974; Kolla et al, 1976a), ocean floor features, e.g. ripple and scour marks (Kolla et al, 1976a, 1976b; Ewing et al, 1968) and the existence of extensive manganese nodule fields (Payne and Connolly, 1972; Kennett and Watkins, 1975, 1976; Watkins and Kennett, 1977). The major bottom currents at present operating in the Indian Ocean - the Antarctic Bottom Water (AABW) and Circum Polar Current and associates - together with some minor currents and shallower water features are described below (see Fig. 1.3.1),, following a brief survey of previous observations in this field.

1.3.2. Previous Work on Indian Ocean Currents

It has only been in recent years, that physical oceanographic data on the Indian Ocean have been forthcoming. This is particularly true 44

for the south east Indian Ocean (Kennett and Watkins, 1976), where evidence now exists for the presence of the Circum Polar and the Antarctic Bottom Water currents (Kennett and Watkins, 1975; 1976; Watkins and Kennett, 1977; Kolla et al, 1978). In the western Indian Ocean, Le Pichon (1960) proposed the northward flow of bottom water against Africa into the Mozambique Basin as far as the Mozambique Channel and its turning and southward movement down the west coast of Madagascar. However, the data were too few to indicate an unambiguous flow pattern. The cruises of the International Indian Ocean Expedition have provided additional data on potential temperature and salinity (Wyrtki, 1971), which in general have confirmed this pattern. More recent work by Warren (1974), using hydrographic data, has in general confirmed the existence of a northward deep flow and southward return flow in the Madagascar and Mascarene Basins.

The Indian Ocean, unlike the Atlantic, has no northern source of cold deep water (i.e. more than 2km depth) (Warren, 1974). Hydro- graphic considerations place constraints on the source of deep water in that it occurs in the high southern latitudes and moves northwards as a narrow boundary current in western and eastern basins (Warren, 1974; Kennett and Watkins, 1976). The principal deep ocean current in the lower latitudes is the Antarctic Bottom Water (AABW) which is linked to the Circum Polar Current of higher southern latitudes (Watkins and Kennett, 1977; Kolla et al, 1978). Other currents of minor significance and shallower depths of movement have also been recognised.

1.3.3. Antarctic Bottom Water (AABW)

The AABW is the major circulating water mass and transporter of detritus in the Indian Ocean. It is probably responsible for the high turbidity due to detrital influx to the north of the equator (Kolla et al, 1976a).

The AABW originates on the Antarctic continental shelf. The temperature/ salinity behaviour of water masses to the east and west of the Kerguelen Plateau is generally similar, although the water in these two areas is of different origin. To the east of the Kerguelen Plateau the deep water is derived from the Weddell Sea, while to the west it is derived from two water masses, one near the Adēlie Coast and one from the 30°E 4O°E 6O°E 8O°F 100E 12OoE

15°N Fig. 1.3.1. Deep Ocean Currents in the Indian Ocean. (after Kolla et al, 1976a; 1976b; Kennett and Watkins. 1975; 1976; Watkins and 0111116 00 Kennett, 1977)

North Atlantic Deep Water

4000 0 0 100 20 S

40 S miR

5O0S 6 3o0E 40°E OoE 8O°E 100E 120 E 46

Ross Sea (Kolla et al, 1976a). There is also some minor influx of deep water from the Atlantic-Indian Basin between Antarctica and Kerguelen although this is of minor significance.

In the western Indian Ocean, the AABW enters the Agulhas Basin from the Southern Ocean through the Prince Edward fracture zone (48 - 50°S/35°E) in the Mid-Indian Ocean Ridge system (see Fig. 1.3.1), while minor currents pass between the African Continental margin and the Agulhas Plateau (Kolla et al, 1976a). As the AABW moves northwards into the Mozambique Basin it gets warmer and saltier due to mixture with circum polar and other deep currents and acts as an important means of sediment transport. In the Mozambique Basin it produces giant ripples in the ocean bottom sediments (4km wave length) which migrate northwards due to the movement of the AABW. The AABW extends to about 25°S and then turns southwards and flows down the west flank of the Madagascar Ridge. Considerable sediment reworking and the existence of low carbonate sediments in the Crozet Basin are reported to result from the current activity of, and corrosive dissolution by, the AABW (Kolla et al, 1976a, 1976b). The AABW flows northwards across the Crozet Basin from the Southern Ocean and crosses into the Madagascar Basin through the Mid-Indian Ocean Ridge System at about 26 - 29°S/60-64°E. The existence of such a passage through the Mid-Indian Ocean Ridge System for the AABW is suggested by the presence of low carbonate sediments, which have been dissolved by the corrosive effects of the AABW flow. The AABW then flows northwards through the Madagascar and Mascarene Basins, where giant ripples have been reported (Ewing et al, 1968), as a boundary current (Kolla et al, 1976a) into the Somali Basin. Its northward extension is into the Arabian Sea, which it enters by flow through the Owen Fracture Zone in the Carlsberg Ridge. In the northern basins the vigour and corrosive power of the AABW is greatly-diminished.

In the eastern Indian Ocean, the AABW moves eastwards from the Adelie Coast/Ross Sea area, along the Antarctic continental margin and then turns north to the east of the Kerguelen Plateau (Kolla et al, 1976a; Kolla et al, 1978). Sediment erosion has been noted on the Kerguelen Plateau and probably results from the flow of the Antarctic Circum Polar Current (see below), as well as the.AABW (Kennett and Watkins, 1976; Kolla et al, 1978), the direct northward movement of the latter being inhibited by the Kerguelen Plateau. The AABW moves eastwards along the 47

southern boundary of the South East Indian Ocean Ridge, with local southward flows at about 85 - 100°E and 100 - 110°E, which are probably caused by topographic highs (Kolla et al, 1976a; Kolla et al, 1978). This eastward flow is strongly corrosive, probably due to enhancement by the high velocity Antarctic Circum Polar Current, with which it mixes (Kennett and Watkins, 1975; 1976; Kolla et al, 1978). The flow of the AABW divides, some corrosive water flowing eastwards into the South Tasman Basin (Kennett and Watkins, 1975; 1976; Kolla et al, 1978), while the remainder flows westwards and northwards into the Wharton Basin. This northward flow is erosive and flows over the South East Indian Ocean manganese nodule pavement (Payne and Connolly, 1972; Kennett and Watkins, 1975; 1976; Watkins and Kennett, 1976) and may also account for erosion on the Naturaliste Plateau (Kennett and Watkins, 1976). In the Wharton Basin, there is very little current activity north of 20°S, while the presence of a northward moving current off the western Australian continental margin is inferred from the evidence of Kennett and Watkins (1976).

Although the Ninety East Ridge forms an effective barrier to the passage of the AABW from the Wharton to the Central Indian Basin, Kolla et al (1976a) suggest that the AABW enters the Central Indian Basin by flow along the Amsterdam fracture zone. The presence of disconformities in sediments in the eastern Indian Ocean, e.g. the Wharton Basin, which expose Miocene sediments, have been reported by Kennett and Watkins (1976) as being evidence for major bottom current activity during the middle and early Cainozoic. The AABW is probably the cause of this deep water erosion.

The AABW began to circulate about 38 m.y. B.P., in the Oligocene, according to evidence from oxygen isotopes (Kennett and Watkins, 1976). The source of such Cainozoic bottom waters was not the Ross Sea since Antarctica/Australia had not separated until the late Oligocene (Kennett et al, 1974). The source was probably the Weddell Sea (Kennett and Watkins, 1976). The exact history of the AABW is not clear, but it is probable that glaciation in and around Antarctica has an effect on the amount of AABW reaching the basins of the Indian Ocean. 48

1.3.4. Antarctic Circum Polar Current (or Deep Water).

The Antarctic Circum Polar Current (ACPC) is a fast, eastward flowing bottom current which circulates around the Antarctic continent and transports at a rate of 200 x 1060 water/sec (Kennett et al, 1974). It is divisible into several components, among them the AABW, which has a greater effect in the southern and western Indian Ocean than in the eastern Indian Ocean (Payne and Connolly, 1972; Kolla et al, 1976a). Parts of the ACPC have been reported as being channelled through the Tasman fracture zone into the Tasman Basin, in the extreme south east corner of the Indian Ocean. The effect of the ACPC is most markedly observed in the south eastern Indian Ocean between the Australian and Antarctic continents. Current activity leading to sediment erosion over the Naturaliste Plateau (Kolla et al, 1976a) and the flanks and crest of the Kerguelen Plateau at depths of 1700m

(Kennett and Watkins, 1975) have been attributed to the eastward movement of the ACPC. The ACPC was formed as a result of plate tectonics in the high southern latitudes and Antarctic glaciation over the last 60 m.yrs (Kennett et al, 1974; Kennett and Watkins, 1976). During the late Cretaceous and Early Tertiary, Antarctica and Australia were joined together, and no ACPC existed (Kennett et al, 1974). Following the separation of these continents, movement of the ACPC was still prevented by the existence of the South Tasman Rise, a continental block (Kennett et al, 1974). However, during the late Eocene to the late Oligocene, condensed sediment sequences formed indicating bottom current activity, coincidentally with the separation of the South Tasman Rise from Australia. This bottom current activity marked the initial active development of the ACPC. During the Neogene and Oligocene, little sediment erosion occurred due to northward deflection of the ACPC up the east coast of New Zealand (Kennett et al, 1974). Current activity, since the Oligocene by the ACPC, together with AABW current activity has been evidenced by ocean floor features such as sediment disconformities, and manganese nodule development in the area between Australia and Antarctica (Kennett and Watkins, 1976; Watkins and Kennett, 9977).

1.3-5 Minor Ocean Currents.

Kolla et al (1976a) have reported bottom features on the Agulhas Plateau which occur at depths too shallow to be caused by the AABW. 49

They suggest that this represents the penetration of, and erosion by, the North Atlantic Deep Water (NADW) (see Fig. 1.3.1). The NADW moves eastwards, mixes with the ACPC and then moves northwards, before turning southwards to the west of Madagascar, but does not generally effect the bottom sediment in these areas.

Kolla et al (1976a, c) have observed high levels of turbidity, redistribution of smectite-rich sediments from around Indian river mouths and sediment waves with 5m amplitudes and wave lengths of 1 - 3km in the western Bay of Bengal. The features they report are associated with valleys in the western Bengal deltaic fan on the floor of the Bay of Bengal. They are suggestive of southward and south-eastward flowing waters, i.e. bottom currents, which occur at depths too shallow to be associated with the AABW (Kolla et al, 1976c).

Swallow (1964) and Krauss and Taft (1964) have made observations on the Equatorial Under Current (EUC) in the Indian Ocean. This is a seasonal current, occurring at a shallow depth, from 75m down to a maximum of 400m, whose movement is probably linked with wind velocities and their directions (Swallow, 1964). It is a high salinity current which moves at speeds of up to 120m/sec (Swallow, 1964). It has its maximum development during the N.E. Monsoon, but due to its shallow extent, has little or no direct effect on Indian Ocean equatorial bottom sediments.

Sharma (1970) has reported data on the Transequatorial Current (TEC) which is generally deeper than the EUC, 100 - 1000m, but undergoes seasonal variations. Sharma (1970) has shown it is present off the East African coast, in the Arabian Sea and Bay of Bengal to depths of 100 - 500m, but like the EUC, due to its general shallow nature has little or no direct effect on the bottom sediments in the area being considered in this study.

1.3.6 Concluding Remarks.

Although minor surfaces and ocean floor currents do occur in the Indian Ocean, and may also effect bottom sediments to a limited extent, the main bodies of water responsible for bottom current effects are the AABW and the ACPC. 50

1.4 SEDIMENTATION IN THE INDIAN OCEAN

1.4.1 Introduction

In a study of the geochemistry of Indian Ocean sediments, both Recent and those from the geological past, it is important when discussing sedimentation in the Indian Ocean to consider, not only the present sedimentary regimes and sediment types, but also those which developed during the evolution of the Indian Ocean in the geological past. The results of Legs 22 to 27 of the Deep Sea Drilling Project are important in this respect because they provide evidence on past sediment distributions and aid in palaeogeographic reconstructions (Kidd and Davies, 1978). A study of past sediment distributions provides an understanding of the development of the relationship between the sedimentary and tectonic histories of the Indian Ocean. Data from empirical subsidence curves, sediment sequences at DSDP sites and palaeobathymetric reconstructions may be used in establishing such sediment distributions (Kidd and Davies, 1978; Luyendyk and Davies, 1974).

In order to understand better the sedimentation patterns of the Indian Ocean, sediment distributions are discussed since the Late Jurassic before a review of the present day sedimentation is given in terms of sediment type and thickness, sedimentation rates and clay mineral provinces, with particular reference to calcium carbonate sedimentation.

The account of past sedimentation patterns is based on the data from those volumes of the Initial Reports of the Deep Sea Drilling Project that refer to the Indian Ocean (von der Borch et al, 1974; Whitmarsh et al, 1974; Fisher et al, 1974; Simpson et al, 1974; Davies et al, 1974; Veevers et al, 1974). The sediment distributions are described in terms of six main sediment groups: terrigenous (clastic, detrital) sediments; volcanogenic sediments (ashes, clays, etc (Vallier and Kidd, 1977)); calcareous ooze (foraminiferal, coccolithic, etc); siliceous ooze (diatomaceous, radiolaran, etc); pelagic clays (sedimentation rate of less than O.5cm/1O3yrs; no bedding; no significant coarse fraction; accumulated very slowly; negligible organics and pyrite); and 'other' deep sea clays (Kidd and Davies, 1978). 51

1.4.2 Sedimentation in the Indian Ocean since the Late Jurassic

1.4.2a Pre - Late Cretaceous Sediments.

The Jurassic is represented at only one Site, 261, off north west Australia where the sediments are predominantly clays (Kidd and Davies, 1978). The Early Cretaceous is little better represented. Volcanogenic sediments occur on the Mozambique Ridge while off Australia detrital clays were accumulating. To the north and west siliceous sediments are recorded at DSDP Site 260. Albian/Aptian detrital clays occur on the Naturaliste Plateau from the erosion of basaltic rocks in the Perth Basin (Kidd and Davies, 1978). The separation of Australia from Antarctica, which began at this time (Luyendyk, 1974) is reflected in the volcanogenic sediments of the Mozambique Ridge and Naturaliste Plateau.

1.4.2b. Late Cretaceous Sediments (see Fig. 1.4.2b, Sediment Distribution, 75 - 70 m.y.B.P., Campanian-Maastrichtian).

During this period, in the now Wharton Basin, detrital clays were deposited on its margins, as at DSDP Site 258, from the erosion of volcanic rocks around Perth, while in the centre under deep water conditions, pelagic clays were deposited (Luyendyk and Davies, 1974; Kidd and Davies, 1978). On the Ninety East Ridge, the shallow water sediments and volcanic ashes at DSDP Site 216 reflect the volcanism occurring on the ridge. The northern part of the Ninety East Ridge (DSDP Site 217) together with the Broken Ridge and Naturaliste Plateau were receiving pelagic carbonates at this time (Luyendyk and Davies, 1974; Kidd and Davies, 1978). During this period the Ninety East Ridge, the Indian Subcontinent, the Broken Ridge and the Naturaliste Plateau formed a topographic barrier separating the enclosed eastern basin (described above) from the western basin which was open to the South Atlantic and the Tethys Ocean (Luyendyk and Davies, 1974; Kidd and Davies, 1978). Despite this, however, the sediments were relatively similar in both (Luyendyk and Davies, 1974). In the western basin detrital clays occur off E. Africa (Mozambique Basin) while elsewhere unfossiliferous clays occur in the Mascarene and Madagascar Basins. The presence of montmorillonite (from the erosion of basaltic material) in clays from the Madagascar Basin (DSDP Site 239) may indicate nearness to a Fig 1.4.2b

Sediment Distribution in the Indian Ocean, 75 - 70m m.y.B.P.

(Campanian - Maastrichtian)

(after Kidd and Davies, 1978; Sclater et al, 1977; Luyendyk and Davies, 1974)

LEGEND

Terrigenous Sediments

Other types, Deep Sea Clays

Pelagic Clays

Carbonate ooze

Siliceous ooze vvv Volcanogenic Sediments

spreading ridge (Kidd and Davies, 1978). Volcanogenic sediments occur on the Mozambique Ridge (Kidd and Davies, 1978) while calcareous oozes were deposited off the West coast of India and off Madagascar (Luyendyk and Davies, 1974). The calcite compensation depth (CCD) was shallower than at present, at a depth of between 3000 - 4000m (Kidd and Davies, 1978), with the deepest calcareous sediments forming at a depth of 3500m at DSDP Site 211. Such a pattern indicates that the developing basins were generally the sites of detrital clays formed by high sedimentation rates and that only at distance from the continents in the deepest ocean did pelagic clay develop (Kidd and Davies, 1978).

Calcareous Palaeocene sediments occur on the present ridges and plateaux while pelagic clays are developed at the centre of the basins. Terrigenous or silty detrital clays were deposited off E. Africa and the western coast of India at this time, while volcanogenic sediments occur on the Ninety East Ridge and Owen Ridge. Elsewhere Palaeocene sediments are generally absent (Kidd and Davies, 1978).

1.4.2c. Early Eocene Sediments (see Fig. 1.4.2c. Sediment Distribution, 53 m.y.B.P., Early Eocene).

During this period circulation between the eastern and western basins was still restricted by the Ninety East and Broken Ridges. The presence of littoral gravels at DSDP Site 255 indicates that the Broken Ridge was above sea level at this time (Luyendyk and Davies, 1974). Volcanism at the southern end of the Ninety East Ridge gave rise to volcanic ashes and shallow water sediments at DSDP Site 253 and was accompanied by the deposition of volcanogenic sediments on the Mozambique and Owen Ridges (Luyendyk and Davies, 1974; Kidd and Davies, 1978). The northern Ninety East Ridge, the Naturaliste Plateau and submerged margins of the Broken Ridge were receiving carbonate sediments, while calcareous sediments were also being deposited on the Chagos-Laccadive Ridge and Mascarene Plateau in the north western basin (Luyendyk and Davies, 1974; Kidd and Davies, 1978). Sedimentation in the western basin was dominated by the mid-ocean ridge system, while the northern connection with Tethys was becoming more restricted (Kidd and Davies, 1978). The CCD was probably at a depth of 3 - 4000m and as a result all the major basins - Wharton, Central Indian, Madagascar, Mozambique - were areas of non- or slow deposition of pelagic clays (Luyendyk and Davies, 1974; Kidd and Fig 1.4.2c

Sediment Distribution in the Indian Ocean, 53 m.y.B.P. (Early Eocene)

(after Kidd and Davies, 1978; Sclater et. al, 1977; Luyendyk and Davies, 1974)

Legend as for Fig 1.4.2b. 55

Davies, 1978). Detrital sediments, silty clays, occur off West Africa during this period. The Eocene is generally absent in the Somali Basin (DSDP Site 240), the N. Mascarene Basin (DSDP Site 248) and on the Mozambique (DSDP Site 249) and Madagascar (DSDP Site 246/7) Ridges due to thin accumulations and possible removal during the Oligocene unconformity (Luyendyk and Davies, 1974).

The Middle to Late Eocene is generally patchy and discontinuous, or generally similar to the sediment regime of the Early Eocene. However, two striking features occurred. The Broken Ridge subsided below sea level and the first major influx of terrigenous sediments into the Arabian Sea occurred along the Indus Trough due to the uplift of the Himalayas (Kidd and Davies, 1978).

1.4.2d. Early Oligocene Sediments (see Fig. 1.4.2d. Sediment Distribution, 36 m.y. B.P., Early Oligocene).

The Oligocene is generally poorly represented in the Indian Ocean due to the presence of the Oligocene unconformity (Luyendyk and Davies, 1974). The geography is very similar to that of the present day with three main regions: a northwest enclosed basin containing the Carlsberg Ridge, with a more restricted connection with Tethys; a central region dominated by the inverted 'Y' of the mid-ocean ridge system which is similar to the present one and was becoming active; and an eastern basin which was open to the Pacific due to separation of Australia and Antarctica thus providing a clear, narrow circum Polar passage and connections through the Ninety East Ridge between the Central Indian and Wharton Basins (Lu)endyk and Davies, 1974; Kidd and Davies, 1978). Volcanogenic (subaerial?) detrital sands and silts at DSDP Site 254 at the southern end of the Ninety East Ridge, indicate deposition in littoral or lagoonal conditions and this activity may be linked with the separation of the Kerguelen Plateau. Evidence of volcanic activity also occurs on the Carlsberg Ridge in DSDP Sites 234 and 238 (Kidd and Davies, 1978). Elsewhere on the Ninety East Ridge, the Broken Ridge and the Kerguelen Plateau pelagic carbonates occur. Calcareous sediments are also found between Madagascar and Africa, on the Chagos-Laccadive Ridge and Mascarene Plateau and on the area to the north of and west of them in the Arabian and Somali Basins. The CCD was probably at a depth of approximately 4000m during this period (Kidd and Davies, 1978). 56

Fig 1.4.2d.

Sediment Distribution in the Indian Ocean, 36 m.y.B.P. (Early Oligocene)

(after Kidd and Davies, 1978; Sclater et al, 1977; Luyendyk and Davies, 1974)

Legend as for Fig 1.4.26. 57

Detrital sediments occur off the north east coast of Africa and in the Arabian Sea where the first Indus fan sediments occur at DSDP Site 221. The build up of the Bengal Fan is also believed to have begun during this period. The Oligocene is generally absent from other areas, and in the ocean basins, thin, unfossiliferous clays, deposited at very slow sedimentation rates occur and represent dissolution facies related to the Oligocene unconformity (Luyendyk and Davies, 1974; Kidd and Davies, 1978).

1.4.2e. Neogene to Recent Sediments.

It would appear that very little has changed in terms of sedimentation since the Oligocene (Kidd and Davies, 1978). Terrigenous sedimentation has increased with the formation of the Zambezi Fan (DSDP Sites 248 and 250) during the Miocene; the continued progradation of the E. African continental margin and deposition of detrital clays in the E. Somali Basin; and the influx of Bengal Fan sediments from the Ganges/Brahmaputra River system during the Middle Miocene at DSDP Site 218 and also at DSDP Site 211 on the Ninety East Ridge. The development of the Sunda - Java Trench by the Miocene resulted, by middle - Pleistocene times, in the starving of sediments from the Nicobar Fan, transported from the N.E. and their deposition along the Java Trench instead. The development of the Indonesian island arc has resulted in the build up of volcanogenic sediments in this area (Kidd and Davies, 1978). India continued to move northwards during this period, the Gulf of Aden opened (Blow and Hamilton, 1975) and the Indus Fan continued to build up. By the Middle Miocene slumped detrital sediments, calcareous and volcanogenic sediments were deposited in the new Gulf of Aden. Siliceous sediments make their first appearance in the Wharton and Crozet Basins between Miocene and Pliocene times, while calcareous sedimentation, already extensive over the South East Indian Ocean Ridge was increased due to further lowering of the CCD, which caused calcareous sediments to succeed clays in the Somali Basin during the Miocene (Kidd and Davies, 1978). Kidd and Davies (1978) estimate that general oceanic conditions and circulation patterns have been similar to those of the present for the last 10 m.yrs. 58

1.4.3 Present-Day Sedimentation in the Indian Ocean.

1.4.3a. Present-Day Sediment Distributions.

Indian Ocean sediment distributions have been discussed by Ewing et al (1969) in terms of terrigenous, pelagic, aseismic ridge and plateau and mid-ocean ridge sediments mainly on the basis of seismic data, while Goldberg and Griffin (1970) have described the distribution of mainly terrigenous sediments in the northern Indian Ocean on the basis of sampling data. The results of the International Indian Ocean Expedition (IIOE) and data from the six legs of the Deep Sea Drilling Project (DSDP) concerned with Indian Ocean drilling have provided further data on present Indian Ocean sediment distributions. These later works have generally confirmed the original findings of Ewing et al (1969) and Goldberg and Griffin (1970) but have drawn attention to details which were not apparent in those earlier studies.

Fig. 1.4.3a is a map of present-day sediment distributions compiled mainly on evidence published on the IIOE (Udinstev et al, 1975) and on the results of the DSDP work as summarised by Luyendyk and Davies (1974) and Kidd and Davies (1978). CALCAREOUS SEDIMENTS Calcareous sedimentation is better developed at the present day in the Indian Ocean than in the past, due to the occurrence of the CCD at depths of between 4500 and 5000m, its deepest point during the course of evolution of the Indian Ocean (Kolla et al, 1976b; Luyendyk and Davies, 1974; Sclater et al, 1977 in press; Kidd and Davies, 1978). Calcareous sediments occur along the shallow continental shelves of Africa and Australia and along all major elevated areas, such as the mid-ocean ridge system, the aseismic ridges and plateaus, as well as in the shallow Somali Basin (Ewing et al, 1969 Luyendyk and Davies, 1974; Kidd and Davies, 1978). Ewing et al (1969) noted a general lack of sediments on all portions of the mid Indian Ocean Ridge system except the south-west Indian Ocean Ridge where pockets of sediment up to 300m thick occur in this area of complex topography. The occurrence of sediments on the South West Indian Ocean Ridge may be attributed to the late initiation of spreading along this ridge segment (Schlich, 1974). The absence of thick accumulations of calcareous sediments elsewhere on Fig 1.4.3a.

20°N Terrigenous Present Day Sediment Sediments Distribution in the Indian Carbonate Ocean Sediments (after Udintsev et al 1975; Siliceous Luyendyk and Davies, 1974; Sediments 10°11 - Kidd and Davies, 1978) 1000m and 4000m contours shown. Non Pelagic Clay Volcanogenic 0° Sediments

10° S~

20° S -

30°

T T 30°E 40°E 50°E 60°E 70°E 80°E 90°E 100°E 110°E 120°E 60

the mid-Indian Ocean Ridge system, despite the relatively high, if rather variable, sedimentation rate (see below) can be attributed to the young age of the ocean floor in these areas allowing little time for sediment 'accumulation (Ewing et al, 1969). Calcareous sedimentation is considered in more detail below (1.4.3d). DEEP SEA SEDIMENTS Below the CCD, the deep oceans are receiving two types of sediments. Siliceous oozes occur below the areas of equatorial and sub polar high biological productivity in the parts of the Crozet and Mascarene Basins and the central parts of the Wharton and Central Indian Basins. Such siliceous oozes have only been accumulating over the last 10 m.yrs (Kidd and Davies, 1978). Elsewhere in the oceans where the sedimentation rate is minimal or deposition may have ceased altogether, deep sea pelagic clays occur. These are found in the Wharton, Central Indian and South Australian Basins, where these areas of almost non- deposition may be related to the activity of ocean bottom currents, such as the Antarctic Bottom Water (AABW) (Kolla et al, 1976a; Kennett and Watkins, 1975; Watkins and Kennett, 1977). Deep sea clays, containing more detrital material, also occur in the Mozambique, Madagascar and Mascarene Basins, where their deposition is also strongly related to bottom current activity (Kolla et al, 1976a). TERRIGENOUS SEDIMENTS Superimposed on the above pattern of oceanic sedimentation is the influence of land derived sediments. Terrigenous sediment input has increased during geological time (Kidd and Davies, 1978) and now such sediments represent between 46 and 69% of the total sediment cover of the Indian Ocean (Ewing et al, 1969). The main accumulations of terrigenous sediments are in the north and west of the Indian Ocean, which have been deposited by rivers and turbidity currents (Ewing et al, 1969; Goldberg and Griffin, 1970) in the Indus Fan in the Arabian Sea and Bengal Fan in the Bay of Bengal, associated with the erosion of the Himalayas (Kolla et al, 1976c; Curray and Moore, 1971), and in the Zambesi Fan off west Africa, from the erosion of a wide area of southern Africa (Kidd and Davies, 1978). Little terrigenous sediment is at present removed from Australia by river erosion due to the general lowlying nature of the land mass during the Mesozoic (Kidd and Davies,

1978). 61

VOLCANOGENIC SEDIMENTS

The major development of volcanogenic sediments is to the south of the Indonesian volcanic arc, which contains 14% of the world's active volcanoes, and from which widespread dispersal by aeolian transport of pyroclastic deposits, occurs. However, volcanogenic sediments are also found on the South East Indian Ocean ridge around the St. Paul's and St. Peter's Rocks, on the Central Indian Ocean Ridge to the east of Mauritius and Reunion, on the Comores Islands to the N.W. of Madagascar and in the Gulf of Aden along the Arabian continental margin (Udintsev et al, 1975; Kidd and Davies, 1978).

1.4.3b. Present-Day Sedimentation Rates and Sediment Thicknesses in the Indian Ocean (see Figs. 1.4.3b(i) and 1.4.3b(ii)).

Sedimentation rates and sediment thicknesses of present day Indian Ocean sediments have been described by such workers as Baranov and Kuzmina (1958), Goldberg and Koide (1963), Ewing et al (1969) and Opdyke and Glass (1969) who employed a variety of methods and used data from sampling and geophysical techniques to describe rather limited subsets of data. Not until the advent of the IIOE have fairly extensive sets of data become available for interpretation. The description that follows is based on the data collected during the IIOE and published by Udintsev et al, (1975).

Figs. 1.4.3b(i) and 1.4.3b(ii) show the present day sedimentation rate (in mm/103) and sediment thickness (in km), respectively, for Indian Ocean sediments.

The Mid-Indian Ocean Ridge System is an area of outcrop of basaltic material, has very little sediment cover (Ewing et al, 1969) and is an area of highly variable sedimentation rate, 3 - 7mm/103yrs (Opdyke and Glass, 1969). The lack of sediment in such areas and variable sedimentation rate can probably be attributed to the young age of the basalt allowing little time for sediment accumulation and the variable topography causing uneven distributions, dissolution of carbonate material etc. The mid-ocean ridge system is an area protected from continental detrital sources (Griffin et al, 1968). Other areas of the ocean floor, ie. the Ninety East Ridge, the Broken Ridge, the Chagos-Laccadive Ridge, Mascarene Fig 1.4.3b(i) Areas of 200 N Sedimentation Rate in variable mm/103 years in Indian > 100 Ocean Sediments. 30 - 100

30 - 10 (Sedimentation rates are 100 N averages for 0.69 m.y.B.P Brunhes-Matuyama geomagnetic-polar boundary)

(after Udintsev et al, 00 1975)

1000 and 4000 metre contours shown.

10o S

a, N /no c ,bnn r a no c. .7n0 An0 r nnn r f n r 0 r 63

Plateau and Mozambique and Madagascar Ridges, also have highly variable sedimentation rates but have sediment thicknesses of up to 1km. (Udintsev et al, 1975). This is probably a reflection of the age of the ocean floor in these areas in comparison to that of the mid-ocean ridge system (Opdyke and Glass,1969; Ewing et al, 1969).

In areas of terrigenous sedimentation there is a direct correlation between high sedimentation rates and large thicknesses of sediment. This occurs in the Arabian Sea on the Indus Fan; on the Bengal and Nicobar Fans to the west and east of the Ninety East Ridge, respectively, in the Bay of Bengal; in the Somali Basin; and in the Zambesi Fan between Madagascar and E. Africa. I-lere sedimentation rates are generally in the range of 10 - 10Omm/103yrs but can exceed 100mm/103yrs, as in the Bay of Bengal and Andaman Sea, and produce sediment thicknesses of between 1 and 3km, but thicknesses of up to 12km can occur, as in the Bay of Bengal (Curray and Moore, 1971). These terrigenous sediments are generally river borne, but can also be introduced by aeolian transport from the erosion of such areas as southern India, the Himalayas, Indonesia, Southern Arabia and southern and central Africa. The sources of such terrigenous material will be considered in 1.4.3c (below).

Correlations occur between areas of low sedimentation rates (less than 3mm/103yrs) and shallow thicknesses of sediments (less than 1km) as in the Crozet Basin, the Central Indian Basin, the Madagascar Basin and the Mozambique Basin. Such areas may be isolated from continental detrital input and may be areas of slow or non-deposition. However, there are areas of the Indian Ocean where the sedimentation rate is extremely low (less than 1mm/103yrs) but the sediment thickness may be up to 1lcm, as in the southern Wharton and Crozet Basins, the South Australian Basin and the southern Madagascar Basin, and where the sedimentation rate is moderate, 30 - 3mm/103yrs, but the sediment cover is very thin, less than 0.1km, as in the Central Indian and Wharton Basins. Both such relationships can be explained in terms of ocean bottom current activity. In the latter case (moderate rate, low thickness) bottom current activity in these areas, by the AABW, (Kolla et al., 1976a; Kennett and Watkins, 1975; Watkins and Kennett, 1977) prevents sediment accumulation and causes winnowing and sediment dispersion. In the former case (i.e. low sedimentation rate and fairly thick sediment core), the Fig 1.4.3b(ii)

Sediment Thickness in km of Indian Ocean Sediments (after Udintsev et al, 1975)

1000 and 4000 metre contours shown.

Cr) 30° E 40° E 50° E 60° E 80° E 100° E 1100 E 120°. E 65

recent onset of the operation of the AABW (Kolla et al, 1976a; Kennett and Watkins, 1975) as in the Crozet and southern Wharton Basins prevents further deposition on an existing moderately thick sediment cover. Such interpretations are in good agreement with the oceanographic data for these areas. In some areas in the Crozet, Madagascar and Mozambique Basins where low sedimentation rates are associated with shallow sediment cover, winnowing and dissolution of sediment and non-deposition by the aggressive effect of the AABW is probably in operation (Kolla et al, 1976a; Burkie et al, 19711; Kolla et al, 1976c).

1.4.3c Clay Mineral Provinces of Present-Day Sediments in the Indian Ocean (see Fig. 1.4.3c).

A study of the clay mineral content of the less than 2mm fraction of Indian Ocean surface sediments allows an understanding of the distribution of the clay minerals, and the derivation of the sediments in terms of source and dispersal in the ocean basins. Clay mineral sedimentation, although potentially complex, is influenced by the varied nature of the ocean floor features of the Indian Ocean, the asymmetric distribution of the land masses, together with their divergent climates, topography and geology (Venkatarathnam and Biscaye, 1973).

Various workers have carried out studies of clay mineral distributions in Indian Ocean surface sediments. Griffin et al (1968) studied their distribution in terms of chlorite, mor.tmorillonite, illite and kaolinite for the three major oceans, and postulated oceanic provinces based on transport paths. Rateev et al (1969) studied clay mineral distributions in the Indian and Pacific Oceans in terms of kaolinite, gibbsite, chlorite and illite and noted that their distributions were controlled by such parameters as sedimentation conditions, catchment geology of the continents, climatic zonation, types and degree of weathering, occurrence of ocean currents and transporting waters and the influence of volcanism. The distributionsthey described tended to be latidudinal. ones. Goldberg and Griffin (1970) have reported on the wind blown and river borne clay mineral components of the northern Indian Ocean. However, all these studies were based on rather limited sample sets and more recent work (Venkatarathnam and Biscaye, 1973; Kolla et al, 1976d; Kolla et al 1976a, c) has shown that the distributions are rather more complex than previously described, but generally result from the parameters identified by Rateev et al (1969). 66

Venkatarathnam and Biscaye (1973) and Kolla et al (1976d) have carried out an extensive study, incorporating the data of Goldberg and Griffin (1970), of clay mineral distributions in both the eastern and western basins of the Indian Ocean. Their data provide an extensive survey of the sources and distribution of clay minerals in Indian Ocean surface sediments. Venkatarathnam and Biscaye (1973) and Kolla et al (1976d) recognise four types of provinces - smectite-rich, illite-rich, kaolinite-rich and mixed - covering seventeen province areas. These are shown in Fig. 1.4.3c, together with indications of the direction of sediment transport. SMECTITE-RICH PROVINCES The provinces are distinguished by containing more than 60% smectite in the sediments. The Deccan Province which surrounds southern India in the eastern Arabian Sea and western Bay of Bengal corresponds approximately to the montmorillonite province of Goldberg and Griffin (1970). The sediments also contain less than 2D% illite and less than 12% each of kaolinite and chlorite. The smectite is derived by the rivers of southern India from the erosion of the basaltic rocks of the Deccan Trap lavas in an arid climate, the Krishna and Godavari rivers having their headwaters in areas of smectite-rich soils (Venkatarathnam and Biscaye, 1973; Kolla et al, 1976d). The smectite is dispersed by slumping and turbidity currents along the canyons on the western edge of the Bengal Fan, surface currents and south flowing turbidity currents along the canyons on the western edge of the Bengal Fan. Surface currents and south flowing turbidity currents may also act as a means of dispersion (Venkatarathnam and Biscaye, 1973; Kolla et al, 1976c, 1976d). Sediments of the Indonesian province contain less than 20% illite, less than 10% chlorite, and less than 20% kaolinite as well as more than 60% smectite. The smectite derives from the area on the landward side of the Java Trench and from the Coccos-Roo Rise areas and is distributed to the west as far as the Ninety East Ridge by aeolian transport. Its extent is marked by the occurrence of volcanic ash material in the sediments. In situ alteration of volcanics may contribute some smectite to this province (Venkatarathnam and Biscaye, 1973). The inter Ridge Province extending over areas of the Central Indian Ocean Ridge and South East Indian Ocean Ridge derives its smectite from the in situ alteration of volcanic material, transported over short distances, together with minor amounts from atmospheric fall out. Volcanic ash is common east of 80°E and tephra from the Indonesian arc may be introduced by the N.E. trade winds (Venkatarathnam and Biscaye, 1973; LEG E N D Fig 1.4.3c. Smectite-Rich Clay Mineral Provinces Provinces in the Indian Ocean m Illite-Rich Provinces (after Kolla et al 1976d; Kaolinite-Rich Venkatarathnam and Provinces Biscaye, 1973; Goldberg Mixed Provinces and Griffin, 1970) -4110.-Direction of Sediment Transport PROVINCES: Province Boundaries 1. Deccan ,'t"4000 metre con to 2. Ganges 0° 3. Antarctic 4. S. African 5. Zambezi 6. Arabian 7. Indus 8. Indonesian 100 S 9. Australian 10. Central African 11. Madagascan 12. Inter Ridge 13. Antarctic-Crozet 14. Somali Mixed 15. Zambezi-Madagascan 16. Zambezi In Situ 17. Mixed Illite

30° E 40° E 50° E 60° E 80° E 90° E 100° E 110° E 120° E 68

Kolla et al, 1976d). The Antarctic-Crozet Province is influenced by the transport of smectite-rich clays from the southern ocean volcanic areas of the Crozet and Kerguelen Plateau by the AABW. There is also a local volcanic influence in this province on the Central Indian Ocean Ridge . and from the influx of material from the Madagascar Province (Kolla et al, 1976d). ILLITE-RICH PROVINCES These provinces are distinguished by containing up to 70% illite and greater than 10 - 12% chlorite in their sediments. The sediments of the Ganges Province which occupies the eastern Bay of Bengal and extends southwards to east and west of the Ninety East Ridge as far 0 as 8 S are derived from the erosion of the Himalayas by mechanical weathering and their transport by the Ganges-Brahmaputra river system as a heavy sediment load. The illite-rich sediments are dispersed by turbidity currents for distances up to 3000km from their source.- The absence of illite from the northern Ninety East Ridge modifies the pattern suggested by Goldberg and Griffin (1970) (Venkatarathnam and Biscaye, 1973). The Arabian and Indus provinces are also high in chlorite. The Indus Province occupies the Indus Cone and the Arabian Province the western Arabian Sea and the Carlsberg and Owen Ridges. The two provinces overlap in the Southern Arabian Sea. The difference between the two is that in the Arabian Province, the palygorskite content is high and is correlated with topographic highs, reflecting the dominance of aeolian processes noted by Goldberg and Griffin (1970) which remove material from the deserts of Somalia and Arabia during the monsoon and by N.W. winds (Kolla et al, 1976d). Palygorskite is virtually absent in the Indus province where the sediments are river derived from the erosion of the Himalayas and are dispersed by turbidity and surface currents (Kolla et al, 1976d). In the South African province which extends from the continental margin to the Agulhas Plateau the sediments are derived from the illite- rich soils of southern Africa (Kolla et al, 1976d). In the Zambesi Province the sediments contain 30 - 60% illite and 15 - 25% kaolinite (and gibbsite). The province extends between Madagascar and east Africa, into the Mozambique Basin and onto the Madagascar Ridge, where the sediments are diluted by Madagascar Province clays. The sediments are formed by the erosion of the Zambesi River basin in Africa 69

rich in kaolinite and illite and are transported by the Agulhas Surface current and AABW (Kolla et al, 1976d). The sediments of the Antarctic Province contain 30 - 60% illite and less kaolinite and more chlorite than those of the Zambesi Province. They are transported into the Antarctic Province from the Agulhas and N.W. Antarctic-Indian Basins by the AABW. Ice rafting is a possible source of illite in this province. The ultimate source of the sediments is the continental margin of Antarctica. KAOLINITE-RICH PROVINCES. The Australian Province sediments contain more than 20% kaolinite, less than 60% smectite and less than 20-25% illite. The province extends across the south Wharton Basin to the west of the Ninety East Ridge and south at 150 and corresponds approximately to the high kaolinite area of Goldberg and Griffin (1970). The kaolinite derives from the erosion of laterite soils of western Australia and their transport by the South East trade winds. Coastal erosion may also contribute to the seaward movement and smectite may be added by the alteration of local volcanics and from global fallout (Venkatarathnam and Biscaye, 1973). The kaolinite-rich provinces of Central Africa and Madagascar derive their kaolinite from the African and Madagascar land masses and it is dispersed by surface and deep circulation (Kolla et al, 1976d). MIXED PROVINCES These are the Somali Mixed Province, the Zambesi-Mozambique Province, the Zambesi In Situ Province and the Mixed Illite Province. No specific mineral is dominant in them, but the clay mineralogy of the sediments generally reflects the in situ alteration of basalt and the influx of sediment from adjacent clay mineral provinces (Kolla et al, 1976d).

1.4.3d. Calcareous Sedimentation in the Oceans and the Distribution of Calcium Carbonate in Present Day Indian Ocean Surface Sediments (see Fig. 1.4.3d).

CALCAREOUS SEDIMENTATION IN THE OCEANS The amount of Ca in the ocean input from rivers could all be delivered to the oceans within a million years (Berger,1974). To balance this the oceans precipitate CaCO3 in near surface waters in shell building organisms. Some of these shells settle onto the ocean floor and are preserved while those at greater depth are dissolved because of the undersaturation of seawater with respect to Ca, with increases in pressure and 70

decreasing temperature. Such a process was first recognised by Murray and Renard (1891) who also noted two further factors which control carbonate percentages, namely the supply of shells and dilution by detrital matter and clay. The processes of carbonate dissolution have been widely studied by Berger, 1970, 1971; Edmond, 1974; Broecker and Broecker, 1974; and others (see Berger, 1974). There are two important terms which require clear definition in consideration of calcareous sedimentation in the oceans - the calcite or carbonate compensation depth or CCD and the lysocline. The CCD is the depth in the oceans at which the rate of supply is exactly balanced by the rate of dissolution, there being therefore no net accumulations of CaCO3. In practice, it is the level where the CaCO3 Wt% drops towards zero (Berger, 1974). In a recent study of Ca:03 in Indian Ocean sediments discussed below, Kolla et al (1976b) used Calcite critical depth or CCrD and defined it as the depth below which less than 10% carbonate is present. The depth of the present CCD or CCrD in the Indian Ocean is between 4800 and 5400m (Kolla et al, 1976b). The lysocline as defined by Peterson (1966), Berger (1967) and Ruddiman. and Heezen (1967) is the boundary between well and poorly preserved species of foraminifera or the level at which there is the maximum change in the composition of the calcite fossil assemblage due to the differential dissolution. As such this is the definition of the foraminiferal or coccolith, etc lysocline, which collectively are termed the 'sedimentary lysocline' (Berger, 1974). The term lysocline is also used to describe the level at which drastic increases in the CaCO3 dissolution rates occur and as such is termed the 'hydrographic lysocline'. Kolla et al (1976b) use the term 'carbonate lysocline' which they define as the level at which significant decreases in the CaCO3 percentages occur, i.e. there is an abrupt change in the carbonate-depth trend. Such a definition indicates that this is the hydrographic lysocline. The distinction between these uses of the term lysocline are important because in biologically fertile regions the sedimentary lysocline may become shallow and diffuse (Berger, 1974). The depth of the lysocline in the sample area considered later in this thesis was determined by Kolla et al (1976b) on the basis of carbonate depth plots to be at about 3900 - 4000m. High biological productivity will depress the depth of the CCD. This is because productivity is proportional to calcite accumulation. In shallow areas, near the continents, the relationship is an inverse 71

one, there are high organic contents in the sediments which produce increases in benthonics, leading to an increase in CO2 rich interstitial waters. This causes dissolution of calcite at shallow depths, above the lysocline. In equatorial, deep ocean regions the supply of calcite shells is greater than that of organics, there is less CO2 in interstitial waters as a result and CaCO3 is dissolved at greater depths, thus deepening the CCD (Berger, 1974). Thus carbonate dissolution will be controlled by the supply of calcite and organics which effect interstitial water acidity and control dissolution. Ocean bottom currents such as the AABW can cause dissolution either by their critical undersaturation or by their rates of flow and hence CaCO3 distributions can provide clues to oceanic bottom circulation (see Kolla et al, 1976b) (Berger, 1974). The precipitation of CaCO3 in carbonate shells exceeds the supply of Ca by rivers thus producing depleted surface waters. By cooling, compression and CO2 uptake these become undersaturated bottom waters and therefore can redissolve the supply of CaCO3 and maintain a steady state situation (Berger, 1974). It is important to understand the processes involved in carbonate sedimentation because this has bearing on the geochemistry of a group of surface sediments, described later, which were sampled from sites above and below the hydrographic lysocline along the Central Indian Ocean Ridge. DISTRIBUTION OF CAC03 IN PRESENT DAY INDIAN OCEAN SEDIMENTS The distribution of CaCO3 in Indian Ocean surface sediments is displayed in Fig. 1.4.3d and based on the data reported by Udinstev et al (1975) and Kolla et al (1976b). The CaCO3 distributions display the well known first order correlations with depth. Hence in all the ocean basins - Wharton, Somali, Central Indian, Crozet, South Australian - Ca CO values do not exceed 10%, while on the topographic highs, such as 3 the Mid Ocean Ridge system, the Ninety East Ridge, the Broken Ridge, Naturaliste Plateau, Chagos-Laccadive, Madagascar Ridges and Mascarene Plateau high CaCO3 concentration occur reflecting deposition above the lysocline. Locally low CaCO3 concentrations occur as at 29 - 26°S/60-64°E on the Mid Indian Ocean Ridge System. This is due to the greater depths associated with fracture zones, and dilution by influx-of non-carbonate sediments (siliceous clays and diatoms) from the Antarctic carried by the AABW, as well as the dissolution effects of this ocean bottom current (Kolla et al, 1976b, 1976a). This area marks the passage of the AABW Fig 1.4.3d. Calcium Carbonate Content of Bottom Sediments in the Indian Ocean (after Udintsev et al, 1975, and Kolla et al, 19766)

1000 m and 4000 m contours shown. 73

through the mid Indian Ocean Ridge system between the Crozet and Madagascar Basins. Low carbonate values occur in the Crozet Basin due to the dissolution effects of the AABW and due to dilution by the influx of siliceous sediments. The dilution by siliceous sediments also occurs in equatorial regions of the Central Indian and Wharton Basins (Kolla et al, 1976b). Terrigenous dilution occurs in the Bengal Fan, the Arabian Sea, the Mozambique and possibly also parts of the Crozet Basin (Kolla et al, 1976b). Superimposed on this pattern is a secondary variation with respect to latitude resulting in variations in depth of the CCrD (CCD) and carbonate (hydrographic) lysocline (Kolla et al, 1976b). The trends are similar to those of the Atlantic and Pacific Oceans, with the CCrD (CCD) being deepest in the equatorial zone, shallowing to the south, deepening slightly at 40 - 50°S and becoming shallowest (3900m) in the polar region (60°s). The carbonate (hydrographic) lysocline behaves oppositely to the CCrD (CCD) north of 50°S and similarly between 50 and 60°S. Such variations are explicable in terms of the process related to biological productivity and bottom current activity by high acidity, undersaturated waters described above (Berger, 1974; Kolla et al, 1976b).

1.4.4 Concluding Remark

The foregoing description has broadly summarised the nature of Indian Ocean sedimentation since the Late Jurassic through to the present day in terms of sediment type and distribution, sediment thickness, sedimentation rate and distribution of clay minerals, paying particular attention to the area of calcareous sedimentation. It is within this context that the following discussions of the geochemistry of surface and basal sediments are considered. 74

Section 2

2. THE GEOCHEMISTRY OF INDIAN OCEAN DSDP BASAL SEDIMENTS

2.1 INTRODUCTION AND SAMPLE DESCRIPTION

2.2 BULK GEOCHEMISTRY OF INDIAN OCEAN DSDP BASAL SEDIMENTS

2.3 GEOCHEMICAL PARTITION OF INDIAN OCEAN DSDP BASAL SEDIMENTS

2.4 DISCUSSION OF THE GEOCHEMISTRY OF INDIAN OCEAN DSDP BASAL SEDIMENTS 75

2.1 INTRODUCTION AND SAMPLE DESCRIPTION

2.1.1 Basal Metalliferous Sediments in General

The great attention paid to the geochemistry of metalliferous sediments in recent years has chiefly been concerned with the surface and near surface occurrences of these deposits. Such deposits have been frequently described from localities in the world's oceans (from the Pacific by Bostrdm and Peterson, 1966, 1969; Bostr8m et al, 1969; Horowitz, 1970; Dasch et al, 1971; Piper, 1973; Sayles and Bischoff, 1973; Bertine, 1974; Sayles et al, 1975; Dymond and Veeh, 1975; from the Atlantic by Cronan, 1972; Horowitz, 1974a, b; from the Indian Ocean by Bender and Schultz, 1969; Bostr8m et al, 1969; Horowitz, 1970; Bostr8m and Fisher, 1971; and from the Red Sea by Bignell, 1975 and Bignell et al, 1976). However, since the advent of the Deep Sea Drilling Project (DSDP) in the late 1960's it has been possible to study the geochemistry, and other geological features, of sediments from most of the sedimentary column contained within the ocean basins.

Occurrences of metalliferous sediments from DSDP cores have been reported in the literature since 1970. These sediments are usually found to be directly overlying, the basaltic oceanic crust. The palaeontological ages of such deposits have been found to be in good agreement with the estimated palaeomagnetic ages of the basaltic ocean floor in the same cores. This type of deposit has been described from the Pacific Ocean by von der Borch and Rex (1970), von der Borch et al (1971), Cook (1971), Heath and Moberly (1971), Cronan et al (1972), Cronan (1973), Cronan and Garrett (1973), Dymond et al (1973, 1976), Swanson and Scott (1974), Bostrdm et al (1976), and Cronan (1976); from the Atlantic Ocean by Peterson et al (1970), Bostr8m et al (1972), Horowitz (1974a), Horowitz and Cronan (1976) and Chester et al (1976); and from the Indian Ocean by Cronan et al (1974), Gieskes et al (1974), Fleet and Kempe (1974), Marchig and Vallier (1974), Pimm (1974), Warner and Gieskes (1974) and Fleet (1977).

The apparent lack of post-depositional alteration affecting these sediments (Heath and Moberly, 1971; Bostrfm et al, 1972; Cronan, 1974) has permitted comparisons to be drawn with ridge crest metalliferous sediments. Although such comparisons have been carried out chiefly in 76

the Pacific (Cronan, 1974, 1976) and the Atlantic oceans (Horowitz, 1974a) it has indicated that the two types of deposit are chemically similar (Cronan, 1974; Horowitz, 1974a).

This evidence, together with the now known widespread occurrence of basal metalliferous material has led to the theory that the basal deposits represent the ancient analogues of existing crestal metalliferous sediments. Furthermore, that they have been moved to their present locations, from the crest and flanks of active ocean ridges, by the process of sea-floor spreading (Peterson et al, 1970; von der Borch and Rex, 1970; Cook, 1971; Bostrdm et al, 1972; Cronan et al, 1972; Cronan, 1974, 1976; Horowitz, 1974a; Horowitz and Cronan, 1976). The similarities between the crestal metalliferous sediments and their ancient analogues from the East Pacific Rise and the Northern Mid- Atlantic Ridge tends to suggest that the processes which form them at present are similar to those which operated in the geological past (Cronan et al, 1972; Horowitz, 1974a). This has also been shown to be the case in the South Atlantic Ocean where processes appear to have remained constant since Eocene times (Bostrdm et al, 1972). However, it has been shown (Bostrdm et al, 1972; Cronan et al, 1972; Horowitz, 1974a) that the composition of basal metalliferous sediments may be variable through geological time. For example, Cronan et al (1972) reported that Middle Eocene sediments cored during DSDP Leg 16 were more metal enriched than Oligocene sediments cored during the same Leg. These variations may result from changes in ridge derived metal additions caused by fluctuations in ridge spreading rate or may be due to alterations in the physio-chemical environment of deposition (Horowitz, 1974a), e.g. reducing conditions producing enrichment in Fe, U, V and Cr, etc (Lynn and Bonatti, 1965; Calvert and Price, 1972) or oxidising conditions changing to reducing conditions producing Mn deposition followed by upward migration (Li et al, 1969; Horowitz, 1974a). Furthermore changes in deposition patterns may decrease the proportion of clay minerals present thus decreasing the absorbing capacity of the sediments.

In the Atlantic Ocean it may be possible to suggest whether the processes of formation of metal-rich sediments have remained constant or varied through geological time. Horowitz (1974a) has shown that metal additions from the ridge and detrital sources have remained 77

relatively similar since Aptian/Cenomanian times (106 m.y.B.P.). However, further information on mineralogy, sedimentation, spreading rates and certain physio-chemical parameters will be required before the question can be fully resolved.

Chemical partition studies have been carried out on basal metalliferous deposits from the Pacific (Cronan and Garrett, 1973; Cronan, 1976) and the Atlantic Oceans (Horowitz, 1974a; Horowitz and Cronan, 1976; Chester et al, 1976). These studies have shown that the partition of trace metals in basal and crestal metalliferous sediments is similar. In general about 85% of the Fe, together with major amounts of such trace metals as Cu and Zn are concentrated in the I-ICL-soluble phases, while 98% of the Mn, together with major amounts of the Ni and Co, are concentrated in the hydroxylamine HCL soluble phases of the sediments (Cronan and Garret, 1973; Horowitz, 1974a; Cronan, 1976). The different compositions of basal sediments from the Pacific and the Atlantic Oceans reflect the differences in sedimentation between these basins, i.e. higher detrital sedimentation rates and contribution and lower hydrothermal contributions in the Atlantic as compared to the Pacific Ocean (Horowitz, 1974a; Cronan, 1976). No chemical partitions studies have been carried out to date on basal metalliferous sediments from the Indian Ocean.

Pimm (1974) has described the occurrence of basal metalliferous sediments from DSDP Sites 211, 212 and 213, cored in the Wharton Basin, eastern Indian Ocean during Leg 22 of the DSDP. The sediments were of the basal Fe oxide facies recognised by other authors from other oceans, (e.g. the Pacific, von der Borch and Rex, 1970; Cronan et al, 1972). Although the Fe oxide rich clayey aggregates were between 30 and 40 metres thick at DSDP Sites 212 and 213, it was only in the 5 metres immediately adjacent to the basalt that the total Fe concentrations exceeded 10%. Samples above this contained concentrations of Fe comparable with typical pelagic brown clays. The concentrations of Mn and other trace metals showed no coincident enrichment with Fe. Only at DSDP Site 212 was Zn significantly enriched with Fe, while the concentrations of the trace metals in the basal Fe oxide facies were comparable to those of deep sea clays higher up the stratigraphic column. Pimm (1974) attributes the origin of these basal metal-rich sediments to 78

volcanic hydrothermal exhalations following the Bostrdm and Peterson (1966) model for East Pacific Rise sediments. He further shows that there is an absence of Fe enrichment in calcareous sediments at the base of DSDP Site 211, in a metamorphosed limestone between basalt pillows at DSDP Site 212 and in calcareous ooze horizons interbedded with metal enriched clay horizons of DSDP Site 213. This he suggests supports the hypothesis that the Fe oxide material is contemporaneous with the sediments and has not been derived by the leaching of basaltic basement. The analyses of samples of igneous rocks cored from these sites show no Fe or trace metal depletion, thus supporting his hypothesis. It should be noted that examination of the plots of trace metal concentration against depth (Fig. 2 and 3, Pimm, 1974) shows that Fe, V, Cu, Ni, Pb and Zn (and to a lesser extent Mn at DSDP Site 213 only) increase in concentration with depth at these three sites. Pimm (1974) has also noted the occurrence of Mn enriched layers, 10 - 30cm thick, at a depth of 18 to 50 metres below the sea floor at DSDP Site 213. These layers contain up to 8% Mn and although the Fe value is low, they are enriched in Cu, Pb and V. Pimm (1974) offers no explanation as to their mode of formation. However, since the sediments are diatomaceous siliceous oozes, it may be possible that the Mn has been remobilised at depth and then concentrated in this horizon by the mechanisms proposed by Bonatti et al (1966), Bostrtim (1967) and Lynn and Bonatti (1969).

Cronan et al (1974) whilst reporting on the sedimentology of sediments from the Gulf of Aden and western Indian Ocean during Leg 24 of the DSDP, noted the presence of basal metalliferous sediments at a number of sites. Although their development is less extensive than in sediments from the Wharton Basin (Pimm, 1974) they were found at DSDP Sites 235, 236 and 238. The lowermost sediments of DSDP Site 235 at 580.7 metres contain up to 8% Fe (CFB). However, Cronan et al (1974) report that it is not known if these sediments extend to the basement cored at 646 metres, since in the intervening interval no sediment was recovered. At DSDP Site 236 the basal Palaeocene or Eocene nannofossil clayey ooze is reported (Cronan et al, 1974) to contain a thin layer (a few cms) of yellow-green ferruginous clay which is poor in calcium carbonate and contains 23% Fe. This layer occurs 40cm above the basalt-sediment interface. At DSDP Site 23$, Cronan et al (1974) report the presence of Oligocene Fe-rich layers which contain more than 10% Fe. These sediments are clays with indurated carbonate oozes containing globules of amorphous 79

Fe-oxide. They are yellow to reddish-brown in colour and contain Mn micro-nodules which occur in association with zeolites (Phillipsite up to 35%). These sediments also contain up to 64% montmorillonite together with plagioclase and alkali feldspars. Cronan et al (1974) suggest that the Fe in these deposits is of hydrothermal origin. They also suggest that if Fe concentrations in basal sediments reflect ridge activity, then the Carlsberg Ridge at the time of deposition of the DSDP Site 236 basal sediments was similar in activity to the present East Pacific Rise, since the concentrations of Fe in DSDP Site 236 sediments and the East Pacific Rise crestal sediments are similar. By the same argument, the Central Indian Ocean Ridge at the time of deposition of the basal sediments at DSDP Site 238 was less active than the Carlsberg Ridge when the DSDP Site 236 sediments were deposited, because of the lower Fe values in the basal sediments at DSDP Site 238.

A further deposit of basal metalliferous sediments has been described by Warner and Gieskes (1974) and Gieskes et al (1974) from DSDP Site 245 (and 245A)which was drilled in the southern Madagascar Basin during Leg 25 of the DSDP. They report that this deposit overlies a diabase sill and contained sharply defined volcanic ash layers overlying a basal sequence of black manganiferous clay ooze. The sediments are Eocene and Palaeocene in age and the Fe-Mn material contained in them occurs as amorphous aggregates of very fine grains suggesting rapid formation. The sediments contain about 50 - 55% calcium carbonate, and are Al-poor and Fe-Mn rich in keeping with other basal metalliferous sediments (Cronan et al, 1972) and present active ridge sediments (Bostr8m et al, 1966, 1969). The metal concentrations reach about 11% Fe and about 3% Mn. Copper and zinc show moderate concentrations of up to 250 and 290ppm respectively. Warner and Gieskes (1974) reported that the high Al:Ti ratios in these sediments is the result of the high contribution of volcanic ash. They also calculated ratios of Al/A1 + Fe + Mn, similar to those used by BostrJm et al (1972), and found values comparable to those obtained in South Atlantic DSDP sediments (Bostr8m et al, 1972). On the basis of SiO :Al 203 and Al:Ti and other ratios, 2 Gieskes et al (1974) were able to distinguish three mineralogical zones in the sediments. Warner and Gieskes (1974) suggest that the Fe and possibly the Mn at DSDP Site 2115 was associated with volcanic ash, either in layers or in disseminated fragments, throughout the carbonate ooze. These elements appear to have been supplied by alteration of the volcanic 80

material rather than being supplied by hydrothermal emanations. Gieskes et al (1974) suggest that changes in the composition of the interstitial waters indicate post-depositional chemical reactions such as clay mineral and volcanic ash diagenesis. These data would tend to support a non- hydrothermal origin for the enriched elements in DSDP Site 245.

Marchig and Vallier (1974) have described sediments from DSDP Sites 248 and 249 cored during Leg 25 of the DSDP, and report the presence of basal metalliferous sediments in DSDP Site 248 only. DSDP Site 248 is in the western part of the Mozambique Basin and the sediments are believed to have accumulated near or below the calcite compensation depth (CCD). By contrast, the sediments from DSDP Site 249, located on the Mozambique Ridge, are believed to have accumulated above the CCD. The basal sediments from DSDP Site 248 are thin bedded brown and reddish-brown clays. They are similar to some sediments from the crest of the East Pacific Rise and show enrichments in Mn (0.6%), Fe (6.2%), V (600ppm) and Ti (1.2%). However, it should be noted that the metal concentrations are considerably lower than the concentrations of these metals in other basal metalliferous sediments from other DSDP Sites in the Indian Ocean (Pimm, 1974; Warner and Gieskes, 1974; Cronan et al, 1974). Furthermore, there is no enrichment in Cu, Ni and Mo as might be expected (Bostr8m et al, 1966; Cronan et al, 1972). The authors suggest that these metals have been affected by post-depositional remobilisation related to redox reactions and have hence been removed from the basal unit (Bonatti et al, 1971). They further suggest that this unit of about 40 metres was formed near a spreading ridge and that the present distribution patterns result from changes in the redox conditions which affected metal mobilities. No explanation of the exact source of the metals is given by the authors, but it is presumed that they originated from the hydrothermal leaching of basalts.

Fleet and Kempe (1974) and Fleet (1977) have carried out preliminary geochemical studies of sediments (91 in all) from DSDP Sites 250 to 258, drilled in the southern Indian Ocean during DSDP Leg 26. The sediments have been analysed for a wide range of elements. However, only at DSDP Site 254 do they report basal sediments which have Fe (12%) values comparable with those previously reported from basal metalliferous sediments from other sites in the Indian Ocean (Pimm, 1974; Gieskes et al, 1974; Warner and Gieskes, 1974; Cronan et al, 1974; Marchig and 81

Vallier, 9.974). The sediments here are volcanoclastic but do not contain enriched amounts of Mn and other trace metals except Cr and V. The authors suggest that these sediments have a basaltic provenance, and hence infer such a source for the enrichment in Fe. The authors also record that the non-carbonate material (basically clays) of these Leg 26 DSDP cores, shows enrichment in V, Co, Ni, Cu, Pb, and Zn with respect to near-shore sediments and igneous rocks. Furthermore, that the average values of V, Co and Ni are intermediate between those reported from the Atlantic and Pacific Oceans, while the Mn values are about half that reported for ocean bottom sediments (Fleet and Kempe, 1974; Fleet, 1977).

The studies to date carried out on Indian Ocean metalliferous sediments have proved extremely useful in describing their characteristics and in providing a preliminary basis for further work. However, a number of problems have been highlighted and are in need of clarification. Not least amongst these is the need for evidence on the geochemical partition patterns of trace and major elements in basal sediments from the Indian Ocean. To this end 154 samples from 22 DSDP Sites which recovered basaltic basement and were collected during DSDP Legs 22, 23, 24, 25 and 26,have been obtained and analysed for a wide range of elements (Ca, Al, Ti, Mg, Ba, Fe, Mn, Ni, Co, Cr, Cu, Cd, Pb, Zn, Li and As). A selected group of these sediments (64 in number) have also been analysed using the chemical partition techniques of Chester and Hughes (1966, 1969) and Cronan (1976). The data are presented in the succeeding sections in order to provide an overall view of the geochemistry of basal metalliferous sediments in the Indian Ocean. However, before describing and discussing the geochemical data it is necessary to set them in their geological context by describing the salient geological features of the various DSDP cores from which the samples for analysis have been taken.

2.1.2. Indian Ocean DSDP Site Location and Geological Setting, Sample Descriptions.

2.1.2a. Introduction.

The purpose of the present survey was to study the geochemistry of as many occurrences of basal metalliferous sediments as possible from the Indian Ocean. To this end samples were collected from the lowest litho- 82

logical units of those DSDP sites which struck basaltic basement since it has been shown that such sediments will be more likely to occur adjacent to the basaltic basement than elsewhere (Pimm, 1974; Cronan et al, 1974; Gieskes et al, 1974; Warner and Gieskes, 1974; Marchig and Vallier, 1974; Fleet and Kempe, 1974; Fleet, 1977). Therefore no samples have been studied from Indian Ocean DSDP sites which did not recover basaltic basement. Fig. 2.1.2a shows the main bathymetric features and locations of all DSDP sites in the Indian Ocean; those sites from which samples have been obtained are indicated in Fig. 2.1.2a. Table 2.1.2a contains relevant site data, and brief sample descriptions together with sample position within each core are given in Table 2.1.2b. The major features of the DSDP sites will be described in the. following sections. For more detailed information the reader should consult the pertinent volume of the DSDP Initial Reports (Leg 22 - von der Borch et al, 1974; Leg 23 - Whitmarsh et al, 1974; Leg 24 - Fisher et al, 1974; Leg 25 - Simpson et al, 1974; and Leg 26 - Davies et al, 1974).

2.1.2b. Leg 22 (Darwin - Colombo, 1972).

The sites (211-218) drilled on this leg were occupied between January and March 1972 and are all in the eastern basin of the Indian Ocean (von der Borch et al, 1974).

DSDP Site 211 is sited on the relatively smooth topography of the Cocos Basin, about 3300km WNW of Christmas Island and 1800km south of the Java Trench, in a water depth of 5528m. Basement was cored at a depth of 428.5m below the sediment-seawater interface. The basement is an altered amphibole-bearing basalt, considered to be possibly extrusive, i.e. pillow lavas (von der Borch et al, 1974), and is intruded by an amphibolite sill. The alteration was probably caused by the intrusion of the sill and subsequent submarine weathering. Two sediment samples (see Table 2.1.2b) were taken from this site and come from Unit 6, which immediately overlies the basaltic basement. This unit is a variegated cream and red-brown nannofossil-bearing clay of early Campanian-Maa.strichtiait age. It is 18m thick and contains a poor assemblage of nannofossils and planktonic foraminifera. 4o°E 60°E 8o°E 100°E 120°E

Fig. 2.1.2a. Location of DSDP Sites In the Indian Ocean.

Map after Luyendyk and Davies. 1974

TABLE 2.1.2a. DSDP INDIAN OCEAN SITE DATA

SITE NO LEG NO. SITE POSITION WATER DEPTH TO AREA OF DEPTH BASEMENT INDIAN OCEAN (M) (M)

211 22 09°46.53'S 102°41.95'E 5528 428.5 Cocos Basin 212 22 19°11.34'S 99°17.84'E 6240 516 Wharton Basin 213 22 10°12.71'S 93°53.77'E 5609 154 Wharton Basin 214 22 11°20.21'S 88°43.087 E 1665 490 Ninety East Ridge 215 22 08°07.30'S 86°47.50'E 5309 151 Central Indian Basin 216 22 01°27.73'N 90012.48'E 2237 457 Ninety East Ridge

220 23 06°30.97'N 59.02'E 4036 70° 329 Arabian Sea 221 23 07°58.18'N 68°24.37'E 4650 261 Arabian Sea 223 23 18°44.98'N 60°07.78'E 3633 717 W Flank of Owen Ridge 224 23 16°32.51'N 59°42.10'E 2500 787 Owen Ridge

235 24 03°14.O6'N 52°41.64'E 5130 646 Somali Basin 236 24 01°40.62'S 57°38.85'E 4487 306 SW of Carlsberg Ridge 238 24 11°09.21'S 70031.56'E 2832 506 NE end of Argo Fracture Zone

239 25 21°17.67'S 51°40.73'E 4971 320 Mascarene Basin 245 25 31°32.02'S 52°18.11'E 4857 389 S Madagascar Basin 248 25 29°31.78'S 37°28.48'E 4994 422 NW Mozambique Basin 249 - 25 29°56.99'S 36°04.62'E 2088 408 Mozambique Ridge

250A 26 33°27.74'S 39°22.15'E 5118 725 Mozambique Basin 251A 26 36°30.26'S 49°29.08'E 3489 489 Flank of SW Indian Ocean Ridge 254 26 30°58.15'S 87°53.72'E 1253 301 Ninety East Ridge 256 26 23°27.35'S 100°46.46'E 5361 251 Wharton Basin 257 26 30°59.16'S 108°20.99'E 5278 262 SE Wharton Basin 85

DSDP Site 212 is situated in the deepest part of the Wharton Basin in a water depth of 624Om, at the southern termination of the N/S trending Investigator Fracture Zone (Sclater and Fisher, 1974). Basement was cored at a depth of 516m below the sediment-seawater interface. The basement consists of a sequence of altered and weathered pillow lavas in which the rims are chloritised and contain palagonite glass and smectite. These rocks have apparently suffered hydrothermal alteration and have very similar characteristics to weathered basalts from the Central Indian Ocean Ridge (von der Borch et al, 1974). Five sediment samples (see Table 2.1.2b) were taken from this site and all come from the lowermost sediment unit - Unit 8. This is a zeolitic claystone, Fe-rich in parts, believed to have been deposited below the CCD at a rate of about 1mm/m.yrs (von der Borch et al, 1974). It is brown or yellowish brown Or green-grey in colour, about 34m thick and is probably'of Mid-Late Cretaceous age, although this is by no means certain due to the almost total absence of any fossils. In addition it contains amorphous Fe oxide-rich layers, Mn micronodules and Mn-rich layers, as well as volcanic glass and other heavy minerals.

DSDP Site 213 is situated about 5O0km east of the Ninety East Ridge in the Wharton Basin at the southern end of the distal Nicobar Fan (the extension of the Bengal Fan on the east side of the Ninety East Ridge) in a water depth of 5609m. Basement was cored at a depth of 154m below the sediment-seawater interface. The basement consists of a number of basaltic flows. Each flow is a pillow lava with an altered, glassy palagonitised rim,on which thin manganese coatings have been observed. The basalts of this unit (Unit 5) have distinct affinities with those of the Central Indian Ocean Ridge. Calcareous ooze adheres to the top surface of the basalt suggesting that at the time of formation of the basalt, this site was above the CCD and remained so into the Eocene. One sample (see Table 2.1.2b) taken from this core comes from the lowermost sedimentary unit (Unit 4). This is Early Palaeocene in age and is an Fe oxide-manganese rich facies. The iron oxide is made up of oolitic-like goethite (?) fragments and associated red scales of haematitic and less crystalline limonitic material. Micronodules and Mn aggregates are also visible as well as some palygorskite. The unit is greyish-brown in colour. 86

DSDP Site 214 is situated on the Ninety East Ridge in a water depth of 1665m. Basement was cored at a depth of 490m below the sediment-seawater interface, and consisted of three types of partially weathered basalt interbedded with each other:- a vesicular basalt, an amygdaloidal basalt and a porphyritic basalt. The absence oT pillow structures suggests that these flows were extruded on land or in a shallow water environment. Immediately overlying the basalt is a series of volcanogenic sediments (Unit 3) which have been intruded by rocks (Unit 4), which are interpreted as being sills (von der Borch et al, 1974). The two samples from this station (see Table 2.1.2b) are taken from Unit 3, from an 'horizon' interbedded between the differentiate rocks. This unit consists of volcanoclastic sediments (tuffs), pyritic volcanic clays and lignite horizons, although the lignite was not sampled. The sediments are probably Palaeocene in age, and the volcanic tuffs contain variable amounts of pyrite, volcanic glass and are stained with haematite and other Fe oxides. The unit lacks marine fossils and this together with its lithological characteristics suggests that it was mostly deposited subaerially (von der Borch et al, 1974). The volcanics at DSDP Site 214 are similar to the mildly tholeiitic basalts of the St Paul and New Amsterdam islands. These data suggest that the Ninety East Ridge was an emergent chain of volcanic islands which sank below sea level in Palaeocene times (von der Borch et al, 1974).

DSDP Site 215 is situated about 240km to the west of the median axis of the Ninety East Ridge in the Central Indian Basin in a water depth of 5309m. Basement was cored at a depth of 151m below the sediment- seawater interface and consisted of a succession of fourteen basalt pillows with glassy, palagonitised margins which have undergone various degrees of seawater weathering and have affinities with Centra]. Indian Ocean Ridge basalts (von der Borch et al, 1974). The one sample (see Table 2.1.2b) from this station comes from the sub-unit 4b, immediately overlying the basalt. This unit is about 1m in thickness and is of mid-Palaeocene age. It consists of an iron oxide-rich, dark brown, nannofossil ooze and contains Mn micronodules (von der Borch et al, 1974).

DSDP Site 216 is situated just north of the equator on the crest of the Ninety East Ridge in a water depth of 2237m. Basement was cored at a depth of 457m below the sediment-seawater interface, and consisted of a complex sequence of tholeiitic basalt rocks, whose compositions were 87

similar to suites from the St Paul and New Amsterdam Islands. The sequence consists of chloritised volcanic tuffs, and vesicular, amygdaloidal, and highly oxidised scoriaceous basalts. These rocks are significantly different from mid-ocean ridge basalts. The lack of pillow structures, and the amygdaloidal and vesicular nature of the rocks suggests a subaerial or near-surface lava extrusion (von der Borch et al, 1974). The nineteen samples (see Table 2.1.2b) from this station come from sub-unit 2b, which overlies the basement rocks. The unit is an intermixed sequence of volcanic clays and micro-carbonate chalk, with discrete beds of ash. The unit is much chloritised, greenish-grey in colour and contains volcanic glass, and discrete amounts of glauconite. The sequence is about 60m thick and is of late Maastrichtian age. The lithological characteristics of the succession suggest a shallow water environment of deposition (von der Borch et al, 1974).

2.1.2c. Leg 23 (Colombo - Djibouti, 1972)

The sites (220-230) drilled on Leg 23 were occupied between March and May 1972. They were drilled in two areas: the Arabian Sea and the Red Sea, and it is with samples from the group of sites (220-224) from the former area that this investigation has been concerned.

DSDP Site 220 is situated to the west of the Chagos-Laccadive Ridge in the Arabian Sea, 740km WSW of the southern tip of India in a water depth of 4936m. Basement was cored at a depth of 329m below the sediment- seawater interface and consists of a number of basalt flows with two interbedded layers of calcareous sediment. Vesiculation is uncommon and the presence of glass zones on the top of the flows indicates that the lava was extruded on to the sea floor (Whitmarsh et al, 1974). Although the basalt is broken by fractures and veins which are sometimes filled with calcite and a reddish-brown opaque mineral, the rock is extremely fresh with no detectable secondary alteration. The three samples (see Table 2.1.2b) from this station come from Unit 4 which immediately overlies the basement. This unit is 39m thick and is of lower Eocene age. It consists of massive light Orange, micro-carbonate-rich, nannochalk with thin chert bands and occasional volcanic glass fragments. The lack of alteration of the basalt may indicate little or no contribution to the metal content of the sediments from this source (Whitmarsh et al, 1974). 88

DSDP Site 221 is situated 10O0km due west of the southern tip of India in a water depth of 465Om, on the Arabian Abyssal Plain which terminates 60km to the south against the flank of the Carlsberg Ridge. Basaltic basement was cored at a depth of 261m below the sediment- seawater interface, and is composed of a number of flows, identified by their chilled margins of partially vesicular tholeiitic basalt. Fractures along their surfaces are infilled with calcite and the rock is remarkably fresh and shows no evidence of secondary alteration. The unit (4) immediately overlying the basement was not sampled due to poor core recovery. Therefore the samples from this station (see Table 2.1.2b) come from Unit III which overlies the unit in contact with the basalt. This unit is 43m thick and is of Miocene-Pliocene age. It is a brown, homogeneous, unfossiliferous clay with a low calcium carbonate content. Traces of ferromanganese particles and zeolites are present. The low sedimentation rate calculated from this unit (2mm/m.yrs) suggests deposition under pelagic conditions (Whitmarsh et al, 1974).

DSDP Site 223 is situated on the continental rise off the coast of Muscat and Oman on the west flank of the Owen Ridge. Basement was cored at a depth of 717mbelow the sediment-seawater interface, and consists of a 10m thickness of volcanic breccia overlying 11m of calcite veined fine-grained, vesicular, greyish green amygdaloidal trachy—basalt. The presence of vesicles suggests that extrusion took place at a depth of less than 800m and furthermore that they were formed along a mid-ocean ridge (Whitmarsh et al, 1974). The breccia consists of glass fragments and fine-grained basalt, and is in an advanced state of alteration with the opaque phases altered to Fe oxides (goethite?). The four samples from this station (see Table 2.1.2b) come from the unit immediately overlying the basement sequence (Unit IV). This is of late Palaeocene to Early Eocene age and is about 60m thick. It is a semi-lithified generally unfossiliferous brown montmorillonitic claystone.

DSDP Site 224 is situated on the Owen Ridge to the northwest of the Owen Fracture Zone and to the SSW of DSDP Site 223, in a water depth of 25O0m. Basement was rec overed from a depth of 787m below the sediment- seawater interface and was essentially similar to a lamprophyre. This rock type, normally found as dykes or sills, has not previously been described as occurring on mid-ocean ridges and does not resemble volcanic 89

suites from island arc environments. The rock is black, veined and fine-grained and composed predominantly of titano-augite rosettes rimmed by brown hornblende (barkevikite?) and set in a glassy matrix. Iron oxides, pseudomorphs of serpentine and calcite after olivine, and calcite and chlorite after plagioclase feldspar are also present. Such an assemblage would be termed a monchiquite. The glassy matrix is partially altered and contains brown clay (montmorillonite?). The six samples from this station (see Table 2.1.2b) come from Unit III, which immediately overlies the lamprophyre. This unit is about 107m thick and of Early Eocene to Early Oligocene age. It is composed of greyish micro-carbonate-rich clay nanno-chalk and pale red nanno-rich claystone. The clay mineral is predominantly montmorillonite (63°%- 72%) and volcanic glass is an abundant constituent near the contact with underlying lamprophyre (Whitmarsh et al, 1974).

2.1.2d. Leg 24 (Djibouti - Mauritius, 1972)

The sites (231-238) drilled on this leg were undertaken in May and June 1972. Samples have been studied from three sites in the Western Indian Ocean (DSDP 235, 236 and 238) all of which recovered basaltic basement rocks (Fisher et al, 1974).

DSDP Site 235 is situated in the north-west Somali Basin on the eastern flank of the Chain Ridge in a water depth of 5130m (Fisher et al, 1974). Basement was cored at a depth of 646m below the sediment- seawater interface and consisted of massive, somewhat fractured greenish grey porphyritic basalt. Yellowish, brownish iron hydroxides are common in the upper portion and also at depth (23.5m below upper contact), where they might represent the top of another flow (Fisher et al, 1974). Carbonate-rich, metamorphosed clays are common inclusions in the upper section. The absence of massive rocks in the upper section suggests that the 'basement' has been subject to submarine weathering (Fisher et al, 1974). The single sample (see Table 2.1.2b) from this station comes from Unit 3, which overlies the basement but is not in direct contact with it. This unit of uncertain Maastrichtian to Mid-Miocene age is brown clay. It contains mainly clay minerals (undifferentiated) together with traces of nannoplankton, quartz, heavy minerals and aggregates of Fe oxides are common (Fisher et al, 1974). 90

TABLE 2.1.2b. INDIAN OCEAN DSDP SAMPLE DATA

SAMPLE NO. SITE CORE SECTION INTERVAL NO NO SAMPLED SAMPLE AGE SEDIMENT TYPE (CMS)

211 13 1 69-71 ) Campanian- ) Fe oxide rich 14 1 70-72 ) Maastrichtian ) nanno-clay

212 35 5 47-49 ) Greyish green claystA 36 1 41-43 ) ) Reddish brown zeolite. 1 103-105 ) Middle to ) claystone 37 1 122-124 ) Late Cretaceous Brown to yellowish clay-rich zeolitite 38 1 138-140 ) Light to dark brown clay zeolitite

213 16 4 129-131 Early Palaeocene Fe oxide rich basal facies sediment

214 46 2 141-142 ) Palaeocene ) Volcanic clays 51 1 25-27 ) )

215 16 5 147-149 Mid Palaeocene Fe oxide rich nanno- ooze

216 30 2 141-143 ) ) 5 120-126 ) ) Glass-rich, micro- 31 2 138-140 ) ) carbonate volcanic clay 5 130-132 ) ) 32 1 140-142 ) ) 2 76-78 ) ) 3 140-142 ) ) 4 134-136 ) ) Grey, bluish, olive 5 138-140 ) Maastrichtian ) grey, microcarbonate 6 140-142 ) ) rich volcanic clay 33 1 135-137 ) ) 2 148-150 ) ) 34 1 68-70 ) ) 2 50-52 ) ) 3 128-130 ) ) 4 142-144 ) ) 35 1 130-132 ) ) 2 113-115 ) ) 3 53-55 ) )

220 18 1 121-123 ) ) Light orange, micro- Lower 2 20-23 ) ) carbonate rich nanno ) Eocene 3 142-144 ) chalk with chert bands 91

TABLE 2.1.2b (cont)

SITE CORE SECTION INTERVAL SAMPLE AGE SEDIMENT TYPE NO NO SAMPLED (CMS)

221 14 1 101-103 ) ) 2 90-92 ) ) 3 120-122 4 70-72 ) Miocene to l iocene ) 5 100-102 ) P 6 131-133 )(undifferentiated) ) Brown clay 15 1 125-127 ) ) 2 102-104 ) ) 3 129-131 ) ) 4 130-132 ) ) 223 37 1 144-146 ) ) cc - ) Late Palaeocene to ) Brown, montmorill onit€ 38 1 130-132 ) Early Eocene ) claystone cc - ) )

224 9 1 26-28 Early Oligocene Microcarbonate-rich, clay, nanno-chalk 10 1 18-20 Late Eocene Grey to red claystone 2 50-52 Mid Eocene It " " 145-147 Mid Eocene Greyish nanno-claysta0 11 1 91-93 Mid Eocene Nanno, microcarbonate_ rich claystone 2 130-132 Early Eocene Greyish claystone

235 16 1 - Maastrichtian (?) Brown clay

236 2 15 ) ) Pale greenish grey, 33 t La e ) olive chalk and brown nannp- 3 47 )Palaeocene 6 ) ) 238 51 2 43-46 ) ) ght brown ) ) Li 3 92-94 ) nanno ooze 4 76-78 ) ) Very pale orange 52 1 5-7 ) Early to 2 38-40 ) ) nanno ooze 3 5-9 Late Volcanic ash 76-78 )Oli gocene ) 4 5-7 ) ) Very pale orange 56-58 ) ) to pinkish grey, 5 11-13 ) ) nanno ooze 80-82 ) ) 53 2 46-48 ) ) Greyish orange nanno ) ooze 68-70 ) ) Volcanic ash 115-117 ) ) )Greyish orange 3 21-23 ) nanno ooze 143-145 ) 54 1 95-97 ) ) Volcanic ash 131-133 ) Greyish orange nanno ooze 59 4 20 Pre-Early Oligocene Metamorphosed metal- rich chalk.

TABLE 2.1.2b (cont)

SITE CORE SECTION INTERVAL SAMPLE AGE SEDIMENT TYPE NO NO SAMPLED (CMS)

239 15 1 147-149 ) ) 2 103-105 ) ) Brown silt-bearing 16 2 133-135 ) Late ) clay 3 142-143 ) Palaeocene ) 4 147-149 ) ) 17 1 79-81 ) ) •2 127-129 ) Nanno-rich brown clay 5 139-141 ) Brown clayey nanno oa3e.. 1 39-41 ) Brown clay 125-127 ) Early Palaeocene " II 2 35-37 ) 131-133 ) it 3 37-39 ) It 128-130 ) Nanno-rich brown clay 19 1 38-50 ) Ii U II II 127-129 ) 2 36-38 ) ) Brown to green grey 138-140 Late Cretaceous(L. ) clayey nanno-ooze Senom to Camp)

245 13 5 136-138 ) Early to Late ) Carbonate, clay-rich, 6 143-145 ) Palaeocene ) FeMn oxide-rich nanno- ) chalk 14 2 138-140 ) 4 64-66 ) ) Reddy orange, light 5 123-125 ) ) yellow brown, clay 6 141-143 ) ) rich, FeMn oxide-rich 15 1 146-148 ) Early Palaeocene )nanno chalk 2 101-103 ) ) Brownish, olive black 3 44-46 ) ) FeMn oxide-rich clayey 148-150 ) )nanno chalk 16 1 43-45 ) ) Brownish, olive-black 139-141 ) ) FeMn oxide-rich clayeZr nanno Chalk

248 14 1 129-131 ) ) 2 119-121 ) ) 3 139-141 ) Late Palaeocene )Brown silt-bearing 4 100-102 ) )clay 5 85-87 ) ) 6 40-42 ) ) 147-149 ) ) 249 30 1 144-146 ) ) Grey, olive-black, 2 99-100 ) 3 140-142 ) )calcic, silty volcanic. 4 148-150 )claystone 31 1 130-132 ) )Medium grey to olive 2 137-139 ) Neocomian 3 136-138 ) )black silty claystone 4 52-54 32 1 103-105 ) )Medium dark grey, olive. 139-141 ) )grey, silty-rich claystone 93

TABLE 2.1.2b (cont)

SITE CORE SECTION INTERVAL SAMPLE AGE SEDIMENT TYPE NO NO SAMPLED (CMS)

250A 22 2 120-122 Moderate brown, zeol e 4 69-71 ) bearing clay Coniacian 23 1 67-69 Pale brown clay,zeolik¢, 2 68-70 ) rich, with FeMn oxide, 24 1 139-141 ) Olive grey, greenish 2 131-133 black clay 25 1 50-52 ? Dark yellowish brown? massive clays.

251A 26 4 19-21 ) Yellowish brown, Fe Lower ) oxide stained, micro- 27 2 102-104 Miocene 28 1 39-41 ) ' ) carbonate chalk contains Mn garnets

254 27 3 119-121 Pebbly sandy mudston€ 28 1 139-141 Olive-black, silty claj 29 1 141-143 Late Eocene Olive black, silty cfctj 3o 1 33-35 ) to Black silty 139-141 ) Oligocene ) clays 31 cc Olive black silty cleat' 32 1 136-138 Fe-rich silty clay 33 1 47-49 Fe-rich silty clay

256 6 6 133-135 7 3 148-150 ) Upper ) Brownish grey, 8 4 135-137 Albian detrital clay 6 69-71 9 1 135-137 )

257 7 6 61-63 ) Reddish brown-grey, 8 2 12-14 Mid-Albian ) coccolith bearing, 8 2 110-112 ) detrital cIay 9 3 18-20 ) Semi-lithified 118-120 ) Cretaceous laminated reddish- 10 1 18-20 ) (Mid.Albian?) ) brown detrital clay 118-120 )

NOTE: cc - Core Catcher Sample 94

DSDP Site 236 is situated on the outermost foothills southwest of the Carlsberg Ridge, about 270km northeast of the Seychelles clank in a water depth of 4487m. Basement was cored at a depth of 306m below the sediment-seawater interface and is composed of broadly two types. A lower analcite-bearing, sub-alkali, melano-basalt is overlain by a chloritised, altered olivine tholeiite. The upper rock type is porphyritic, altered and contains glassy seams, as well as basaltic breccia fragments and fragments of slightly metamorphosed carbonate sediments. The three samples (see Table 2.1.2b) from this station are taken from Unit 6 which immediately overlies the basement rocks. This unit is about 4m thick, is of Late Palaeocene age and comprises a pale green to greyish olive green and brown nanno-chalk. It contains clay minerals and up to 5% Fe oxides. Pyrite is concentrated in small fissures and the Fe oxides are concentrated in the oxidised lower 2m immediately in contact with the basalt (Fisher et al, 1974).

DSDP Site 238 is situated at the extreme northeast end of the Argo Fracture Zone, adjacent to the southern end of the Chagos-Laccadive Ridge in a water depth of 2832m (Fisher et al, 1974). Basement was cored at a depth of 506m below the sediment-seawater interface and is composed of normal oceanic tholeiites, some of which are olivine-bearing. The rocks are probably flows, but no pillow structures are discernible. Volcanic glass is common, but alteration is confined to chloritisation, palagonisation and the formation of the hydroxides along the margins of fractures. Inclusions of metamorphosed sediments are common between the flows, and they are often associated with glassy zones. One sample from the station (238-59-4-20) is taken from such a metamorphosed, carbonate sediment. The remaining nineteen samples (see Table 2.1.2b) from this station are taken from Unit 3 which immediately overlies and is in contact with the basement rocks. This unit is about 35m thick, is of Late Oligocene age and is a sequence of intercalated, semi-lithified nanno-chalks with horizons of volcanic debris, zeolitic sands and clays. The volcanic ash horizons are dark yellowish brown in colour, while the colours of the remaining sequence are highly variable - orange, browns, greys,etc. Fe oxide staining is common throughout the sequence, but increases to appreciable globules of Fe oxides near the contact with the underlying basalt. Quartz, micas and feldspars are common, while clay minerals reach up to 80% in certain horizons. Zeolites, palagonite and occasional manganese micronodules are also present. 95

2.1.2e Leg 25 (Mauritius - Durban, 1972)

The sites (239-249) drilled on this leg were occupied between June and August 1972. Samples have been obtained from those sites which struck the basaltic basement (239, 245, 248 and 249). The stations are situated in the western and southwestern Indian Ocean, in the Mascarene (239), Madagascar (245), and Mozambique (248) basins and on the Mozambique Ridge (249) (Simpson et al, 1974).

DSDP Site 239 is situated on the abyssal plain of the Mascarene Basin in a water depth of 4971m, 180 miles east of Madagascar and 200 miles west of the island Rēunion. Basement was cored at a depth of 320m below the sediment-seawater interface and is a fine-grained, glassy tholeiite (Simpson et al, 1974). The chemical analyses of the basement show that the basalt is similar in composition to the low K-tholeiites recovered from the mid-ocean ridges. The rocks show signs of alteration and weathering - zones of Fe oxidation follow irregular fractures in the rock and the filling of such fractures with iron carbonates and cherts suggest that a pillow structure may have been present. Sixteen samples (see Table 2.1.2b) from this station are taken from Unit IIB which immediately overlies the basalt. This unit is 162m thick and is of late Cretaceous (Campanian) and early Palaeocene age. The unit is brown clay with amounts of clay-rich nanno ooze and clayey nanno ooze. Volcanic contributions are present in the form of montmorillonite-rich layers, which may represent devitrified volcanic ash layers (Simpson et al, 1974). Two further samples from this station (see Table 2.1.2b) are taken from Unit IIA which is a brown clay of pelagic and terrigenous detrital origin, and is of late Palaeocene to Early Miocene age. The samples come from the lowermost portion of this unit, where the presence of manganese nodules have been reported (Simpson et al, 1974).

DSDP Site 245 is situated in the southern Madagascar Basin in a water depth of 4857m, about 300mIs. east of the Madagascar Ridge and about 200m1s. northwest of the South West Indian Ocean Ridge. Basement was cored at a depth of 389m below the sediment-seawater interface and is composed of two main types of basalt - a medium grained, dark grey diabasi_c basalt, overlain by a glassy black basalt. Both types are veined, while younger fractures cut across the veins and are marked by reddish-brown Fe oxide staining. The chemical composition of the basalts shows that they 96

are similar to the low K tholeiites from the mid-ocean ridges (Simpson et al, 1974). Warner and Gieskes (1974) consider that this basalt represents a sill rather than true basement material. Twelve samples (see Table 2.1.2b) were taken from this station, six from Unit IVC which immediately overlies the basalt and six from the overlying Unit IVB. Unit IV is broadly speaking a clay-rich nanno-chalk, of Early Palaeocene to Early Eocene age and is 181 metres thick. Sub-unit IVC is distinguished from the overlying sub-units IVB and IVA in having an increased content of clay material (30-40%) and a large proportion of FeMn oxides (10-15%) which impart a dark brownish and olive black colour. This unit has been fully described by Warner and Gieskes (1974) and their work is reviewed in detail above. The overlying Unit IVB is greenish or olive grey in colour and contains less clay material (15-20%) which is montmorillonite and probably originates from the devitrification of volcanic ash layers which occur in this unit. Unit IVB contains less FeMn oxide material than the underlying Unit IVC (Simpson et al, 1974).

DSDP Site 248 is situated in the northwestern Mozambique Basin about 30 miles east of the Mozambique Ridge in a water depth of 4994 m. Basaltic basement was cored at a depth of 422 metres below the sediment- seawater interface and is a dark grey, porphyritic tholeiite and is amygdaloidal in its upper zones. Veins containing chlorite, calcite and goethite occur and there is little evidence of submarine weathering, except for reddish-brown Fe oxide zones adjacent to the fractures in the upper zone. It is believed to have been extruded on the sea floor as lava flows (Simpson et al, 1974). Its chemical composition is unlike other low K tholeiites from mid-ocean ridges and with higher Ba, Sr, Ti, Zr and Nb contents it is more similar to the Karroo(Stromberg) basalts from the Lebombo Monocline and tholeiites from Iceland and Hawaii (Simpson et al, 1974). The seven samples (see Table 2.1.2b) from this station come from Unit III which immediately overlies the basalt. This unit is of Late Palaeocene age and is 15m thick. It consists of a brown clay and silt-bearing clay, which contains reddish-brown and blackish-red bands due to the presence of Fe oxide material (Simpson et al, 1974).

DSDP Site 249 is situated in a small sediment filled basin close to the crest of the Mozambique Ridge, 80 miles west of DSDP Site 248 in a water depth of 2088m. Basement was cored at a depth of 408m below the sediment-seawater interface and is composed of vesicular, amygdaloidal, 97

glassy weathered basalt. The matrix of the basalt shows alteration to a mixture of smectite (montmorillonite) and zeolite. The chemical composition of the basalt shows certain similarities with the low K-tholeiites recovered from the mid-ocean ridges, although they contain higher Ba values. The vesicular nature suggests an oceanic origin, although the glassy nature implies rapid cooling due to subaqueous extrusion. The ten samples from the station (see Table 2.1.2b) were taken from sub-unit IIIB, which immediately overlies the basement. The unit is of Neocomian age and is 56m thick. It is a grey and olive black silty claystone and it contains 55-88% material of volcanic origin, which is mainly in devitrified volcanic ash layers and which appears as clay minerals (montmorillonite). The distinctive dark colour of the sediments is probably caused by the presence of organic carbon and reduced Fe, in the form of Fe sulphides (Simpson et al, 1974).

2.1.2f. Leg 26 (Durban - Fremantle, 1972).

The sites (250-258) drilled on this leg were occupied in September and October, 1972. Samples were collected from those stations where sediments and basement were cored together. The sites were all drilled in the southern part of the Indian Ocean and were in the Mozambique (250) and Wharton (256, 257) Basins, on the Southwest Indian Ocean Ridge (251) and on the Ninety East Ridge (254) (Davies et al, 1974).

DSDP Site 250A is situated in the southeast corner of the Mozambique Basin on the flank of the Southwest Indian Ocean Ridge in a water depth of 5118m. Basement was cored at a depth of 725m below the sediment- seawater interface, is believed to be of Coniacian age (Late Cretaceous) (Davies et al, 1974) and is composed of olivine (oceanic) tholeiites. The unmetamorphosed nature of the overlying sediments suggests that the contact is sedimentary and not intrusive. The olivine-rich basalts show moderate signs of weathering and alteration, the upper zone being traversed by a number of fractures containing calcite and serpentinite. Parts of the matrix of the basalts are devitrified and contain reddish- brown Fe hydroxides, together with granules of Fe oxide. Three samples from this station (see Table 2.1.2b) are taken from Unit 5 which immediately overlies the basalt. This is a semi-lithified, olive grey clay, which is about 22m thick and is of at.least Coniacian age. Four further samples (see Table 2.1.2b) come from Unit 4 which overlies Unit 5 98

and which in its lower portion (from where the samples are taken) is a zeolite-bearing, nanno-fossil-bearing, brown detrital clay. The whole unit is 66m thick and its lower portion is of Late Cretaceous age. Fe or Mn nodules occur in this unit and FeMn oxides are disseminated throughout it imparting a dark colour to certain bands. Siderite is also common in this unit (Davies et al, 1974).

DSDP Site 251A is located 180km north of the Southwest Indian Ocean Ridge in a water depth of 3489m. Basement was cored at a depth of 489m below the sediment-seawater interface and was an olivine-rich (oceanic) tholeiite. The upper portion of the basalt is highly weathered, glassy, partly devitrified and is fractured in places. The fractures contain calcite and the rock is vesicular in its upper portion. One sample from this station (see Table 2.1.2b) is taken from Unit 5 which immediately overlies the basalt. This unit is of lower Miocene age, is about 18m thick and is a yellowish-brown, micro-carbonate chalk. The unit is stained brown by Fe oxides and contains manganiferous garnets (Fleet and Kempe, 1974; Kempe, 1974). The two remaining samples from this station (see Table 2.1.2b) are taken from Unit 4, which overlies Unit 5. This unit is about 14m thick, is of Lower Miocene age and is a yellow to reddish-brown nanno-plankton chalk. The colour has been produced by the oxidation of disseminated pyrite to limonite and associated hydrated Fe oxides. Detrital constituents (quartz, mica, volcanic glass and heavy minerals) are also present (Davies et al, 1974).

DSDP Site 254 is situated on a broad, high, steep-sided plateau at the southern end of the Ninety East Ridge, in a water depth of 12S3m, to the south of the apparent intersection of Broken Ridge and the Ninety East Ridge. Basement was cored at a depth of 301m below the sediment- seawater interface and consists of a number of basaltic flows (Davies et al, 1974), of three main rock types. A coarse, highly altered, ophitic basalt overlies an amygdaloidal, porphyritic olivine basalt, which is underlain by an unamygdaloidal, brecciated, olivine basalt. The basement is overlain conformably by weathered ferruginous silty clays and fine sandstones, containing fragments of weathered basalt, which comprise Unit 4. The eight samples from this station (see Table 2•.1.2b) are taken from this unit. The unit is grey-green or olive-black in colour, about 92m thick and is of probable Late Eocene or Oligocene age. The unit is 99

extremely poorly sorted, contains occasional conglomerates and pyrite, as well as volcanic ash fragments and a restricted littoral fauna. Montmorillonite is the most common mineral phase together with phillipsite, pyrite, glauconite, kaolinite and aggregates of Fe oxides. As the contact with the underlying basalt is approached the sediments become increasingly ferruginous, as well as containing pyrite, lenses of opaque oxides and fragments of altered basalt (Davies et al, 1974).

DSDP Site 256 is located in a water depth of 5361m in the southern Wharton Basin (Davies et al, 1974). Basement was cored at a depth of 251m below the sediment-seawater interface and consists of about four flows of coarse-grained, glassy, olivine-poor basalt. The basalt is brecciated near the contact with the overlying clays, and is highly altered. The five samples taken from this station (see Table 2.1.2b), are from the base of the overlying Unit 1. This unit is a brownish to greyish detrital clay. Coccoliths and zeolites increase in proportion towards the contact. Near the contact with the underlying basalt the predominant clay mineral is montmorillonite. However, above this mica/ illite predominates over this in cores 7 and 8. Quartz, feldspar, glauconite, gibbsite and haematite are also common constituents of the sediments (Davies et al, 1974).

DSDP Site 257 is located in the Southeastern Wharton Basin, northwest of the Naturaliste Plateau in a water depth of 5278m. Basement was cored ata depth of 262m below the sediment-seawater interface, and the oldest sediments (Unit 1b) overlying the basement are of Middle Albian age. The basement consists of seven or eight flows of a fine to medium grained, grey, vesicular, porphyritic olivine basalt. The upper portion of the basalt is highly fractured and veined, the veins being filled with calcite, chlorite, serpentine and goethite, although there is no indication of deuteric or metasomatic alteration or the baking of the overlying sediments. The seven samples taken from this station (see Table 2.1.2b) are taken from Unit 1 which overlies the basalt. This is a dark brown detrital clay which is divided into three sub-units. Four samples are taken from Unit 1C. This is in contact with the basalt, is about 13m thick and of Middle Albian age. It is a reddish-brown semi- lithified detrital clay, composed almost entirely of montmorillonite and quartz, with further amounts of ferruginous material (haematite) and some 100

barytes. A further three samples come from the overlying 64m thick, Unit 1B. This is a reddish-grey brown coccolith-bearing detrital clay. Calcite is a significant component but montmorillonite is the dominant clay mineral and ferruginous material also occurs (Davies et al, 1974). 101

2.2 BULK GEOCHEMISTRY OF INDIAN OCEAN DSDP BASAL SEDIMENTS

2.2.1 Introduction

The DSDP samples have been prepared and analysed according to the procedures described in Appendix A.1. The results have been recalculated on a carbonate-free basis (CFB) to aid comparison with previously published carbonate-free data and to remove the diluting effect of calcium carbonate, where this is appropriate. The method of recalculation and the reasoning. behind it is described in Appendix A.4. The results are reported on a CFB with the exception of Ca. The bulk geochemistry of the basal sediments is described below for each DSDP site, and some general conclusions are then drawn. Table 2.2.2a contains the site CFB averages, together with the maximum and minimum values where appropriate. The average for all the DSDP sediments is also included in Table 2.2.2a. Where there are sufficient samples from the basal units of the DSDP cores, the CFB element values have been plotted against depth. The results are displayed in Figs. 2.2.2a to 2.2.2m and detailed column sections are available in the relevant DSDP volumes. The purpose of studying closely spaced samples immediately above basement is to try to find trends of metals in relation to the underlying basalt, in order to provide some information as to the source of the metals. Since samples have only been taken from those units which immediately overly the basalt, no comparison can be made with the composition of the younger, overlying sediments. Hence all the following comments apply only to the geochemistry of the basal units. For this reason it may only be possible to detect the effects of post-depositional remobilisation in sites where a large number of samples are available, and even then it may be of a very small scale nature, due to the spacing of the samples. The geochemical variations discussed below are only those which are not explicable in terms of the analytical precision of the determination method. Analytical precisions for the various elements are given in Appendix A.2. 102

2.2.2 Geochemical Variations within Indian Ocean DSDP Cores.

2.2.2a Leg 22 (DSDP Sites 211, 212, 213, 214, 215 and 216)

The calcareous clayey sediments from the base of Site 211 show no metal enrichment. The Fe, Mg, Al and Ti are about average for Indian Ocean DSDP sediments (see Table 2.2.2a), while the Mn concentrations of about 0.25% and those of the other trace metals are below the average values. Pimm (1974) has also noted this absence of metal enrichment in these calcareous sediments and has suggested this was caused by no metals being leached from the underlying basalts.

Pimm (1974) has reported the presence of an Fe oxide rich basal facies in the lowermost 10 metres of DSDP Site 212. However, none of his Fe determinations exceeded 10% and there was only a slight increase in the concentrations of Fe and trace metals with depth. Generally speaking the Site 212 sediments show no metal enrichment and only the concentrations of Cu, Li and As are above average. Over the interval studied (see Fig. 2.2.2a) only Mn and possibly Ba increase significantly with depth. However, there is a general tendency for a decrease down the core in the concentrations of Ni, Cr, Li and As.

The basal sediment from Site 213 is extremely metal-rich. It is taken from an Fe oxide/Mn-rich facies, containing goethite and manganese micronodules. It has above average values of Fe, Mn, Ni, Cu, Pb, Zn and As. The low Al and Ti values are similar to other basal and active ridge metal-rich sediments (Bostr8m et al, 1969; Cronan, 1976). The value for As (89ppm) in this sediment is high for marine sediments and shows a factor of enrichment over basalt and seawater of 30 and 30,000 respectively.

Although the basement rocks at Site 214 are weathered and the overlying volcanogenic sediments have been reported as containing pyrite and Fe oxide material, the concentrations of the elements other than Zn and Cu fall well below the average values for Indian Ocean DSDP sediments.

The sediment from Site 215 comes from an Fe and Mn oxide rich facies immediately overlying a weathered and altered basement sequence. It shows

103 .

Fig 2.2.2a.

Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit 8 (Brown Zeolitic Claystone) of DSDP Site 212, Wharton Basin, Indian Ocean

10 1 0.1 / I ... . . a a l 1l • a 483

488

493

498

503

508

513 AI Fe

518

0.01 0.0 01 0.00 01

1. 1• 1, . • 11 1 . a

All data are expressed on a C.F.B. except Ca. Vertical scale is metres beldw sediment-seawater interface. Concentrations are plotted on a log10 scale 104

Fig 2.2.2b Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit Ilb (Intermixed volcanic clay and microcarbonate chalk with discrete beds of volcanic ash) of DSDP site 216, Ninety East Ridge, Indian Ocean.

10 1 0.1 I , , . , . , , It , , , I , ,

399

409

419

429

439

649

659

Ba Mn 105

Fig 2.2.2b (cont'd)

All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface with an expansion factor of 2 for the sample interval.

Concentrations are plotted on a log 10 scale.

0.01 0.0 01 0.0001

404

414

424

434

444

454 106

above average values of Fe, Mn, Ni, Cr, Cu, Pb, Zn, As and Ba, and here, as at site 213, the As value of 81ppm is high and is enriched over basalt and seawater by factors of 27 and 27,000 respectively. The basal sediments at Sites 213 and 215 although showing similar degrees of metal enrichment and being of similar ages (i.e. Palaeocene) are different in that DSDP Site 213 is CaCO3 poor while the basal DSDP Site 215 sediment is CaCO3 rich.

The sediments from Site 216 on the Ninety East Ridge, are generally more metal enriched than those from Site 214, also on this feature, but overall they are not significantly enriched in metals. The values of Fe, Mg and Ti are generally above average, while those of Mn, Ni, Zn, As and Ba are below average. As a study of Fig. 2.2.2b reveals there is very little consistent variation in the metal concentrations with depth. Fe is the only metal significantly enriched in the basal sample, while there is a tendency for the values of Mn, Fe, Al and Mg to be above the core mean in the upper portion of the core.

2.2.2b Leg 23 (DSDP Sites 220, 221, 223 and 224)

The sediments from Site 220 have been reported by Whitmarsh et al (1974) as having no metal contributions from the underlying, unaltered basalts. From the composition of these sediments this would appear to be the case, since the concentrations of Fe, Al, Mg, Ti, Cr and other trace metals are below average. However, the values of Mn, Ba and to a lesser extent Cu are all above average. The Mn values reach 4.25% while those of Ba range from 0.25% to 1.1%. The high Mn and associated Cu values suggest the presence of Mn oxide material, such as manganese micronodules in the sediments; the above average Ba values are anomalous in DSDP sediments.

It was not possible to sample the sediments immediately overlying basement at Site 221 due to extremely poor core recovery. Therefore little can be said regarding the relationship of their trace element content to the underlying basalt. However, the composition of the sediments analysed from this site is similar to that of pelagic clays from the Pacific Ocean (Chester, 1965h; Cronan, 1969). None of the sediments from this core show metal enrichment, except sample 221-15-2-102/ 104, which is enriched in Mn, Ni, Cu and Co. The metal contents of the 107

Fig 2.2.2c.

Vertical Distribution, in weight percent, of Element Concentrations in the carbonate-free fraction of basal sediments from Unit III (Brown Clay) of DSDP site 221, Arabian Sea, Indian Ocean.

10 0.1 • . • 1 A II ! I .1 i 11 . 1 1 t

133

143

153

163

173

Al Fe Mg Mn Ti Ca

258

-263

All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface with an expansion factor of 3 for the sample interval. Concentrations are plotted on a log10 scale. 108

Fig 2.2.2c (coned)

0.0001 0.01 0.001! 1 • . . • • a . • 1 • • • t • t a

138

148

158

168 109

Fig 2.2.2d.

Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IIl (Grey microcarbonate-rich clay nannochalk) of DSDP site 224, Owen Ridge, Indian Ocean

10 1 0.1 t(

7.00

710

720

730

740

750

760

780

790

All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface. Concentrations are plotted on a log 10 scale.

110

Fig 2.2.2d (cont'd)

0.01~ . 0.0 01~ 0.0001

705

715

725

735

71.5

755

775

785 111

other sediments are average or below average, except for Mg which is slightly enriched in all the samples. It seems unlikely that alteration of basalt has contributed greatly to the composition of the sediments. This would support Whitmarsh et al's (1974) opinion that the brown clays were formed under normal pelagic conditions.

The composition of the sediments from Site 223 shows no metal enrichment, despite the fact that the underlying basalt has been reported as being in an advance state of alteration (Whitmarsh et al, 1974). Higher than average values of Al occur, while all the other metal concentrations fall below the average for Indian Ocean DSDP sediments (see Table 2.2.2a).

The sediments from Site 224 are interesting because they overly a rock-type previously unreported from the ocean basins - a 1.amprophyre - which probably forms a sill in this core (Whitmarsh et al, 1974). The composition of the sediments, is similar to those from the nearby Site 223. The Site 224 sediments show no signs of metal nerichment and only the values of Al, Li, Mg and possibly Ti are above average. Only Mn shows any enrichment with depth, but the absolute values are very low (see Fig. 2.2.2d).

2.2.2c. Leg 24 (DSDP Sites 235, 236 and 238)

The lowermost sediment from Site 235 is a clay which is not in contact with the basement. It shows no signs of metal enrichment and only the values of Fe, Al, Ti and As are above average. Although Fe oxides have been reportec{ in this sedimentary unit (Fisher et al, 1974), it would appear that any extensive metal enrichment has occurred in the zone in immediate contact with the highly weathered and altered basalt (below the location of the sample) where Fe oxides occur (Fisher et al, 1974). The As value of 47ppm is an enrichment over basalt by a factor of 16. The carbonate-rich sediments from Site 236 have higher than average values of certain elements (see Table 2.2.2a) and are enriched in Fe (up to 26%), Mn (up to 4.6%), Ni, Cu, Pb, and As (up to 213ppm). Fisher et al (1974) suggest that these sediments contain Fe oxide facies-type sediments in the lower oxidised zone, and that this is suggestive of 'weak, post- eruptive hydrothermal activity'. The Fe value of 26% is higher than that reported by Cronan et al (1974) for this site and is greater than concentrations of Fe reported from East Pacific Rise DSDP basal sediments (Cronan, 1976). It is also the highest Fe concentration reported from Indian Ocean DSDP sediments. 112

TABLE 2.2.2a DSDP Site Bulk Composition Averages C CA=

(N.B. Maximum & Minimum values only given for sites with MORE than 3 samples). Wt% Ppm . I_____ r--- 4 . SITE AVER/MAX/MIN Ca Fe Al Mn Ni Co Cr Cu NO.

211 AVER. 5.16 5.82 6.45 2425 91.6 38.1 121 88.3

212 AVER. 0.47 5.97 8.03 2770 112 51.2 114 144 MAX. 0.50 7.75 9.99 5450 148 63.7 156 180 MIN. 0.44 3.94 5.8o 1250 85.7 40.0 81.1 116

213 AVER. 0.39 13.92 4.56 31800 354 77.5 68.7 410

214 AVER. 1.12 1.90 7.73 129 34.5 37.5 108 163

215 AVER. 33.49 10.48 2.29 55700 624 86.8 329 439

216 AVER. 8.34 8.53 5.83 1327 74.3 43.3 167 133 MAX. 12.85 13.77 8.51 2840 159 62.3 232 204 MIN. 2.08 6.20 3.01 448 35.2 29.8 116 41.5

220 AVER. 20.05 3.49 2.53 21260 111 33.8 129 101

221 AVER. 0.84 5.69 7.67 3654 261 61.3 179 170 MAX. 2.92 6.12 8.55 10600 540 135 218 327 MIN. 0.33 5.38 7.16 630 127 37.8 137 68.2

223 AVER. 1.01 6.01 8.88 421 130 40.9 181 52.0

224 AVER. 4.57 5.98 8.04 816 114 25.0 169 68.1 MAX. 12.07 7.53 9.4o 1640 166 35.5 212 127 MIN. 0.59 4.09 5.19 458 62.3 1.40 133 36.o 235 AVER. nil 7.19 7.23 883 99.3 64.3 144 58.0

236 AVER. 27.72 11.32 3.26 22511 247 74.3 111 276

238 AVER. 11.76 5.96 7.50 1835 124 55.6 140 110 MAX. 23.83 8.98 9.08 5550 213 111 264 214 MIN. 0.84 2.65 4.95 234 24.2 40.5 49.0 24.9

239 AVER. 1.98 7.73 7.28 7232 158 46.9 158 152 MAX. 15.0 10.48 7.90 14400 169 55.0 216 191 MIN. 0.62 6.11 6.75 1370 139 32.1 114 61.8

245 AVER. 26.45 7.97 4.68 20062 183 40.8 172 165 MAX. 30.66 10.43 5.40 35600 252 74.8 268 230 MIN. 18.89 6.54 3.90 5440 104 18.3 91.3 110 113

TABLE 2.2.2a (cont).

Wt°% ppm r ~ SITE AVER/MAX/MIN Ca Fe Al Mn Ni Co Cr Cu NO.

248 AVER. 0.73 6.56 7.15 3436 119 45.8 146 88.8 MAX. 1.63 6.81 8.23 4930 152 60.9 156 97.0 MIN. 0.39 6.25 6.62 769 99.4 37.7 137 76.5

249 AVER. 4.56 5.71 4.96 499 68.1 37.1 167 67.5 MAX. 10.72 9.95 6.44 1230 87.2 53.3 212 96.7 MIN. 0.54 2.97 2.89 133 49.7 25.1 121 27.5

250A AVER. 0.92 5.34 5.36 2760 95.1 47.0 115 201 MAX. 1.77 6.56 6.09 8610 141 73.3 129 626 MIN. 0.25 4.64 4.33 400 60.4 27.1 93.7 28.6

251A AVER. 27.47 5.82 7.80 2330 119 102 313 207

254 AVER. 0.69 11.16 8.35 744 209 102 303 124 MAX. 2.03 13.49 10.92 2250 579 167 375 206 MIN. 0.25 4.16 6.78 218 81.2 52.5 175 76.1

256 AVER. 2.90 4.80 5.19 1701 64.7 45.9 102 85.9 MAX. 9.80 7.14 5.87 4760 77.0 52.8 114 135 MIN. 0.38 3.68 4.18 718 57.0 36.0 93.7 66.1

257 AVER. .342 5.82 6.66 4778 111 63.1 120 101 MAX. 8.83 6.50 8.02 10800 141 73.7 201 140 MIN. 0.63 4.54 4.75 1070 86.3 48.0 87.5 69.8

DSDP AVERAGE (ALL SEDIMENTS) 7.52 6.67 6.49, 5187 134 52.3 160 125 114

Table 2.2.2a(cont)

ppm

SITE AVER/MAX/MIN Cd Pb Zn Li As Mg Ba Ti NO.

211 AVER 1.8 58.0 142 21.3 4.21 25700 820 7820 212 AVER 2.8 63.6 158 69.7 12.36 17200 890 5070 MAX 3.9 70.7 179 104 19.80 19300 1240 6000 MIN 2.0 59.6 127 37.4 2.12 14800 660 3730

213 AVER 2.7 188 326 22.5 89.0 22100 1130 3750 214 AVER 2.2 60.4 218 14.5 1.09 10300 1100 13000 215 AVER 7.3 188 379 36.7 80.8 21700 7020 1470 216 AVER 1.5 53.8 127 29.4 3.28 43000 590 11136 MAX 3.3 59.5 303 52.8 5.94 59200 890 20906 MIN nil 48.0 58.7 8.36 0.60 26900 360 7940 220 AVER 2.2 65.1 140 17.8 2.03 17800 6720 4420 221 AVER 2.3 61.5 165 58.1 8.80 27200 10920 4680 MAX 2.7 68.8 185 63.0 12.14 28700 2180 5910 MIN , 1.3 58.1 150 55.0 7.03 23000 1130 1630 223 AVER 1.9 55.2 149 50.1 7.03 12200 1280 4810 224 AVER 1.7 53.4 135 60.6 3.07 21900 1080 6900 MAX 3.4 56.4 165 77.6 6.51 43400 1920 8710 MIN nil 50.7 71.9 15.2 0.83 11800 720 3800 235 AVER 4.0 54.5 172 70.4 47.1 n.d. n.d. 10400 236 AVER 2.5 132 148 31.7 136 n.d. n.d. 3146

238 AVER 3.3 55.7 167 66.2 10.38 n.d. n.d. 5790 MAX 7.0 106 294 197 28.20 n.d. n.d. 17600 MIN 2.7 44.4 90.1 48_2 2.00 n.d. n.d. 2120

AVER 239 2.7 55.7 173 42.1 18..92 18775 910 7269 MAX 6.o 63.0 306 48.7 58.66 20600 1230 8760 MIN 1.2 39.1 148 24.4 2.38 16200 630 3930 245 AVER 5.4 86.1 233 44.8 45.94 23617 2780 4479 MAX 9.9 138 322 63.9 240.11 32300 4980 6180 MIN 2.1 25.5 180 31.6 13.99 18300 1140 3510 115

Table 2.2.2a (cont) ppm SITE AVER/MAX/MIN Cd Pb Zn Li As Mg Ba Ti NO.

248 AVER 2.7 58.9 140 38.9 5.51 18270 870 7059 MAX 3.9 73.8 1. 48 45.9 7.65 21400 1260 7720 MIN 1.3 45.8 131 33.3 3.17 17300 490 5930 AVER 249 2.6 53.1 100 26.1 5.61 11877 7O0 10283 MAX 4.0 65.8 455 41.3 9.41 16500 1160 15000 MIN 1.0 43.3 65.9 11.2 1.09 5150 410 5620 250A AVER 2.3 68.6 107 28.8 2.59 19713 1440 4468 MAX 3.3 87.9 134 47.4 7.43 24900 2660 5130 MIN 1.4 50.0 89.8 17.5 0.46 14100 590 3670 251A AVER 2.7 100 150 52.4 13.28 18625 3740 3775

254 AVER 3.5 58.6 176 43.8 5.68 10449 810 20250 MAX 5.1 67.9 260 65.3 19.78 21800 990 28200 MIN 2.2 36.8 84.7 19.5 0.42 3850 630 13700

256 AVER 2.9 54.7 94.4 30.2 8.25 15380 1340 4512 MAX 6.6 67.7 117 38.1 17.41 20900 1870 . 5340 MIN 1.0 43.5 75.4 21-5 3.35 11600 210 3360

257 AVER 3.2 57.0 132 64.5 19.06 19013 1870 3766 MAX 3.9 67.2 153 85.7 46.04 20800 3330 5310 MIN 1.8 43.9 98.0 45.7 6.75 16300 790 2370

DSDP AVERAGE (ALL SEDIMENTS) 2.9 65.5 160 48.7 15.43 22133 1390 7476 116

Fig 2.2.2e

Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit 3 (Nannochalk with zeolites) of DSDP site 238, from the North East end of the Argo Fracture Zone, Indian Ocean

0.1

540-

550

All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface with an expansion factor of 2 for the sample interval. Concentrations are plotted on a log10 scale.

117

Fig 2.2.2e (cont'd)

No data are available for Ba and Mg. b = Lowermast sample of Unit 3. a = Intrabasaltic metamorphosed chalk. Zn is plotted as Zn/100. Pb is plotted as Pb/100.

0.01 0.0011' 0.0001

477

487

497

507 r I 1 1 I I I 1 I I 1 1 I I I 1 1 I 1 1 I i

a 11 545- As Cd Zn Pb 118

The sediments from Site 238 are not as strikingly metal-rich as other basal sediments recovered from elsewhere in the Indian Ocean. However, there is a general tendency for some metals to increase in concentration as the contact with the underlying basalt is approached (e.g. Mn, Fe, although the values are low) and for certain metals to have generally higher concentrations in the lower rather than the upper portion of the core (e.g. Cu, Ni). Furthermore, some metals show a decrease with depth, notably Li, Ti and possibly Al, (see Fig. 2.2.2e). The concentrations of Al, Ti and Li only are above average. The metamorphosed carbonate rock from between basalt flows (238-59-4-20) has higher Al, Fe, Ni, Co, Cr, and Pb values than the average for this site, and lower Li values. In comparison to the basal sediment it is enriched in Al and Cr, and is depleted in Zn and Li. The metamorphosed carbonate shows only slightly higher than average values of Fe, Al, Ni, Cr and Ca, when compared with other Indian Ocean DSDP sediments (see Table 2.2.2a). The trends observed above the contact with the basalt and the degree of enrichment of the sediments does not fully support the opinion of Fisher et al (1974) that the sequence was in close proximity to a volcanic vent which acted as a source for the enriched metals.

2.2.2d. Leg 25 (DSDP Sites 239, 245, 248 and 249)

The sediments from Site 239 show few metal enrichment trends. However, in Unit IIA, the high Mn value (1.4%) may possibly be due to the presence of Mn micronodules in the sediments, as reported by Simpson et al (1974). Furthermore, the basal sample shows a higher than average Fe value (11%). However, there is a general tendency for the concentrations of Fe, Al, As and possibly also Cu to be greater than average throughout the core. The concentrations of certain elements tend to increase down the core, notably Fe, Ti and Zn, although these variations with depth are not strikingly evident.

The carbonate-rich basal sediments from Site 245 are extremely similar to basal Fe oxide facies sediments reported from Pacific DSDP

119 Fig 2.2.2f Vertical Distribution, in weight percent, of Element Concentrations in the carbonate-free fraction of basal sediments from Unit II (Brown silty clay and brown nanno clay) of DSDP site 239, Mascarene Basin, Indian Ocean

10 0.1 • I....,

280

290

300

310

320

N.B. The uppermost pair of samples are from Unit Ila; the remainder are from Unit Ilb. 120 Fig 2.2.2f (cont'd)

All data are expressed on a C.F.B. except Ca. Vertical scale is metres below the sediment-seawater interface with an expansion factor of 2 on the sample interval. Concentrations are plotted on a log10 scale.

0.01 0.001 0.0001 ■ A L A ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■

275

285

295

305

315

325 350

360

370

380

390

Fig 2.2.2g. Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from DSDP site 245, southern Madagascar Basin, Indian Ocean. Unit IV (Grey-black, clay-rich, ferromanganese nannochalk) of N All data expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface with an expansion j-Gc.faY 2 122

sites (von der Borch, 1971). The sediments have higher than average (see Table 2.2.2a) concentrations of Fe, Mn, Cu, Zn, As and to a lesser extent Mg and Ba. Furthermore, they are Al-poor in keeping with other basal metal-rich sediments (Bostrdm and Peterson, 1969; Horowitz, 1974a; Cronan, 1976; Horowitz and Cronan, 1976; Dymond et al, 1976). The samples come from two units, one directly overlying the weathered basement (Unit IVC) and from the unit directly overlying this, Unit IVB. (see Fig. 2.2.2Q). Both units have high concentrations of Fe and Mn oxides and volcanic debris but the lowermost unit contains more montmorillonite material (Warner and Gieskes, 1974; Gieskes et al, 1974; Simpson et al, 1974). Furthermore, there is a general tendency for the concentrations of Fe, Mn, Zn and Ba to be higher in Unit IVC than in the overlying Unit IVB, and the concentrations of As, and Cr to be lower in IVC than in IVB. Although the As values are high at the base of Unit IVC, they are highest at the top of Unit IVB. Warner and Gieskes (1974) have suggested that the metal enrichment has resulted from the in situ alteration of volcanic ash material followed by remobilisation. However, the higher concentrations of Mn, Ba, Fe and Zn and the presence of larger amounts of Fe oxides in Unit IVC than in the overlying Unit IVB suggest that there may have been minor differences between metal additions in the Early Palaeocene (Unit IVC) and the late Palaeocene (Unit IVB).

Although Marchig and Vallier (1974) have reported the basal sediments of Site 248 to be metalliferous, the Fe values (up to 7%) are just above average only (see Table 2.2.2a). The sediment, a brown silt bearing clay, is relatively uniform in composition and the metal distributions (see Fig. 2.2.2h) show no trends comparable to other basal metal-rich sediments. Mn decreases in concentration with depth. This has been suggested by Marchi9 and Valuer (1974) as representing processes of remobilisation. However, the variations are extremely small and hence not too great a significance should be attached to them. The slightly higher than average values of Fe are accompanied by a minor increase in Al, up the core.

The basal sediments at Site 249 show lower than average values (see Table 2.2.2a) for all metals except Fe, Al and Ti. The Fe and Al values show a general tendency to increase as the contact with the underlying basalt is approached, while the concentrations of Ti, Cr and Li show minor increases up the core. The higher Ti, Cr and Li values in the upper portions of the core could reflect a higher detrital input. Mn and other 10 i. 0.1 0.011 , * . I 1 , 0.0011 .. • • ,0.0001 1

407

417

427

Al Fe As Cd Fig 2.2.2h

Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit III (Brown silt-bearing clay) of DSDP site 248, northwest Mozambique Basin, Indian Ocean. All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface with an expansion factor of 5 on the sample interval.

Concentrations are plotted on a logi0 scale.

10 01 0.01 0.001 0.0001

370—

38 0-

390-

400-

Fe 'Al Mg Ti 410'-

Fig 2.2.2i

Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IIlb (Grey and olive-black silty claystone) of DSDP site 249, Mozambique Ridge, Indian Ocean. All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface. Concentrations are plotted on a log 10 scale. 125

trace metals show no definite trends with depth and therefore do not lend support to the opinion that remobilisation processes, due to the presence of organic matter may have occurred in these sediments (Simpson et al, 1974) and caused Mn and other trace metals to migrate up the core.

2.2.2e Leg 26 (DSDP Sites 250A, 251A, 254, 256 and 257)

The basal sediments at Site 250A are not metal-rich, except for high Cu value in the lowermost sample (626ppm) and Mn in the uppermost sample (0.80%). All the other values are below average (see Table 2.2.2a). There are very few significant vertical geochemical variations, with most of the metals showing no definite trends. However, Mn, Ba and As are markedly depleted in the basal sediments. The Mn trend may result from remobilisation. Fleet (1977) links high Mn in the sediments from this site with the presence of Mn oxyhydroxide minerals and manganese micronodules. The Ba values may represent a greater proportion of authigenic barytes in the younger sediments.

At DSDP Site 251A, the carbonate-rich sediments are not strikingly metalliferous and only the Ba, Pb, Cu and Cr concentrations are above the average (see Table 2.2.2a). The presence of metasomatic manganiferous garnets has been used to infer a hydrothermal influence at this site, in some sediments (Davies et al, 1974; Kempe and Easton, 197}). The sediments supplied to the author were not those containing garnet minerals . The observed metal concentrations are not comparable to other basal metal- rich sediments from elsewhere in the Indian Ocean. Fleet (1977) has shown that the trace element contents of the carbonate sediments from this site are comparable with ordinary deep sea carbonates (Riley and Chester, 1971), and that the variations which occur can probably be accounted for by the incorporation of varying amounts of volcanogenic, hydrogenous and detrital material.

The basal sediments from Site 254 show enrichment trends for the metals Fe, Al, Co, Cr, Ti and to a lesser extent Mg. The sediments overall have above average values of Fe, Al, Ni, Co, Cr, Pb, Zn, Li, As and Ti. However, the Mn, Ba and Cu values fall below average (see Table 2.2.2a). The distributions of metal concentrations against depth (Fig. 2.2.2k) show•a number of trends. Fe and Mn are enriched as the contact with the underlying basalt is approached, although the Mn values are below average. However, As, Cu, Ti and Mg all decrease with increasing

10 11 0.1 0.O 1 1 0.0001k 1...... L, .. _ . l... ._. s 0.00 L.., .

696

706

716

726

Fig 2.2.2j

Vertical distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit IV (Brown detrital clay) and Unit V (olive-grey, greenish, olive-black detrital clay) from DSDP site 250A, Mozambique Basin, Indian Ocean.

All data are expressed on a C.F.B., except Ca. Vertical scale is metres below the sediment-seawater interface. Concentrations are plotted on a log10 scale The three lowermost samples are from Unit V; the remainder are from Unit IV. 10 0.1 0.01 243 0.001 0.0001

253

263

273

283

293

`Ti Mg Mn ~a Cr Zn Fe Al ~Ca 333 Fig 2.2.2k Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from' Unit IV (Black, grey, brownish sandy and silty clay with pebble conglomerates) of DSDP site 254, Ninety East Ridge, lodian:.0cean All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface. Concentrations are plotted on a log10 scale. 1\3 128

depth to values which are in most cases only equal to or slightly above average. The remaining metals show no definite trends. The concentrations of Fe, Mg, Al, Ti, Cr, Li, Ni and Co, together with the large amounts of montmorillonite and other altered volcanic debris may support the opinion of Davies et al (1974), that this sedimentary sequence has been formed by the erosion of a basaltic terrain in shallow water conditions. This is interesting since only at this site, of those described here that were drilled on the Ninety East Ridge (also Sites 214 and 216) did the basal sediments show significant trends of metal enrichment. This is supported by the findings of Fleet (1977) who attributes the inconclusive metal variations with depth to the highly variable nature of the alteration of the basaltic material.

The basal detrital clay sediments at Site 256 show no signs of metal enrichment (see Fig. 2.2.2 1) and the metal concentrations are below the average values of Indian Ocean DSDP sediments (see Table 2.2.2a). There is an overall tendency for the metal concentrations to increase with depth, but the values do not become high. The increase in the values of Fe, Al, Mg, Ti, Cr and possibly Li and Ni may in part be correlated with the presence of montmorillonite from the alteration of volcanic material in the lower portion of the sequence (Davies et al, 1974).

Although showing higher concentrations of certain metals than the sediments at the nearby Site 256, those basal sediments from Site 257 are not as metal-rich as some other DSDP Indian Ocean sediments. The sediments contain concentrations of Mn, Co, Li, As and possibly Ba which are above average (see Table 2.2.2a). However, of these only the Mn value could be considered to be high, and is probably caused by the presence of micronodules. The moderately high Ba value has been attributed to the presence of barytes (Davies et al, 1974). The variations in metal concentrations with depth (Fig 2.2.2m) are fairly constant and hence few conclusions can be drawn from them. However, Mn decreases with depth as do possibly Ni and Co. This probably reflects a decreasing content of micronodules down the core. Ba shows a similar trend which may be due to variations in the amount of barytes present. 10 0.1 0.01 0.001 0.0001

193

203

213

233

21.3

253 Fig 2.2.21 Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit I (Brownish-grey detrital clay) of DSDP site 256, Wharton Basin, Indian Ocean. All data are expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface. Concentrations are plotted on log 1 ' scale.

10 1. 0 1 0.011 . . 1 1t .. `. . 1 . 1... , l I s . 0.001 L .: . 0.00011

210

220

2L0

250

260

Fig 2.2.2m Vertical Distribution, in weight percent., of Element Concentrations in the carbonate-free fraction of basal sediments from Unit 1 (Reddish-brown, laminated, coccolith, detrital clay) of DSDP site 257, south east Wharton Basin, Indian Ocean. All data expressed on a C.F.B. except Ca. Vertical scale is metres below sediment-seawater interface. Concentrations are plotted on a log10 scale. 131

2.2.2f. General trends.

It is possible on the basis of the bulk geochemistry of the carbonate-free fraction of the sediments examined to divide them into a number of groups.

The sediments fall into two main groups, those with above average concentrations (see Table 2.2.2b) of Fe, Mn, Ni, Cr, Cu, Pb, Zn, As, Ba etc and those which have below average or average concentrations of these metals, but generally average or above average concentrations of Al, Ti, Mg, Li, etc. The first group have been termed 'metal-rich sediments' and the second group 'non-metal-rich sediments'.

The sediments of the latter, non-metal-rich group are further divisible on the basis of their Ca and Al values into three sub-groups - carbonates, clayey-carbonates and clays. (The average compositions are given in Table 2.2.2b). The carbonates occur at DSDP Sites 220, 238 and 251 and result from the process of calcareous sedimentation above the lysocline. Variations in their compositions are probably caused by the incorporation of variable amounts of volcanogenic, detrital and siliceous materials. The clays, the largest group of Indian Ocean DSDP basal sediments occur at DSDP Sites 211, 212, 214, 216, 221, 223, 224, 235, 238, 239, 243, 249, 250, 256 and 257, and result from the normal processes of pelagic sedimentation. Their compositions are in general similar to those of pelagic sediments from other oceans (Riley and Chester, 1971; Fleet, 1977), and at certain sites the bulk geochemistry and sedimentology suggest a basaltic provenance for the sediments, e.g. DSDP Sites 214 and 216 on the Ninety East Ridge; DSDP Site 256 in the Wharton Basin (Davies et al, 1974; Fleet, 1977). The clayey carbonates occur at DSDP Sites 216, 224, 238, 239, 249, 256 and 257 and result from a mixture of processes responsible for the carbonates and clays described above.

The metal-rich sediments are underlain by weathered and highly altered basalts and contain high values of Fe, Mn, Ni, Cr, Cu, Pb, Zn, As, Ba, Mg etc and are divisible into two major sub-groups - metal-rich carbonates and metal-rich clays. The metal-rich carbonates occur at DSDP Sites 215, 220, 236 and 245. As well as being CaCO3-rich and poor in Al, Ti, etc, 132

TABLE 2.2.2b. Average Composition of the Carbonate-Free Fraction of Indian Ocean DSDP Basal Sediment Groups

1. 2. 3 4. 5

( *CaCO3 36.67 60.81 21.98 1.70 17.71

( *Ca 15.0 24.50 9.32 1.27 7.54 ( Al 5.47 6.68 6.38 7.01 6.49 Wt % ( Mg 1.92 2.02 3.10 1.92 2.21 ( ( Ti 0.83 0.62 0.83 0.67 0.75

( Fe 9.25 6.16 6.4o 6.05 6.67

( Ba 2010 4350 880 1170 1390

( Mn 15220 3470 2310 2620 5190

( Ni 203 123 92 130 134

( Co 61 71 47 50 52 ( ppm ( Cr 193 219 145 149 160 ( ( Cu 175 135 98 119 125

( Cd 4 2 3 3 3 ( ( Pb 84 82 59 59 66

( Zn 199 178 150 148 160 ( ( Li 39 72 55 46 49

( As 42 16 6 9 15

* CaCO3 and Ca expressed on an uncorrected TSB

1. Metal-rich sediments (32) 2. Basal Carbonates (7) 3. Basal Clayey-Carbonates (46) 4. Basal Clays (69) 5. All Indian Ocean DSDP Basal Sediments (154) 133

they have the highest values of Mn recorded in Indian Ocean DSDP basal sediments, lower amounts of Fe and high values of Cr, Ni, Cu, As, Zn and Pb. The underlying basalts at the sites at which they occur often have Mn oxide coatings. The enriched trace metals (Ni, Cr, Cu, Zn, Pb etc) are probably associated with Mn oxide material in the form of coatings, aggregates and micronodules which are common in metal-rich carbonates. The metal-rich clays occur at DSDP Sites 213, 216, 236, 239, and 254, they are associated with altered basalts, are CaCO3 - poor and have higher concentrations of Fe and lower concentrations of Mn, Ni, and Cr than the metal-rich carbonates, but about equal concentrations of Cu, Pb, Zn, As etc. The lower values of certain trace metals in the metal-rich clays reflects the lower proportions of Mn oxide material which occur in them. The metal-rich clays from DSDP Sites 213, 216 and 236 have low Al values, those from DSDP Site 239 have average Al values, and those from DSDP Site 254 have high Al values and are associated with high Fe, Mg, Ti, Cr, Li and low Mn. This increase in Al and associated metals may indicate a basaltic provenance in these sediments. Such a conclusion is supported by the presence in them of large amounts of volcanic alteration products, i.e. montmorillonite (Davies et al, 1974; Fleet, 1977). Metal-rich clayey- carbonates occur at DSDP Site 239 and have metal compositions intermediate between those of metal-rich carbonates and metal-rich clays.

2.2.3 Metal Accumulation Rates

Metal accumulation rates have been used in the past to determine if sediments are metal-enriched (e.g. McMurtry, 1974; McMurtry and Burnett, 1974). The metal accumulation rate is derived by multiplying together the metal concentration, the sedimentation rate and the dry bulk density of the sediment.

In the past, various authors have employed a constant value of 0.70 gm cros-3 for the dry bulk density of marine sediments (McMurtry, 1974; McMurtry and Burnett, 1974; Dymond and Veeh, 1975; Sayles et al, 1975). However, some caution should be exercised in using this approach since considerable errors can be introduced in the calculation of the metal accumulation rates for water-rich, low carbonate pelagic sediments, if such a value is assumed (Dymond and Veeh, 1975; Sayles et al, 1975; 1976). Lyle and Dymond (1976) have reported errors TABLE 2.2.3 Metal Accumulation Rates of Indian Ocean DSDP Basal Sediments

SAMPLE NO. CaCO3 WET BULK WATER DRY BULK SEDIMENT- AGE OF CONTENT DENSITY CONTENT DENSITX AT ION RATE SEDIMENTS SEDIMENT TYPE Wtjō (g/cm ) (g/cm- ) (cm/1000yrs) 1. 215-16-5-147/9 83.32 1.40 0.56 0.62 1.50 Mid-Palaeoc. M.R.0O3

2. 238-54-1-131/133 42.42 1.22 0.73 0.33 1.90 Late Oligoc. M.R.0O3

3. 245-13-5-136/138 76.12 1.36 0.59 0.56 1.00 Early-Late Palaeoc. M.•R.0O3

4. 245-16-1-139/141 46.18 1.23 0.72 0.34 1.00 Early Palaeoc. M.R.00 3 5. 251A-26-4-19/21 71.94 1.34 0.61 0.52 2.90 Early.Mioc. (M.R.)CO 3 6. 236-33-3-67 10.89 1.13 0.79 0.24 1.00 'Late Palaeoc. M.R.Clay (low Al)

7. 254-32-1-136/138 nil 1.10 0.83 0.19 1.20 Late Eoc-Oligoc. M.R.Clay (high Al)

8. 238-52-5-80/2 58.66 1.28 0.62 0.49 1.20 Late Oligoc. N.M.R.0O3

9. 257-8-2-12/14 20.59 1.15 0;75 0.29 1.00 Mid. Albian N.M.R. Clayey CO 3 10. Site 224 Average 5.85 1.11 0.81 0.21 1.00 Mid.Eoc - Early N.M.R. Clay Oligoc.

From Lyle and Dymond (1976) M.R. = Metal Rich N.M.R. = Non-Metal Rich CO = Carbonate 3 TABLE 2.2.3. (cont'd) Metal Accumulation Rates (gm/cm3/1000 yrs)

Fe Mn Ni Cu Cr Zn As Ba Al Ti

1. 1.63 0.86 0.010 0.007 0.005 0.006 0.001 0.109 0.35 0.023 2. 4.33 0.10 0.005 0.007 0.008 0.004 0.0006 3.78 0.505 3. 1.20 0.27 0.003 0.003 0.003 0.003 0.003 0.034 0.56 0.056 4. 1.91 0.65 0.005 0.004 0.002 0.006 0.0009 0.036 0.72 0.079 5. 3.05 0.10 0.005 0.016 0.015 0.006 0.0005 0.133 3.30 0.216 6. 0.56 0.02 0.005 0.003 - 0.002 0.005 - 0.39 7. 4.20 0.05 0.007 0.002 0.008 0.005 0.00002 0.017 2.49 0.454

8. 1.03 0.07 0.004 0.001 0.002 0.004 0.00002 2.06 0.429 9. 1.36 0.12 0.002 0.001 0.003 0.003 0.0006 0.076 , 1.80 0.087 10. 1.04 0.01 0.002 0.003 0.004 0.003 0.0001 0.025 1.77 0.181

A. 9.00 3.30 0.06 0.08 0.04 0.80 B. 5.90 2.10 0.07 0.08 0.03

A. = Ba1er Deep Sediment (Sayles et al, 1976) B. = East Pacific Rise Sediments (McMurtry and Burnett, 1974; Lyle and Dymond, 1976) 136

of between 300 and 400% in the calculation of metal accumulation .rates for southeast Pacific sediments using this method. Broecker and Broecker (1974) recognised that there is a relationship between the CaCO3 content and the dry sediment density of marine sediments and thus Lyle and Dymond (1976) were able to derive an expression for estimating the dry bulk density of marine sediments from determinations of their CaCO3 content. The work of Lyle and Dymond (1976) based on southeast Pacific sediments is used as the basis for the following discussion of metal accumulation rates.

Accumulation rates have been calculated for elements in those basal sediments for which sedimentation rate data are available. The sedimentation rates are based on palaeontological ages and are taken from the relevant site chapters of the DSDP Initial Reports (von der Borch et Al, 1974; Whitmarsh et al, 1974; Fisher et a1,1974; Simpson et al, 1974; Davies et al, 1974). The dry bulk density data are calculated from the graph and equations presented by Lyle 'and Dymond (1976). The results are presented in Table 2.2.3. Metal accumulation rates are shown for metal-rich clays and carbonates and non-metal-rich varieties of clays, clayey-carbonates and carbonates.

The metal-rich sediments from DSDP Sites 215, 238, 245 and 254 (see Section 2.2.2f) have higher accumulation rates of Fe, Mn, C , Ni, Cr, As and in some cases Ba than the non-metal-rich sediments. In addition, carbonates from DSDP Site 251A have accumulation rates of Fe, Mn, Ni, Cu, Cr, Zn, As and Ba which are greater than those for non-metal-rich sediments and comparable with those of other metal-rich sediments..

In the metal-rich clays with high Al concentrations from from DSDP Site 254 , described in Section 2.2.2f, only the Fe accumulation rate is comparable with that in certain metal-rich carbonates from DSDP Site 245. Furthermore, the Mn and trace metal accumulation rates are less than or equal to those of non—metal-rich sediments. The low trace metal accumulation rates at DSDP Site 254 are accompanied by moderate to high accumulation rates of Al and Ti.

High accumulation rates of Mn, Ni, Zn, Ba and possibly Fe occur in Palaeocene sediments from several DSDP Sites (215 and 245) (see Table 2.2.3) and it would appear on the basis of the sites studied 137

that the Palaeocene in the Indian Ocean was a period of high rates of accumulation of Mn, Ba, As, Ni and Zn. The period of maximum accumulation of Cu and Cr was during the Early Miocene, as seen at DSDP Site 251A, while that of Fe was during the Late Eocene to Oligocene, as seen at DSDP Site 254 (see Table 2.2.3).

The metal accumulation rates in Indian Ocean metal-rich sediments are generally lower than those of the Batter Deep (Sayles et al, 1975; 1976) and on the East Pacific Rise (McMurtry and Burnett, 1974; Lyle and Dymond, 1976) (see Table 2.2.3). The Palaeocene, when the high metal accumulation rates occur, was a period of rapid sea floor spreading and increased ridge activity in the Indian Ocean (McKenzie and Sclater, 1971; Johnson et al, 1976). The high accumulation rate of Fe during this period would tend to support Cronan et al's (1974) suggestion that Fe concentrations may be taken as an indicator of ridge activity. On this basis one could speculate that the Carlsberg and South East Indian Ocean Ridges were about one quarter as active in the Palaeocene as the East Pacific Rise is at present. Furthermore, the high accumulation rate of Fe in Early Miocene sediments at DSDP Site 251A might be correlated with ridge activity and be related to the initiation of sea-floor spreading which occurred along the South West Indian Ocean Ridge at this time (Schlich, 1974; Luyendyk and Davies, 1974).

Although high metal accumulation rates have been recorded at particular locations and times in the history of the Indian Ocean, they have, in general, been lower than the rates of metal accumulation recorded at present on the East Pacific Rise. 138

2,2.4 Data Handling - Correlation Coefficients and Factor Analysis

2.2.4a Introduction

The use of computers in the manipulation and interpretation of geological and geochemical data is now well established. Multivariate statistical techniques (correlation coefficients and factor analysis) have been employed to interpret the interelement associations in the Indian Ocean DSDP sediments.

2.2.4b. Correlation Coefficients

In order to understand the inter-element associations within Indian Ocean DSDP basal sediments, correlation coefficients have been calculated between all pairs of elements (see Appendix A.4, for details of computer programme used). In order to see if interelement associations were specific to particular groups of samples or common throughout the whole data set, correlation coefficients have been calculated for each group of sediments - metal-rich; carbonates (> 55% CaCO3); clayey- carbonates (5-55% CaCO3); and clays (<5% CaCO3) as well as all Indian Ocean DSDP basal sediments. The results are displayed in Tables 2.2.4a, b, c, d and e in which only correlations significant at the 95% confidence level are recorded.

Interelement associations for all Indian Ocean DSDP basal sediments will be discussed first and the differences which are peculiar to particular groups of sediments are discussed separately.

ALL BASAL SEDIMENTS (Table 2.2.4a)

Manganese is positively correlated with trace metals, such as Ni, Cu, Cd, Pb, Zn, Ba and As and is negatively correlated with Al, Ti. The trace metals themselves (Ni, Cu, Cd, Pb, Zn, Li, and Ba) are all positively correlated with each other and negatively with Ti. Iron is positively correlated with Al, Ti, Cr, Cu, Ni, Zn and Co. There is no association of Fe and Mn in these sediments. These associations strongly suggest that Fe and Mn, together with their associated trace metals, are contained in different mineral phases in the sediments. Calcium shows a negative correlation with Al and Ti while Mn and As are TABLE 2.2.4a. Interelement Associations in Indian Ocean DSDP Basal Sediments

Fe n = 154 Al .19 Al

Mn - -.26 Mn Ni .32 .21 .42 Ni Co .23 .29 - .46 Co Cr .48 .26 - .36 .18 Cr cu .32 - .35 .55 .25 •31 cu Cd - - .22 .20 .21 - - Cd Pb - - .33 .22 .25 - .23 .18 Pb

Zn .20 .29 .34 .47 .20 - .22 .28 .29 Zn

Li .39 .23 .18 - -.20 .25 - .39 Li

Mg - - .24 - .17 - .31 - - - - Mg

Ba -.21 - .57 .40 .19 - .24 .20 .29 .22 .18 - Ba

Ti .39 .37 -.43 - - .37 - -.20 - - -.47 Ti

As - .48 .42 - - .34 .27 .35 .38 .3o - .28 -.19 As

Ca - -.35 .34 - - -.18 .22 Ca Table 2.2.4b. Interelement Association in Indian Ocean DSDP Metal-Rich Sediments

Al .54 Al n = 32 Mn -.37 -.56 Mn

Ni - - - Ni

Co .40 .48 - .58 Co

Cr .42 .58 -.48 - .39 Cr

Cu - - .48 .80 - - Cu

Cd - - - .28 - .41 - Cd Pb - .38 - - - .41 - Pb

Zn - .44 .68 - - .63 - - Zn

Li - .43 - .39 - .46 - - - .46 Li

Mg - -.54 .50 ------Mg

Ba -.51 -.61 .76 - - - .47 - - - - - Ba

Ti .60 .83 -.82 - .47 .61 ------.68 Ti

As - -.44 .45 - - - .53 - - - - .65 - -.38 As

Ca -.49 -.65 .78 ------.76 -.70 .49 Ca 141

positively correlated with Ca. It has been suggested that Hg may substitute in the lattice of carbonate minerals (Horowitz, 1974a), and Erickson and Wollin (1973) have indicated a similar behaviour for Mn. However, the association with Ca, and that of As, may also be explained in terms of the high concentrations of these metals (i.e. As, Mn) in the large proportions of the oxide material present in the metal-rich carbonate sediments. The association of Mn with other trace elements, see above, may be explicable in terms of their coprecipitation with Mn oxides.

METAL-RICH SEDIMENTS (Table 2.4.2b)

A feature of these sediments is the negative correlation between Fe and Mn. A positive correlation between these two metals has been reported from basal and surface metal-rich sediments from other areas (BostrBm and Peterson, 1969; Horowitz, 1974a; Cronan, 1976; Dymond et al, 1976). BostrBm and Peterson (1969) have shown that where detrital sedimentation is low, e.g. on the East Pacific Rise, Fe and Mn are strongly positively correlated. However, where detrital sedimentation is higher, e.g. in North Atlantic surface sediments (Horowitz, 1974a, b), this correlation between Fe and Mn becomes low or may become insignificant. The negative correlation of Fe and Mn in metal-rich sediments may be due to independent processes of enrichment and incorporation of Fe and Mn in different mineral phases. The Mn in metal-rich sediments is associated with such trace metals as Cu, Pb, Zn, Ba and As, as well as Ca, and it is negatively correlated with Fe, Al, Cr and Ti. This probably results from the coprecipitation of these metals with Mn oxides. The Mn oxides are particularly enriched in carbonate-rich, metal-rich sediments, thus giving an association of Mn with Ca, although some Mn may also be held in the lattice of carbonate minerals (Ericson and Wollin, 1973). Iron in the metal-rich sediments is associated with Al, Co, Cr and Ti. Such an association, and the negative correlation of these elements with Mn, suggests that Fe is held in different mineral phases from Mn, probably in oxides and silicates (clay minerals, etc). The positive correlations of Al with Co, Cr, Li, etc indicates that these metals may be held in clay minerals. An association between Fe and As is absent in these metal-rich sediments, As being positively correlated with Mn. Cronan (1972), on the basis of positive correlations between Fe and As for Median Valley sediments from the Mid-Atlantic Ridge Table 2.2.4c. Interelement Associations in Indian Ocean DSDP Basal Carbonates

AI - Al n - 7

Mn - - Mn

Ni - .76 - Ni

Co - .76 -.77 - Co

Cr - - - Cr Cu - - -.76 - - .83 Cu

Cd ------Cd Pb - - -.83 - .93 - - - Pb

Zn ------Zn

Li ------Li

Mg - -.96 .8o - - -.98 -.92 - -.98 -.99 Mg

Ba - -.88 .97 .98 -.86 -.92 -.81 - -.93 -.85 -.87 .82 Ba

Ti ------.90 - - -.95 Ti

As - - - -.81 - - - - .85 - - -.87 .87 As

Ca ------.88 - - - - -.91 -.89 - Ca Table 2.2.4d. Interelement Associations in Indian Ocean DSDP Basal Clayey-Carbonates Fe

Al - Al

Mn - .46 Mn n =46 Ni - .64 - Ni

Co .44 .44 .44 Co

Cr .61 - - - -.33 Cr

Cu .83 - - .37 - .64 cu

Cd - - - .58 -.43 - Cd

Pb -.36 - - - .49 -.54 -.35 .38 Pb

Zn - .57 - .36 .46 - - .30 .50 Zn

Li -.47 .40 .36 - .37 -.51 -.46 .41 .47 .47 Li

Mg .58 - - - .62 - - - - Mg

Ba .47 .51 - - -.30 - - -.35 - .45 -.47 Ba

Ti .55 .39 .46 - -.39 - - Ti

As - - .31 .37 - - .37 - - •37 -.39 .45 -

Ca - .36 .38 - .34 - - Ca 144

suggested that these elements may originate from a hydrothermal source. Calvert and Price (1977) have suggested that As may be scavenged by, and coprecipitate with Fe. The association of As with Mn in the sediments studied here, however, suggests that this element is more probably coprecipitated with authigenic Mn oxides in these metal-rich sediments.

BASAL CARBONATES (Table 2.2.4c).

No trace metals correlate positively with Mn except Mg and Ba in basal carbonates. There is no correlation between Fe and Mn in basal carbonates. Calcium is positively correlated with Cu and is negatively correlated with Mg and Ba. The association of Cu with Ca may indicate that this metal is to some degree held in the lattice of the carbonate minerals (Greenslate et al, 1973; Oldnall, 1975). Cobalt is positively correlated with Al, and this together with the positive correlations of Co, Pb, Cu and Cr, may indicate their incorporation into clay minerals.

BASAL CLAYEY-CARBONATES (Table 2.2.4d)

Iron is correlated positively with Cr, Ca, Mg and Ti, while Mn is positively correlated with Co, Li and Ba. There is no positive correlation of Fe and Mn in the basal clayey-carbonate. In these sediments there are also positive correlations between Ca, Fe, Al, Cr, Cu and Zn and between Al, Mn, Ni, Co, Zn, Li and Ba. These associations, particularly of Al with Mn and of Al and Mg with Ca suggest that in part these elements which are present in low or average absolute concentrations may be held in the lattice of the clay minerals present in these sediments. The associations of Mg and Zn with Ca may in part be due to the incorporation of these metals in the lattice of the carbonate minerals present in these sediments.

BASAL CLAYS (Table 2.2.4e)

Iron is correlated positively with Al, Ni, Cr, Mg, Ti and As, while Mn is positively correlated with Ni, Co, Cu, Pb, Zn, Mg, Ba and As. There is no association of Fe with Mn. Such associations suggest that the Fe and Mn and associated elements are held in the different mineral phases. Table 2.2.4e. Interelement Associations in Indian Ocean DSDP Basal Clays Fe

Al .27 Al

Mn - - Mn n=69

Ni .43 .41 .47 Ni Co - - .30 .46 Co

Cr .42 .50 - .52 - Cr

Cu - .31 .45 .25 - Cu

Cd -.24 - - - .29 - - Cd

Pb .24 - Pb

Zn - .48 - .27 - - - - - Zn

Li - .43 - .41 - - - - - .31 Li

Mg .27 - .46 .52 .25 - .33 - - - Mg

Ba -.34 - .34 .38 .31 - - - .33 - - - Ba Ti .30 .35 -.36 - - .46 ------.31 Ti

As .36 .27 .41 .40 .30 - .24 - - - .42 - - - As

Ca -.25 -.26 - - - -.27 Ca 146

The associations of Fe, Al, Ti, etc suggest that Fe may be held in the lattice of clay minerals. The associations of such trace metals as Co, Cu, Pb and Ba etc, with Mn and the absence of a correlation between these trace metals with Fe and Al may suggest that they were coprecipitated with authigenic Mn oxides.

2.2.4c. Factor Analysis

Factor analysis is a multivariate statistical technique which is used to describe the total variance of a complex data set in terms of a few, statistically independent factors, which are composed of groups of variables. The R-mode factor analysis programme used in this study and the procedures involved in factor analysis are described in Appendix A.4.

A recent example of the application of factor analysis to geochemical data from DSDP basal sediments is provided by Dymond et al's (1977) study of sediments from DSDP Site 319 in the south-eastern Pacific Ocean. Using what Dymond et al (1977) state is Q-mode factor analysis, but is in fact R-mode factor analysis, they were able to describe the variation in metal contents in terms of three factors - a biogenic factor, a hydrothermal factor and a detrital-hydrogenous factor. Dymond et al (1977) were also able to show how these factors vary as a function of depth down the core.

R-mode factor analysis has been applied to the metal concentrations of the carbonate-free fraction of Indian Ocean DSDP basal sediments according to the procedures described in Appendix A.4.

Such analysis has yielded three factors which together account for 92% of the variance within the data set. From the rotated factor loadings, which are displayed in Fig. 2.2.4a, it is possible to determine which variables, i.e. elements, are important in each of the factors. From the factor scores, i.e. the sample compositions re-expressed in terms of the factors (themselves a linear transformation of the original variables), it is possible to determine the relative importance of each factor in each sample. The factor scores have been plotted for the three factors on a ternary diagram in Fig. 2.2.4b and on scatter plots for each pair of factors in Figs. 2.2.4c, d and e. 147

Fig 2.2.4+a Rotated Factor Loadings Matrix resulting from R-Mode Factor Analysis on data from the carbonate free fraction of Indian Ocean DSDP Basal Sediments

Factor 1 -- Mn oxide +1

Factor 2 — Volcanic

Factor 3 — Biogenic/Clay mineral

Ca Fe Al Mn Ni Co Cr Cu Cd Pb Zn Li Mg Ba Ti As 148

Factor 1, which accounts for 42.4% of the total variance, is composed of Mn, Ni, Ba, As, Pb and Zn, with lesser amounts of Co, Cu, Cd and Li. This factor is of major significance in metal-rich carbonates from DSDP Sites 215, 220 and 245, where it probably represents precipitation of authigenic Mn oxides. It is of significance in metal- rich clays from DSDP Sites 213, 216 and 236 in which Mn is not enriched to the same degree as Fe. Factor 1 is of less significance in basal carbonates from DSDP Site 238 and also in basal clayey-carbonates from DSDP Sites 216, 224, 238, 249 and 257. It is of only minor significance in basal clays from such DSDP Sites as 216, 214, 221, 235, 238, 249 and 250. In both non-metal-rich sediments and metal-rich sediments, this factor probably represents the precipitation of authigenic Mn oxides from seawater, while in the metal-rich sediments the increased significance of this factor may probably be caused by the addition of some oxides by precipitation from hydrothermal fluids.

Factor 2, which accounts for 26.7% of the total variance, is composed of Fe, Cr and Ti, with lesser amounts of Al, Ni, Co, Cu and Zn. This is probably a volcanic factor and may represent the combination of the removal of Fe and associated trace metals from the underlying basalts by hydrothermal leaching, followed by their precipitation as Fe oxides and silicates, as well as the incorporation into the sediments of basaltic detritus and chemically altered volcanic basaltic material. Factor 2, is of major significance in metal-rich clays from DSDP Sites 213, 236 and 254 and of lesser significance in other metal-rich sediments. Else- where it is of minor significance, as in basal carbonates from DSDP Site 251A, in basal clayey-carbonates from DSDP Sites 216, 238 and 249 and of local significance in some basal clays from DSDP Sites 212, 238, 250, 256 and 257. In the non-metal-rich sediments, this factor probably represents incorporation in the sediments of the alteration and erosion products of the underlying basalts rather than a hydrothermal leaching influence.

Factor 3, which accounts for 22.7% of the total variance, is composed of Ca and Mg with lesser amounts of Mn, together with opposed amounts of Al, Co, Ni and Li. This is the biogenic factor and represents the incorporation of Mn, Mg and possibly minor amounts of trace elements also into biogenic carbonate material, as well as a non-biogenic component in the form of clay minerals, authigenic or detrital, represented by the 149 Fig 2.2.4b

Distribution of Factor Scores for Factor 1 (Mn oxide), Factor 2 (volcanic) and Factor 3 (Biogenic/clay mineral) and Indian Ocean DSDP Sediments.

;-(Factor scores have been calculated from the data of the carbonate- free fraction of the sediments).

FACTOR 2 - Volcanic

FACTOR 1 - Mn oxide FACTOR 3 - Biogeni%lay mineral

LEGEND

O 1 Metal-Rich Sediments - clays

p2 Metal-Rich Sediments - carbonates

O 3 Basal Carbonates - DSDP site 238

04 Basal Carbonates - DSDP sites 220, 238 and 251

O 5 Basal Clayey Carbonates - DSDP sites 216, 224, 238, 249 and 257

O 6 Basal Calyey Carbonates - DSDP sites 216, 238 and 249

07 Basal Clayey Carbonates - DSDP sites 216 and 238

e Basal Clays 150

Al and trace metals. This factor is of major significance in metal- rich carbonates from DSDP Sites 215, 220, 236 and 245 and in basal carbonates from DSDP Sites 220, 238 and 251A. It is of less significance in some carbonate-rich clayey-carbonates from DSDP Sites 216 and 238 and it is of occasional significance in basal clays from DSDP Sites 212, 221, 224, 235, 238, 248 and 250. In the latter case of basal clays, its significance is probably due to the local increases in the amounts of carbonate material in these sediments.

A study of Fig. 2.2.4b shows how the sediments can be grouped on the basis of these factors. It illustrates the importance of factors 1 and 3 in metal-rich carbonates, and the importance of factors 1 and 2 in metal-rich clays. The clayey-carbonates are split into three main groups, in which factor 1 (DSDP Sites 216, 224, 238, 249, 257), factor 2 (DSDP Sites 216, 238, 249) and factor 3 (DSDP Sites 216 and 238) dominate. The basal clays cluster as a group near the centre of the plot representing the non-significance of any particular factor in their composition, while the basal carbonates show a loose grouping nearer to factor 3, but spreading between factor 1 (DSDP Site 238) and factor 2 (DSDP Sites 220,238 and 251A). The scatter plots of pairs of factors against each other tend to reinforce these conclusions (see Figs. 2.2.4c, 4d and 4e).

The separation of Fe from Mn into different factors is markedly dissimilar to the findings of Dymond et al (1977) for DSDP Site 319 from the southeastern Pacific Ocean. The results reported by Dymond et al (1977) may be in error from the sampling point of view, since the number of samples used (23) for the factor analysis was an order of magnitude lower than the threshold for reliability of a correlation matrix (i.e. number of samples C< 10x number of variables; Howarth, 1978, person. commun.). This may have given rise to misleading results and may not be indicative of the general trends in basal DSDP Sediments as a whole. Theseparation of Fe and Mn in this study of Indian Ocean DSDP basal sediments is probably a reflection of the well-known fractionation of the elements in the marine environment (Krauskopf, 1956, 1957). 151 Fig 2.2.4c, 2.2.4d and 2.2.4e

Distribution of Factor Scores for Factor 1 (Mn oxide), Factor 2 (Volcanic) and Factor 3 (Biogenic/clay mineral) of Indian Ocean DSDP sediments

Factor 2

Facto r 1

Legend as for Fig 2.2.4b

Factor 3 •• <§) • ~ @ 2 (§) (j) (j) III @l @ (§) (Factor scores have been calculated from (j)(j) • • •• ® •• • ., •• the data of the ®3® ••• 0 : • carbonate-free fraction 0000 •• ·0·· . of the sediments) 0~6 ° ° 0. • 0 0 • ° 0 • o ' 0 •••• • •••• • • •• • ~ ••• 0° • 0'0 ••••••••• o. 0 ° 0 ° 0 •••• • • 000, 5 0 @@ ° ° 00

Factor

" . ~ .

Factor 3

Factor 2 152

2.2.5 Summary

It is possible at this stage to draw some preliminary conclusions regarding the nature of the various groups of DSDP Indian Ocean basal sediments and the possible sources of the metals that they contain.

All the non-metal-rich basal sediments (carbonates, clayey- carbonates and clays) have average or below average concentrations of Fe, Mn, Ni, Cu, Cr, Pb, Zn, As and Ba and generally average or above average concentrations of Al, Ti, Li, Mg and Ca. The metal accumulation rates in non-metal-rich sediments are lower than those for Indian Ocean DSDP basal metal-rich sediments and are generally equal to or less than those for Pacific pelagic sediments. This suggests that their metal concentrations can be accounted for largely in terms of normal oceanic sedimentation processes.

The geochemistry of the basal carbonates is dominated by the biogenic factor, Factor 3, although interelement associations suggest that only small amounts of Cu may be concentrated in the biogenic carbonate material, while the remaining trace metals are associated with clay minerals and Mn oxides present in the sediments. Variations in the composition of the basal carbonates are exemplified in the dominance of Factor 1, the Mn factor, in carbonates from DSDP Site 238 and of Factor 2, the Fe factor, in carbonates from DSDP Site 251A. This variability is probably a reflection of variations in the non-carbonate, i.e. volcanogenic, detrital and siliceous components of the basal carbonates.

The composition of the basal clays is very similar to that of normal pelagic sediments (Riley and Chester, 1971). Iron and manganese in these clays are both correlated with Al, Mg, Ti and certain trace metals. The association of Mn with certain trace metals, e.g. Ni, Cu, Pb, suggests that these sediments include proportions of authigenic Mn oxides precipitated from seawater. Variability within this group of sediments does occur and is exemplified by the importance of Factor 1 at DSDP Sites 214, 216, 221, 235, 238, 249 and 250 indicating a higher proportion of Mn oxides and associated trace metals precipitated from seawater; the importance of Factor 2 in clays from DSDP Sites 212, 238, 250, 256 and 257 indicating a higher proportion of basaltic detritus and basaltic-volcanic alteration products; and the increased significance of Factor 3 in the clays from DSDP Sites 212, 221, 235, 224, 239, 248 and 250, possibly indicating 153

the presence of locally increased proportions of carbonate material.

The geochemistry of the clayey carbonates probably represents a combination of the processes responsible for the composition of carbonates and clays, since their composition is in general not dominated by a single factor. The intere.lement associations in these sediments indicate that proportions of Zn may be concentrated in the carbonate material, while Factor 3, the biogenic factor, is of importance in the carbonate-rich clayey carbonates from DSDP Sites 216, and 238. The association of Mn with certain trace metals (Co, Li, Ba) may indicate the presence of proportions of these elements in authigenic Mn oxides in some sediments. The presence of higher proportions of Mn oxides is supported by the importance of Factor 1, the Mn factor, in clayey-carbonates from DSDP Sites 216, 224, 238, 249 and 257. The positive correlations of Mn, Fe, Al, Ti, Mg and Ca and certain trace metals, indicates that in the clay-rich clayey-carbonates these metals may be contained in clay minerals which may be the alteration products of basaltic material.

Metal-rich sediments, which contain above average concentrations of Fe, Mn, Ni, Cr, Cu, Pb, Zn, As, Ba and Mg, are associated with highly weathered and alterec( basalt sequences, often containing identifiable lava flows and pillow structures. The accumulation rates of Fe, Mn, Ni, Cu, Cr, Zn, As and Ba in these sediments are higher than those in non-metal-rich and Pacific pelagic sediments and are generally equal to or less than those in metal-rich sediments from the East Pacific Rise and Bailer Deep. The patterns of metal enrichment and accumulation rates indicate that there is an additional supply of Fe, Mn, Ni, Cr, Cu, Pb, Zn, As, Ba and Mg in Indian Ocean DSDP basal metal-rich sediments over those supplied by the normal processes for pelagic sedimentation.

The metal-rich carbonates from DSDP Sites 215, 220, 236 and 245 are Al, Ti poor and have values of Mn, Ni, Cu, Cr, Zn, Pb and As higher than those of metal-rich clays. The underlying basalts they are associated with are often coated in Mn oxide material and the sediments contain Mn oxide aggregates and micronodules. The positive correlation of Mn with Ni, Cu, Pb, Zn, Ba and As in the metal-rich carbonates and clays suggests that these metals are coprecipitated with authigenic Mn oxides. This is supported by the dominance of Factor 1, the Mn factor, in the metal-rich 154

carbonates from DSDP Sites 215, 220 and 245. The Fe is positively correlated with Al, Ti, Mg, Cr, Ni and Co in metal-rich carbonates and clays. Factor 2, the Fe factor, is of less importance in the metal-rich carbonates. Factor 3, the biogenic factor, is also of major significance in the metal-rich carbonates from DSDP Sites 215, 220, 236, and 245.

The metal-rich clays contain high concentrations of Fe and associated trace elements, but lower concentrations of Mn, Ni and Cr than the metal-rich carbonates. The metal accumulation rates in metal-rich clays are also lower than those of metal-rich carbonates but are higher than metal accumulation rates in non-metal-rich sediments. The metal- rich clays from DSDP Sites 213, 216 and 236 have low Al concentrations, those from DSDP Site 239 moderate Al concentrations and those from DSDP Site 254 high Al concentrations accompanied by high Ti, Mg, Cr, Li and low Mn concentrations. The DSDP Site 254 clays also have high accumulation rates of Al and Ti. This may be due to an increased proportion of enriched metals supplied by the inclusion of unaltered basaltic detrital fragments and leached basaltic residues in these sediments. The geochemistry of the metal-rich clays is dominated by Factor 2, the Fe factor, which is of major significance in those metal-rich clays from DSDP Sites 213, 236 and 254. The Mn factor, Factor 1, is also of importance in metal- rich clays from DSDP Sites 213, 216 and 236, but it is of generally less importance than Factor 2 in these sediments.

There are high accumulation rates of Mn, Ba, As, Ni and Zn in Palaeocene metal-rich sediments from DSDP Sites 213, 215, 236, 239 and 245 while there are high accumulation rates of Fe, Cu and Cr in the Miocene metal-rich sediments from DSDP Site 251A. Periods of increased seafloor spreading rates and ridge activity along. the Carlsberg and South East Indian Ocean Ridges, and South West Indian Ocean Ridge, respectively, appear to be related to those two periods Of increased metal accumulation rates.

Manganese occurs in all the sediments as authigenic Mn oxides. However, in metal-rich sediments Mn may be precipitated from hydrothermal fluids, which could be derived from the leaching of the underlying basalts. The trace metals enriched with Mn may originate in a 155

similar way. Iron and its associated metals are probably supplied in all the sediments by the incorporation of basaltic detritus (unaltered) and chemically altered basaltic material. However, the increased amounts of Fe and its associated metals in metal-rich sediments may be derived by leaching of these metals from the underlying basalts and their reprecipitation in the sediments as Fe oxyhydroxides and silicates. Portions of the leached basaltic residue, in the form of clay minerals, may also be incorporated into these metal-rich sediments. The separation of Fe and Mn in these sediments probably reflects the well-known fractionation of these metals in the marine environment (Krauskopf, 1956, 1957). Barium in most of the basal sediments appears to be supplied from biogenic sources, but in metal-rich sediments it may in part be hydrothermally introduced also. Arsenic, although being reported to coprecipitate with Fe (Calvert and Price, 1977) would appear to be precipitated with Mn in all the sediments examined here. Biogenic concentration may account in part for Cu and Zn in some of the sediments.

At this stage further discussions on the sources and modes of enrichment of elements in the sediments examined in this work, will be deferred until after discussion of the geochemical partition patterns of the elements described in the next section. 156

2.3 GEOCHEMICAL PARTITION OF INDIAN OCEAN DSDP BASAL SEDIMENTS

2.3.1 Introduction

Marine sediments are composed of a number of mineralogical components, some of which were initially recognised by Murray and Renard (1891), which were classified according to their origin by Goldberg (1954) and which have become more well known in recent years (Chester and Hughes, 1967). A study of the bulk chemical composition of sediments gives an overall picture of the relations of elements to one another in the sediment as a whole. However, it gives no indication of how the various elements are partitioned between the various components of the sediment. A knowledge of the partition of elements is important in understanding the complex geochemistry of marine sediments and may give an indication of the processes by which elements are incorporated into sediments.

It would appear best to separate the various minerals physically and then analyse them separately. However, the physical separation of the various mineralogical components of marine sediments by such processes as heavy mineral analysis, magnetic separation or hand-picking individual grains under the microscope is extremely time consuming and may not be wholly successful in the case of very fine-grained sediments, e.g. clays. To overcome this problem, chemical separation, using particular reagents to dissolve particular mineral components, has been widely adopted. The techniques are relatively straight forward but the results are more difficult to interpret than the chemical analyses of individual mineral components.

Various workers have used a number of reagents to deal with specific problems of chemical partition. Hirst and Nicholls (1958) used 25% (v/v) acetic acid and complexing organic reagents to differentiate detrital and non-detrital fractions of carbonate rocks. Goldberg and Arrhenius (1958) used 5% EDTA (ethylene diamine tetra-acetic acid) to examine the partition of trace elements in Pacific pelagic clays. Chester (1965c)used a similar method to Hirst and Nicholls (1958) in a study of reef and non-reef limestones. Lynn and Bonatti (1965) used a one molar hydroxylamine solution to examine the partition of Mn in marine 157

sediments. However, it was not until 1967, that Chester and Hughes (1967) developed a technique to study the partition of trace metals in pelagic sediments. This technique, although modified in some respects has been widely applied since. They used a series of chemical attacks involving various acids, reducing agents and combined acid/ reducing agents in an effort to determine the partition of Fe, Mn, Ni and V between ferromanganese minerals, carbonates (excluding dolomite) and adsorbed elements on the surfaces of other minerals. They were able to show that in sediments of low carbonate content (i.e. pelagic clays) a 25% (v/v) acetic acid (HAC) solution will remove the carbonate minerals, trace metals from adsorbed sites and those contained in the soluble Fe oxide minerals (Chester and Hughes,1967). Furthermore, that a combined acid/reducing (A/R) agent, hydroxylamine hydrochloride in 35% HAC, will remove all the above plus the reducible FeMn oxide minerals and their associated trace elements. Cronan and Garrett (1973) added a third leaching procedure, hot 50% (v/v) hydrochloric acid (HCl). This removed all but the most resistant silicates and alumino-silicates and those insoluble Fe oxides (e.g. well crystalline Fe203).

Further work (Chester and Hughes, 1969; Cronan and Garrett,1973) on a variety of sediments, mainly from the Pacific Ocean, showed that Fe and Mn are removed by essentially different leaches. Manganese with its associated trace metals is almost completely removed (98%, Cronan and Garrett,1973) by the A/R agent. However, Fe with its associated trace metals will be removed (nearly 85%, Cronan and Garrett, 1973) by the hot HCL leach. More recent work on both surface metal-rich (Sayles and Bischoff, 1973; Horowitz, 1974a, b) and on DSDP basal metal-rich sediments (Horowitz, 1974a; Horowitz and Cronan, 1976; Chester et al, 1976; Cronan, 1976), from the Atlantic and Pacific oceans has supported these findings. Partition studies on Atlantic DSDP sediments from the Bermuda Rise (Chester et al, 1976) have shown that there is a significant change in the distribution of trace elements in Upper and pre-Upper Cretaceous sediments, probably as a result of a different origin of these metals at different geological periods. Further partition studies on Pacific Ocean DSDP sediments (Cronan, 1976) have tended to draw attention to the higher proportion of FeMn oxide material present in these sediments, with which elements such as Ni, Co, Pb and to a lesser extent Cu are strongly associated. However, work on North Atlantic DSDP sediments (Horowitz,

1974a; Horowitz and Cronan, 1976). has shown that in this area there is 158

less oxide material than in the Pacific Ocean and that trace metals such as Cu, Zn, Ni and Pb are more associated with the silicates and detrital minerals, as well as oxides.

No partition analyses have hitherto been reported on Indian Ocean DSDP basal sediments. In this work sixty-four samples, representing the major groups of sediment types recognised in the Indian Ocean DSDP sample population (basal carbonates (more than 55% CaCO3), basal clayey carbonates (5-55% CaCO3), basal clays (less than 5% CaCO3 ) and basal metal-rich sediments (sub-divided on a CaCO3 basis, as above)) were subjected to chemical partition studies. The purpose was to investigate the element partition patterns in these sediments, to see how these compare with basal sediments from other areas and how they compare with their recent equivalents in the Indian Ocean and elsewhere. The techniques used are those of Chester and Hughes (1967, 1969) as modified by Cronan (1976).

2.3.2 Results and Discussion

The sediments were analysed according to the techniques described in Appendix A.1. The results have been recalculated to a carbonate-free basis, with the exception of Ca. Table 2.3.2a contains the average compositions of the major sediment types, together with the percentage of the bulk composition of each metal removed by each partial attack. Only significant variations which are not explicable in terms of the analytical precision of the determination methods are discussed below.

The sediments analysed consist of : one basal carbonate (DSDP Site 220), five basal clayey carbonates (DSDP Sites 211, 249, 256 and 257), forty-four basal clays (DSDP Sites 212, 214, 216, 221, 223, 224, 239, 248, 249, 250A, 254, 256 and 257), six metal-rich basal carbonates (DSDP Sites 215, 245 and 251A), one metal rich basal clayey carbonate (DSDP Site 239) and seven metal-rich basal clays (DSDP Sites 213, 216 and 254). Within each sediment group where there is more than one sample, a certain amount of variation occurs about the stated averages in Table 2.3.2a. The variations occur in certain elements only, and are generally of minor significance. The variations are generally a reflection of differences in mineralogy which may in itself be a reflection of variations in the mode of formation of sediments within a particular sediment group. 159

IRON (See Fig. 2.3.2a)

The partition of Fe is generally similar in all types of sediments.

Little Fe is removed by the HAC leach, but in metal-rich clays about 5% of the total is soluble in HAC. Cronan (1976) has suggested that this attack does not dissolve the Fe oxides and associated trace metals to the extent as originally suggested by Chester and Hughes (1967). However, the presence of 5% of the total Fe in this leach may indicate that the Fe oxide minerals are soluble to a limited extent. Six percent of the total Fe is soluble in the A/R agent leach in metal-rich clays which may indicate the presence of an acid- reducible Fe mineral, such as a mixed FeMn oxide or a reducible Fe oxide mineral in these sediments. (This is discussed more fully in Section 3 in the light of recent work in AGRG). In certain metal-rich clays (DSDP Sites 213 and 254), metal-rich carbonates (DSDP Site 215) and basal clays (DSDP Site 214) more Fe (up to 15% of the total) is soluble in the A/R agent leach emphasising the possible presence of such a mineral phase. Horowitz (1974a) has suggested that the clay mineral lattice may be attacked by the A/R agent releasing Fe and other metals. However, the lack of Al in the A/R agent leach (see below) would suggest that this is not occurring in these sediments. The majority of the Fe (75-99% of the total) is soluble in the HCL leach. This supports the results from other areas, e.g. East Pacific Rise, Bailer Deep (Cronan and Garrett, 1973; Dymond et al, 1973; Sayles and Bischoff, 1973; Cronan, 1976), suggesting that Fe is primarily associated with Fe-bearing smectites (Sayles and Bischoff, 1973) and oxides, e.g. goethite (Buser and Grutter, 1956). The percentage soluble in HCL is lower in both groups of clays (metal-rich and basal), in metal-rich carbonates and in basal clayey carbonates. In metal-rich and basal clays, metal-rich carbonates and basal clayey-carbonates, 14 - 16% of the total Fe is in the HCL insoluble minerals suggesting the presence of Fe-bearing detrital minerals in these sediments. 160

Fig 2.3.2a and 2.3~2b Distribution of Fe and Mn in the Partial Chemical Leaches of Metal-Rich and Basal. Indian Ocean DSDP Sediments

(Ail d~ta are expressed on a C.F.B.)

mre mree mrcl be bee bel

<,~ .. Mn LEGEND

HAC Leach

AIR Agent Leach only

HCI Leach only

Insoluble Residue

mrc t1e tal - Ric h Carbonate

mrcc Metal-Rich Clayey-Carbonate

mrcl Metal-Rich Clay

be Basal Carbonate bee Basal Clayey Carbonate bel Basal Clay 161

MANGANESE (see Fig.2.3.2b)

Although the partition of Mn is generally similar within each group of sediments, there are marked contrasts between partition patterns of the various groups of sediments. Previous work has shown that Mn is concentrated in different mineral phases from those which are enriched in Fe, i.e. the oxides such as psilomelane, todorokite and birnessite which occur as coatings, encrustations and micronodules (Chester and Hughes, 1967, 1969; Cronan and Garrett, 1973; Sayles and Bischoff, 1973; Horowitz, 1974a; Chester et al, 1976; Cronan, 1976). In keeping with previous findings, 83% of the total Mn is soluble in the A/R agent leach in metal-rich carbonates, little is soluble in HCL oris in the HCL insoluble phases, while only 10% of the total is soluble in HAC. By contrast, in the basal carbonates, 67% of the total Mn is soluble in the HAC leach, little is soluble in the A/R agent leach or is in the HCL insoluble phases, while 25% of the total Mn is soluble in the HCL leach. High proportions of Mn soluble in the HAC leach, have been reported from the North Atlantic (Horowitz, 1974a - 14% of the total Mn), although not as high as those reported here. Goldberg and Arrhenius (1958) and Chester and Hughes (1967) showed that dilute HAC will release Mn from an unidentified metal hydrate. Furthermore, Chester and Hughes (1967) showed that although dilute HAC will not physically break down Mn oxides, it has the capacity to leach out a certain amount of Mn and remove Mn from adsorbed sites in the surfaces of clay minerals. Copeland (1970) has shown that in carbonate-rich sediments Mn may be released by an excess of HAC due to the breakdown of Mn coatings on calcareous organisms. Ericson and Wollin (1973) have reported the presence of Mn in the lattice of carbonate minerals in the tests of foraminifera from the Caribbean Sea. The dissimilarity between partition patterns of Mn in metal-rich and basal carbonates can probably be predominantly accounted for by the presence of Mn oxide aggregates and micronodules in the metal-rich carbonates (e.g. DSDP Site 245 - Simpson et al, 1974) which will only be partially attacked by the HAC, whereas in basal carbonates the smaller total amount of Mn may be held on adsorbed sites or as coatings, and in the carbonate minerals of, organisms which are susceptible to attack by HAC. Thus a greater proportion of this Mn will be liberated by the HAC leach. 162

In metal-rich and basal clays there is a tendency for the Mn to be roughly evenly distributed between all the attacks (12-15% in the HAC; 25-31% in the A/R agent leach; 36-48% in the HCL leach; and 12-21% in the MCL insoluble phases). This may reflect the poly- mict nature of these sediments, in that the mineralogy probably reflects input from detrital, biogenic, authigenic as well as possible volcanic sources. Horowitz (1974a) has suggested that Mn may be held in the lattice of clay minerals. This may well be the case,since 36% of the total Mn is soluble in the HCL leach in metal-rich clays from DSDP Sites 216 and 254 where the sediments are enriched in montmorillonite and other volcanic material of a basaltic provenance. It is also probable that the small, absolute concentrations of Mn in the HCL leach o basal clays could be accommodated in the lattices of the clay minerals contained in the sediments (Arrhenius, 1963). However, in the metal-rich clays from DSDP Site 213, where the absolute concentration of Mn is higher, Mn is almost entirely present in the A/R agent leach (99%), indicating that the Mn at this site is present in the Mn oxide aggregates and micronodules. The presence of Mn in the HCL insoluble minerals of all the sediments (5-21%) may reflect its presence in detrital minerals. Such has been reported from North Atlantic sediments (Horowitz, 1974a - up to 6%). The proportion of detrital Mn is highest in metal-rich clays from DSDP Sites 216 and 254. In the two types of clayey carbonates (metal-rich and basal) there is a general tendency for the Mn to be partitioned predominantly between the HAC soluble (26-48%) and the HCL soluble (27-61%) phases, with lesser amounts in the A/R agent leach and the HCL insoluble phases. This probably reflects the presence of Mn in a mixture of carbonate components (with Mn coatings), Mn oxides, clay and detrital minerals.

NICKEL (see Fig. 2.3.2c).

The partition of Ni is generally similar in all. types of sediment. Although small amounts of Ni are soluble in HAC, the proportions of the total Ni (up to 16%) are higher than previously reported values from North Atlantic (Horowitz, 1974a, 14%), and Pacific Ocean (Cronan, 1976, negligible or nil) sediments. In the sediments which contain 163

significant amounts of carbonate material, this may reflect the incorporation of Ni into the tests of carbonate organisms (Bostr8m et al, 1974), whereas in sediments where biogenic carbonate material is absent, Ni may be removed from adsorbed sites on the surfaces of clay minerals by the HAC leach (Chester and Hughes, 1967, 1969). In metal-rich sediments from the Pacific (Cronan, 1976) and the North Atlantic (Horowitz and Cronan, 1976) Ni has been shown to be strongly positively correlated with Mn, being soluble in the A/R agent leach. This behaviour is most marked in Pacific basal sediments where 90% of the Ni is soluble in the A/R agent leach, while in the Atlantic Ocean this figure drops to 41%. However, on average in Indian Ocean basal sediments only up to 16% of the total Ni is soluble in the A/R agent leach. However, the exception to this is in the metal- rich carbonates from DSDP Site 215, and the metal-rich clays from DSDP Site 213, where the percentage of Ni in the A/R agent soluble minerals is 40 and 80% respectively, showing a strong correlation with Mn. This probably reflects the incorporation of this metal into Mn oxide aggregates and micronodules which have been reported in these sediments (Pimm, 1974; von der Borch, 1974). The majority of the Ni in all Indian Ocean DSDP basal sediments is contained in the HCL soluble minerals (35-80%). Horowitz (1974a) has reported in North Atlantic DSDP sediments that 35% of the total Ni is soluble in the HCL leach. Furthermore, Chester et al (1976) have reported proportions of Ni, as high as 80% of the total, in this leach, in DSDP North Atlantic Upper Cretaceous sediments. The presence of high concentrations of Ni in the HCL soluble fraction of the present sediments suggests that it has been removed from the lattices of clay minerals and/or Fe oxides which are soluble in this reagent. Chester and Hughes (1967, 1969) have shown that although Ni was predominantly associated with Mn in their pelagic sediments, it may also in part have been present in HCL soluble minerals. Up to 33% of the total Ni is contained in the HCL insoluble phases. This is highest in both groups of clays and probably reflects the presence of detrital minerals in the sediments. This would appear to be particularly so for the metal-rich volcanogenic clays, from the sites (DSDP Sites 216 and 254) on the Ninety East Ridge which have been reported as having a strong basaltic provenance (von der Borch et al, 1974; Davies et al, 1974). A high proportion of Ni in the HCL insoluble phases has not been reported from the Pacific Ocean (Cronan, 1976), although in the North Atlantic about 12% of the total Ni is associated with such mineral phases (Horowitz, 1974a). 164

24- NĪ 22."

20- Fig 2.3.2c and 2.3.2d

Distribution of Ni and 18- Co in the Partial Chemical Leaches of 16- Metal-Rich and Basal Indian Ocean DSDP 14" Sediments ppm 12- (All data are expressed x10 on a C.F.B.) 10-

mrc . mrcc mrcl bc bcc bcl

11* CO 10~

Legend as for Fig 2.3.2a

5^ x10 4' 165

COBALT (see Fig. 2.3.2d)

The partition of Co in the Indian Ocean DSDP sediments is similar to that of. Ni and is generally similar throughout all the sediment types. Up to 46% of the total Co is soluble in HAC. This is highest in metal-rich carbonates and clayey carbonates and moderately high in the case of metal-rich clays. (The proportions of Co held in the HAC leach of basal sediments (carbonates, clayey carbonates and clays) is negligible). In the case of the carbonate-rich sediments this may reflect its incorporation into biogenic carbonate material (BostrOm et al, 1974) but it may also, with Ni and Mn be leached from the Mn oxide aggregates in these sediments (Chester and Hughes, 1967). In the case of clay-rich sediments Co may be removed from adsorbed sites on the surfaces of clay minerals (Chester, 1965a; Chester and Hughes, 1967, 1969). Low concentrations of Co (up to 16% of the total Co) are on average contained in the A/R agent soluble phases of all the sediments. This contrasts strongly with previously reported data for Co, which link it with Ni and Mn, it being contained in the FeMn oxide minerals (Cronan, 1976). However, in metal-rich clays, on average 16% of the total Co is associated with the FeMn oxides, and this rises to a figure of 80% in such sediments from DSDP Site 213. Basal carbonates contain no detectable Co in the A/R agent leach, whereas in the metal-rich carbonates from DSDP Site 215 up to 12% of the Co occurs in the A/R agent leach. The higher proportion of Co, and also Ni and Mn, in the A/R agent soluble minerals of the metal-rich clays and carbonates probably reflects the incorporation of Ni and Co into Mn oxide aggregates and micronodules which occur in these sediments. The majority of the Co in all the sediments (up to 90% of the total), like the Ni, is held in the HCL soluble minerals - Fe oxides and clay minerals, e.g. Fe smectites. Chester (1965a) has shown that Co, when it is in the monovalent state as CoCl or Co (OH) in seawater can be incorporated into the lattice of clay minerals, by substitution for hydrogen in the OH groups as well as being adsorbed onto the surfaces of other minerals. Such lattice held Co would be removed by the HCL attack. In North Atlantic sediments, Chester et al (1976) have reported high proportions of Co (40-50%) as well as Ni, in the HCL soluble minerals. However, unlike Ni, they suggest that the Co has been initially precipitated from seawater and incorporated into the silicate lattice of 166

minerals, like zeolites during diagenesis. Certain of the present Indian Ocean basal sediments contain zeolites in their clay fractions, e.g. DSDP Site 212 (von der Borch et al, 1974), in which some of the Co may be entrained. Up to 42% of the total Co is held in the HCL insoluble fraction, i.e. detrital mineral phases, this being most marked in metal-rich and basal clays and basal carbonates. Such a partition of Co in detrital phases of clay-rich sediments is similar to data reported from the North Atlantic (Chester et al, 1976). It may reflect the incorporation into the sediments of material of a basaltic provenance.

CHROMIUM (see Fig. 2.3.2e)

The partition of Cr shows little variation within each group of sediments, and with the exception of both types of carbonate (metal-rich and basal) is similar in all groups of sediments. In the basal carbonates(metal-rich and basal) very little Cr (up to 8% of the total) is contained in the HAC leach and negligible amounts are in the HCL soluble minerals. In metal-rich carbonates from DSDP Site 215 the proportion held in the HAC increases to 31% of the total Cr. The Cr in the HAC leach may be held in biogenic carbonate material in the sediments (Bostr8m et al, 1974; Oldnall, 1975) and/or may be removed from adsorbed sites (Chester and Hughes, 1967). The remainder of the Cr in basal carbonates is partitioned between the A/R agent soluble minerals, i.e. FeMn oxides (25-26%) and the HCL insoluble, i.e. detrital, mineral phases (65-68%). Bostrdm and Peterson (1969) found that in E.P.R. metal-rich sediments the Cr is correlated with Mn, both of which are enriched in Mn oxide aggregates and micronodules and Horowitz (1974a) has reported that up to 13% of the Cr in North Atlantic DSDP sediments may be contained in FeMn oxides. Chester and Messiha-Hanna (1970) and Chester et al (1976) have found little Cr associated with the A/R agent soluble mineral phases in North Atlantic sediments. In the metal-rich and basal carbonates the majority of the Cr (65-68% of the total) is held in the HCL insoluble phases and can be attributed to Cr being linked with detrital material. The Cr-rich detrital material probably reflects the inclusion of unaltered basaltic fragments in the sediments (Bertine, 1974) as well as the inclusion of residues from the alteration and weathering of basaltic material. 167

Fig 2.3.2e and 2.3.2f Distribution of Cr and Cu in the Partial Chemical Leaches of Metal-Rich and Basal Indian Ocean DSDP Sediments

(All data are expressed on C.F.B.)

mre mree mrcl be bee bel Cu

Legend as for Fig 2.3.2a , 168

In clays and clayey-carbonates (metal-rich and basal) negligible amounts of Cr are soluble in the HAC leach. Between 17 and 27% Cr is associated with the A/R agent soluble minerals and may be associated with an amorphous Fe oxide mineral which is reducible by the A/R agent. Up to 22% of the total Cr is soluble in the IHL leach, indicating its removal from the lattices of clay minerals or Fe oxides. The absence of Cr in the HCL soluble minerals of the carbonate-rich sediments is probably a reflection of the lower proportions of clay minerals in these sediments. The majority of Cr in the basal clays and clayey carbonates (53-57%) is held in the detrital mineral phases and is in keeping with findings from the North Atlantic (Chester and Messiha-Hanna, 1970; Horowitz, 1974a; Chester et al, 1976). This is probably the result of the inclusion of basaltic fragments in the sediments.

COPPER (see Fig. 2.3.2f).

The partition of Cu is generally consistent within each group of sediments, but is generally variable between groups of sediments. A tendency common to all the basal sediments, is the occurrence of the majority of the Cu in the HCL leach. This varies from 52 - 59% for the metal-rich sediments to 62-68% for the basal sediments. This confirms previous work from the North Atlantic and Pacific Oceans, which suggests that Cu is predominantly associated with the Fe oxide minerals and to a lesser extent with authigenic clay minerals (Horowitz, 1974a; Chester et al, 1976; Cronan, 1976). Chester et al (1976) suggest that this points to a hydrothermal source of Cu. In basal clays and clayey carbonates negligible amounts of Cu are associated with the HAC and A/R agent leaches and the remainder of the Cu (15-26%) is held in the HCL insoluble, i.e. detrital, mineral phases, suggesting the incorporation of Cu in insoluble silicates and other detrital minerals, possibly in part from the breakdown of tholeiitic material. In basal carbonates negligible amounts of Cu are contained in the A/R agent soluble and HCL insoluble mineral phases, while the remainder of the Cu (25%) is associated with the HAC leach. This may reflect its removal from the biogenic carbonate material in these sediments (Bostrdm et al, 1974; Oldnall, 1975). Further amounts of Cu may be removed from adsorbed sites of the surfaces of other minerals, e.g. clays (Chester and Hughes, 1967, 1969). 169

In metal-rich sediments the small proportions of Cu (11-14%) soluble in the MAC leach in the carbonate-rich sediments are probably removed from biogenic carbonate material. The Cu in the HAC leach may be removed from adsorbed sites in the clay-rich sediments (Chester and Hughes, 1967, 1969). Minor amounts of Cu (up to 14%) are associated with basaltic detrital fragments in the HCL insoluble residue, partic ularly in the metal-rich clays. A feature common to all metal-rich sediments is the relatively high proportions of Cu (up to 27% of the total) held in the A/R agent leach. Such a pattern has been recognised from the Pacific Ocean (Cronan, 1976) and the North Atlantic (Horowitz, 1974a). In metal- rich carbonates from DSDP Sites 213 and 251A the proportion of Cu in the A/R agent leach rises to a maximum of 32% and 45% respectively. This probably reflects the association of. Cu with authigenic Mn oxides which form a high proportion of the sediments at DSDP Sites 213 and 251A.

CADMIUM

Previously published data on Cd in marine sediments are rather limited. Aston et al (1972a) recorded an association of Cd with calcareous shell material in North Atlantic sediments. In the case of the basal sediments, the levels of Cd recorded are so low and so near to the detection limit of the method used (see Appendices A.1 and A.3), that the observed variations are explicable purely in terms of the analytical precision. Therefore no further conclusions can be drawn. This is also true for more metal-rich sediments with the exceptions that the levels of Cd recorded in metal-rich carbonates and clayey carbonates are higher. In these sediments the variation in Cd observable may indicate that the Cd is partitioned between the MAC leach and the HCL insoluble residue. This would in part support the observations of Aston et al (1972a) for North Atlantic sediments.

LEAD (see Fig. 2.3.2g)

The partition patterns of Pb are generally consistent within each grout of sediments but there are differences between the groups. In basal carbonates and metal-rich clayey carbonates the majority of the Pb is contained in the HAC soluble minerals (98 and 96% respectively). This represents the association of Pb with biogenic carbonate material 170

which has been reported previously (Cronan and Garrett, 1973; Greenslate et al, 1973; Bostrbm et al, 1974; Horowitz, 1974a). In the case of clayey-carbonates, the Pb in the HAC leach may in part be removed from adsorbed sites on the surfaces of other minerals. In metal-rich carbonates, the majority of the Pb is still removed by the HAC leach, but it only represents 49% of the total, and is probably removed from biogenic carbonate material. In the metal-rich carbonates from DSDP Site 215, where elevated concentrations of Pb occur, 75% of the total Pb is soluble in the HAC leach, while the remainder is associated with the A/R agent soluble mineral phases, and is probably removed from reducible Fe and Mn oxides which are enriched in these sediments. In other metal-rich carbonates, minor amounts of Pb may be contained in the A/R and HCL leaches, while small proportions of Pb (27%) are held in the HCL insoluble phases. The association of Ph with FeMn oxides of the A/11 agent leach has been reported from elsewhere (Horowitz, 1974a; Chester et al, 1976; Cronan, 1976). The levels reported here are almost equal to those recorded by Horowitz (1974a) for the North Atlantic Ocean, but below those reported by Cronan (1976) from the Pacific Ocean. This would indicate that the presence of FeMn oxide material in these sediments, with which Pb, like Ni and Co are associated is confined to those DSDP sites where hydrothermal metallogenesis is likely to have taken place, e.g. metal-rich carbonates from DSDP Site 215 and metal-rich clays from DSDP Site 213 (von der Borch et al, 1974). In basal clayey carbonates and clays the Pb is predominantly associated with the HCl insoluble phases (44 and 49%), while negligible amounts are contained in the HAC, A/R agent and HCL leaches. In metal-rich clays, the highest proportion of Pb (33% of the total) is in the HCL insoluble residue, and further minor amounts are contained in the HCL, A/R agent and HAC leaches although the levels recorded are very low. The presence of Pb in the HCL leach of Pacific Ocean basal sediments are very small or insignificant. The Pb in the HCL insoluble minerals of these clay-rich sediments would appear to indicate the incorporation of detrital minerals in the sediments from the weathering and breakdown of basaltic material. It has been shown that Pb in metalliferous sediments may be present in minerals derived from the alteration of tholeiitic basalt (Piper, 1973; Bertine, 1974), and this may partly account for its presence in the sediments studied here. 111

11 Pb 10

7 ppm 6 Fig 2.3.2g and 2.3.2h

5 Distribution of Pb and Zn in the Partial x10 ·Chemical Leaches of 4 t1eta ]-R i ch and Basal Indian Ocean DSDP 3 Sediments

2 (A 11 data are expressed on a C. F. B. )

mre mrcc mrel be bee bel Zn

Legend as for Fig 2.3.2a

14 ppm 12 x10 10

2 172

TABLE 2.3.2 Summary Table of Partial Composition of Major Types of Indian Ocean DSDP Basal Sediments

FE MN NI CO CR 1 2 1 2 1 2 1 2 1 2

BASAL A 300 41 3452 67 22.1 15 nil 0 11.5 6 CARBONATE B 100 <1 189 4 nil 0 nil 0 1 9.7 26 (1) C 71500 98 1272 25 90.9 61 23 90 nil 0 (220) D 1000 1 267 5 36 24 2.5 10 129 68 E 7.29 5180 149 25.5 190

BASAL A 600 1 2263 48 11.2 14 3.0 6 4.9 4 CLAYEY- B 300 <1 896 19 8.0 10 3.9 8 33.2 27 CARBONATE C 45300 81 1273 27 41.5 52 20.7 42 14.8 12 (5) (211, D 10100 18 283 6 19.2 24 21.6 42 70.1 57 249,256, E 5.63 4715 79.9 49.2 123 257)

BASAL A 100 <1 280 15 10.1 9 4.2 8 1.6 1 CLAY B 3400 6 466 25 11.2 10 4.2 8 39.1 24 (44) C 46600 82 896 48 57.1 51 21.9 42 35.9 22 D 6800 12 224 12 33.6 30 21.9 42 86.4 53 E 5.69 1867 112 52.2 163

All data expressed C.F.B. except Ca. Column 1 - Absolute Concentration (Average), Column 2 - Proportion of Bulk Value A- HAC Leach B- A/R Agent Leach only (i.e. A/R agent leach - HAC leach) ) All in ppm, ) except Ca in C- HCL Leach only (HCL leach - A/R agent leach) ) Wt % D- Insoluble Residue (Bulk-HCL leach) ) ) All in ppm, except Ca, Fe, Al, T:,- Bulk Composition ) in Wt. % 173

TABLE 2.3.2 (cont.)

CU CD PB ZN LI

1 2 1 2 1 2 1 2 1 2

BASAL A 14 25 1.0 42 24.9 98 21 19 2.3 11 CARBONATE B 4.2 7 nil 0 nil 0 9.2 8 16 8 (CONT) C 35 62 0.8 33 nil 0 80.9 72 9.2 44 D 3.3 6 0.6 25 0.4 2 1 <1 7.7 37 E 56.5 2.4 25.3 113 20.8

A 6.9 10.8 9 2.0 4 BASAL 2.4 3 0.8 23 13 B 8 8.0 6 CLAYEY- 6.4 0.4 13 15 7.2 1.5 3 C 50.2 63 0.5 14.9 28 80.4 CARBONATE 14 67 33.8 67 D 50 23.4 44 21.6 18 13.2 26 (CONT) 20.7 26 1.7 E 79.6 3.4 53.2 120 50.5

A 5.6 5 0.4 16 7.7 12 8.2 6 1.9 4 B 12 0.8 32 15.5 24 15.0 11 0.5 1 BASAL 13.3 C 11 68 32.6 68 CLAY 75.5 68 0.3 9.7 15 92.5 D 20.4 13.0 27 (CONT) 16.6 15 1.0 41 31.6 49 15 E 111 2.5 64.5 136 48.0 174

TABLE 2.3.2 (cont.)

BA CA AL MG TI 1 2 1 2 1 2 1 2 1 2

A 22.69 92 1700 5 4471 18 BASAL B 0.05 <1 nil 0 nil 0 CARBONATE C 1.84 7 28700 '76 20229 82 (CONT) D nil 0 7200 19 nil 0 E 39100 24.58 3.76 24700 1850

A 1531 17 5.02 85 600 1 2421 14 nil 0 BASAL- B 180 2 0.59 10 300 41 346 2 112 2 CLAYEY- C 3512 39 0.18 3 37000 60 10720 62 948 17 CARBONATE D 3782 42 0.12 2 23500 38 3804 22 4518 81 (CONT) E 9004 5.91 6.19 17290 5578

A 1234 20 0.36 36 700 1 1113 6 nil 0 BASAL B 679 11 0.35 35 700 1 1298 7 nil 0 CLAY C 3828 62 0.10 10 44700 61 11315 61 2582 30 (CONT) D 432 7 0.19 19 27100 37 4823 26 6025 70 E 6174 1.00 7.33 18549 8607 175

TABLE 2.3.2 (cont)

FE MN NI CO CR 1 2 1 2 1 2 1 2 1 2

METAL- A 800 1 2634 10 22.7 9 29.7 46 18.9 8 RICH B 4100 5 21862 83 40.3 16 3.6 CARBONATES 6 59 25 C 67000 82 527 2 139 55 12.1 20 4.7 2 (6) (215,245, D 9800 12 1317 5 50.4 20 16.9 28 153 65 251A) E 8.17 26340 252 60.3 236

METAL-RICH A 200 <1 994 26 17.7 12 14.5 43 nil 0 CLAYEY- CARBONATE B 200 <1 236 6 nil 0 nil 0 44.3 24 (1) C 104400 99 2315 61 119.3 80 19.4 57 39.2 21 (239) D nil 0 275 7 12 8 nil 0 102.5 55 E 10.48 3820 149 33.9 186

METAL-RICH A 6500 5 632 12 39.5 16 12.6 12 12.5 5 CLAY (7) B 7800 6 1633 31 39.5 16 16.8 16 42.4 17 (213,216, C 977 00 75 1896 36 86.5 35 33.6 32 54.7 22 254) D 18200 14 1106 21 81.5 33 42.0 40 139.4 56 E 13.02 5268 247 105 249

All data expressed on a C.F.B., except Ca. Column 1 - Absolute Concentrations (Average), Column 2 - Proportion of Bulk Value A - HAC Leach ) All in ppm, B - A/R Agent Leach only (i.e. A/R Agent Leach - HAC leach) ) except Ca in C - HCL leach only (i.e. HCL leach - A/R agent leach) ) Wt % D - Insoluble Residue (Bulk - HCL Leach) E - Bulk Composition ) All in ppm, except Ca, Fe, ) Al in Wt % 176

TABLE 2.3.2. (cont)

CU CD PB ZN LI 1 2 1 2 1 2 1 2 1 2

METAL A 24.2 11 3.5 57 53.9 49 32.1 13 3.7 8 RICH - CARBONATE B 52.8 24 0.1 2 13.2 12 39.5 16 7.8 17 (CONI) C 114 52 1.6 27 13.2 12 153 62 21.7 47 D 28.6 13 0.9 14 29.7 27 22.2 9 12.9 28 E 220 6.1 110 247 46.1

METAL-A 17.1 13 3.1 52 37.4 96 5.8 3 1.6 4 RICH B 0 4 3 2 0.9 2 CLAYEY 34.5 27 nil 1.7 CARBONATE C 75.4 59 nil 0 nil 0 165 88 28.5 75 (CONT) D 1 1 2.9 48 nil 0 13.2 7 7.2 19 E 128 6.o 39.1 187 38.2

METAL- A 21.8 14 0.8 22 15.3 20 23.8 13 1.8 5 RICH B 23.4 15 0.4 10 15.9 21 21.9 12 0.7 2 CLAY C 88.9 57 1.o 28 19.7 26 109.8 60 20.2 56 (CoNT) D 21.8 14 1.4 40 25.0 33 27.5 15 13.3 37 E 156 3.5 75.9 183 36.o

177

TABLE 2.3.2 (cont)

BA CA AL MG TI

1 2 1 2 1 2 1 2 1 2

METAL- A 1061 5 23.94 98 1900 4 3205 15 RICH B 1061 5 nil 0 1400 3 214 1 CARBON- C 12949 61 0.01 <1 30200 63 13461 63 ATE D 6156 29 0.48 2 14400 30 4487 21 (CONT) E 21228 24.43 4.80 21367 3425

METAL- A 14.32 95 1200 2 1315 7 RICH B nil 0 nil 0 nil 0 CLAYEY- CARBON- C o.68 5 44400 64 14727 75 ATE D nil 0 23900 34 3558 18 CONT) E 4230 15.0 6.95 19600 7070

METAL- A 1751 40 0.29 44 700 1 3037 22 nil 0 RICH B 731 3 0.07 10 2100 3 1381 10 nil 0 CLAY C 1619 37 0.17 26 51600 72 6213 45 3450 22 (CONT) D 875 20 0.12 18 17200 24 3176 23 12234 78 E 4377 0.66 7.16 13807 15684 178

ZINC (see Fig. 2.3.2h)

The partition of Zn is similar in all groups of sediments, it is consistent within each sediment group and shows many similarities to the partition of Cu noted above. Minor amounts of Zn (up to 19% of the total) are normally soluble in the HAC leach. However, in the metal-rich carbonates from DSDP Site 213 it rises to 34/0 of the total Zn. This may reflect the incorporation of Zn in biogenic material (Oldnall, 1975) in these metal- rich carbonate sediments, whereas elsewhere it is probably removed from adsorbed sites of the surface of clay minerals, where it may be concentrated in a similar way to Co (Chester, 1965a). Smaller amounts of Zn (up to 16% of the total) are normally soluble in the A/R agent leach. This proportion is highest in metal-rich carbonates in general and in the metal-rich carbonates from DSDP Site 215, 30% of the Zn is associated with the A/R agent soluble FeMn oxides. This probably reflects the presence of Mn oxide material with which Zn may be associated in these sediments. Cronan (1976) has reported a similar association of Zn with FeMn oxides from Pacific Ocean basal sediments. The majority of the Zn is soluble in the HCL leach (62-88%) of all the sediments. This probably reflects an association, which is common to Cu also, of zinc with the Fe oxide minerals and to a lesser extent silicates, such as smectites (Cronan and Garrett, 1973; Horowitz, 1974a; Chester et al, 1976; Cronan, 1976). Minor amounts of Zn (up to 18%) are associated with the HCL insoluble detrital mineral phases (this being highest in the clay-rich sediments, e.g. DSDP Sites 216 and 254). This suggests that like Pb, Zn may be added to these sediments by the inclusion of unaltered basaltic detrital fragments from the breakdown of tholeiitic material.

LITHIUM (see Fig. 2.3.2i)

The partition of Li is generally similar within all the groups of sediments and between the various groups of sediments. Minor amounts of Li (up to 11%) are removed by the MAC leach in all the sediments. This probably reflects the removal of Li from adsorbed sites on the surface of clay minerals. 179

60-

55. Li

504

45-

40- Fig 2.3.2i and 2.3.2j 354 Distribution of Li and Ba in the Partial ppm 30- Chemical Leaches of Metal-Rich and Basal 25• Indian Ocean DSDP Sediments 20-

(All data are expressed 15, on a C.F.B.)

10-

m rc mrcc mrcl bc bcc b ci

22-

20- Ba

16- Legend as for Fig 2.3.2a.

16,

N.B. No partial chemical 14• data for Ba in ppm metal-rich clayey- 12~ carbonates and bas carbonates are 10• available. x1000

S MZI S 180

In general, little Li (up to 8%) is soluble in the A/R agent leach of all the sediments and it is only in the metal-rich carbonates that significant amounts (17% of the total Li)are soluble in this leach. The A/R agent may remove Li from the surface of clay minerals and in the case of the metal-rich carbonates additional amounts may be leached from the larger proportions of FeMn oxides present in these sediments, as for example, at DSDP Sites 215 and 245. Work on North Atlantic basal sediments (Horowitz, 1974a) has shown that Li is evenly partitioned between the HCL soluble and insoluble phases. In Indian Ocean basal sediments Li is predominantly associated with the HCL soluble minerals - alumino-silicates (44-75%). The values are lowest in the carbonate-rich sediments (44-47%) as compared to the less carbonate-rich sediments (56-75%) in which larger amounts of montmorillonite and other clay minerals are present. The amounts of Li in the I-ICI, insoluble phases are similar in all groups of sediments (19-39%). These data would support the view of Horowitz (1974a) that Li in basal sediments is predominantly of detrital origin.

BARIUM (see Fig. 2.3.2j)

The partition data for Ba are incomplete since no results are available for basal carbonates and metal-rich clayey carbonates. However, it is possible from the data available on the remaining sediment groups to draw some conclusions on the partition patterns of Ba. A feature common to all groups of sediments is the negligible or low values of Ba in the A/R agent leach. Cronan (1976) has reported a similar feature for Pacific Ocean basal sediments. This may possibly reflect unsuitability of this large cation to substitute in the lattices of reducible FeMn oxides. In metal-rich carbonates, in which Ba is most enriched of the groups studied, it is divided between the HCL soluble (61%) and insoluble (29%) phases. This pattern is different to that reported by Cronan (1976) for Pacific Ocean basal, non-carbonate sediments, where Ba was principally held in the acid insoluble phases. The partition of Ba in metal-rich carbonates may reflect the presence of authigenic barytes, the Ba in which may be derived from biogenic and/or hydrothermal sources, together with Ba in unaltered basaltic detrital material. 181

In metal-rich clays, the majority of the Ba (45%), which occurs in a lower absolute concentration as compared to metal-rich carbonates, is held in the HCL soluble phases, some of it probably in the clay minerals. The remainder of the Ba in metal-rich clays is divided between the HAC leach and the HCL insoluble phases. This probably reflects the stripping of Ba from adsorbed sites on the surface of clay minerals and its presence in unaltered basaltic detrital fragments. In basal clays the majority of the Ba (62% of the total) is removed by the HCL leach, probably from the lattice of clay minerals. Minor amounts are associated with the HCL insoluble residue, while the remainder (20%) is held in the MAC leach, where it is probably removed from adsorbed sites on the surface of clay minerals. In basal clayey carbonates, Ba partition patterns are the most similar to those reported by Cronan (1976). The majority is divided between the HCL insoluble (42%) and HCL soluble (39%) phases. The remainder (17%) is held in the HAC leach, which in the more carbonate-rich sediments may represent removal of Ba from biogenic carbonate material (Bostrtlm et al, 1973a; Horowitz, 1974a; Oldnal.l, 1975) as well as removing from adsorbed sites on the surfaces of clay minerals.

CALCILM (see Fig. 2.3.2k)

The partition of Ca is consistent within each group of sediments and it is only different in clays (metal.-rich and basal) as compared to the other groups of sediments. As might be expected in the carbonate-rich sediments (metal-rich and basal carbonates and clayey-carbonates) the majority of the Ca (85-98%) is removed by the HAC leach from the abundant biogenic carbonate material of these sediments. Minor amounts of Ca are associated with the HCL insoluble residue (up to 2%), the HCL leach (up to 7%) and the A/R agent leach (up to 10%), of the metal-rich and basal carbonates and clayey carbonates, probably for the reasons suggested below. In the metal-rich and basal clays, where the absolute concentrations of Ca are very small indeed, lesser proportions of Ca occur in the HAC leach (36-44%). This Ca is probably present in the small amounts of biogenic carbonate which are undoubtedly present, as well as in exchange sites on the clays and in interstitial water evaporates. 182

24-

22-

20.4

18- Fig 2.3.2k and 2.3.21

16- Distribution of Ca and Al in the Partial Chemical Leaches of Metal-Rich and 140 Basal Indian Ocean DSDP wt. Sediments 12-

10-

(All data are expressed on a C.F.B., except data for Ca which are expressed on a T.S.B.)

m rc mrcc mrcl bc bcc bcl

Al szmmd

Legend as for Fig 2.3.2a

Wt. 183

Up to 35% of the total Ca occurs in the A/R agent leach and this may be a reflection of amounts of phosphatic fish debris, reported as a common constituent of metalliferous sediments (Dymond et al, 1973; Cronan, 1976), especially in the case of those DSDP Sites (214, 216 and 254) on the Ninety East Ridge, where the basal sediments have been reported as having been formed in a shallow water lagoonal environment (von der Borch et al, 1974; Davies et al, 1974). Up to 26% of the total Ca occurs in the HCL soluble minerals and this may reflect its incorporation into authigenic clay minerals. Similar concentrations of Ca have been reported for the HCL leach of sediments from the North Atlantic (Horowitz, 1974a). The proportions of Ca (18-19% of the total) in the HCL insoluble phases of these clay-rich sediments may indicate the presence of Ga-bearing detrital mineral phases, such as Ca-plagioclase feldspars, which are liberated on the breakdown of basaltic material.

ALUMINIUM (see Fig. 2.3.21).

The partition of Al is generally similar within each group of sediments and in all the groups of sediments. The majority of the Al is held in the HCL soluble (61-76%) and the HCL insoluble (19-38%) mineral phases. This pattern suggests that like Fe, AI occurs in authigenic and detrital clay minerals and other detrital components of all the sediments. This pattern is broadly similar to that reported from the Pacific Ocean (Cronan, 1976) but is slightly different to that reported from the North Atlantic (Horowitz, 1974a). In North Atlantic basal sediments the majority of the Al (52%) is in the HCL insoluble phases while there is less (38%) in the HCL soluble phases. This may reflect the greater input of continental detrital material into the North Atlantic (Horowitz, 1974a), as compared to the Indian Ocean, for obvious geographical reasons. Small amounts of Al are removed by the HAC and A/R agent leaches. Horowitz (1974a) has reported that in North Atlantic sediments the A/R agent may breakdown the lattice of clay minerals releasing Al and other metals. However, this process, if operative is of minor importance in these Indian Ocean sediments, as reflected by the small proportions of Al (up to 3%) contained in this leach. It is more probable that the Al in the A/R agent, and possibly also the HAC leach is removed from colloidal A100H and from the surfaces of clay minerals. 184

MAGNESIUM (see Fig. 2.3.2m)

The partition patterns of Mg are similar within each group of sediments and are generally similar in all the sediment groups. A common feature of all the sediments is the partitioning of the majority of the Mg in the HCL soluble minerals (45-82%) and in the HCL insoluble mineral phases (18-26%). This is similar to the pattern reported for North Atlantic basal sediments (Horowitz, 1974a), and reflects the incorporation in the sediments of the products of basaltic alteration, e.g. clay minerals from the leached basaltic residues and FeMg-rich detrital minerals from the inclusion of basaltic detrital fragments. Up to 10% Mg is removed by the A/R agent, while up to 22% of the Mg is soluble in the HAC leach. The Mg removed by the HAC leach is probably removed from adsorbed sites on the surface of clay minerals in clay-rich sediments; in the carbonate-rich sediments it is possible that more is removed from high-Mg calcite which is a common constituent of Recent carbonate-rich sediments from the Indian Ocean (Wiseman, 1965). In all groups of sediments Mg may be removed by the HAC leach from amounts of sea salt present in the sediments. The partition of Mg in the HAC and A/R agent leaches of the sediments, and particularly in metal-rich clays, is similar to the partition patterns reported from Pacific Ocean basal sediments by Cronan (1976).

TITANIUM (see Fig. 2.3.2n).

The partition data for Ti are incomplete and results are only available for three groups of sediments - basal clays and clayey carbonates and metal-rich clays. However, it is possible to draw some conclusions about the partition pattern of Ti in these sediments. The partition of Ti is similar both within and between the groups. No Ti is soluble in the HAC leach and negligible amounts are soluble in the A/R agent leach (only 2% of the total in,the basal clayey- carbonates). Ti is predominantly partitioned in the HCL insoluble minerals (70-81%), while up to 30% of the Ti is held in the HCL soluble phases. The partition of the majority of the Ti in the HCL insoluble residue probably results from its presence in the sediments in basaltic fragments. 185' 24 Mg 22

18 m=sm Fig 2.3.2m and 2.3.2n

16 Distribution of Mg and Ti in the Partial 14 Chemical Leaches of Metal-Rich and Basal ppm Indian Ocean DSDP 12 Sediments x1000 10

(All data are expressed on a C.F.B.)

mrc mrcc mrcl bc bcc bcl

Ti 20*

18-

16• Legend as for Fig 2.3.2a

14' N.B. No partial chemicai data for Ti in 12 metal-rich carbona,. ppm metal-rich clayey carbonates and base 10. carbonates are x1000 available. Ti was 8~ not determined in the HAC Leach. 186

Titanium in the HCL leach may be associated with clay minerals formed by the chemical alteration of the basaltic terrain. This latter piece of data supports the conclusion of Emelyanov (1974), who suggested that Ti will be concentrated in areas of deposition of volcanogenic sediments associated with submarine weathering of basaltic material.

2.3.3. Summary

In general, similarities in the partition patterns of metals exist in all groups of sediments. Pb and Ca are predominantly associated with HAC soluble minerals, Mn is mainly associated with the A/R agent soluble Mn oxides (where the Mn concentrations are high) and with all phases (where the Mn concentrations are low) and Cr is mainly associated with the acid insoluble phases. Fe, Ni, Co, Zn and Cu are predominantly associated with the HCL soluble phases, while Al, Mg, Ti, Li and Ba, are associated in the main with the HCL soluble and insoluble mineral phases. The variations in these general patterns are consistent with variations between metal-rich and non-metal-rich (basal) sediments and between carbonate-rich and clay-rich sediments.

A feature common to all metal-rich sediments is the increase in the proportion of metals (concommitant with their increases in absolute bulk concentrations) held in the A/R agent leach. This is true for such metals as Mn, Ni, Co, Cr, Cu, Pb, Zn and to a lesser extent Li and Fe. It results from the presence of larger amounts of reducible Mn (y Fe) oxides occurring as aggregates, coatings and micronodules into which these metals can be incorporated and in this sense the composition of Indian Ocean metal-rich sediments may be similar to metal-rich sediments from the Pacific Ocean.

The largest proportions of reducible Mn (+ Fe) oxides are present in the metal-rich carbonates from DSDP Sites 215, 220, 236 and 245 and these sediments contain the highest proportions of metals in the A/R agent leach. A feature common only to metal-rich carbonates is the increase in the proportions of such metals as Ni, Co, Cu, Cr, Pb and Mn in the HAC leach. While in part this may be caused by the leaching of these metals from the carbonate material itself, additional amounts of these metals may also be leached from the Mn 187

oxides which are abundant in these metal-rich carbonates. Further minor amounts may be removed from adsorption sites on the surfaces of reducible Fe oxides and clay minerals (Chester and Hughes, 1967, 1969).

The metal-rich clays are divisible into three groups on the basis of their Al contents. The metal-rich clays with low Al concentrations (DSDP Sites 213, 216 and 236) have, like the metal-rich carbonates, increased proportions of certain metals (Mn, Ni, Co, Cr, Cu, Pb, Zn and Li and Fe) in the A/R agent leach due to the presence of Mn + Fe oxides. However, in the low Al clays and to a greater degree in the metal-rich clays with moderate Al contents from DSDP Site 239, and most strikingly in the metal-rich clays with high Al contents from DSDP Sites 216 and 254, /'here is an increased proportion of such metals as Fe, Mn, Ni, Co, Cr, Pb, Zn and Cu, together with Al, Ti, Mg, Ba, Li and Ca, in the HCL soluble and insoluble phases, i.e. in clay minerals (e.g. smectites) detrital minerals and Fe oxides. The increasing proportion of trace elements in the HCL leach and HCL insoluble residue with increasing major element proportions of these same phases and increasing Al concentration in various groups of clays is probably a reflection of incorporation of Ba and associated trace metals in authigenic barytes, possibly coprecipitated from hydrothermal fluids; of Fe, Ni, Cu, etc in Fe oxides, possibly precipitated from hydrothermal fluids; of metals (Al, Mg, Fe, Ti, Ba, Ni, Co, Cr, Cu, Zn, etc) in clay minerals which originate from the chemical alteration of basalt and metals (Al, Ti, Mg, Ba, Cr, Pb, etc) in unaltered basaltic detrital fragments which may be included in the sediments. An increase in trace metals in the HCL soluble and insoluble phases in the metal-rich clays from DSDP Site 254 on the Ninety East Ridge, may be a reflection of the presence of these elements in basaltic material eroded in a shallow water, lagoonal environment (Davies et al, 1974).

Metal-rich clayey-carbonates from DSDP Site 239, show partition patterns intermediate between those of metal-rich carbonates and metal- rich clays, although the patterns are more similar to those observed for metal-rich clays.

A feature common to all basal (i.e. non-metal-rich) sediments is the lower proportions of trace metals in the A/R agent leach than in metal- rich sediments which with lower proportions of A/R agent soluble Mn + Fe may reflect the absence of large quantities of Fe and Mn oxides. 188

In basal carbonates (DSDP Site 220) and basal clayey-carbonates (DSDP Sites 211, 249, 256 and 257) there are higher proportions of such metals as Ca, Mn, Pb and to a lesser extent Ni, Cu, Zn and Li in the HAC leach. This may in part reflect the removal of these metals from the biogenic carbonate material as well as from Mn oxide coatings on carbonate material, and in the more clay-rich sediments (i.e. clayey- carbonates) also from adsorbed sites on the surface of clay minerals.

In basal clays the minor amounts of trace metals associated with the HAC leach are probably removed from adsorbed sites on the surface of clay minerals. The majority of all the metals in basal clays are held in the HCL soluble and insoluble phases, with lesser amounts in the A/R agent leach. Such partition patterns reflect the processes of normal oceanic sedimentation by which metals are contained in clay minerals from the weathering of basaltic material, in unaltered basaltic and possibly continental detrital minerals, and in authigenic Mn oxides and other phases precipitated from sea water.

There is an overall tendency in the Indian Ocean basal sediments for the majority of the elements (Fe, Al, Mg, Ti, Cr, Li, Cu, Zn, Ni, Co). to be held in the HCL soluble and insoluble phases. This emphasises the importance of the input into these sediments of the alteration products of basalt and of detrital (basaltic and possibly continental) material. The majority of these metals are held in the HCL soluble phases, while metals like Cr and Ti are predominantly held in the HCL insoluble phases. This is in contrast to metal-rich sediments from the Pacific Ocean in which the majority of the trace metals are held in the A/R agent soluble minerals, such as the FeMn oxides. In North Atlantic DSDP sediments the Al, Ti, Cr, Li and also Pb are predominantly in the HCL insoluble residues, while Fe, Mg, Cu and Zn are held in the HCL soluble phases. This may further emphasise the recognised position of the Indian Ocean between that of the Pacific Ocean on the one hand, where hydrothermal input is strongest and detrital input is lowest and the Atlantic Ocean on the other where hydrothermal, ridge input is lowest and the detrital input is highest. 189

2.4 DISCUSSION OF THE GEOCHEMISTRY OF INDIAN OCEAN DSDP BASAL SEDIMENTS

2.4.1 Geochemical. Comparisons Between Groups of Sediments

2.4.1a Introduction

Having discussed the geochemistry of Indian Ocean basal sediments, it is now useful to consider geochemical comparisons with other sediments from the Indian Ocean, and with basal sediments, from other oceans. In order to do this a number of averages have been calculated. All the data used, with the exception of Ca and CaCO3, are expressed on a carbonate free basis (CFB) to facilitate comparisons with previously published data. The results are displayed in Table 2.4.1a for Indian Ocean DSDP sediments and in Table 2.4.1b for Indian, Atlantic and Pacific metal-rich basal. sediments. The comparisons discussed below are shown graphically on ternary diagrams for Indian Ocean DSDP sediments only (Figs. 2.4.1a, b, c) and for Indian, Atlantic and Pacific Ocean metal-rich DSDP sediments (Figs. 2.4.1d, e, and f).

2.4.1b Comparison of Indian Ocean.DSDP Metal-Rich Sediments with Indian Ocean DSDP Basal Carbonates, Clayey-Carbonates and Clays (Table 2.4.1a, Figs. 2.4.1a, b, c).

The principal differences between the compositions of these groups of sediments have already been described in the sections of bulk (2.2) and partition (2.3) geochemistry. However, a few further points are of interest here. The pattern of enrichment of metal-rich sediments is similar to that reported from metal-rich sediments from active oceanic ridges (Bostr8m et al, 1966; Piper, 1973) and for other basal sediments found in direct contact with the underlying basalt (Pimm, 1974; Warner and Gieskes, 1974; Cronan, 1976; Horowitz and Cronan, 1976). The high values of As, Mn and Fe may indicate hydrothermal influences in these basal metal-rich sediments. However, although being depleted in Al the Indian Ocean metal-rich sediments have high values of Ti, which is not a characteristic of metal-rich sediments from elsewhere. Jenkyns and Hardy (1976) have attributed high Ti values in basal sediments from DSDP Site 215A (Line Islands, Central Pacific) to the incorporation in the sediments of detrital clino-pyroxene and anatase, formed by the mechanical Fe 190

Fig. 2.4.1a. 2.4.lb and 2. .1c

Distribution of Ca, Al, Fe and Mn • in the Carbonate-Free Fraction p 0 0 of Indian Ocean D SA) P Sediments

0 00 p o O o% ®4~ 0 0-,00 0 Q000 + + O 8 oco 0

0 + + + +++ + Op O 0 O 1/1 RI + o 0 ® + + O O0 0 O 0 ~~ + + + + ++ + ++ 0 Oo O o ° • 0 o O 8 0 0 00 ©0 III ' ❑ + + + + 00

Ca Al

Indian Ocean Al Metal-Rich Clays

• Indian Ocean Metal-Rich Clayey Carbonates

Indian Ocean Metal-Rich Carbonates

° Indian Ocean Basal. Clays

+ Indian Ocean Basal Clayey Carbonates

O Indian Ocean Basal Carbonates

Fe Mn

Ca Mn 191

TABLE 2.4.1a Composition of DSDP Sediments from the Indian Ocean

1 2 3 lt

( CaCO3* 36 .67 60.81 21.98 1.70 Ca* 15.00 24.50 9.32 1.27 ( Al 5.47 6.68 6.38 7.01 ( Ti 0.83 0.62 0.83 0.67 Wt% ( Mg 1.92 2.02 3.10 1.92 Fe 9.25 6.16 6.40 6.05

( Ba 2010 4350 880 1170 ( Mn ( 15220 3470 2310 2620 ( Ni 203 123 92 130 ( Co 61 71 47 50 ( Cr 193 219 145 149 ppm ( Cu 175 135 98 119 ( Cd 4 2 3 3 ( Pb 84 82 59 59 ( Zn 199 178 150 148 ( Li 39 72 55 46 ( As 42 16 6 9

1. Metal-Rich DSDP Sediments (32) 2. DSDP Basal Carbonates (NMR) (7) 3. DSDP Basal Clayey Carbonates (NMR)(46) 4. DSDP Basal Clays (NMR)(69)

All data CFB, except * which are TSB.

!L 192

breakdown of the underlying oceanic rocks. Such a source may contribute to the high Ti values in the present Indian Ocean DSDP metal-rich sediments. The enriched Mg, particularly in clayey carbonates (column 3, Table 2.4.1a) may be attributable to the inclusion of detrital silicates (e.g. pyroxenes (Jenkyns and Hardy, 1976) and clay minerals (Horowitz, 1974a) ). The division of metal-rich and basal (i.e. non-metal-rich) sediments into various sub-groups described in sections 2.2.2f and 2.2.5 is well illustrated in Figs. 2.4.1a, b and c. The division of metal-rich carbonates from metal-rich clays is a reflection of the large amounts of Mn oxide material and lower amounts of Fe oxides in the metal-rich carbonates as compared to the metal-rich clays. The partition patterns of metals are also different between these two groups of sediments (see Section 2.3).

2.4.1c. Comparisons of DSDP Indian Ocean Metal-Rich Sediments with DSDP Metal-Rich Sediments from the Atlantic and Pacific Oceans (see Table 2.4.1b, Figs. 2.4.1d, e and f).

The composition of. Indian Ocean DSDP metal-rich sediments is generally intermediate between those from the Pacific and Atlantic Oceans. Indian Ocean DSDP metal-rich sediments have concentrations of Al, Mg, Fe, Mn, Ni, Co, Cu, Zn and to a lesser extent Pb which are less than those reported for the Pacific Ocean (columns 4-7, Table 2.4.1b) and greater than those from the Atlantic Ocean (columns 8-10, Table 2.4.1b). Locally Ti and Cr show high concentrations in Indian Ocean DSDP metal-rich sediments, while Pb is most enriched in North Atlantic basal carbonates. Horowitz and Cronan (1976) have suggested that the enriched Pb value in North Atlantic basal carbonates is due to its incorporation in biogenic carbonate material. The enrichment of. Ti and Cr in Indian Ocean sediments over those from the Atlantic and the Pacific Oceans may reflect the addition to Indian Ocean DSDP metal-rich sediments of unaltered basaltic, detrital fragments. The intermediate composition of Indian Ocean DSDP metal-rich sediments between those of the Atlantic and Pacific Oceans probably results from the interplay of two independent processes - ridge activity and detrital sedimentation. It has been shown (Heirtzler et al, 1968) that seafloor spreading rates and ridge activity are highest in the Pacific, intermediate in the Indian and lowest in the Atlantic ocean. Hence it is probable: that Fe 193

Fig. 2.4.1d, 2.4.1e and 2.1i.1f

Distribution of Ca, Al, Fe and Mn it the Carbonate-Free Fraction of DSDP Metal-Rich Sediments from the Pacifi Indian and Atlantic Oceans.

Ca Al

A Pacific Ocean Metal-Rich Al Carbonates

▪ Indian Ocean Metal-Rich Carbonates ry Atlantic Ocean Metal-Rich Carbonates

• Pacific Ocean.Metal-Rich Clays

® Indian Ocean Metal-Rich Clays

O Atlantic Ocean Metal-Rich Clays

Fe Mn

Ca Mn Table 2.4.1b. Chemical Composition of DSDP Metal-Rich Sediments.from the Indian, Pacific and Atlantic Oceans.

1 2 3 4 5 6 7 8 9 10 ( CaCO * 36.67 59.86 0.70 - 12.12 68.62 80.07 19.36 - 3 86.87 ( Ca* 15.00 24.43 0.66 - 5.60 28.30 32.90 8.28 - 35.40 ( Al 5.47 4.80 7.16 - 2.48 0.23 6.16 6.53 4.16 wt ( Ti 0.83 0.34 1.57 - 0.14 - 0.34 0.96 - 0.77 ( Mg 1.92 2.14 1.38 - 1.01 - 2.30 - 3.38 ( Fe 9.25 8.17 16.56 17.50 14.46 9.19 10.15 6.65 7.00 2.30

( Ba 2010 3730 774 - 15100 8500 3800 - - - ( Mn 15220 26340 5670 45000 41000 27600 31400 1800 2990 460o ( Ni 203 252 247 535 693 465 513 83 112 370 ( Co 61 6o 105 82 212 40 71 - 34 - ( Cr 193 236 249 - 24 - 67 118 111 162 ppm ( Cu 175 220 156 917 993 390 605 85 92 119 ( Cd 4 6 4 ------( Pb 84 110 76 145 - 7o - 95 68 495 ( Zn 199 247 183 358 494 180 389 148 151 267 ( Li 39 46 36 _ - - - 38 - 33 ( As 42 46 89 ------

All data CFB, except * which are TSB 1. Indian Ocean Basal Metal-Rich Sediments (32) ) 2. Indian Ocean Basal Metal-Rich Carbonates (18) ) This study 3. Indian Ocean Basal Metal-Rich Clays (14) 4. Pacific Ferruginous Sediments, DSDP Sites 159, 160, 162 (Cronan et al, 1972; Cronan, 1973) 5. Pacific Non-Carbonate Sediments, DSDP Site 319 (BostrLm et al, 1976; Dymond et al, 1976) 6. Pacific Basal Carbonates, DSDP Sites 37, 38, 39, 66, 77B, 162 (Cronan; 1976) 7. Pacific Basal Carbonates, DSDP Sites 320, 321 (Bostr8m et al, 1976; Dymond et al, 1976) 8. Basal North Atlantic Sediments, DSDP Sites 9A, 112, 114, 117A, 118, 136, 137, 138, 141 (Horowitz and Cronan, 1976) 9. Basal Sediments, Bermuda Rise, N. Atlantic DSDP Site 9 (Bruty et al, 1973; Chester et al, 1976) 10. North Atlantic Basal Carbonates, DSDP Sites 10, 136, 137 (Horowitz and Cronan, 1976) 195

the degree of hydrothermal metal addition follows a similar pattern and could account for the intermediate concentrations of Fe, Mn, Ni, Co, Cu, Zn and Pb in Indian Ocean DSDP metal-rich sediments. Ku et al (1968) have shown that the rate of detrital sedimentation is highest in the three oceans along the Mid-Atlantic Ridge, while the Mid-Indian Ocean Ridge system and more so the East Pacific Rise receive a lower proportion of detrital components. A variation in the rate of detrital sedimentation probably accounts for the intermediate values of Al, Ti, and Mg and possibly minor amounts of other trace metals in the Indian Ocean DSDP sediments. The intermediate composition of both Indian Ocean I)SDP metal-rich clays and carbonates is well-illustrated in Figs. 2.4.1d, e and f, where the higher Fe in clays and Mn in carbonates is also shown. This pattern of higher Mn in metal-rich carbonates and higher Fe in metal-rich clays from the Indian Ocean is also observed in North Atlantic DSDP metal-rich sediments but there are no such consistent differences in Pacific Ocean DSDP metal-rich sediments. Furthermore, while in Pacific Ocean DSDP sediments the majority of the trace metals are partitioned in the A/R agent soluble minerals - MnFe oxides (Cronan and Garrett, 1973; Cronan, 1976) and in Atlantic Ocean DSDP sediments the majority of the trace metals are held in the HCI soluble and insoluble phases - Fe oxides, Al silicates and detrital minerals (Horowitz and Cronan, 1976), in Indian Ocean basal sediments there is a mixture. In metal-rich Indian Ocean sediments (particularly the carbonates) most trace metals are partitioned in a similar fashion to Pacific Ocean DSDP sediments, while in the less metal-rich sediments (and to a certain extent the metal-rich clays also) the trace metals are partitioned in a similar way to the North Atlantic DSDP sediments. This has been recognised previously from the Pacific and Atlantic Oceans (Horowitz, 1974a; Cronan, 1972; Horowitz and Cronan, 1976) and is explicable in terms of the interplay of the two independent mechanisms described above.

Local variations do occur in the Indian Ocean DSDP metal-rich sediment! e.g. Fe and Mn and some other elements are comparable in concentrations with metal-rich sediments from the Pacific Ocean. This is probably the result of local variations in metal additions and does not illustrate any major differences other than those described above, between the oceans. 196

2.4.2 The Composition of Metal-Rich Sediments from Locations of Special Interest in the Indian Ocean.

2.4.2a. Introduction.

The geochemical investigations described in the sections on bulk and partition geochemistry have indicated that metal-rich sediments occur in two types of location, other than overlying basaltic basement. The first such type of location is in association with volcanic material which is not oceanic basement and is intrusive, i.e. a sill. The examples of this type of location are in association with a diabase sill at DSDP Site 211 (von der Borch et al, 1974), a lamprophyre sill at DSDP Site 224 (Whitmarsh et al, 1974) and basaltic material at DSDP Site 245

(Simpson et al, 1974; Gieskes et al, 1974; Warner and Gieskes, 1974). The second type of location is on an uplifted asiesmic area, i.e. the Ninety East Ridge (DSDP Sites 214, 216 and 254; von der Borch et al, 1974; Davies et al, 1974) unassociated directly with seafloor spreading. It would therefore seem appropriate to describe the geochemistry of these sediments in some detail and to discuss their modes of formation.

2.4.2b Metal-Rich Sediments Associated with Intrusive Sills.

At DSDP Site 211, a diabase sill has been intruded into the sediments at a depth of 401.5 metres below the sediment-seawater interface (von der Borch et al, 1974). Unfortunately it was not possible to analyse samples from the intruded sediments above the sill due to the extremely poor sample recovery (core catcher samples only). However, those overlying sediments, brown clays, from the lower part of Unit 4, are reported to contain much amorphous Fe oxide, Fe oxide-rich ash and pyrite-rich material (von der Borch et al, 1974). The sediments from below the sill show no signs of metal-enrichment (see Section 2.2). It has been suggested (Pimm, 1974; von der Borch et al, 1974) that the mineralisation that has occurred in the lower portion of Unit 4 at DSDP Site 211 is the result of the intrusion of the diabase sill, from which the enriched Fe in the sediments is derived.

DSDP Site 224, drilled on the Owen Ridge, recovered as basement a rock-type which is extremely rare in the ocean basins and has not been previously reported from the Indian Ocean - a lamprophyre (Whitmarsh et 197

al, 1974). It is of the variety known as monchiquite, which Wells et al (1972) have described as representing an abnormal magma fraction, in this case containing many ferromagnesian minerals, such as olivines, amphiboles and pyroxenes. The rock is highly altered, a common feature of lamprophyres (Wells et al, 1972), with the glassy matrix containing brown clay, but the overlying sediments have low concentrations of Fe, Mn and other trace metals and are enriched in Al, Ti and Mg. The sediments contain volcanic glass and the dominant clay - mineral is montmorillonite (Whitmarsh et al, 1974). The partial chemical composition of the sediments shows that the majority of the trace metals are held in the HC1 soluble and insoluble phases suggesting the presence of chemically altered volcanic material and detrital volcanic fragments. The presence of unmetalliferous sediments, with average or below average metal accumulation rates in contact with this sill, together with the partition data, seem to indicate that hydrothermal leaching of the lamprophyre has contributed little to the metal content of these overlying sediments. The results of the factor analysis (see Section 2.2.4c) support such a conclusion and show the importance of factor 1 (Mn factor) and factor 3 (clay mineral factor) in these sediments indicating the importance in them of authigenic precipitation from seawater and inclusion of clay minerals. The presence of volcanic debris together with the other evidence tends to suggest that the metals in the basal sediments result from the alteration, by denitrif- ication, of the pyroclastic material present in these sediments, together with authigenic precipitation of trace metals from seawater.

Metal-rich sediments, similar to the basal Fe oxide facies common in Pacific Ocean DSDP sediments, have been reported in the carbonate-rich sediments from DSDP Site 245 (Simpson et al, 1974;. Gieskes et al, 1974; Warner and Gieskes, 1974). The basement rocks show many similarities to the low-K tholeiites found on the mid-ocean ridges (Simpson et al, 1974). Warner and Gieskes (1974) state that the volcanic material represents a sill, the age of which they report as 27- 3 m.y. old (Miocene/Oligocene). No sediment has been recovered from below the sill, nor has the true basement been encountered (Warner and Gieskes, 1974), and the sediments into which the sill is intruded are of Early Palaeocene age ( approx. 64 m.y. B.P.) (Simpson et al, 1974). Warner and Gieskes (1974) report the predicted age of the basement as being 68 m.y. B.P. (Schlich, 1974), which means that the sill intersects the sediment column not more than 50 metres above true basement, assuming a constant sedimentation rate of 1cm/1000 yrs. 198

This poses the question as to whether the metallogenic activity exemplified in these sediments is contemporaneous with the sediments and related to a period of increased seafloor spreading during the Palaeocene, or Whether it is related to the intrusion of the sill in the Early Miocene. Warner and Gieskes (19711) prefer the former possibility and suggest that the metal enrichment is the result of metals. being leached by the in situ alteration of the abundant volcanic ash debris present in the sediments. Such an interpretation is supported by the changes in composition of the interstitial waters (Gieskes et al, 1974). The metal ratios reported by Warner and Gieskes (1974) and Gieskes et al (1974) - Al:K, each of Fe, Al and Mn to (A1 + Fe 4 Mn) are characteristic of metal-rich sediments from other areas. The basal sediments from DSDP Site 245, show high concentrations of, and enrichment with depth of, Fe, Mn, Ni, Cu, Zn, Ba and As and are also Al and Ti-poor, in keeping with metal-rich sediments from other areas (Bostr8m and Peterson, 1966; Horowitz, 1974a). The sediments show high accumulation rates of Fe, Mn, Ni, Cu, Cr, Zn, As and Ba, supporting the conclusion of Warner and Gieskes (1974) that the Mn/Fe oxide material in the sediments was deposited rapidly. The geochemistry of the metal-rich carbonates at DSDP Site 245 is dominated by the significance of Factor 1, the Mn Factor, and Factor 3, the biogenic factor (see Section 2.2.4d). The characteristics of these sediments are similar to other Palaeocene metal-rich sediments from other locations in the Indian Ocean. Furthermore, Fe and Mn are partitioned in different phases in the sediments. The Mn and associated trace metals are held in the A/R agent soluble Mn oxide minerals, while the Fe, Al and associated trace metals are held in the UCl soluble phases, in keeping with DSDP metal-rich sediments from other areas (Cronan, 1976). The above characteristics of these sediments indicate that the enriched metals are probably derived from the hydrothermal leachinc of basaltic material related to increased ridge activity, i.e. rapid seafloor spreading, as is occurring on the East Pacific Rise today. The Miocene/Oligocene (i.e. age of the intrusion of the sill) was a period of low or negligible spreading in this area (McKenzie and Sclater, 1971) while the Palaeocene (i.e. the age of the sediments) was a period of rapid seafloor spreading in the Indian Ocean (McKenzie and Sclater, 1971; Johnson et al, 1976) with which other occurrences of metal-rich sediments are associated. Such evidence suggests that the metallogenic activity at DSDP Site 245, as exemplified by the geochemistry of the basal sediments. was associated with the period of increased seafloor spreading and ridge activity in the western Indian Ocean during the Palaeocene, and not with the 199

intrusion of the diabase sill during the Miocene.

It would appear that where metal-rich sediments are found in close association with volcanic material that does not represent true oceanic basement, in one case (DSDP Site 211, Pimm, 1974; von der Borch et al, 1974) metal enrichment due to post basement volcanic activity has taken place. This indicates that processes of hydrothermal metal enrichment may not be confined to the crest of the mid- ocean ridge system. This is similar to the findings of Horowitz (1974a) and Horowitz and Cronan (1976) who found metal-rich sediments above a sill at DSDP Site 138 in the eastern North Atlantic Ocean. This also confirms the findings of several workers (Corliss, 1971; Piper, 1973; Bertine, 1974) who have suggested that metals can be leached from any basalt by interaction with seawater.

The data from DSDP Site 224 suggest that a lamprophyre may not provide a suitable source of metals for the overlying sediments.

The geochemical characteristics of the metal-rich sediments at DSDP Site 245, which are similar to the characteristics of metal-rich sediments from other locations in the Indian Ocean and from elsewhere, indicates that the metallogenic activity was contemporaneous with the deposition of the sediments and related to increased ridge activity and sea floor spreading in the Palaeocene. Only minor amounts of certain metals may have been contributed to the basal sediments by the alteration of volcanic ash debris and the weathering of a diabase sill, the intrusion of which in the Miocene post-dates the major metallogenic event.

2.4.2c. Metal-Rich Sediments from the Ninety East Ridge.

Samples were collected from three stations on the Ninety East Ridge which encountered basement - DSDP Sites 214, 216 and 254. The age of the basement and hence the oldest sediments increases to the northward - late Eocene/Early Oligocene at DSDP Site 254; Palaeocene at DSDP Site 214 and Maastrichtian at DSDP Site 216 (Davies et al, 1974; von der Borch et al, 1974). The nature of the basement at these sites bears little relation to those basalts extruded on the mid-ocean ridges, although flows are distinguishable, the basalts are amygdaloidal and vesicular, are devoid of pillow structures (DSDP Site 214) and have compositions similar to lava 200

suites found on the volcanic islands of St Pauls and New Amsterdam (DSDP Site 216) (von der Borch et al, 1974; Davies et al, 1974). The features suggest that the basalts were extruded in a subaerial or shallow water environment, possibly as a series of volcanic islands (von der Borch et al, 1974; Davies et al, 1974).

The sediments which overlie the basement are volcanogenic, contain ash horizons (DSDP Sites 214 and 216, von der Borch et al, 1974) and have Rare Earth Element contents resembling those of the underlying Ninety East Ridge basalts (DSDP Site 254, Fleet et al, 1976; Fleet, 1977). The sediments have been described in detail in Section 2.1.1., and from these and other data (von der Borch et al, 1974; Davies et al, 1974) it could be suggested that they result from the subaerial erosion of a basaltic terrain, followed by the rapid deposition of poorly sorted sediments in a littoral or lagoonal environment.

The chemical composition of the Ninety East Ridge DSDP sediments is interesting in a number of ways. In the sediments from DSDP Site 214, the metals are mainly held in the i1C1 insoluble mineral phases, emphasising the importance of the input of metals from basaltic detrital sources. Factor analysis has shown that the geochemistry of sediments from this site are not dominated by any single factor.

The sediments from DSDP Site 216 only have concentrations of Fe and Mg which are greater than the levels recorded in sediments from DSDP Site 214. Iron is the only metal which shows marked basal enrichment. The majority of the metals, Fe included, are held in the HCI soluble mineral phases, with some in the HClinsoluble phases and little in the A/R agent soluble minerals. This would tend to indicate that the increased values of Fe (up to 13.8%) in these sediments result from the chemical alteration of the underlying basalts. Factor analysis has shown that the geochemistry of the sediments from this site is not dominated by any single factor, but that the Fe factor (factor 2) is of greater significance than the others, indicating that the metals are supplied by the inclusion of the basaltic alteration products in the sediments. Local high values of Fe and associated trace metals in this core may be contributed by localised hydrothermal leaching processes. 201

The basal volcanogenic sediments from DSDP Site 254 have the highest values of Fe (up to 18.4%) of basal sediments on the Ninety East Ridge. However, there is a concomittant increase in Al and Ti, together with high accumulation rates of these metals. There is no enrichment in, or high accumulation rates of Ba, Mn, Cu and As and other trace metals, and the majority of the metals are partitioned in the HC1 soluble and HC1 insoluble mineral phases. These data suggest the.incorporation into the sediments of the products of the chemical alteration of the underlying basalts, in the form of authigenic alumino silicates, e.g. Fe smectites and other clay minerals, as well as unaltered basaltic detrital fragments.

The question remains as to the mode of formation of the sediments on the Ninety East Ridge. The origin of the Ninety East Ridge has been greatly discussed in the past and has been fully summarised in Section 1.2. The DSDP drilling has shown that throughout its history, the Ninety East Ridge had been extruded in a shallow water environment, and subsided to oceanic depth as it moved northwards (Pimm et al, 1974; Davies et al, 1974; Luyendyk and Davies, 1974; Luyendyk and Rennick, 1977). Such a conclusion is supported by the nature of the volcanic rocks and volcanogenic sediments recovered from the Ninety East Ridge (von der Borch et al, 1974; Davies et al, 1974; McKelvey and Fleet, 1974; Luyendyk and Davies, 1974; Fleet, 1977). The three physiographic provinces of the Ninety East Ridge (see Section 1.2) are believed to have different origins all of which can be linked to the evolution of the Eastern Basin of the Indian Ocean as a whole and can be described in terms of a model, which involves the movement of the Australian, Antarctic and Indian plates over two mantle hot spots during the Cretaceous and Tertiary. The geochemical data available from the Ninety East Ridge sediments are not conclusive, but based on these data a possible mode of formation, within the above framework can be suggested. The portion of the Ninety East Ridge north of 7oS was formed during the Late Cretaceous and Early Tertiary by localised volcanic activity in shallow water above a volcanic point source (Luyendyk and Rennick, 1977). Erosion of such material, coupled with subsidence, probably gave rise to the relatively unmetalliferous sediments at DSDP Site 216. However, localised hydrothermal circulation and leaching associated with such a process could account for the locally high values of Fe in the sediments at DSDP Site 216. The portion of the Ridge between 7oS and the Osborn Knoll (14-16oS) was formed by the extrusion 202

of volcanic material along a transform fault immediately to the east of the Ninety East Ridge, during the Palaeocene. The weathering of this material in shallow water conditions probably gave rise to the basal sediments at DSDP Site 214. The portion of the Ninety East Ridge and the overlying sediments to the south of the Osborn Knoll are influenced by the continued extrusion of volcanic material along the transform fault to the east of the Ridge. The rapid erosion of this volcanic material, which was probably initiated by the separation of Antarctica and Australia along the South East Indian Ocean Ridge about 53 m.y.B.P., was coincident with the breakup of an aseismic massif, composed of the Naturaliste Plateau, the Broken Ridge, the Kerguelen Plateau and the Ninety East Ridge (Luyendyk and Rennick, 1977). The breakup of such a massif may have initiated minor localised hydrothermal leaching of basalts along the southern portion of the Ninety East Ridge. This together with the erosion of the volcanic material in a shallow water environment could account for the development of Mid Eocene Al- rich metal-rich clays at DSDP Site 254.

2.4.3 Metallogenesis and Regional Geochemical Variations in the Indian Ocean.

2.4.3a. Introduction.

In previous studies of basal metal-rich sediments is has been possible to draw conclusions regarding variations in metallogenesis through time. Work on basal sediments from a number of DSDP Sites associated with spreading about the Mid-Atlantic Ridge (Horowitz, 1974a) by the use of the ratios of Bostr8m et al (1972) has shown that over the last 106 m.yrs (from Aptian/Cenomanian times), with the exception of a period during the Palaeocene, ridge additions and detrital input have remained relatively constant. Work by Cronan (1976) on basal sediments associated with the spreading of the East Pacific Rise, has shown that development of metal-rich sediments may follow a cyclic pattern. Cronan (1976) has suggested, by using Fe as an indicator of ridge activity, that there have been two complete cycles of metallogenesis since the Cretaceous, with the most metal-rich sediments being of Mid Miocene and Oligocene age coincident with periods of rapid seafloor spreading. In a preliminary study of Indian Ocean sediments, Cronan et al (1974) attempted to use Fe values again as an indication of the degree of ridge activity in the western Indian Ocean. 203

In order to investigate the relationship of variations in metallogenesis with time in the Indian Ocean, compositional data on basal sediments Of different ages and from different locations have been plotted against time for the western (Fig.2.4.3a) and the eastern (Fig.2.4.3b) Indian Ocean. The data are presented in Tables 2.4.3a and 2.4.3b. Although the Al/(Al + Fe + Mn + Ti) ratios of Bostrdm et al (1972) have been employed in the past by several authors (Horowitz, 1974a; Horowitz and Cronan, 1976) to indicate the importance of ridge derived and detrital derived metal inputs to marine sediments respectively, their use can be misleading and does not necessarily indicate areas of volcanogenic metal input. McArthur and Elderfield (1977) have shown for Central Indian Ocean Ridge sediments, that although the sediments may have a ratio within the range used to classify them as volcanogenic, examination has shown them not to be volcanogenic and that their metal contents can be accounted for by hydrogenous precipitation from seawater. The mis- interpretation arises from the fact that the ratio of Bostriim et al (1972) links two processes - ridge crest metal addition (Fe and Mn) and detrital metal additions (Al and Ti) - which operate coincidentally in some marine sediments, but are in fact entirely independent of each other. The use of this ratio is further complicated by the data of Heath and Dymond (1977) on East Pacific Rise sediments which suggest that some Al in these sediments may be of hydrothermal origin. In a similar respect the suggestion by Strakhov (1976) that the Fe and Mn /Ti ratio can be used to indicate areas of volcanogenic metal input to marine sediments can also be misleading because this ratio links two independent processes - ridge crest metal additions (Fe and Mn) and detrital sedimentation (Ti). For these reasons neither of these ratios were used in the following discussions.

The metal values plotted are AI and Ti, which are in general taken to be indicative of detrital sedimentation, while the metals plotted which have been suggested by various authors as representing ridge crest hydrothermal activity in particular instances are:- Fe (Corliss, 1971; Cronan et al, 1972; Cronan, 1974, 1976); Mn (Horowitz, 1974a); As (Cronan, 1972); Ba (Bonatti et al, 1972; Heath and Dymond, 1977) and Mg (Bischoff and Rosenbauer, 1977). 204

2.4.3b Metallogenesis in the Western Indian Ocean.

Seafloor spreading in the western Indian Ocean, at the present time is occurring about three ridge segments - the Carlsberg Ridge, the Central Indian Ocean Ridge, and the South West Indian Ocean Ridge. In the Early Cretaceous seafloor spreading occurred about the proto South West Indian Ocean Ridge (DSDP Site 249) as Africa separated from Antarctica/India/Australia (see section 1.2). From then, until 36 m.y. B.P. seafloor spreading in the western Indian Ocean was related to the proto Carlsberg Ridge only (DSDP Sites 250A, 239, 245, 236, 220, 224 and 238). At about 36 m.y.B.P. the present pattern of spreading along the Central Indian Ocean Ridge and Carlsberg Ridge was established and spreading since then has occurred about these two segments (DSDP Site 221). In addition at about 20m.y.B.P. spreading began on the South West Indian Ocean Ridge (DSDP Site 251A), thus completing the pattern of three actively spreading ridge segments which exists in the western Indian Ocean at present.

From Table 2.4.3a and Fig. 2.4.3a it can be seen that basal sediments of Neocomian age occur at DSDP Site 249. These can probably be related to the slow separation of Africa from the southern continents along the proto South West Indian Ocean Ridge, between 130 and 80 m.y. B.P. After the cessation of spreading along this segment in about the Mid. Albian, when Africa reached its present position relative to Antarctica (see Section 1.2) no further spreading occurred until the Miocene (see below) (Le Pichon and Heirtzler, 1968).

The major development of metal-rich sediments, with high Fe, Mn, As, Ba, low Al, Ti and moderate Mg occur in the Palaeocene at DSDP Sites 236 and 245, with a further minor development in the Eocene at DSDP Site 220. The Palaeocene in the western Indian Ocean was a period of rapid seafloor spreading, which reached a peak rate of 12.2cm/yr (Schlich et al, 1974). This spreading occurred in a N/S direction along the proto- Carlsberg Ridge which allowed the separation of Arabia/India from Antarctica. The western boundary of the Indian plate during this period was the Chagos fracture zone (i.e. at the eastern end of the Carlsberg Ridge - see Section 1.2 for a more detailed description of the geological. evolution of this area). The development of metal-rich sediments at DSDP Site 220, during the lower Eocene (54 m.y.B.P.), to the north of the 205 Fig. 2.4.3a Metal Concentration Variations with Time in DSDP Basal Sediments from the Western Indian Ocean

P^|000i 60

Maas B.

ars Ye

n lio l Mi

~ ' ' 12 11 10 9 o wt% Fe. Mn, Ba, A17

' . Ti, Mg. c~ 22xz 20 18 16 14 12 m o o * 2 0 ppm x 10 As

M Fe

0 Mn

A As

— Ba

o Mg

, Ti

w Crestal Sediment, Central Indian Ocean Ridge

All data expressed on CFB Table 2.4.3a. Metal Contents of DSDP Basal Sediments from the Western Indian Ocean. Wt% PPm M.Y. Site No. Fe Mn Mg Al Ti Ba As Age of Sediments B.P.

Central Sed., C.I.O.R.* 4.31 0.77 2.61 2.38 0.18 6690 20 Recent 0

CARLSBERG RIDGE

221 5.69 0.37 2.72 7.67 0.47 1960 9 Miocene-Pliocene 7

SOUTH WEST INDIAN OCEAN RIDGE 251A 5.82 0.23 1.86 7.80 0.38 3740 13 Early Miocene 23

PROTO-CARLSBERG RIDGE 238 5.96 0.18 n.d. 7.50 0.56 n.d. 10 Late Oligocene 29 224 5.98 0.08 2.19 8.04 0.69 1080 3 Early Eoc.- Early Oligoc. 38 220 7.29 2.13 1.79 2.53 0.44 6723 2 Early Eocene 53 236 11.32 2.35 n.d. 3.26 0.31 n.d. 136 Early Palaeocene 56 245 10.43 3.56 2.36 4.68 0.45 2220 240 Early Palaeocene 65 239 7.73 0.72 1.88 7.78 0.73 910 19 Late Cret-Early Palaeocene 70 250A 5.34 0.28 1.97 5.36 0.45 1440 3 Coniacian 85

PROTO-SOUTH WEST INIDAN OCEAN RIDGE 249 5.71 0.05 1.19 4.96 1.03 700 6 Neocomian 125 All data CFB n.d.= not determined "Average Crestal Sediment, Central Indian Ocean Ridge - This study 207

Carlsberg Ridge in the Arabian Sea, with high values of Fe, Mn and Ba and low Al. and Ti, probably marks the final period of this rapid seafloor spreading and increased ridge activity. The association of metal-rich sediments with increased rates of seafloor spreading strongly suggests that the increased concentrations'of Fe, Mn, As and Ba in them result from increased ridge activity and ridge derived metal additions during these periods.

The period of fast seafloor spreading in the western Indian Ocean in Palaeocene to Eocene times was followed by one of slow spreading from about 53 to 36 m.y.B.P., at a rate of less than 4.Ocm/yr. The basal sediments deposited during this period - DSDP Sites 224 and 238 - have low concentrations of Fe, Mn, Ba and As, moderate Mg and elevated values of Al and Ti. The composition of these sediments probably owes very little to ridge crest. hydrothermal activity and probably results from slow weathering of basaltic material and hydrogenous precipitation of metals from sea water.

At about 36 m.y.B.P. the present system of seafloor spreading in the western Indian Ocean about three ridge segments was initiated, as described in Section 1.2. Spreading along the Carlsberg Ridge and the Central Indian Ocean Ridge in a NE/SW direction occurred at rates of 1.2 - 1.3cm/yr and 2.3 cm/yr respectively (McKenzie and Sclater, 1971). These rates have remained constant since the initiation of this spreading regime (at 36 m.y.B.P.) and the basal sediments deposited at DSDP Site 221, during this period have low Mn, Fe, As, Ba values and moderate Mg, Al and Ti. They probably result from slow basaltic weathering and hydrogenous precipitation of metals from sea water and probably owe little in their compositions to ridge crest hydrothermal activity.

Seafloor spreading did not begin along the South West Indian Ocean Ridge until about 20 m.y.B.P. in the Lower Miocene (Schlich, 1974). The spreading was in a NNE/SSW direction and has been occurring at a rate of 1.0cm/yr since that period (see Section 102). The basal sediments at DSDP Site 251A deposited on the flanks of this ridge do not show signs of metal enrichment. However, the accumulation rates at this site of Fe, Mn, Ba, Cr and Cu are high and similar to those of other metal-rich sediments of Palaeocene age. (See Section 2.2.3). These high accumulation 208

rates and moderate increases in the Ba concentrations may be due to localised hydrothermal activity at this site related to the initiation of seafloor spreading during the Lower Miocene along the South West Indian Ocean Ridge.

2.4.3c. Metallogenesis in the Eastern Indian Ocean.

Seafloor spreading in the eastern Indian Ocean is less complex than in the western basin because it relates to spreading about a single ridge segment - the South East Indian Ocean Ridge. However, the history of seafloor spreading to the east and west of the Ninety East Ridge is quite different (see Section 1.2).

From Table 2.4.3b and Fig. 2.4.3b it is evident that the most metal-rich sediments are those of Palaeocene age at DSDP Site 215. The Palaeocene in the Central Indian Basin was a period of rapid seafloor spreading along an E/W trending South East Indian Ocean Ridge (see Section 1.2), as India separated from Antarctica. The rate of separation reached a peak of 12.2cm/yr (Sclater and Fisher, 1974; Sclater et al, 1976) at about 64 m.y.B.P. The metal-rich sediments at DSDP Site 215 have high concentrations and accumulation rates of Fe, Mn, Ba, As, Mg and low Al and Ti concentrations. The association of metal- rich sediments in the Central Indian Basin with increased rates of seafloor spreading, suggests that the increased metal concentrations are the result of increased ridge crest hydrothermal activity during the Palaeocene.

The situation in the Wharton and Coccos Basins to the east of the Ninety East Ridge is less clear, due to the general absence of correlatable magnetic anomalies. However, the results of the DSDP drilling (discussed in Section 1.2) have greatly helped in the understanding of this area. The age of the ocean floor in the Wharton Basin youngs to the north, in the opposite direction to the aging of the Ninety East Ridge and the ocean floor in the Central Indian Basin. Such a direction of younging, together with other evidence from the DSDP sites in this area and from the geology of the southern continents (see Section 1.2), has led to the suggestion that a spreading centre existed in the N.W. Wharton Basin between Santonian and Oligocene times (Luyendyk and Davies, 1974; Johnson et al, 1976; Luyendyk and Rennick, 1977). Seafloor spreading along this centre, if extant, allowed the separation, in a N/S direction, of India from Australia. A fracture zone, immediately to the east of the Ninety Table 2.4.3b. Metal Contents of DSDP Basal Sediments from the Eastern Indian Ocean.

Site No. Fe Mn Mg Al Ti Ba As Age of Sediments M.Y. B.P.

WEST OF NINETY EAST RIDGE

215 10.48 5.57 2.17 2.29 0.15 7020 81 Mid-Palaeocene 56

EAST OF NINETY EAST RIDGE

*Recent Sedim 3.77 0.64 1.71 6.38 0.18 Boo 12 Recent 0 213 13.92 3.18 2.21 4.56 0.37 1130 89 Early-Palaeocene 64 211 5.82 0.24 2.57 6.45 0.78 820 4 Camp/Maastrichtian 70 256/257 5.30 0.32 1.74 5.70 0.41 1450 13 Albian 100 212 5.97 0.28 1.72 8.03 0.51 890 12 Mid-Late Cretaceous 115

All data CFB *Recent sediment - Surface sediment, Wharton Basin (unpubl. data, Moorby person. commun. 1977). 210 Fig. 2./1.3b. Metal Concentration Variations with Time in DSDP Basal Sediments from the Eastern Indian Ocean

9 8 7 6 5 4 3 2 1 0 ppm x 10 As ▪ Fe

o Mn (For period names see Fig. 2.4.3a) A As

• A]

o Mg

wb Wharton Basin Sediment All data expressed on CFU 211

East ,Ridge allowed movement between the Indian and N.W. Wharton Basin plates (Johnson et al, 1976). Such an interpretation correlates well with the nature (von der Borch et al, 1974) and age of the basement from DSDP Sites in this area. Seafloor spreading along this E/W trending ridge segment between the Santonian and Early Palaeocene occurred at a rate of 3.5 - 4.8cm/yr. Basal sediments developed during this period at DSDP Sites 212, 256, 257 and 211 have low concentrations of Fe, Mn, Ba, As, and high to moderate Al and Ti values. This suggests that their origin is not related to ridge activity but probably results from slow weathering of basaltic material and hydrogenous precipitation of metals from sea water. During the Palaeocene, the rate of seafloor spreading increased to over 6.0 cm/yr (Johnson et a1,1976) as India separated more rapidly from Australia. The increase in spreading is correlated with a development of metal-rich sediments at DSDP Site 213, which have high Fe, Mn and As values and low to moderate Al, Ti and Mg values. They do not, however, show the same metal enrichment trends as the Palaeocene sediments at DSDP Site 215 to the west of the Ninety East Ridge in the Central Indian Basin which have higher Mn, Ba values and lower Al and Ti values than the metal-rich sediments at DSDP Site 213 in the Wharton Basin. Although the association of metal-rich sediments with increased seafloor spreading rates in the Wharton Basin suggests that the enriched metals may be added by ridge crest hydrothermal activity, the increase in seafloor spreading rate is not as marked as that observed in the Central Indian Basin at DSDP Site 215. Furthermore, the differences in the nature of the metal-rich sediments at DSDP Sites 215 and 213 does not fully support the conclusion of Pimm (1974) who suggested a similar mode of formation for them. The spreading centre in the N.W. Wharton Basin was offset during this period by fracture zones, e.g. the Investigator Fracture Zone. Localised hydrothermal circulation along one or other of these fracture zones may well also contribute towards the metal enrichment in the basal sediments at DSDP Site 213. Movement along this spreading centre in the. N.W. Wharton Basin ceased during the Oligocene when the Indian and Australian plates ceased to exist as separate plates. (See Section 1.2). The present regime of spreading along a continuous South East Indian Ocean Ridge in the eastern Indian Ocean dates from the separation of Australia from Antarctica in a NE/SW direction which began during the Miocene. The South East Indian Ocean Ridge is an example of a fast spreading ridge and is at present spreading at a rate of 4.5-6.4cm/yr (Johnson et al, 1976; Sclater et al, 1976). 212

2.4.3d Concluding Remarks

It has been possible to show by the use of metal concentration plots against time, that mid-ocean ridge metallogenesis in the Indian Ocean has been variable, in a similar manner to that of the Pacific Ocean (Cronan, 1976), but that there have been long intervening periods of little variation in ridge derived and detrital metal inputs in a similar manner to the North Atlantic Ocean (Horowitz, 1974a).

2.4.4. Sources of Metals in Indian Ocean DSDP Basal Sediments.

The source or sources of metals in basal metal-rich sediments is still a matter of some controversy. However, it is now generally recognised that the enrichment of a particular metal may result from several processes rather than a single process of enrichment (Cronan, 1976; Heath and Dymond, 1977). The general view is that processes related to ridge crest volcanism, i.e. hydrothermal processes, are the main source of the majority of the metals in basal metal-rich sediments. However, opinions differ as to the exact nature of these processes. It has been suggested that metal enrichment can be caused by the introduction of deep seated hydrothermal fluids, possibly of carbonatite composition (Bostr8m et al, 1966, 1969, 1973h,1976; Pimm, 1974). It has also been suggested that metal enrichment can occur due to the interaction of freshly extruded basalt with sea water (Corliss, 1971; Dymond et al, 1973; Piper, 1973; Heath and Dymond, 1977), which results in the initiation of hydrothermal circulation through cracks and fissures in the basalt and the leaching of metals from the basalts. The heat source necessary to drive such a circulation is provided by the proximity to the volcanic centre on an active mid-ocean ridge (Bonatti, 1975). The metals precipitate from higher temperature, acidic hydrothermal fluids as they mix with cooler, more alkaline bottom waters,near the spreading centre. The residues of the hydrothermally leached basalts, which are probably composed of clay minerals (Gorbunova, 1974; Bischoff and Dickson, 1975; Hajash, 1975), when incorporated into the basal sediments may constitute an additional source of metals. Metals may also be added to basal sediments in the products of the slow weathering of volcanic pyroclastics and basalts by sea water (Sayles and Bischoff, 1973; Bertine, 1974; Horowitz and Cronan, 1976), i.e. as clay minerals, particularly montmorillonites (Davies et al, 1974; Warner and Gieskes, 1974; Gieskes et al, 1974). The inclusion in 213

the basal sediments of unaltered basaltic detrital fragments, from the mechanical breakdown of the basement rocks, may also enrich certain metals (Horowitz, 1974a; Bertine, 1974). Hydrogenous precipitation from sea water has been suggested as a further source of metals in the lattices of, and adsorbed onto the surfaces of, such authigenic minerals as clays and FeMn oxides (Dymond et al, 1973; Cronan, 1976; Heath and Dymond, 1977). The influence of metal addition from continental detritus is probably of minor importance in basal metal-rich sediments due to the general geographical location of spreading centres at great distances from the adjacent land- masses. Certain trace metals may be incorporated into the tests of calcareous marine organisms, such as foraminifera, coccoliths and nannoplankton which are found in basal sediments (Turekian and Imbrie, 1966; Aston et al, 1972a; Belyayev, 1973; Bostr8m et al, 1974; Oldnall., 1975). However, the contribution from biogenic sources is probably of minor importance in comparison to the proportion of metals supplied by hydrothermal processes.

Generally, the metal concentrations and accumulation rates of basal non-metal-rich sediments are similar to those of normal pelagic sediments, while basal metal-rich sediments from DSDP Sites 213, 215, 216, 220, 236, 239, 245 and 254 are enriched in Fe, Mn, Ni, Cr, Cu, Pb, Zn, As and Ba. The higher metal concentrations and accumulation rates in metal-rich sediments are greater than those of basal and normal pelagic sediments. The accUmu- lati.on rates of metals in Indian metal-rich sediments may be similar to accumulation rates in similar sediments from the crest of the East Pacific Rise. Although minor local variations do occur in these patterns of enrichment, the above observations indicate that the enriched metals in DSDP metal-rich sediments are probably supplied by additional sources to those of normal oceanic sedimentation. The additional source of most of the metals is probably hydrothermal activity.

Geochemical partition studies in DSDP metal-rich sediments are relevant to any discussions of proportions of metals supplied by different sources. Carbonate-rich sediments make up only 15% of the total suite of Indian Ocean DSDP basal sediments studied here. Concentrations of certain metals in the lattice of carbonate minerals of the biogenic material have been reported previously - Ba (Bostr8m et al, 1973b); Mg in high Mg calcite of Indian Ocean surface sediments (Wiseman, 1965); Cd in shells of organisms from North Atlantic surface sediments (Aston et al, 1972a); 214

Pb in micro-organisms and calcareous material (Cronan and Garrett, 1973; Greenslate et al, 1973; Bostr8m et al, 1974); Cu released from the tests of foraminifera upon dissolution of the carbonate on the floor of the Baiier Deep (Oldnall, 1975); Mn as oxide coatings in carbonate sediments (Copeland, 1970) and in the lattice of carbonate minerals of foraminifera from the Caribbean Sea (Ericson and Wollin, 1973); and also minor concentrations of Li, Co, Ni and Zn in carbonate sediments (Greenslate et al, 1973; Belyayev, 1973; Bostr8m et al, 1974; Oldnall, 1975). All these metals are taken up during the life processes of the organisms (Oldnall, 1975) and become incorporated into the sediments when the organisms die and their tests settle onto the ocean floor. The correlations with Ca indicate that amounts of Cu and Zn may be associated with biogenic material. The partition studies have shown that proportions of Mn, Co, Cr, Cu, Pb, Zn and Mg (and obviously Ca) and to a lesser extent Ni, Li, Cd and Ba are removed by the HAC leach and that these proportions are highest in the carbonate-rich sediments. In these sediments it probably represents the removal of proportions of the metals from the lattices of carbonate minerals. However, in the carbonate-poor sediments, amounts of such metals as Ni, Co, Cu, Zn, Li, Cr and Ba are also soluble in the HAC leach. Here these metals are probably removed from adsorbed sites on the surface of clay and oxide minerals, and represent the authigenic fraction of these metals (Chester and Hughes, 1967, 1969). However, Although biogenic concentration may account for proportions of certain metals in the sediment e.g. Pb, Ca, Zn, it is only likely to be of minor importance as a means of metal enrichment in the present DSDP sediments as a whole because of the relatively minor proportions of carbonate material in the majority of the sediments studied.

In the carbonate-rich sediments, the source of Ca is the biogenic carbonate material. In other sediments where there are lower amounts of Ca it is probably contained in phosphatic fish debris, as well as in included Ca-rich detrital minerals, formed from the breakdown of basalt and in the lattice of clay minerals derived from the weathering of basaltic material. 215

Barium in all the sediments is probably in the form of }{Cl soluble barytes, although it may also be associated with the lattices of clay minerals. The positive correlation of Ba and Mn in all the sediments suggests that Ba is probably also authigenically precipitated from sea water, although in the carbonate-rich sediments high proportions may also be linked to biogenic sources (Heath and Dymond, 1977). In the metal-rich sediments, the higher concentrations and increased accumulation rates of Ba, together with its positive correlation with Mn and its inclusion in the Mn factor (factor 1) indicates that it may be hydrothermally enriched in these sediments.

Aluminium in all the sediments is associated with the HC1 soluble and }ici insoluble minerals, i.e. the clay minerals, silicate and detrital minerals. This indicates that it is supplied to the sediments in the form of clay and detrital minerals which may result from slow weathering of basalt. Heath and Dymond (1977) have suggested that Al may be hydrothermally enriched in Nazca Plate sediments. The association of Al with Fe is probably due to inclusion in clay minerals as the leached residues of basalt in the sediments, rather than the hydrothermal removal of Al from the basalts. In the basal (i.e. non-metal-rich) sediments the Al in the clay minerals is probably supplied by the slow weathering of basaltic material. It is unlikely that the Al in Indian Ocean DSDP sediments has a hydrothermal source.

Titanium in the present sediments is probably supplied by the inclusion of unaltered basaltic detrital fragments containing such minerals as anatase and ilmenite. Slow weathering of basaltic material, particularly in basal sediments, may also supply Ti to the clay minerals of all the sediments.

It has been suggested (Bischoff and Rosenbaiier, 1977) that Mg may be of hydrothermal origin in Pacific sediments. However, in all the Indian Ocean DSDP sediments it would appear that Mg is supplied in clay minerals, as well as in basaltic detrital fragments containing Mg-rich detrital minerals such as FeMg silicates. In the carbonate-rich sediments the association of Mg with Ca in the biogenic factor (factor 3) suggests that it may be held in the lattice of high Mg-calcite contained in the sediments. Minor amounts of Mg are probably contained in proportions of sea salt present in all the sediments. 216

Lithium in the sediments is associated with the HC1 soluble minerals, i.e. the clay mineral component of factor 3. Such associations indicate that the Li is probably derived by the slow weathering of basaltic material. Lithium in unaltered detrital fragments would also appear to be an additional source of this metal. The association of. Li with Mn in the Mn factor may indicate that it is co-precipitated with authigenic Mn oxides from sea water.

A striking feature of Indian Ocean DSDP metal-rich sediments is the separation of Fe and Mn into different mineral phases as well as the marked negative correlation of Fe and Mn in metal-rich sediments. This separation of Fe and Mn probably reflects the well-known fractionation of these metals in the marine environment, which has led Cronan (1976) to suggest that in Pacific Ocean DSDP metal-rich sediments they may be supplied by independent processes. A negative correlation of Fe and Mn has been reported by Bostr6m and Peterson (1969) as occurring in areas of high detrital sedimentation. The evidence presented here, does not support high detrital sedimentation rates in metal-rich sediments, and it would appear therefore that this negative correlation of Fe and Mn is possibly due to other factors.

In the basal sediments, the observed metal concentrations and accumulation rates are similar to those of normal pelagic sediments. It would appear therefore that the metals in these sediments are supplied by normal processes of oceanic sedimentation. In the basal carbonates proportions of certain metals may be associated with the lattices of carbonate minerals, Pb, Cu, Zn, Co and Ni, while in the non-carbonate sediments these metals are probably present on adsorbed sites on the surfaces of clay minerals. The Mn and associated trace metals (Co, Cr, Cu, Pb, Ni, Li) are precipitated from sea water as authigenic Mn oxides. In the more clay-rich sediments it is probable that the Mn and associated trace metals may be incorporated into the same mineral phases as Fe. The Fe and associated metals (Al, Ti, Mg, Ni, Ca, Zn, Cr, Li) are probably supplied by the slow weathering of basalt by sea water and are contained in clay minerals. Proportions of certain metals, Ti, Fe, Mn, Ni, Cr, Co, Cu, Zn, Pb,.Li, Mg and Al are contained in detrital minerals due to incorporation in the sediments of unaltered basaltic detrital fragments. 217

In metal-rich sediments the increased concentrations of Fe and Mn and associated trace metals and their increased accumulation rates are probably due to hydrothermal processes. The Fe and associated trace metals (Fe, Ba, Ni, Ca, Co, Cr, Zn) in metal-rich sediments are probably leached from the underlying basalts and are precipitated as Fe oxides and silicates in these sediments. The dominance of Factor 2, the Fe factor, in metal-rich sediments and the proportions of the enriched metals held in the HC1 soluble (and insoluble) phases indicates that the leached residues of the basalt, i.e. clay minerals, and unaltered basaltic fragments are also probably included in these sediments and constitute a further source of certain metals, e.g. Mg, Al, Ti, Cr, Li, Pb. In metal-rich sediments increased proportions of Pb, Cu, Ni, Co, Zn, Mg and Ba are associated with the A/R agent leach indicating their removal from the increased proportions of Mn oxides present in the metal-rich sediments. A proportion of the Mn oxides in the metal-rich sediments is authigenically precipitated from sea water in a similar way to those in basal sediments. The excess Mn is probably supplied by hydrothermal leaching of basalt. However, Bostrdm et al (1972) have suggested that more basalt would have to be hydrothermally leached to supply the excess Mn in basal metal-rich sediments than that required to supply the Fe in these deposits, if Fe is supplied by the same source. When allowance for the sea water precipitated component of the total Mn has been made, the excess Mn in metal-rich sediments can probably be accounted for by removal from basalt by hydrothermal leaching. It may also be possible where Mn is very high that it may in part be accounted for by local addition of Mn from sub-crustal sources as well as from removal of Mn, with Fe, by hydrothermal leaching from the underlying basalts.

It has been suggested that As in metal-rich sediments may possibly be of local volcanic origin in Median Valley sediments from the Mid- Atlantic Ridge (Cronan, 1972), and that in shallow water, marine ferromanganese nodules As may occur as an absorbed arsenate ion or replace phosphate ions in a ferric phosphate mineral (Calvert and Price, 1977). Only in the basal clays does As show a positive correlation with Fe (very low) as well as with Mn, which in these sediments might indicate its scavenging and co-precipitation with Fe, as suggested by Calvert'and Price (1977). In all the other sediments groups, and particularly in the metal- 218

rich sediments, there are strong positive correlations of As with Mn and no correlations are observed with Fe. Arsenic is also a major component of the Mn factor in the basal sediments. These data suggest that the increased concentrations and accumulation rates of As in metal- rich sediments cannot be accounted for in terms of co-precipitation of As with Fe, as suggested by Calvert and Price (1977). It would therefore appear that the As in the metal-rich and certain other groups of sediments is co-precipitated with Mn in authigenic Mn oxides. The concentration of As in basalts is 3ppm (Krauskopf, 1970). The hydrothermal leaching of this As, together with amounts of Fe and Mn from the basalts may on its own be insufficient to account for the enriched values and high accumulation rates of As observed in metal-rich sediments. It therefore seems probable that in metal-rich sediments, the increased concentrations of As may be added by sub crustal, volatile, volcanic sources,and that it is then co-precipitated with the Mn in authigenic Mn oxides.

2.4.5 Conclusions.

1. The basal (non-metal-rich) sediments have concentrations and accumulation rates of Fe, Mn, Ni, Ca, Cr, Pb, Zn, As and Ba which are below average for Indian Ocean DSDP sediments as a whole and are comparable with normal pelagic sediments, while concentrations of Al, Ti, Li, Mg and Ca are generally above average. The geochemistry of the basal carbonates is dominated by the biogenic factor (factor 3). In them above average proportions of Ca, Mn, Pb and also Ni, Cu, Zn and Li are removed by the IIAC leach, indicating the incorporation of proportions of these metals into biogenic carbonate material. The geochemistry of the basal clayey-carbonates is not dominated by any single factor, although each of the three factors may be of local significance. In the clayey- carbonates increased proportions of certain metals, as in basal carbonates, are held in the HAC leach, reflecting in part removal from biogenic carbonate material, but also in part removal from adsorbed sites on the surfaces of clay and oxide minerals. The remainder of the metals are divided between the HC1 solubl.e'and insoluble phases with minor amounts in the A/R agent leach. The overall geochemistry of the basal clays, whose composition is very similar to that of normal pelagic sediments, is not dominated by any single factor. In the basal clays, minor amounts of trace metals are removed from adsorbed sites by the IIAC leach while the majority of the metals are held in the HC1 soluble and insoluble phases, with minor amounts in the A/R agent leach. 219

2. The metal-rich sediments have concentrations and accumulation rates of Fe, Mn, Ni, Cu, Cr, Pb, Zn, As, Ba and Mg which are above average for Indian Ocean DSDP basal sediments as a whole, while the concentrations of Al, Ti and Li are generally below average. Local variations to this pattern do occur and may reflect the localised nature of the processes of metal addition along an active mid-ocean ridge. The metal accumulation rates are comparable with, or just below those for East Pacific Rise crestal sediments. These increased metal concentrations and accumulation rates in metal-rich sediments suggest that they are caused by additional sources to those operating in the basal sediments and are probably related to ridge-crest hydrothermal activity. A common feature of the partition patterns of all the metal-rich sediments is the increase in the proportions and absolute concentrations of such metals as Mn, Ni, Co, Cr, Cu, Pb, Zn and to a lesser extent Li and Fe in the A/R agent leach suggesting that they are authigenic in origin. The metal-rich carbonates contain the highest concentrations of Mn and associated trace metals of all the metal-rich sediments and their geochemistry is dominated by factor 1 (the Mn factor) and factor 3 (the biogenic factor) and to a lesser extent by factor 2 (the Fe factor). A feature of the metal-rich carbonates, apart from the increase in certain metals in the A/R agent leach, in common with all metal-rich sediments, is the higher proportions of Ni, Cu, Co, Pb and Mn in the HAC leach, partly due to their removal from biogenic carbonate and partly due to their removal from adsorbed sites on the surfaces of Fe and Mn oxides and clay minerals. The metal-rich clays have high concentrations of Fe, but lower concentrations of Mn and other trace metals when compared to metal-rich sediments as a whole. The geochemistry of metal-rich clays is dominated by factor 2 (the Fe factor) while the Mn factor, factor 1, is of less significance, especially in the low-Al metal-rich clays, as at DSDP Site 213, 216 and 236. The low-Al metal-rich clays have particular patterns similar to those of metal-rich carbonates, with increased proportions of certain metals in the A/R agent leach due to the higher proportions of Mn oxides present in the sediments. However, in the moderate-Al and high-Al metal-ridh clays (DSDP Sites 239, and 216 and 254 respectively) there are increased proportions of metals such as Fe, Mn, Ni, Co, Cr, Cu, Pb, Zn, together with Al, Ti, Mg, Ba, Li and Ca in thelICl soluble and insoluble minerals, i.e. the Fe oxides, clay minerals (e.g. smectites) and detrital minerals. The metal-rich clayey-carbonates from 220

DSDP Site 239, show partition patterns intermediate between those of metal-rich carbonates and clays, but with greater similarities to those of metal-rich clays. The geochemistry of metal-rich clayey- carbonates is not dominated by any single factor.

3. The differences in partition patterns between the various groups of Indian Ocean DSDP basal sediments result from variations in sediment type between carbonates and clays. In the case of carbonates higher proportions of certain metals and associated with carbonate material and in clay sediments they are associated with clay minerals, either as adsorbed ions or as lattice held ions. In metal-rich sediments, higher absolute concentrations of certain metals, e.g. Mn, Pb, Co, Cu, Zn, Ni, etc are associated with the A/R agent soluble mineral phases, whereas in basal sediments the lower absolute concentrations of these metals are associated with the HC1 soluble and insoluble mineral phases.

4. In comparison with basal metal-rich sediments from the Pacific and Atlantic Oceans, the Indian Ocean DSDP metal-rich sediments are of intermediate composition. The Indian Ocean metal-rich sediments are similar to those from the Pacific Ocean in having proportions of such metals as Mn, Ni, Co, Cr, Cu, Pb, Zn held in the A/R agent leach in association with Mn oxide minerals, while the more Al-rich Indian Ocean metal-rich sediments show similarities with metal-rich sediments from the Atlantic Ocean in having high proportions of such metals as Fe, Al, Ti, Mg, Ba, Cr, Cu, Ni, Zn and Li in the HC1 soluble and insoluble phases. These data emphasise the intermediate composition of Indian Ocean metal-rich sediments between those from the Pacific and Atlantic Oceans which probably results from the interplay but independence of two processes. The processes are firstly, the higher seafloor spreading rates and degree of ridge activity in the Pacific Ocean than in the Indian Ocean (Heirtzler et al, 1968) and secondly the higher detrital input in the Atlantic than the Indian Ocean (Ku et al, 1968). The interplay of these two independent processes in the geochemistry of metal-rich sediments has been recognised previously (Cronan, 1972; Horowitz, 1974a).

5. The possible presence of metal-rich sediments above the diabase sill at DSDP Site 211 would confirm the findings of Horowitz and Cronan (1976) from the North Atlantic, that metal enrichment processes may not 221

necessarily be confined to hydrothermal and volcanic centres along the mid-ocean ridges. However, the data from DSDP Site 224 indicates that the lamprophyre sill has not acted as a major source of metals found in the overlying sediments, indicating that this rock type may be un- important in this regard. Furthermore, the evidence from DSDP Site 245 indicates that the metallogenic activity at this site was probably related to increased rates of seafloor spreading and ridge activity during the Palaeocene, and not to the intrusion of the diabase sill in the Miocene.

6. The nature of the metal-rich sediments on the Ninety East Ridge indicates that they have been formed in a shallow water environment and have subsided to their present depths. The basal sediments at DSDP Site 216 were probably formed during the Late Cretaceous and Early Tertiary by erosion of volcanic material. Localised hydrothermal leaching may have given rise to the increased Fe values in the sediments at DSDP Site 216. The basal sediments at DSDP Site 214 were probably formed during the Palaeocene by the erosion and weathering of the volcanic material of the Ninety East Ridge under shallow water conditions. The Al-rich metal-rich volcanogenic clays at DSDP Site 254 were probably formed by the rapid erosion of volcanic material of the Ninety East Ridge, which had been initiated by the separation of Australia from Antarctica along the South East Indian Ocean Ridge, about 53 m.y.B.P.

7. By the use of metal concentration plots against time it has been possible to relate high concentrations (and also high accumulation rates) of such metals as Fe, Mn, As and Ba in the eastern and western Indian Ocean to periods of increased rates of seafloor spreading and ridge crest activity during the Palaeocene and Early Miocene. Hence, it would appear that mid- ocean ridge metallogenesis in the Indian Ocean has been variable in a similar manner to that in the Pacific Ocean, but that there have been long intervening periods of little variation in ridge derived and detrital metal inputs in a similar manner to the North Atlantic Ocean.

8. It is possible to account for the observed metal concentrations in basal sediments in terms of normal oceanic sedimentation by such processes as authigenic precipitation of Mn oxides (Mn and associated trace metals - As, Co, Cr, Pb, Zn, etc), precipitation of authigenic clay minerals (Al, Fe, Mg and trace metals), basaltic (and continental?) detrital input (Li, Cr, Ti, Mg) and biogenic concentration (Pb, Cr, Cu, Ni, Zn, Li, Ba, Mg, Co.). 222

9. Although there may be an authigenic precipitation from seawater of the Mn oxide component in DSDP metal-rich sediments, the data suggest that the increased concentrations of Fe and Mn and associated trace metals in some sediments are supplied from additional sources. The Fe and associated metals can be accounted for in terms of the leaching of these metals by hydrothermal fluids from the underlying basalts. Although some Mn is supplied with the Fe by hydrothermal leaching of basalts, it is possible that locally high concentrations of this metal, together with As, with which it is associated may be supplied by sub-crustal, volatile volcanic sources to the hydrothermal fluids and then precipitated as Mn oxides. The inclusion of the hydrothermally leached residues of the basalts as clay minerals, and inclusion of unaltered basaltic detrital fragments, may contribute to the concentrations of Al, Mg, Ti, etc. in basal metal-rich sediments. Biogenic concentration of certain metals is of minor importance in metal-rich carbonate sediments. 223

Section 3

3. THE GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE

3.1 INTRODUCTION AND SAMPLE DESCRIPTION

3.2 BULK GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE

3.3 PARTITION GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE

3.4 DISCUSSION OF THE GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE 224

3.1 INTRODUCTION AND SAMPLE DESCRIPTION

3.1.1 Previous Geochemical Studies of Sediments from the Indian Ocean.

The general nature and modes of formation of metal-rich sediments from the world's active mid-ocean ridge system have been described in Section 1. Here, attention will be drawn to those studies and geological features pertinent to this survey of the geochemistry of sediments from the Indian Ocean in general and from the Central Indian Ocean Ridge in particular.

The Indian Ocean, although the most geologically complex of the three major oceans (McKenzie and Sclater; 1971), has undergone the least investigation. This is particularly true of previous geochemical work on the sediments from the Central Indian Ocean Ridge. Geochemical studies have been carried out on the waters of the western Indian Ocean (see Topping, 1969; Chester and Stoner, 1974) and on the ferromanganese encrustations and nodules of the ocean as a whole (see Glasby et al., 1974; Cronan and Tooms, 1969; Moorby, 1978). However, few studies have concerned themselves with the geochemistry of Indian Ocean sediments.

A Swedish Deep Sea Expedition sampled sediments in the northern Indian Ocean and the results available, reported by Landergren (1964), are in general agreement with later data. More recent work has been concerned with the distribution of specific elements in the Indian Ocean as a whole.

In 1969, Bostrdm and his co-workers described the distribution of the Al/A1+Fe+Mn ratio in five hundred and forty-three sediments from the three major oceans. They showed that this ratio was lowest on the mid- ocean ridges and highest near the continents or at points furthest from the mid-ocean ridges. They suggested that this could be explained in terms of a simple model which was the result of the interplay of two factors. Firstly, that the sedimentation rate of the carbonate-free (i.e. detrital) portion of pelagic sediments decreases to a value of zero from the continents towards the mid-ocean ridge. Secondly, that the rate of deposition of the ferromanganese component of these sediments was 225

greatest along the mid-ocean ridges and decreased to the lowest value at points furthest from them. They suggested that other processes must contribute to the observed enrichment of ferromanganese oxides along the mid-ocean ridges. They considered that the most likely process was submarine volcanism producing hydrothermal emanations which when released into the bottom waters precipitate Fe and Mn (and probably other metals). They showed that this seemed a likely source since FeMn-rich sediments are poorly developed on inactive ridges (e.g. Ninety East and western portion of the South East Indian Ocean ridges), where the carbonate-free sedimentation rate is low. They also showed that the aerial extent of these FeMn-rich sediments is a reflection of the intensity of volcanic activity along the mid-ocean ridges (i.e. the higher the spreading rate, the more extensively developed the sediment). This, they suggested, provided further evidence of the view that the metals enriched in these sediments were supplied by volcanic sources. They concluded by suggesting that the composition of the volcanic emanations may be similar to the carbonatite magmas which occur in the continental rift system of East Africa (Girdler, 1964; Heinrich, 1966).

In 1969, Bender and Schultz, reported values of Mn, Ni, Co, Cu, Zn, Fe, and CaCO3 in a series of twenty-two surface sediment samples from the southern Indian Ocean (18-40oS). These samples were collected on the 'Vema 18' traverse from off the coast of southern Africa to the continental shelf off Australia (23-114°E). The sediments showed moderately high concentrations of Fe (up to 8% on a carbonate-free basis (CFB)) and high concentrations of Zn (up to 4% CFB), caused by contamination due to storage in galvanised steel pipes (Bender and Schultz, 1969). Contamination of other elements was minimal. The authors observed that the concentrations of Mn, Co, Ni and Cu were greatest in the centre of the basin and lowest on its fringes. Furthermore, that the trace metal values were three times higher in the eastern than in the western basin. They concluded that this enrichment was due to the presence of a fine-grained silicate fraction which preferentially concentrated these trace metals. This hypothesis was suggested by Turekian and Imbrie (1966) to explain the high trace metal values in a core from the centre of the Atlantic Ocean. Bender and Schultz (1969) suggest this hypothesis to explain their reported metal distributions and support it by showing that the average mean settling diameter for sediments from the eastern basin is 0.65}L, compared to 3.9u, for sediments from the western basin of the Indian Ocean. 226

Horowitz (1970) analysed a set of eighteen, widely spaced surface sediments from the Central Indian, Madagascar, Somali and Wharton Basins, the Mascarene Plateau and the Ninety East and flanks of the Southwest Indian Ocean Ridges, for the elements Pb, Zn, Sn, Ag and Ti. He calculated correlation coefficients between these elements and Mn, Fe and As, and between these elements and distance from the ridge crest. From this he was able to make three observations for the Indian Ocean. Firstly, there was an association between high concentrations of Pb, Zn, Ag, and to a lesser extent Tl, with high heat-flow, proximity to the ridge and high Fe, Mn and V, etc, indicating that these elements were derived from an active ridge source. Secondly, the association of these elements with Fe and Mn indicated that they were in part concentrated by adsorption on hydrous ferromanganese oxides (Krauskopf, 1956). Thirdly, the negative correlation of Pb, Zn and Ti with CaCO3 suggested that these elements were not concentrated by deposition of skeletal remains. However, he pointed out that the active ridges probably only contribute 10% or less to the concentrations of these elements, due to their greater concentration from such other sources as continental outwash, aeolian dust, glacial outwash, biogenic components, scavenging from seawater, post-depositional alteration and submarine weathering, all of which tend to dilute the portion supplied by the active ridges. Using sedimentation rates calculated by Ku et al (1968) he derived sedimentation rate-weighted element concentrations (for each of the three major oceans) to give more weight to samples from areas of high sedimentation. For the elements Pb, Ag, Ti and Zn this value was greatest for the Pacific, intermediate for the Indian and lowest for the Atlantic Oceans. He used this as an estimate of crestal activity, i.e. the highest element concentrations deriving from the most active mid-ocean ridge. This he showed was well-supported by geophysical evidence from heat flow measurements (von Herzen and Langsetn, 1965) and sea-floor spreading rates (Heirtzler et al, 1968). This variation in geochemical activity between the mid-ocean ridges from the three major oceans has been recognised by later workers and is supported by more recent data (Bostrbm et al, 1972; Cronan, 1972; Horowitz, 1971Ea, b).

Bostrdm and Fisher (1971) analysed a set of a hundred surface sediments from sites throughout the Indian Ocean for the elements Fe, V and U. They showed that Fe was concentrated in two areas. Firstly, along the active part of the Indian Ocean Ridge system, where it is probably supplied by volcanic activity. Secondly, on elevated inactive areas, e.g. the Broken Ridge, where there is a tendency for authigenic Fe deposits to 227

form in the absence of terrigenous material due to a low terrigenous sedimentation rate. They showed that the high concentrations of V and U occur on different parts of the active mid-ocean ridge system from Fe, and not in the other areas of Fe enrichment, thus ruling out a direct relationship of V and U with the precipitation of Fe hydroxides and oxides. They suggest that although V and U are also concentrated near the continents in sediments rich in organic matter, the high concentrations of V and U close to the active portion of the mid-ocean ridge system points to a source of these elements associated with submarine volcanism.

The existing studies have concerned themselves with the distributions of particular elements over very wide areas. In as much as this has provided a general survey of the geochemistry of Indian Ocean sediments as a whole, they have not provided a detailed study of a relatively small area of the active part of the mid-ocean ridge system in the Indian Ocean. This study is concerned with an active portion of the Central Indian Ocean Ridge. The sediments studied have been analysed for a wide range of elements (Ca, Si, Al, Ti, Mg, Ba, Fe, Mn, Ni, Co, Cr, Cu, Cd, Pb, Zn, Li and As). A selected group of these sediments have been analysed using the partial chemical techniques of Chester and Hughes (1967, 1969) and Cronan (1976). It is hoped that this will provide information about metallogenic processes along an active mid-ocean ridge and the nature of the mechanisms of incorporation of those enriched metals. Furthermore, it is hoped that a greater understanding will be gained of the sources of those metals which are enriched in Indian Ocean metal-rich sediments. Finally, it is hoped that a knowledge will be gained of the dispersive behaviour of metals about an exhalative feature such as an active mid-ocean ridge, which may be of significance in the field of marine mineral exploration.

3.1.2 Geological Setting, Sample Location and Description of Recent Sediments from the Central Indian Ocean Ridge.

3.1.2a Introduction

Eighty Recent surface sediments have been obtained for this study from the following three sources. 228

A set of sediments obtained from the West German mining firm of Pruessag and collected by the West German Research Vessel 'Valdivia' in 1973/(t, using piston and gravity coring techniques (for details of these techniques see Rona, 1972). A set of sediments obtained from the Lamont-Doherty Geological Observatory, New York and collected using a piston corer. A set of sediments obtained from the Scripps Institution of Oceanography California, collected during a series of cruises between April 1960 and October 1971 in the Indian Ocean. These were collected using gravity and piston corers.

All the samples are of Recent sediments from the crest and flanks of the Central Indian Ocean Ridge. The sample stations lie on a number of traverse which cross the ridge crest and generally trend at right angles to the crestal magnetic anomaly (Fisher et al, 1971; McKenzie and Sclater, 1971). Fig. 3.1.2a shows the location of the sample area within the Indian Ocean and Figs. 3.1.2b and 3.1.2c show the main bathymetric features and the location of the sample stations in relation to the bathymetry of the sample area. Table 3.1.2a contains the relevant station data, i.e. station position, water depth, depth sampled and brief sample descriptions.

3.1.2.b Geological Setting of the Central Indian Ocean Ridge.

The Central Indian Ocean Ridge in this area is composed of nine northwest trending, seismically active ridge segments offset by nine, subparallel, east-north-east trending active fracture zones, composed of transform faults giving rise to a complex bathymetry within the sample area (Fisher et al, 1971; McKenzie and Sclater, 1971). It has been possible using the results of investigations into bathymetry, magnetic anomalies and seismic activity to define the position of the ridge segments and the fracture zones and to determine the direction and amount of motion along the transform faults in the fracture zones (Fisher et al, 1971; McKenzie and Sclater, 1971).

North of 10°S, three fracture zones occur (see Fig. 3.1.2d), E-E', the Vityaz fracture zone (Udintsev, 1965); F-F'; and G-G', the Verna fracture zone (Fisher et al, 1971). They all have right lateral offsets and are seismically active. Between 10°S and 20°S four fracture zones occur, H-H'; I-I', the Argo fracture zone which has an offset of 90km w[s odE odE m01 ' ' 1e01

Fig 3.1.2a.

Location of the sample area within the Indian Ocean

' 50.E 701 901 230

Table 3.1.2a. Station Data of Recent Surface Sediments from the Central Indian Ocean Ridge.

Station/ Station Station Depth Sediment

Sample Position Depth Sampled Type

Number (m) (cros)

Valdivia Sediments (R.V. 'Valdivia', 1973/4)

451 H-01 14°29'S 61°23'E 2971 0-4 Pale to medium cream -02 35-40 foram-rich, calcareous -03 65-70 ooze -04 95-100 46GK-01 14°21'S 62°18'E 3724 0-3 Pale cream, foram-rich -02 40-45 calcareous ooze. Some SiO - radiolaria (?) 2 50GK-01 14°15'S 63°12'E 3440 0-3 Cream, foram-rich calcareous ooze -02 27-30 55GK-01 14°o4'S 64°31'E 3070 0-5 Pale cream, foram-bearing calcareous ooze -02 23-28 60GK-01 13°56'S 65°39'E 2883 0-5 Pale cream, foram-rich, calcareous ooze -02 21-26 69P-01 13°46'S 66°31'E 3856 0-10 Pale cream, foram and radiolarian bearing -02 50-60 calcareous ooze -03 140-150 73P-01 13°35'S 66°2o'E 4516 0-12 Medium cream to pale brown foram and radiolarian bearin5 -02 12-18 calcareous ooze Occasional -03 18-24 Mn micronodules. Detrital material and Fe oxide material. -04 24-30 and coatings -05 30-38 -06 38-48 80GK-01 13°16'S 69°32'E 3382 0-5 Pale cream foram-bearing calcareous ooze -02 27-32

Lamont Doherty Geological Observatory Sediments

V29-47 06°31's 64°31'E 4206 0-25 Pale orange, foram-rich chalk ooze V29-48 06°16'S 63°26'E 3882 0-41 Pale orange, foram-rich chalk ooze V29-49 10°00'S 63°33'E 3630 0-65 Pale yellowish brown chalk °o r,

RC12-320TW 06°36'S 47'48'E 4784 0-37 Grey orange radiolarian clay 231

Table 3.1.2a (cont)

Station/ Station Station Depth Sediment

Sample Position Depth Sampled Type

Number (m) (cms)

Scripps Institution of Oceanography Sediments 'Monsoon' Expedition (August 1960-April 1961)

MSN5OG 12°58'S 75°01'E 5226 0-3 Brown clay MSN51G 14°05'S 72°15'E 5197 0-3 Brown clay with lighter material MSN52G 14°54'S 70°12'E 3980 0-3 Buff, foram calcareous ooze MSN55G 17°48'S 62°40'E 3730 0-3 Buff, foram calcareous ooze

'Lusiad-Argo' Expedition (May 1962-August 1963)

LSDA-105G 05°40'S 66°36'E 4365 0-2.5 Watery White clay LSDA-106G 05°34'S 63°43'E 4090 0-2.5 Light buff calcareous ooze LSDA-107Ga 05°26'S 59°15'E 4010 0-2.5 White calcareous foram ooze LSDA-107Gb 05°26'S 59°15'E 3930 0-2.5 White calcareous foram ooze LSDA-108G 05°30'S 57°56'E 2550 0-2.5 White calcareous foram ooze LSDA-111G 09°53'S 56°32'E 3843 0-2.5 Watery buff granular foram oog, LSDA-113G 10°21'S 58°31'E 3495 0-2.5 White calcareous foram ooze LSDA-114G 10°36'S 59°52'E 2270 0-2.5 White calcareous foram ooze LSDA-117G 13°41'S 59°41'E 3900 0-2.5 Buff calcareous foram ooze

'Lusiad-Horizon' Expedition (May 1962-August 1963)

LSDH-9G 05°43'S 65°58'E 4250 0-3 Light buff calcareous foram ooze LSDH-11G 05°31'S 63°o4'E 4070 0-3 Light buff calcareous foram ooze LSDH-12G 05°38'S 6o°o2'E 4100 0-3 Light buff calcareous foram ooze LSDH-18G 09°49'S 56°29'E 3890 0-3 Light buff calcareous foram ooze LSDH-19G 10°26'S 57°52'E 3938 0-3 Light buff calcareous foram ooze LSDH-20G 10°29'S 59°23'E 2870 0-3 Light grey calcareous foram ooze 232

Table 3.1.2a (cont).

Station/ Station Station Depth Sediment

Sample Position Depth Sampled Type

Number (m) (cms)

'Dodo' Expedition (May 1964-December 1964)

DODO-117PG 18°21'S 64°04'E 3398 0-3 Pale buff calcareous foram ooze DODO-119PG 14°02'S 62°30'E 3698 0-3 Pale buff calcareous foram ooze DODO-121PG 12°16'S 62°50'E 3990 0-3 Pale buff calcareous foram ooze DODO-148V 23°20'S 67°05'E 3597 0-3 Pale buff calcareous foram ooze DODO-149V 22°28'S 68°03'E 3141 0-2 Light buff-cream foram calcareous ooze DODO-151G 21°14'S 69°26'E 3103 0-2 Light buff-cream foram calcareous ooze DODO-152V 20°38'S 70°08'E 3324 0-2 Light buff-cream foram calcareous ooze DODO-153V 20°23'S 70°20'E 3502 0-2 Light buff-cream foram calcareous ooze DODO-154V 19°24'S 70°47'E 3673 0-2 Light buff-cream foram calcareous ooze DODO-155V 18°32'S 70°13'E 3399 0-2 Light buff-cream foram calcareous ooze DODO-156V 18°02'S 69°30'E 3787 0-2 Light buff-cream foram calcareous ooze DODO-158V 17°29'S 67°29'E 3000 0-2 Light buff-cream foram calcareous ooze DODO-161V 16°17'S 65°21'E 3196 0-2 Light buff-cream foram calcareous ooze DODO-165V 12°55'S 64°10'E 4110 0-2 Buff calcareous foram ooze

DODO-166G 11°29'S 65°0o'E 3981 0-2 Light brown calcareous foram ooze DODO-167V 10°55'S 65°37'E 4065 0-2 Light brown calcareous foram ooze DODO-168V 10°31'S 66°03'E 3719 0-2 Light brown calcareous foram ooze DODO-169V 09°56'S 66°42'E 4290 0-2 Light brown calcareous foram ooze DODO-171V 09°12'S 67°28'E 3758 0-2 Light buff calcareous foram ooze DODO-172V 08°46'S 68°12'E 3452 0-2 Light buff calcareous foram ooze DODO-173G 08°19'S 69°o1'E 3977 0-2 Light buff calcareous foram ooze 233

Table 3.1.2a (cont)

Station/ Station Station Depth Sediment

Sample Position Depth Sampled Type

Number (m) (cms)

DODO-175V 06°22'S 69°53 1 E 3500 0-2 Light grey buff foram ooze

-DODO-177V 07°10'S 68°47'E 3816 0-2 Light grey buff foram ooze

DODO-179V 08°o6'S 67°34'E 3257 0-2 Light brown calcareous foram ooze DODO-180V 08°24'S 66°54'E 3339 0-2 Light brown calcareous foram ooze DODO-181V 08°49's 66°15'E 3512 0-2 Light brown calcareous foram ooze DODO-182V 06°23'S 65°34'E 4020 0-2 Light brown calcareous foram ooze DODO-184V 05°54'S 66°47'E 4359 0-2 Light brown calcareous foram ooze DODO-185G 05°43'S 67°27'E 3646 0-2 Light brown calcareous foram ooze DODO-186v 05°28'S 68°08'E 3562 0-2 Light brown calcareous foram ooze DODO-188V -05°01'S 69°23'E 3381 0-2 Light brown calcareous foram ooze

'Antipode' Expedition (June 1970-October 1971)

ANTP-12OPG 11°08'S 70°31'E 2827 0-3 White calcareous foram ooze

ANTP-123PG 11°08'S 70°291 E 2827 0-3 White calcareous foram ooze 55•E 57.E 59E 61.E 63'E 65.E 67.E 69.E 7 ~E 73.E ...... / _...._..... :7:, S'S SEYCHELI Fig. 3.1.213. PHYSIOGRAPHIC FEATURES _ ^:~ \ BANK r 6.S -0 ,,1 AGO t BANK of the

100 00 7's ~iego ~ CENTRAL INDIAN OCEAN RIDGE, arcia

5. to 2465

10.S

LEGEND

M AS CAR EN E

BASIN tt-13 S

14S

/NAZARETH BANK 15.S

I6S CARGADOS CARAJOS BAN K AL

~17 S IIJ AN k 18S A S nI N / ~ 0/1 r,■. 19'S 1011 Areas abave sea level

20S Trench-lake features of the ocean I floor, bellow 4000 metres

Submarine contours in r . union ∎ 21 S \---,-3000 thousand metre intervals

,//_ 22S /j,,~~'8~ S Map after Fisher et at (1971) les' /7 4, I S 6-E 58E 60.E 62'E 64% 66'E 70'E r 23'S

24S 235

55E • 57.E 59•E 61.E 63.E 6S'E 67'E 69.E 7L E 73• E S'S p1fki06 Fig. 3.1. 2c. LOCATION o LH 11 'i: '-'-i‘ :i ..1: 0 V29-48 SAMPLE STATIONS on V 0175 1) the

CENTRAL INDIAN OCEAN RIDGE

0181 J„ i' 9•S LH 18 ;3PLA114" _!! LH 19

NTP 120 11 4S 4TP 123 LEGEND

12 4', awl

SAMPLE STATIONS 13 S

• Valdivia Sediments

0 Lamont-Doherty Sediments 15.S

a ® Scripps Institution of Oceanography Sediments

MSN - Monsoon D - Dodo r 17•S ANTP - Antipode LA - Lusiad-Argo LH - Lusiad-Horizon MSN 55

M • 19 S 0 Areas above sea level

Trench- like features of the ocean dv_ floor, below 4000 metres

Submarine contours in 21.S ✓ ..3000 thousand metre intervals

1 Map after Fisher et al (1971) /rVP

I 56'E 58E 60'E 62'E 64‘E 66'E ` 14 70'E • ®0148 23. S

24S

68°E 236

(Fisher et al, 1971); J-J'; and K-K', the Marie Celeste fracture zone which has an offset of 220km (Fisher et al, 1971). All four are seismically active and have right lateral offsets (Fisher et al, 1971; McKenzie and 0 Sclater, 1971). The two southernmost fracture zones (south of 20 S), L-L' and M-M', the Rodriguez fracture zone, are different from the others to the north in having left lateral offsets. They are both seismically active (Fisher et al, 1971). The fracture zones terminate in the northeast against the Chagos-Laccadive Ridge and in the southwest against the Mascarene Plateau, suggesting that recent spreading along the Central Indian Ocean Ridge has separated these features (Fisher et al, 1971; McKenzie and Sclater, 1971; Deitrick et al, 1977). The Central Indian Ocean Ridge in this area is spreading at present at a rate of 2.3cms/yr. in a NE/SW direction thus giving rise to magnetic anomalies 1-5 (McKenzie and Sclater, 1971).

The present structure of this area and the rate and direction of spreading of the Central Indian Ocean Ridge result from the alteration in the direction of relative motion of the Indian and African plates. This changed from N/S to NE/SW about 35 m.y.B.P. (Anomaly 6, Oligocene/Miocene) (Fisher et al, 1971; McKenzie and Sclater, 1971, 1973). Prior to this date the line of the Central Indian Ocean Ridge was occupied by a N/S trending fracture zone, the Chagos fracture zone (Fisher et al, 1971), which together with a similar N/S trending feature further to the east along the line of the present Ninety East Ridge, allowed the northward movement of the Indian Plate (Fisher et al, 1971; McKenzie and Sclater, 1971, 1973). This change in direction of movement from N/S to NE/SW brought about the generation of a number of spreading ridge crest segments offset by fracture zones, i.e. the Central Indian Ocean Ridge, as has been suggested for the Pacific Ocean by At. water and Menard (1970) (See Section 1 for a fuller explanation of the geological evolution of this part of the Indian Ocean). The fracture zones with the right lateral offset were generated from the original Chagos fracture zone, while those with a left lateral offset derive from an original spreading centre (McKenzie and Sclater, 1971). The reconstruction of this part of the Indian Ocean (McKenzie and Sclater, 1971, Fig. 38) clearly shows the presence of such a ridge segment bounded by fracture zones which gave rise to those left laterally offset fracture zones (L-L' and M-M') on the present Central Indian Ocean Ridge. 237

S SE S 1~E 59 F 614E 6S.E 67.E 69•E 7 E 13E I i 1 lV Fig . 3.1.2d. STRUCTURAL FEATURES \\

of the

• CENTRAL INDIAN OCEAN RIDGE ,

•-•

`4000 5. to 24I S • 9S -% ti 10'` .••• ` ~p -too

11 5 LEGEND

Active fracture zones

13 S E-E Vityaz G-G' Verna I - I ' Args 145• K-K' Marie Celeste M-M' Rodaiiguez

• 15'S

16'S — a

175

Active ridje crest segments

• 19'S Area above sea level

6-21 Trench-like features of the ocean ?05— floor, below 4000 metres

Submarine contours in 21'S X3000 thousand metre intervals

12'S Map after Fisher et 4l(1971) r • 1 56'E 5dbE 60'E 62'E 64'E 66'E 70 E 72 E w 2;5

245 238

3.1.2c Valdivia Surface Sediments

These sediments lie on a traverse across the Central Indian Ocean Ridge in the vicinity of the Argo fracture zone (13°S). The sediments, with the exception of one core, are biogenic calcareous oozes, rich in foraminifera. The length of the cores is commonly less than 1 to 1.5 metres and they are uniform in composition. The sediments are poorly lithified and show little colour variation, being pale to medium cream throughout. It would appear that the sediments in these cores are the result of the accumulation of the calcium carbonate (CaCO3) skeletons of dead organisms on an elevated portion of the ocean floor (i.e. the Central Indian Ocean Ridge) lying above or near the level of the lysocline (Berger, 1967, 1976; Kolla et al, 1976b). A value of between 1 and 2cms/yr has been reported as the rate of carbonate deposition in this area of the Indian Ocean (Kolla et al, 1976b; McArthur and Elderfield, 1977). The crest of the Central Indian Ocean Ridge lies above the reported depth of the lysocline, 3900-4000 metres, for this area (Kolla et al, 1976b).

Station 73P is on the floor of the Argo fracture zone. The sediments from this core are less carbonate-rich than those samples from other stations taken on this traverse. Some poorly preserved species of foraminifera were observed, indicating that deposition below the lysocline (Kolla et al, 1976b) on the floor of the Argo fracture zone has been coincident with some partial dissolution of the calcareous tests, as has been observed here (Henderson, 1976, person. commun.) and reported previously (Berger, 1967; Andelseck and Berger, 1975). The samples from station 73P are pale to medium brown in colour and contain foraminifers,` although their proportion is lower in comparison to samples taken elsewhere on this traverse. Clayey material, unidentifiable detrital fragments (possibly mafic rock fragments), amorphous Fe oxide material and the occasional manganese micronodule are also present in this core. The core is less uniform, although not significantly so, than the other cores, and the sediments are poorly lithified. It would appear that these sediments are transitional between the more carbonate-rich varieties found on the crest of. the Central Indian Ocean (Kolla et al, 1976b; McArthur and Elderfield, 1977) and other mid-ocean ridges (Bostrtim et al, 1966, 1969) and those reported from the Red Sea by Bignell (1975) and the amorphous goethite facies described by Bischoff (1969) from the same area. 239

The predominant biogenic component is planktonic foraminifera which are present in all the samples, varying from 75 to 95%. Benthonic foraminifera are also common in samples from stations 45KH, 60GK, 69P and 73P. Radiolaria are relatively abundant (10%) in samples from station 73P and common in samples from stations 45KH, 60 GK, 69P and 80GK. They are absent from the samples of station 55GK. Sponge spicules, ostracod fragments diatoms, bryozoans and coccoliths (see Section 3.1.2g for further details and S.E.M. photographs of biogenics) make up the remaining biogenic components of the sediments.

Tables 3.1.2b and 3.1.2c give details of the species of foraminifera and other biogenic components present, and their frequency of occurrence. Further information on the biogenic components of all the Recent sediments together with Scanning Electron Microscope (S.E.M.) photographs, are given in Section 3.1.2g below.

3.1.2d. Lamont-Doherty Geological Observatory Sediments.

These samples are composed of a set of three samples from the western flank of the Central Indian Ocean Ridge and one from the Somali Basin. All are surface sediments. The sample from the Somali Basin (RC12-320TW) is a radiolarian clay. It is greenish brown in colour and is CaCO3-poor. The predominant organisms are radiolaria, together with poorly preserved, unidentifiable species of planktonic foraminifera. Manganese micronodules have been reported as occurring at this station although none were contained in the sample supplied to the author. The samples from the western flank of the Central Indian Ocean Ridge are foraminiferal-rich chalk oozes. They are poorly lithified and relatively uniform in colour, being pale cream to very pale yellow, orange or brown. They are of a similar origin to those carbonate-rich sediments collected by the research vessel 'Valdivia'. The main biogenic component is planktonic foraminifera and similar well-preserved species of Globorotalia, Globigerina and Globigerinoides to those found in the Valdivia sediments were recognised. Traces of benthonic foraminifera are also present.

3.1.2e Scripps Institution of Oceanography Sediments.

These sediments were collected on a series of five expeditions to the sample area between August 1960 and October 1971. All are surface sediments. 240

Table 3.1.2b. Frequency of Occurrence of Species of Planktonic Foraminifera in the Valdivia Sediments

Sample Numbers

ri C.) r'') ' r-{ CV H C'.: ri CV r- I CV r-1 CV rn ri CV t'r'1 SY lf\ N 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ::n` ,— I I I I I I I I I 1 1 1 ni I I I I I I I I I) I ;pedes L0 . U) 'p 'cc

G.lobi;erina bulloiFes A011 A A AA bulloices(d'Orbimny) A A A A A A A A A A A G. praecalida(Blow) A m A m S 0 m A A A A A A A A A <; G. e ,^'eri eggeri A A A AA AA A ® A Mumbler) --__ _— — "_.- G. bra.rlyi(Uietzler) A A A A A A A A G. cf•falconeneis A G. rubescens(Hofker) AA A A A —

Globirerinita glut n ta A A A A AA • (I.gter)

0lobigerinoides rulcr A A A A A A A A A A A A A A A m A A „ G. conglo:)_;tusa y) A a 12.A A A A® m A • G. cuadrilobatus A A @ a AA A A A U A A • • • A ims turus (Le Roy) G. quadrilobatus A m m A trilobus(Reuss) G. cuadrilobatus A A A Im m AM AA sacculifer( sra.d.y)

A A Sp_haeroidinella A A A A A m ® m m AA dehiscens(Parker S; Jones)

Globorotalia tumida AA A A • A a A m•• u a A• (Brady) G. oumilio(Parker) m a m n• • A m A■•• 0 A• m• m• nu G. fir riata(3r-,riy) m m a m ID e m A m m O a A m 0 Da 0 G. scituia(rady) • • A A A • A A A• A G. cultrata mon+.rdii • ♦ AA AA AA UU A U (Parker, Jones &. ī3rz. G. obesa. • A

G. ;.cost ensi_5 A A Saito)

G. t3.COStaen31S 11ermE-7.' )'".° A A A a, A (TakayanLgi & ito` G. hirsut'. Ir r) r.2r'bi rrn. A A A G. trunctuloides A

(d'Crbi mnv)■ 241

Table 3.1.2b. (cont.)

Sample Numbers

--1 :^.i M 'd' H N H CV H CJ H CV H N r•"1 H i\1 M V" if\40 H C•1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 I I 1 I I I 1 1 I 1 I I I I i l i t I i 1 i l Species C`J C C7 0 CN M V) 0 u, 0 w r- v In L l \-0

Globorotalia truncatulinoides p:.:.chy- AA theca(Blow) G. inflata(d'Orbi?;ny) A ♦ ®® A ♦

Globorotaloides hc: agon:.. R I ♦ ■ ® il e n n m ® ® I (Na-bland)

Turborotalita hu?nilis ® ♦ ♦ ♦ A A m 0 n 1si n MA (d'Orbizny)

Hatigerina siphoni era A A ♦ ♦ A A (ml'Orbigny ) H. nelagica(d'Orbi ny) ♦ A AA A A M M AA

Orbulin;. univerc ;. (m'.' Orbir'ny) Biorbulina b.._l;.ot . ® ♦ ♦ ♦ - (d'Orbigny)

Candiena nitida(n itida ) (d'orbic-ny)

Pulle niat ina. ♦ ♦ ♦ ♦ A ♦ ♦ obliauiloculata A AM M A ♦ (Parker, Jones & "rady)

Abundant -

C on-irnon -

Present - A

Absent -

242

Table 3 1.2c. Fr cu ncy of Occurrence of other Bio tie Components in the Valdivia Scdie:mte

Sample Numbers

we m w cwm wwwamm ww ry^) 0000000000000C)0000009 o 2 , i , , / , \ \ i / I l i j , , ! ! % Syeoiee 2 ' e i s e0 w cLC1 ;: ±

B] Sonic Foraminifera

Ammonia Cf. incica A A A Ammonia ammonia A Loticarinina p.uperata A s Cornuopira cor=u9ira a • 7isnurina fissurina Snistominella egistomin ll Bolivin bolivina A • A w Cancrio canons A . A Rosalint rolalina & & Discor ic siocorbis A & AA cassi.ulimm cassidulina A. • zmphicoryna »sicoryna Sp cell« eggerella ")mall 7a771utinating foraminifera

ot ' rs

Radio» ria ■ ■ A A A ■ ■ 0 ■ e Ootr co „,c7,-ments A e A

Diatoms A A S2on: npiculee A A Miliolid(intermcii e)

y as for Table 3.1.2b. 243

The samples collected during the Monsoon Expedition lie on a traverse to the south of the Valdivia sediments at about 15oS. One sample, MSN15G, is from the western flank of the Central Indian Ocean Ridge and two are from the eastern flank. A fourth sample is from the western edge of the Central. Indian Basin. The most easterly sample, MSN50G, is a medium brown radiolarian clay. Manganese micronodules were not observed to be present. The two westernmost samples, MSN52G and 55G, are foraminiferal.-rich calcareous oozes. They are pale buff in colour and are poorly lithified. Similar, well-preserved species of foraminifera to those found in the Valdivia sediments can be recognised in these samples. The sediments are similar to those found on the Valdivia traverse and are probably formed by similar processes. Sample MSN51G, lying between the carbonate-rich and carbonate-poor sediment types can be described as being transitional between them. It is a pale brown clay containing poorly preserved foraminifera as well as radiolarian remains. A stage of partial dissolution of the foraminifera is probably represented here, caused by the movement of the sample site through the lysocline towards the carbonate compensation depth (CCD) as a result of the process of sea-floor spreading (Berger and Winterer, 1974; Berger et al, 1976). The sediments collected during the Lusiad-Argo Expedition lie on two traverses down the western flank of the Central Indian Ocean Ridge at 5°S and 10oS. A further sample lies at 13oS on the western flank, on the edge of the Mascarene Basin. All the samples are white to light buff calcareous oozes. They are poorly lithified and contain abundant foraminifera of similar species to those recognised in the Valdivia sediments. The samples collected on the Lusiad-Horizon Expedition come from the crest and western flank at 5oS and the western flank at 10oS of the Central Indian Ocean Ridge. They are carbonate-rich, light buff coloured, foraminiferal oozes, similar to those samples collected by other Scripps Institution of Oceanography surveys in this area. They are poorly lithified and vary little in their sedimentological properties. The sediments collected by the Dodo Expedition come from a wide variety of sites within the sample area, ranging from 21°S in the south 244

of the area to the northern portion on almost the same latitude as the Seychelles Islands (4oS). The samples come from the flanks and in some cases the crest of the Central Indian Ocean Ridge and represent a number of traverses across the crest which are generally perpendicular to the strike of the central magnetic anomaly (McKenzie and Sclater, 1971). The samples are generally similar throughout the sample area and are light tan, buff or cream calcareous oozes, which are poorly lithified and rich in the tests of foraminifera. In the south of the area a certain amount of amorphous oxide material is detectable in the sediments, while elsewhere clayey material and detrital fragments make up the non-carbonate fraction of the sediments but the oxide material is generally lacking. The species of foraminifera contained in the sediments are similar to those reported from the samples taken along the Valdivia traverse. The two samples collected by the Antipode Expedition come from the eastern flank of the Central Indian Ocean Ridge in the vicinity of the Deep Sea Drilling Project Site 238, which lies at the eastern end of the Argo fracture zone. These sediments are white calcareous, chalk oozes composed almost entirely of foraminiferal tests and coccoliths. The foraminiferal species are generally similar to those recognised elsewhere in the sample area, although the cooler water forms referred to in 3.1.2g are relatively abundant in these samples.

3.1.2f General Comments.

The sediments from the sample area are generally similar in that they are carbonate-rich and contain abundant foraminifera thanatocoenoses (i.e. death assemblages). The non-carbonate portion of the sediments is generally clayey material and detrital fragments, which are probably of mafic rocks and volcanic glass, and occasional Fe (--Mn) oxide material and manganese micronodules. The siliceous material is predominantly radiolaria and occasional diatoms, but these together rarely make up more than io% of the biogenic components. These similarities strongly suggest that the sediments in this area result from the accumulation of the calcareous (and siliceous) tests of dead marine organisms on an elevated portion of the ocean floor, i.e. the mid-ocean ridge. The non-biogenic component of the sediments probably results from the combined processes of detrital sedimentation, weathering of volcanic material and the precipitation of metal oxide/ hydroxides from seawater under the influence of volcanic activity occurring along the crest of this active mid-ocean ridge. The exceptions to the above 245

general comments on the nature of these sediments have already been described in the preceding paragraphs.

3.1.2g Biogenic Components of Central Indian Ocean Ridge Sediments.

The biogenic components of the Valdivia sediments are in a better state of preservation than those in the other sediments. For this reason, they have been studied in more detail since the biogenic components of all the sediments are generally similar. Plates 3.1.2a to 3.1.2u are S.E.M. photographs of various biogenic components from the Valdivia sediments, and show the excellent preservation of these specimens. (It should be noted that in this context preservation does not refer to specimens which do or do not show any signs of partial dissolution). The captions to these Plates describe fully the subject matter they contain and therefore no further explanation is given here.

Examination of the foraminifera, using binocular and scanning electron microscopes, in all the samples has revealed a number of interesting features. Firstly, the forms generally show no signs, partial or otherwise, of dissolution, and are in an excellent state of preservation (see Plates 3.1.2a to 3.1.2u; Henderson, 1976, person. commun.). It would therefore seem that within this set of samples, which straddle the lysocline in this area, little detectable dissolution of calcareous material has occurred. The possible excpetions to this are the sediments from station 73P on the floor of the Argo fracture zone and sample MSN51G from the eastern flank of the Central Indian Ocean Ridge at about 14°S, in which partially dissolved forms occur. The depth of the lysocline, calculated on the basis of CaCO3 : Depth plots is put at between 3900 and 4000 metres in this area (Kolla et al, 1976'0 „ Secondly,there are present those species of foraminifera which are representative of water temperatures which are marginally cooler than those of the present day (i.e. temperate as opposed to warm temperate). The forms are Globigerina eggeri eggeri (Rhumbler), Globorotalia hirsuta (d'Orbigny) and Globorotalia truncatulinoides (d'Orbigny). These forms are present in the sediments from stations 46GK, 50GK, 55GK, 60GK, 69P and 8OGK, and are abundant in sediments from the floor of 73P, and the eastern end of ANTP-126PG and ANTI'-123PG, the Argo fracture zone. The 246

presence of such eurythermal forms as Globigerina bradyi Metzler) and Globigerina bulloides bulloides (d'Orbigny) in sediments from some of the stations listed above (see Table 3.1.2b) may also indicate cooler water conditions. The presence of those fauna indicative of cooler water conditions may result from the inflow of bodies of cooler water carrying these organisms, along such features as the fracture zones of the Central Indian Ocean Ridge. Kolla et al (1976a) has shown that bodies of Antarctic bottom water may possibly pass through parts of the Central Indian Ocean Ridge System at 20-25°S. A feature such as the Argo fracture zone may provide a zone along which such cooler waters may pass. Thirdly, many of the organisms are covered in a coating of a pale buff carbonate material which often obscures the otherwise opalescent tests of the foraminifera. This is not observed to occur on the non-biogenic components of the sediments and it is probably a feature of the life processes of the organisms, rather than the result of the precipitation from the pore fluids of the sediment after deposition (Henderson, 1976, person. commun.). Be (1965) has shown that planktonic foraminifera, e.g. Globigerinoides sacculifer (Brady), thicken their tests as an adaption to a deeper habitat by the secretion of a calcite crust on the test. This has its maximum development between 300 and 2000 metres (Be, 1965) and may account for the carbonate coating observed in these sediments. Coccoliths also occur on the surface of the foraminiferal tests in these samples, and these together with bryozoan fragments and plates of clay minerals are contained within the chambers of some of the species of foraminifera (see the relevant plates included with this section). These may have been ingested by the foraminifera during 'their life processes (Berger, 1974) or may have become entrained into the dead foraminifera as they settled through the water column, or during their period of accumulation on the ocean floor. 247

Plate 3.1.2a. (i) Globigerina bulloides bulloides (d'Orbigny) from station VA-45KH

Plate 3.1.2a. (ii) Surface of G. bulloides bulloides (d'Orbigny) showing the pores and postulae(a), with coccoliths (b) and diatom fragments adhering to it. 248

Plate 3.1.2b. (i) Globigerina praecalida (Blow) from station VA-50GK

Plate 3.1.2b (ii) Blocky surface of G. praecalida (Blow) showing coccoliths adhering to it, from station VA-73P 249

Plate 3.1.2c. (i) Globigerina eggeri eggeri (Rhumbler) from station VA-69P

Plate 3.1.2c (ii) Blocky surface of G. eggeri eggeri (Rhumbler) showing coccoliths (a) and diatom fragments (b) adhering to it. Plate 3.1.2d (i) Globigerina bradyi (Weitzler) from station VA-50GK

Plate 3.1.2d (ii) Coccoliths and small plate-like minerals (?clays) adhering to the surface of G. bradyi (Weitzler) 2b

Plate 3.1.2e. (i) Globigerina rubescens (Hoftier) from station VA-80GK

Plate 3.1.2e. (ii) Hexagonal pores containing coccoliths and diatom debris in the test of G. rubescens (Hoftier). Small plate-like minerals present - ? clays. Plate 3.1.2f. (i) Globigerinita gluttinata (Egger) from station VA-55GK

Plate 3.1.2f. (ii) Helmet shaped radiolarian in the valve of G. gluttinata (Egger) z~~

Plate 3.1.2f. (iii) Interior of G. gluttinata (Egger) showing benthonic forams (?)

Plate 3.1.2f. (iv) Coccoliths in the interior of G. gluttinata (Egger) Plate 3.1.2g. (i) Globigerinoides quadrilobatus immaturus (Le Roy) from station VA-80GK.

Plate 3.1.2g. (ii) Hexagonal pores in the test of G. quadrilobatus immaturus (Le Roy) Plate 3.1.2h. (i) G. quadrilobatus trilobus (Reuss) from station VA-69P

Plate 3.1.2h. (ii) Hexagonal pore surface of G. quadrilobatus trilobus (Reuss) with coccoliths filling the pores 256 I

Plate 3.1.2i (i) G. quadrilobatus sacculifer (Brady) from station VA-80 GK

Plate 3.1.2i (ii) Surface of G. quadrilobatus sacculifer (Brady) showing coccoliths, diatom fragments and diatom spicules (?) 257

Plate 3.1.2j (i) G. ruber (d'Orbigny) from station VA-S5GK

Plate 3.1.2j. (ii) Pores of the tests of G. ruber (d'Orbigny) showing radiolaria, diatoms, coccoliths and plate-like minerals-clays (?) 25u

Plate 3.1.2j. (iii) Pores of the tests of G. ruber (d'Orbigny) showing radiolaria, diatoms, coccoliths and plate-like minerals-clays (?),

Plate 3.1.2k. (i) Globorotalia pumilio (Parker) from station VA-69P 25. 9

Plate 3.1.2k. (ii) Postulae on the test of G. pumilio (Parker), showing coccoliths and diatom fragments

Plate 3.1.21. (i) G. cultrata menardii (Parker, Jones & Brady) from station VA-69P 266

Plate 3.1.21. (ii) Crystalline structure of the test of G. cultrata menardii (Parker Jones & Brady) with coccoliths adhering to it

Plate 3.1.2m. (1) G. fimbriata (Brady) from station VA-45KB L61

Plate 3.1.2m. (ii) Fragment, interior of G. fimbriata (Brady)

Plate 3.1.2m. (iii) Fragment, interior of G. fimbriata (Brady), with coccoliths and clay minerals. 262

Plate 3.1.2n. (i) G. tumida (Brady) from station VA-55GK

Plate 3.1.2n (ii) Surface of G. tumida (Brady), showing postulae and coccoliths. 263

Plate 3.1.2o. (i) Hastigerina siphonifera (d'Oribgny) from station VA-69P

Plate 3.1.2p. (i) Orbulina universa (d'Orbigny) from station VA-4511 264

Plate 3.1.2p. (ii) Surface of 0. universa showing coccoliths, fragments of diatom frustales, diatom spines

Plate 3.1.2p. (iii) Surface of 0. universa, station VA-8OGK, showing solution marks 265 '

Plate 3.1.2p. (iv) Orbulina universa (d'Oribgny) from station VA-80GK

Plate 3.1.2p. (v) Surface of O. universa, station VA-80GK, showing solution marks. . LO.:

Plate 3.1.2q. (i) Pulleniatina obliquiloculata (Parker, Jones and Brady) from station VA-69P

• Def

1 • J•

1W.,W .4

- Y

Plate 3.1.2q. (ii) Surface of P. obliquiloculata (Parker, Jones and Brady) showing dendritic solution marks, following crystal face edges, and coccoliths. a

'felt r iloPt lAgtAir -gt

Plate 3.1.2q. (iii) Surface of P. obliquiloculata (Parker, Jones and Brady) showing dendritic solution marks, following crystal face edges, and coccoliths.

Plate 3.1.2q. (iv) Pulleniatina obliquiloculata (Parker, Jones and Brady) from station VA-46GK 268

Plate 3.1.2q. (v) surface of P. obliquiloculata (Parker, Jones and Brady) showing postulae, fragments of diatoms and coccoliths

Plate 3.1.2r (i) Sphaeroidinella dehiscens (Parker and Jones) from station VA-80GK 269

Plate 3.1.2r (ii) Surface of S. dehiscens (Parker and Jones) showing pores

Plate 3.1.2r (iii) Surface of S. dehiscens (Parker and Jones) showing diatom fragments, coccoliths and plate-like clay minerals 210

Plate 3.1.2r (iv) Surface of S. dehiscens (Parker and Jones) with coccoliths

Plate 3.1.2s (i) l;adiolaria from. station VA -45Kx 271

Plate 3.1.2s. (ii) Close up of diatom, Stephanodiscus stephanodiscus

Plate 3.1.2t. (i) Radiolaria from station VA-69P

-J 272

Plate 3.1.2t. (ii) Surface of radiolaria from station VA-45KH showing diatom fragments

Plate 3.1.2u. (i) Coccoliths on surface of iron oxide fragment from station VA-73P plate 3.1.2u. (ii) Coccoliths on surface of iron oxide fragment from station VA-73P with diatom spines and clayey material

Plate 3.1.2u (iii) Coccoliths and diatom fragments on interior surface of unknown foraminifera from station VA-45KH 274

3.2 BULK GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE.

The samples were prepared and analysed according to the procedures described in Appendix A.1. The results have been recalculated to a carbonate free basis (CFB) according to the procedure described in Appendix A.4. The recalculation has been carried out for two reasons. Firstly, the presence of the high percentages of biogenic carbonate material in these sediments tends to dilute the trace metal values. It is therefore necessary to correct for this effect. (A fuller discussion of the carbonate free basis correction is given in Appendix A:4.). Secondly, to facilitate the comparison of the results of this study with already published carbonate free data from other areas. The results are reported on a CFB, with the exception of Ca. The geochemical variations discussed below are only those which are not explicable in terms of the analytical precision of the determination methods. Analytical precisions for the various elements are given in Appendix A.2.

3.2.1. Geochemical. Variations Across the Central Indian Ocean Ridge.

3.2.1a. Introduction.

The variations in composition of the carbonate-free fraction of the surface sediments across the Central Indian Ocean Ridge are discussed in terms of six traverses. The traverses are perpendicular or near perpendicular to the strike of the ridge crest (Fisher et al, 1971; McKenzie and Sciater, 1971). The positions of the six traverses, ridge crest segments and fracture zones which offset them in the sample area are shown in Fig:. 3.1.?c F. 3.1.?d. The distributions of metals in the carbonate free fraction of the sediments across the ridge crest along the traverses are plotted in Fig. 3.2.1b to 3.2.1g.

3.2.1b. Traverse 1. (23°16'S 66°50'E to 19°33'S 71°18'E).

Traverse 1 cuts the ridge crest to the south of the Rodriguez fracture zone, in the south of the sample area, but does not intersect this or any other fracture zone. The sediments of the ridge crest are enriched in Fe, Mn, Ba, Zn, As, Cr, Cu, Pb and Cd, the concentrations being some of the

• Ca

10-

Al Mg

1— Mn

0.1— •

•

• r • Cu

0.01—

0.001-

•

Thousands of metres below sea level D149 D1 1 0154 D152 D153 D11!

MOR

Fig 3.2.1b

Concentrations of Elements, in weight percent., in the Carbonate-free fraction of surface sediments along Traverse 1 (230 10'S 660 50'E ko 190 33'S 71 0 48'E) across the Central Indian Ocean Ridge. sl = Sea Level MOR = Mid Ocean Ridge Crest Segment

(Concentrations are plotted on log10 scale) 276

highest throughout the sample area. Chromium, Cu, Pb and Cd and to a lesser extent Mn also show a peak to the east of the ridge crest. Lithium and to a lesser extent Al and Ti are depleted over the ridge crest. The remaining elements show no significant variations.

3.2.1c. Traverse 2 (18°S 62°E to 13°50'S 72°30'E).

Traverse 2 cuts the ridge crest and the fracture zone J-J', a right lateral transform fault (Fisher et al, 1971). The fracture zone appears to have little or no effect on the metal distributions across the ridge in this area, the main controlling feature being the ridge crest itself. The elements Fe, Mn, Ba, Cr, Cu, Pb, Zn, Li and As are enriched in the crestal region with respect to either flank. Cadmium and also As show a peak to the west of the ridge crest, while Al, and to a lesser extent, Ti and Mg, are depleted in the crestal region.

3.2.1d. Traverse 3 (14°30'S 61°30'E to 13°S 70°45'E).

Traverse 3 is oblique to the strike of the axial magnetic anomaly (Fisher et al, 1971). It cuts the fracture zone I-I' and the crest of the Central Indian Ocean Ridge. This fracture zone, the Argo fracture zone, is a right lateral transform fault and offsets the crest of the Central Indian Ocean Ridge by 90km in this area (Fisher et al, 1971; McKenzie and Sclater, 1971). Unlike Traverse 2, the fracture zone in this area has a marked effect on the metal distributions. Iron, Al, Mg, Si02, Ni, Cr and to a lesser extent Ti are all enriched in the fracture zone, with respect to both the ridge crest and flanks. However, Ca, Ba, Co, Li and to a lesser extent Mn, As, Cd are depleted in the fracture zone with respect to both the ridge crest and flanks. The fracture zone appears to have little effect on the distributions of Zn and Cu, although these are higher in the fracture zone than on the ridge crest. Chromium, Cu, Zn and to a lesser extent Pb and Co are enriched over the ridge crest while Ti, SiO and to a lesser extent Al are depleted over the crest with respect 2 to the flanks. This enrichment is not as marked as the enrichments associated with the Argo fracture zone. The distributions of Fe, Mn, Ba, Mg, Ni and Li show relatively little variations between the ridge crest and flanks. Fig 3.2.1c. Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 2 (18°S 620E to 130 50'S 720'30'E) across i the Central Indian Ocean Ridge •

D158 J D181 MSN55 \ism\ \ ' 4 /4\ M S N 51 l

MOR

MOR = Mid Ocean Ridge Crest Segment FZ = Fracture Zone

Vertical Scale on section is thousands of metres below sea level. (Concentrations are plotted on login scale).

0.1

Co 0.01 Pb

Li Cd

0.001

0.0001

1 VA69 VA45 2 n55 VA60 VA80 VASO VA46 i19__...... A.. VA73 Ii ,si ii i/ e°

Fig 3.2.1d Concentrations of Elements, in weight percent., in the carbonate-free fraction of surface sediments along Traverse 3 (14° 30'S 61° 30'E to 130S 70° 45'E) across the Central Indian Ocean Ridge. MOR = Mid Ocean Ridge Crest Segment FZ = Fracture Zone sl = sea level

Vertical Scale on section is thousands of metres below sea level. (Concentrations are plotted on a log10 scale). 279

3.2.1e. Traverse 4. (11°30's 64°30'E to 5°10'S 70°40'E).

Traverse 4 intersects the ridge crest but does not cross any fracture zone. Mg, Mn, Cr, Cu, Zn and to a lesser extent Fe, Al, Ni and Co are enriched over the ridge crest. No elements are markedly depleted over the ridge crest, while the distributions of the remaining elements do not vary significantly. An unusual feature is the slight enrichment in Al over the ridge crest.

3.2.1f.. Traverse 5. (10°30'S 63°30'E to 6°S 70°30'E).

Traverse 5 cuts no fracture zones and is perpendicular to the ridge axis. Mg, Ba, Cr, Cu, Pb and to a lesser extent Fe are enriched in the sample to the west of the ridge crest with respect to the flank sediments. However, Ni (which is depleted over the crest) and Zn also show an enrichment on the western flank. Cd and Zn also show enrichment in the sample to the east of the ridge crest. The remaining elements are not significantly enriched across the ridge crest.

3.2.1g. Traverse 6 (7°S 63°E to 5°S 70°50'E).

Traverse 6 cuts both the ridge crest and the Vityaz fracture zone (Udinstev, 1965) which is a right lateral feature (Fisher et al, 1971; McKenzie and Sclater, 1971). On this traverse, as on traverse 3, the fracture zone appears to affect the element distributions. According to Fisher et al (1971) the portion of the ridge crest intersected by this traverse is to the east of the fracture zone. Nickel, Cr, Cu, Zn and to a lesser extent Pb, Li and Fe are enriched to the west of the crest, while Co and Cd are enriched to the east as well as to the west. Barium is depleted in this area and the remaining elements show no significant variations. No elements are depleted over the fracture zone, while the majority show no significant variations related to it. However, Mn and Ni, and to a lesser extent Cr, Pb and Zn are enriched in the fracture zone.

3.2.1h. General Trends.

It is possible to recognise from the metal distributions along the six traverses, three main groups of sediments. Fig

= emen: ,vei jht percent., in the carbonate-rrae fraction .3f ;urf,ica sediments along Traverse 4 (11 0 30'S 640 30'E to 50 10'S 700 40'E) across the Central Indian Ocean Ridge 100- •

10-

0.1- Ti

0.01-

0.001y

MOR MOR = Mid Ocean Ridge Crest Segment Vertical Scale on section is thousands of metres below sea level

(Concentrations are plotted on a log10 scale) 9 g.2.If Concentrations o' elements, in weight percent., in the carbonate-free fracti)n of surface sediments along Traverse 5 (10° 30'S 63° 30'E to 60S 70° 30 1 E) across the 100 Central Indian Ocean Ridge

Mg Al Fe

0.1-^

Ni Co 0.01— Zn

0.001

0.0001

D177 2- 0179 D1Q0

MOR

MOR = Mid Ocean Ridge Crest Segment Vertical Scale on section is thousands of metres below sea level

(Concentrations are plotted on a log 10 scale) PI Concentrations or Elements,. in 4eight percent., in :he carbonate--!--!e -action of surface sediments along Traverse 6 (7°S 63°E to 5°S 70° 50'E) across 100-' the Central Indian Ocean Ridge

10-

1—. • • •

•

0.1-

•

0.01-

0.001—

0186 D 8 D182 D185 V29/48 I n V!9/47 0184 ,!

FZ -E-E' MOR MOR = Mid Ocean Ridge Crest Segment FZ = Fracture Zone Vertical Scale on section is thousands of metres below sea level

(Concentrations are plotted on log10 scale) 283

The sediments occurring on the crest of the ridge segments, crestal sediments, are enriched in such elements as Fe, Mn, As, Ba, Ni, Cu, Cr, Zn, Mg and Pb and are also depleted in Al, Ti, SiO and Li. The sediments 2 from Traverse 1 are the best examples of crestal sediments in the area, although the pattern of enrichment is observed in total or in part along nearly all the traverses. The crestal sediments have characteristics similar to other active ridge sediments from other oceans, e.g. Pacific (Bostrdm and Peterson, 1966; Piper, 1973) and the Atlantic (Bostr8m et al, 1972; Horowitz, 1974a, b). The slight enrichment in Al, in the crestal sediments on Traverse 4 is atypical-of active ridge sediments. Horowitz (1974a, b) recorded high Al values in sediments from the Reykjanes Ridge and attributed them to the presence in the sediments of the products of basaltic weathering. The high Mg values in crestal sediments on traverses 4 and 5 may have a similar origin to the enriched Al values. High values of enriched elements, such as Cr, Mn, Cu, Pb and Cd on Traverse 1, Cd and Co on Traverse 6 and Cd, Zn and Ni on Traverse 5, to either side of the crestal zone, has been suggested by Horowitz (1974a, b) as being possibly caused by localised' metal addition on the ridge flanks.

Three traverses crossed fracture zones which offset the Central Indian Ocean Ridge (i.e. Nos. 2, 3 and 6). Fracture zone J-J' on Traverse 2 appears to have no effect on the metal distributions across the ridge. However, the sediments from the Argo and Vityaz fracture zones are distinctive and form the second group of sediments - fracture zone sediments. The fracture zone sediments from the Argo and Vityaz fracture zones are depleted in Mn, Ba, Ca, Co, Li, Cd and As and Mn, Ni, Cr, Pb and Zn respectively. In the case of the Argo fracture zone, a depletion in these elements, which might be associated with the lattice of the carbonate minerals, may be due to the accelerated dissolution of CaCO3 below the lysocline at the depths in the fracture zone (Kolla et al, 1976b). Such a process would then remove these elements from the sediments. The depletion of Mn and such trace elements h as Ni, Co, Cr, Pb and As may reflect unsuitable conditions (i.e. low E h p ) for the precipitation of authigenic Mn oxides in the fracture zone. The sediments from the Argo fracture zone are enriched in (and contain high values of) Fe, Al, SiO2' Mg, Cr and to a lesser extent Ti, Cu, Zn and Cd. Such enrichment may reflect the incorporation in the fracture zone sediments of the products of the weathering and breakdown of basalts. 284

TABLE 3.2.1a. Bulk Chemical Composition of Sediment Groups from the Central Indian Ocean Ridge.

1 2 3 4

( CaCO3* 65.61 85-47 75.73 76.96 ( Ca* 26.88 34.36 30.56 31.07 ( Al 8.12 2.38 2.70 3.18 Wt% ( Ti 0.39 0.18 0.18 0.20 ( SiO 16.05 9.14 11.09 11.04 2 ( Mg 7.31 2.35 2.29 2.81 ( Fe 6.86 4.31 2.24 3.19

( Ba 3650 6690 6990 6580 ( Mn 396o 7730 3840 4750 ( Ni 481 178 133 179 ( Co 68 118 77 85 ( Cr 533 539 375 ~r29 ppm ( Cu 306 319 223 254 ( Cd 7 13 10 11 ( Pb 71 92 71 76 ( Zn 168 161 130 141 ( Li 24 31 27 28 ( As 7 20 7 10

All data CFB, except * which are TSB

1. Fracture Zone Sediments (7) 2. Crestal Sediments (16) 3. Non-Crestal Sediments (46) 4. All Surface Sediments (69). 285

The third group of sediments, non-crestal sediments occur on the flanks of the ridge segments, have similar concentrations of CaCO3 to crestal sediments but contain higher concentrations of Al, Ti, Si02, Mg, etc and lower concentrations of Fe, Mn, Ba, As and other trace elements.

The average compositions of the three groups of sediments are tabulated together with the average of all surface sediments in Table 3.2.1a.

3.2.2 Vertical Geochemical Variations

All the sediments studied, with the exception of those collected by the R V 'Valdivia', are surface sediments. Samples other than from the surface have been obtained from those 'Valdivia' cores on Traverse 2 (see Fig. 3.2.1a). However, of the eight stations occupied by the R V 'Valdivia' only three had more than two samples taken from them and only one core is over a metre in length. Since all the cores are generally uniform in lithology (see Section 3.1.2c) any observable geochemical variations down the cores will be extremely small scale and of limited geochemical significance.

The bulk composition of the surface and sub-surface 'Valdivia' sediments, together with the mean value for each core and where applicable (i.e. where there are more than two samples per core) and the maximum and minimum values for that core are reported on a carbonate free basis, with the exception of Ca. Observation of these data show that although the metal distributions down the cores are not uniform, the variations which do occur, if not explicable in terms of analytical error of the determination method, are of little geochemical significance. Furthermore, in view of the lengths of the cores in question, such variations will not be considered any further. 286

3.2.3. Geochemical Variations within the Sample Area.

3.2.3a. Introduction.

Although the sample coverage of the area is far from detailed it has been possible to draw some conclusions regarding the regional variations of particular elements. The regional distributions of elements in the carbonate free fraction of the surface sediments are displayed in Figs. 3.2.3b to Fig. 3.2.3p. The regional distribution of CaCO3 is shown in Fig. 3.2.3a.

3.2.3b. Metal Distributions within the Sample Area.

CALCIUM CARBONATE (see Fig. 3.2.3a). The distribution of CaCO3 is relatively uniform and reflects the position of the lysocline throughout the area. The CaCO3 concentration is slightly higher over those uplifted portions of the ridge crest which are above the lysocline, such as in crestal sediments. The CaCO3 content is lower along the fracture zones and in the eastern part of the area. In these two areas the ocean floor lies below the level of the lysocline and the sediments are depleted in CaCO3 due to the accelerated rate of dissolution of CaCO3 below this level. In the east of the area, the ocean floor approaches the level of the calcite compensation depth (CCD), where the rate of dissolution exceeds that of precipitation of CaCO3 and siliceous and clay material predominate in the sediments.

CADMILM and LEAD (see Figs. 3.2.3b and 3.2.3c). The distribution of Pb and to a lesser extent that of Cd, are similar to that of CaCO3. Lead follows CaCO3 by showing lower values along the fracture zones and in the eastern part of the area, while it is highest in the crestal sediments in the southern portion of the area. Throughout the remainder of the area it has a relatively uniform distribution. The distribution of Cd is similar to that of CaCO3 in the north and the east of the area, however, throughout the remainder its relationship to CaCO3 is less clear, it being high along the fracture zones and being slightly depleted over certain parts of the ridge crest. The distribution patterns of Ph and Cd suggest that they may in part be incorporated in biogenic material. 287

71.E 55-E 57 E 59E 61'E 63'E 65'E 67'E 69.E 73'E 5'5 Fig 3.2.3a. DISTRIBUTION of CaCO3 in

SURFACE SEDIMENTS from the CENTRAL el INDIAN OCEAN RIDGE , 5' to 24°S

9.5

'—▪ _40 0 0

11 '5 LEGEND

moo Wt. percent. CaCO3

1; 5

© >85

15 S • Q 85 -75

IR~ — d © 75 -55

17 S 55 -35

• 35 -5

• < 5

• • 19 S

Trench-like features of the ocean floor, below 4000 metres

Submarine contours in 21'S ..,3000 thousand metre intervals

Map after Fisher et al(1971)

I i ' T • i 1 56'E 58 E 60fE 62'E 64'E 66'E•+!que J 9°~~ 70 E 72'E ' 23.5 •

24.5 55t 57E 59° E 61°E 63°E 65°E 67.E 69°E 71°E 73°E S • Fig. 3.2.3b. DISTRIBUTION of CADMIUM

• in the CARBONATE- FREE FRACTION

of SURFACE SEDIMENTS from tt,~

CENTRAL INDIAN OCEAN RIDGE , torro

0 400 5° to 24° S 9° S

4000 o? (>° 11'S LEGEND

1 2's—

ppm cadmium .p 13's

It'swm 0 > 30

✓ 1 s°s 0 30 — 20

20— 10 1F°ti — O • 10- 5 17.S . < 5

1»S—

Areas above sea level r 14'S idt

Trench- like features of the .41 ocean floor, below 4000 metres .) Submarine contours in j 2 I'S t ~3000 thousand metre intervals

Map after Fisher et al (1971)

1 1 56°E 58°E 60°E 62'E 64 E 66~E 70 E 72 E • • 23 S

24 5 289

55 E 57.E 59.E 61.E 63.E 65°E 67.E 69.E 71.E 73.E 1 ■ I t • i 1 S'S • l A / Fig. 3.2. 3c. DISTRIBUTION of LEAD

in the CARBONATE - FREE FRACTION

7.S of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S 9.5

11.S LEGEND

13.S ppm lead

>150

CO 150 - 100 15 S • 100 - 50 • < 50

17•S

Areas above sea level 19S

Trench- like features of the ocean floor, below 4000 metres

Submarine contours in 21*S 3000 thousand metre intervals

Map after Fisher et al (1971) 290

BARIUM (see Fig. 3.2.3d). Barium is relatively evenly distributed throughout the whole area. It is low in fracture zone sediments suggesting an association with biogenic material in a similar way to Pb, Cd and. Li. Barium is also high in crestal sediments in the north and south of the area, which suggests it may be related to ridge crest metal additions, as well as to biogenic sources.

COBALT (see Fig. 3.2.3e). Cobalt is not generally enriched in crestal sediments in this area. However, high values do occur in the north and south of the area, but these occur in the flank region and not along the ridge crest. Low values of Co occur in the Vityaz and Argo fracture zones, suggesting that if Co is associated with biogenic material it may be removed from fracture zone sediments by dissolution below the lysocline, in a similar way to Pb, Cd, Li and Ba.

LITHIUM (see Fig. 3.2.3f) Lithium is relatively evenly distributed throughout the area with two exceptions. It is enriched on the ridge flanks, but is low in the crestal region, in the north and south of the area. It is also low along the Vema and Argo fracture zones. The low values in the crestal regions may reflect a lack of detrital material, with which this element may be associated (Morowitz, 1974a, b). Such an association with detrital material may account for the moderately high values of Li in the northwest of the area, from the input of detrital material from the erosion of the Seychelles Plateau. Low values along the fracture zones, may be caused by loss of Li due to dissolution of CaCO3 below the lysocline in these regions. Oldnall (1975) has suggested that Li may be incorporated in biogenic material.

MANGANESE (see Fig. 3.2.3g). Manganese is relatively high throughout the sample area although it is enriched in crestal sediments to the south of the Rodriguez fracture zone and in the north central region. It is however, lower along the Argo, Vema and Vityaz fracture zones.

IRON (see Fig. 3.2.3h) The distribution of Fe contrasts strongly with that of Mn, it being enriched in crestal sediments throughout the whole area. This may reflect 291

S'S Fig. 3. ,3d. DISTRIBUTION of BARIUM

in the CARBONATE - FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24'S 9'S

11'S LEGEND

ppm barium

13.S © >10,000

© 10,000 - 7,500

IS•S © 7,500- 5,$00

Q 5,000 - 3,500

17.S • 3,500 - 200

Areas above sea level 19.S 41,

Trench-like features of the ocean floor, below 4000 metres

Submarine contours in 21'S 1/4,..3000 thousand mere intervals

Map after Fisher et al((1971) 292

S.5 Fig 3.2.3e. DISTRIBUTION of COBALT

in the CARBONATE- FREE FRACTION

of -SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S 9- S

11-S LEGEND

ppm cobalt

13-S >200

© 200-150

s's ® 150-100

• 100- 50

• <50 17•S

Areas above sea level 19-S

Trench-like features of the ocean floor, below 4000 metres

Submarine Lontours in 21.5 X1000 thousand metre intervals

Map after Fisher et al(1971) 293

Fig. 3.2.3f. DISTRIBUTION of LITHIUM

in the CARBONATE - FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S

LEGEND

ppm lithium

0 >40

40 - 30

• 30- 20

• 20- 10

Areas above sea level

Trench-like features of the ocean floor, below 4000 metres 4041

Submarine contours in 3000 thousand mete intervals

Map after Fisher et al(i1971) 55•F 57.E 59E 61.E 63.E 65.E 67.E 69'E 73.E 5S Fig. 3.2.3g. DISTRIBUTIION of MANGANESE

in the CARBONATE - FREE FRACTION

7.S of SURFACE SEDI MENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24'S 9~S

11.S LEGEND

ppm manganese

1 .J.S >10,000

© 10,000 — 7,530

1 s's 7,500— 5,010

• 5,000— 3,510

2,010 1 - s • 3,500— • < 2,000

Areas above sea level 19'S

Trench- like features of the ocean floor, below 4000 metres

Submarine contours in 21.S u3000 thousand metre intervals

Map after Fisher et al(1971) 295

S'S Fig.3.2.3h. DISTRIBUTION of IRON

in the CARBONATE- FREE FRACTION

1'S of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGL ,

5. to 24.S 9' S

11'S LEGEND

Wt. percent. iron 13 S

0 >6

O 6-3 15 S

• 3-1

• < 1 17.S

Areas above sea level 19'S

Trench- like features of the ocean floor, below 4000 metres

Submarine contours in 21'S t. 3000 thousand metre intervals

Map after Fisher et al (1971) 296

the well-recognised process of fractionation of Fe and Mn in the marine environment (Krauskopf, 1956, 1957). Iron is, however, at its highest concentration in fracture zone sediments from the Vema, H-H', Argo and Marie Celeste fracture zones, indicating that Fe-rich metal additions may be preferentially concentrated along the fracture zones with respect to the ridge crests.

ARSENIC (See Fig. 3.2.3i). Arsenic is distributed in a similar fashion to Fe, being highest along the ridge crest, particularly in the south of the area. This suggests that it may be concentrated along the ridge crest by processes similar to those responsible for the concentration of Fe. It is relatively evenly distributed on the ridge flanks but unlike Fe is lowest along the fracture zones, suggesting that like Mn, conditions were unfavourable for its precipitation along the fracture zones.

COPPER and ZINC (see Figs. 3.2.3j and 3.2.3k). Both Cu and Zn, show a very similar distribution and exhibit a marked enrichment along the ridge axis and to a lesser extent in the fracture zones. This is very similar to the distribution pattern of Fe, and suggests that all three elements may be concentrated in crestal sediments by similar processes.

MAGNESIUM (see Fig. 3.2.31) Magnesium is distributed relatively uniformly over the ridge flanks, it is slightly enriched on the ridge crest, as in the south and central parts of the sample area, but is highest along the fracture zones, in a similar pattern to Fe. Its enrichment in fracture zone sediments with Fe, may be related to the incorporation of the products of basaltic weathering in the sediments.

NICKEL and CHROMIUM (see Figs. 3.2.3m and 3.2.3n) Nickel and chromium show a fairly similar pattern of enrichment. They are concentrated in crestal sediments, particularly in the south of the area for Cr, while Ni is relatively uniformly distributed throughout the whole area. Although this pattern is similar to that of Cu, Zn, Fe, suggesting a similar source of enrichment, both Cr and Ni are concentrated most highly in sediments from the Argo and other fracture zones. The concentration of these elements in fracture zone sediments is accompanied by high Mg, Fe, Al and Ti values and their enrichment may be caused by the incorporation of the 297

Fig.3.2.3i. DISTRIBUTIION of ARSENIC

in the CARBONATE - FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE

5. to 24"S

LEGEND

ppm arsenic

Q > 40

Q 40 - 20

A 20 -15

15 -10

• 10 - 5 . c5

Areas above sea level

Trench- like features of the ocean floor„ below 4000 metres

Submarine contours in 3000 thousand mitre intervals

Map after Fisher et all (1971) 55L 57.E 59• E 61'E 63.E 65.E 6fE 69.E 71'E 73.E • ig.3.2.3j. DISTRIBUTION of COPPER

in the CARBONATE - FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S

LEGEND

ppm copper

> 400

400 - 300

• 300- 200

• <200

Areas above sea level

Trench- like features of the ocean floor, below 4000 metres

Submarine contours in 3000 thousand mitre intervals

Map after Fisher et at(.1971) ■ 56'E 58'E Fig. 3.2.3k. DISTRIBUTION of ZINC

R in the CARBONATE- FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S

10.5

LEGEND

17.S ppm zinc

0 >300

11'5 300 — 200

411 200 — 150

• 150 — 100

• <100

Areas above sea level

Trench- like features of the 1)is ocean floor, below 4000 metres

Submarine contours in U,3000 thousand mitre intervals

Map after Fisher et al (1971) 300

`, c Fig 3.2.31. DISTRIBUTION of MAGNESIUM

in the CARBONATE- FREE FRACTION

's of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S

11.S LEGEND

Wt. percent. magnesium • >6

• 6-3 • 3-1

Areas above sea level I 9•S

Trench-like features of the .41 ocean floor, below 4000 metres Submarine contours in 21'5 3000 thousand metre intervals

Map after Fisher et al (1971) ■ 55 E 61.E 63.E 65°E 67.E 69.E 71.E 1 t j I • Fig. 3.2.3m. DISTRIBUTION of NICKEL

in the CARBONATE - FREE FRACTION

of SURFACE SEDIMPITS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S

LEGEND

ppm nickel

>400

400 -300

300- 200

• 200- 100

▪ < 100

41P Areas above sea level

Trench-like features of the ocean floor, below 4000 metres

Submarine contours in 3000 thousand metre intervals

Map after Fisher et at((1971) S's ig. 3.2.3n. DISTRIBUTION of CHROMIUM

in the CARBONATE- FREE FRACTION

7'S of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGF ,

5. to 24.S 9.5

11'S LEGEND

ppm chromium 13.S Q >600

600 - 500

• 500-400

• 400- 300

17.S • < 300

Areas above sea level 19'S

Trench- like features of the ocean floor,. below 4000 metres

Submarine contours in 21'S '.-.3000 thousand metre intervals

Map after Fisher et al (1971) 303

products of weathering of basic and ultrabasic rocks which are exposed along the fracture zones (Engel and Fisher, 1975).

ALUMINIUM and TITANIUM (SILICA) (see Figs. 3.2.3o and 3.2.3p) Aluminium, titanium and silica, like Fe and Mg are concentrated in fracture zone sediments. Aluminium and to a lesser extent Ti are generally low in crestal sediments, probably due to the lower proportions of detrital material in these sediments. This is characteristic of active ridge sediments from other oceans (Bostr8m et al, 1966, Piper, 1973)- However, in the north of the sample area, Al and to a lesser extent Ti are enriched in the crestal sediments. This may be due to incorporation of the products of the alteration of basaltic material in these sediments.

It would appear that the geochemical variations within the area may be accounted for in terms of metal enrichment associated with the ridge crest and its fracture zones, and in the case Of some metals with distribution of CaCO3 and its dissolution due to variations in depth of the sediments with respect to the lysocline.

3.2.4 Metal Accumulation Rates.

The metal accumulation rates have been calculated using the method of Lyle and Dymond (1976) which is fully described in Section 2.2.3.

The dry bulk density data are calculated from the graph and equations presented by Lyle and Dymond (1976). The rates of sedimentation are those reported by Kolla et al (1976b) and McArthur and Elderfield (1977) for the deposition of carbonate sediments in the Indian Ocean. The results are presented in Table 3.2.4. Metal accumulation rates are shown for fracture zone, crestal and non-crestal sediments.

A striking feature of the metal accumulation rates (Table 3.2.4) is the high rates of accumulation of Fe, Ni, Al, Ti, Mg and to a lesser extent Cr in sediments from the Argo fracture zone. These values are comparable to those reported from the Marie Celeste fracture zone by McArthur and Elderfield.(1977)• The high accumulation rates from the Argo fracture zone, coupled with the bulk geochemistry of the sediments, strongly suggests the incorporation in them of the products of alteration and weathering of basaltic material. The high accumulation rates of these metals may also be 304

'S Fig.3.2.3o. DISTRIBUTION of ALUMINIUM

in the CARBONATE-FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5° to 24'S 9°S

11'S LEGEJID

13'S Wt. percent aluminium

> 6

15°S • 6-3

• <3

17.5

Areas about sea level 19'S

Trench- like features of the .41 ocean floor, below 4000 metres J. Submarine contours in 21.S ' 3000 thousand nitre intervals

Map after Fisher et 44(1971) • 55•E 57.E 59.E 61.E 63.E 65.E 67 E '.( 69.E 71.E 73•E s'S Fig.3.2.3p. DISTRIBUTION of TITANIUM

in the CARBONATE- FREE FRACTION

of SURFACE SEDIMENTS from the

CENTRAL INDIAN OCEAN RIDGE ,

5. to 24.S 9'S

11 S LEGEND

13.S Wt. percent, titanium

® >0.4

15S 0.4 —0.3

• 0.3 —0.2 • <0.2

17•S

Areas above sea level 19S

Trench-like features of the .0=5( ocean floor, below 4000 metres

$,' Submarine Qontours in 21.S 3000" thousand metre intervals

Map after Fisher et al(1971)

30€

TABLE 3.2.4 Metal Accumulation Rates of Indian Ocean Surface Sediments

SAMPLE NO. CaCO WET BULK WATER DRY BULK SEDIMENT= SEDIMENT CONTENT DENSITY DENSITY ATION CONTENT (P cm TY E Wt.% ( g cm- ) g -3) RATE (cros~ 1000 yrs)

1. 73P (Core 65.61 1.32 0.59 0.54 1.9 FZS from the Average) Argo Fracture Zone 2. Average C S 85.47 1.41 0.52 0.68 1.5 Average of 6 Crestal Sedime:

3. DODO-151 86.34 1.42 0.52 0.68 1.5 Crestal Sedim- ents from 4. DODO-152 90.29 1.44 0.50 0.72 1.5 ) Traverse 1 5. 69P-01 87.62 1.43 0.51 0.70 1.5 Crestal sediment from Trav- erse 3 6. Average NCS 75.95 1.35 0.56 0.59 1.5 Average of 46 non-crestal sediments

i From Lyle and Dymond (1976) From Kolla et al (1976b) and McArthur and Elderfiel. (1977)

FZS = Fracture Zone Sediments, C S Crestal Sediments; NCS = Non -crestal Sediments.

METAL ACCUMULATION RATES (gm/cm3/1000 yrs)

Metal 1. 2. 3. 4. 5. 6. 7. 8. Meto.&.

Fe 2.18 1.10 0.704 0.918 0.347 0.354 3.80 6.00 Fe Mn 0.114 0.166 0.140 0.156 0.051 0.071 0.55 0.65 Mn Ba 0.087 0.100 0.045 0.070 0.094 0.131 Ba Ni 0.019 0.002 0.001 0.002 0.004 0.001 Ni Cu 0.010 0.005 0.004 0.005 0.006 0.002 Cu Cr 0.022 0.007 0.008 0.008 0.007 0.004 - Cr Zn 0.006 0.003 0.002 0.002 0.003 0.003 Zn Pb 0.003 0.002 0.001 0.002 0.002 0.0005 Pb As 0.0002 0.0007 0.0004 0.0008 0.0002 0.00008 - As

Al 2.49 0.255 0.214 0.248 0.221 0.552 2.05 11.0 Al Ti 0.144 0.026 0.022 0.025 0.042 0.028 0.14 0.55 Ti. Li 0.0008 0.0004 0.0003 0.0004 0.0004 0.00004 - Li Mg 2.87 0.330 0.231 0.302 0.441 0.291 Mg

7. Marie Celeste Fracture Zone (Average), McArthur E Elderfield, 1977) 8. Pacific Pelagic sediments (Bostrl3m et al, 1973b) 307

due to the hydrothermal supply of these elements along the fracture zone. The low Mn and other trace element accumulation rates is the same order of magnitude as that reported from the Marie Celeste fracture zone by McArthur and Elderfield (1977) and lower than those reported from Pacific pelagic sediments (BostrOm et al, 1973b). The low Mn accumulation rate in the Argo fracture zone is also similar to that reported for Mn in Mn nodules from the world's oceans (Bender et al, 1970). This would suggest that the Mn is hydrogenous, being precipitated from seawater. This may be due to the absence of a hydrothermal Mn component along the fracture zone as is suggested by McArthur and Elderfield (1977) for sediments from the Marie Celeste fracture zone.

Although the accumulation rate of Fe is high in the Argo fracture zone sediments, it is of equal value or higher on other parts of the ridge crest of the Central Indian Ocean Ridge (see Table 3.2.4). The rates of accumulation of Mn, Ba, Cu, Cr, Zn, Pb and As are all highest in crestal sediments while the Al, Mg, and Ti accumulation rates are lower than in fracture zones sediments. The crestal sediments also exhibit higher accumulation rates of Fe, Mn, Ni, Cu, Cr, Zn, Pb, As and possibly Ba, Mg and about equal accumulation rates of Al and Ti than non-crestal sediments. The data in the case of Mn and other trace metals, suggest that these elements are accumulating in the sediments at rates that exceed those of normal hydrogenous precipitation. The same is true of iron. The values of Al and Ti suggest a similar degree of enrichment of these metals, from volcanic material, in both crestal and non-crestal sediments. The variability in metal accumulation rates in crestal sediments from different parts of the area suggests that metal-rich deposits on the Central Indian Ocean Ridge may be localised, which is in keeping with the findings from other mid-ocean ridges (Cronan, 1972; 1974; Horowitz, 1974a, b). In the south of the area, along Traverse 1, to the south of the Rodriguez fracture zone, the crestal sediments are enriched in Fe, Mn and associated trace elements and exhibit high accumulation rates of Fe and Mn which are slightly,greater than those reported from Pacific pelagic sediments (which may be taken as reflecting normal hydrogenous precipitation).

The metal accumulation rates in non-crestal sediments are equal to or less than those reported from the Marie Celeste fracture zone (McArthur and Elderfield, 1977), and Pacific pelagic sediments (Bostrtim et al, 1973b)as well as being lower than the rates reported for fracture zone and crestal sediment. 308

This suggests that the rates of metal accumulation in non-crestal sediments can be accounted for in terms of hydrogenous precipitation and basaltic weathering and have no hydrothermal components.

3.2.5 Data Handling - Correlation Coefficients and Factor Analysis

3.2.5a. Introduction.

The use of computers in the manipulation and interpretation of both geological and geochemical data is now well-established. Multivariate statistical techniques (correlation coefficients and factor analysis) have been employed to understand the interelement associations in the Indian Ocean surface sediments.

3.2.5b. Correlation Coefficients.

In order to understand the interelement associations within surface sediments from the Central Indian Ocean Ridge, correlation coefficients have been calculated for all pairs of elements (see Appendix A.4, for details of the computer programme used). In order to see if the interelement associations were specific to particular groups of sediments or common throughout the whole data set, correlation coefficients have been calculated for each group of sediments - fracture zone, crestal and non- crestal sediments, as well as for all surface sediments. The results are displayed in Tables 3.2.5a, b, c and d, in which only significant correlations at the 95% confidence level are recorded. Interelement associations for all surface sediments will be discussed and then differences which are specific to particular groups of sediments are discussed separately.

ALL SURFACE SEDIMENTS (see Table 3.2.5a). The most striking associations are the positive correlations between Fe and Mn, and of other trace elements, such as Ni, Cu, Co, Zn and As, with Fe and Mn, and the positive correlation of these trace elements with each other. This is similar to the association for Fe and Mn reported from the East Pacific Rise (Bostrdm and Peterson, 1969), and suggests that Fe and Mn and associated trace metals may be contained in the same mineral phases or may be in separate phases which are coprecipitated with each other e.g. TABLE 3.2.5a. Inter Element Associations in Surface Sediments from the Central Indian Ocean Ridge.

Fe Fe Al .65 Al Mn .75 Mn n = 69 Ni .62 .69 .37 Ni Co _ _ .36 - Co Cr - - - .26 .29 Cr Cu .75_ .38 .73 .46 .37 cu Cd - -.34 - - .29 .30 - Cd Pb - - - - .28 .35 - .24 Pb Zn .44 .29 .35 .51 .25 .36 .36 - .30 Zn Li - - .24 - •3 1 - - .25 .44 Li Mg .29 .44 - .70 - .44 - - - .43 - Mg Ba -.44 -.49 - -.41 - - -.26 - - - - _ Ba Ti .32 - .46 - .33 - - - .42 - .71 - Ti As .67 - .67 .34 .40 .38 .55 - - .44 .37 - - - As TABLE 3.2.5b Inter Element Associations in Fracture Zone Sediments from the Central Indian Ocean Ridge

Fe Fe Al .96 Al n = 7 Mn - - Mn Ni .82 .90 - Ni Co -.95 -.93 - -.87 Co Cr - - - .83 - Cr Cu - - .86 - Cu Cd ------Cd Pb - - - - - _ _ _ Pb Zn .93 .96 - .88 -.91 .78 _ _ _ Zn Li -.90 -.89 - -.89 .98 _ _ _ _ -.86 Li Mg .76 - .87 ------Mg Ba -.89 -.95 - -.91 .88 _ _ _ _ -.94 .88 -.86 Ba Ti .80 .86 - .92 -.86 _ _ - _ -.88 _ -.83 Ti As .99 .99 -.99 .99 Table 3.2.5c. Inter Elemert Associations in Crestal Sediments from the Central Indian Ocean Ridge.

Fe Fe Al - Al

Mo .93 Mn n = 16 Ni - Ni Co .43 - Cr .48 .53 - Cr Cu .49 - .40 .53 .49 • Cu Ccl - - Cd Pb - - - - .57 .47 - Pb Zn - .59 - Zn Li .54 - .65 .68 .72 - - Li Mg - - .48 - - - - Mg

Pa -.49 -.51 .40 - - - -.60

Ti - - - ' - .46 Ti

As .92 .86 - .53 .45 - .41 .S3 - -.44 As Table 3.2.5d. Inter Element Associations in Non-Crestal Sediments from the Central Indian Ocean Ridge

Fe' Fe Al .72 Al n=46 Mn. .71 .39 Mn Ni .45 .37 .69 Ni Co - .36 Cr, •.44 -.58 -.33 - - Cr Cu .67 .46 .73 .47 - -.41 Cd - -.44 - .30 .35 - Cd

Pb .. - - 30 0 .39 Pb .Zn .33 .30 .33 .58 .30 - .34 Zn Li .30 .49 _ - - - - .58 Li Mg - - - .33 .58 - - .48 .38 Mg Ba - .30 .31 .44 Ba Ti. - - - .33 - .44 .38 .61 - Ti As .54 .46 .43 .32 - - .40 RS 313

Pe ar.d Mn oxides, or may IA- „2 4 :=r,i e by :bz deposition of a thira phase, e.g. CaCO1. The positive correlation of Fe with Al, Mg and certain trace metals, e.g. Ni, Cu, Zn. is similar to that reported from the North Atlantic by Horowitz (1974a, b). This may reflect the incorporation of these elements in clay minerals by the alteration of volcanic material, end also in detrital basaltic fragments included :.n the sediments. The

negative correlation of Ba and Cd with Fe, Al and other metals may indicate. that these metals are associated with mineral phases, other than those mentioned above, such as barytes.

FRACTURE ZONE SEDIMENTS (see Table 3.2.5b) The striking feature of these sediments is the absence of a positive correlation between Fe and Mn, and that only Cu is positively correlated with Mn. It has been suggested that where the detrital. sedimentation rate increases, the correlation between Fe and Mn may disappear (Bostr8m et al, 1969), because of the diluting effect of the detrital phases. The absence of trace element correlations with Mn, may indicate that there 4s'less Mn oxide material present, due to the physiochemical conditions in the fracture zone being unsuitable (i.e. low Eh and p1') for Mn oxides to precipitate. There are strong positive correlations between Fe, AI, Mg, Ti and trace elements such as As,'Ni, Zn and Cr in fracture zone sediments. Such an association would tend to indicate the inclusion of detrital and volcanic material in these sediments. The association of trace elements with Fe, Al, Mg, Ti, etc may indicate their incorporation in clay minerals. Such associations indicate that Fe and Mn may be supplied by different sources in fracture zone sediments. Lithium, reported as being an indicator of detrital phases in active ridge sediments (Horowitz, 1974a, b) is negatively correlated with Fe, Al and Ti and is only positively correlated with Ba and Co.

CRESTAL SEDIMENTS. (see Table 3.2.5c) In crestal sediments, Fe and Mn are strongly, positively correlated together and also with such trace elements. as Cr, Cu, Li and As. There are also positive correlations between Ni, Co, Cr, Cu, Pb and Zn. Only Co is positively correlated with Al and positive correlations between Al, Fe, Ti, and Mg are absent. Such associations suggest that in crestal sediments, Fe and Mn, may be supplied by the same source, and that this is not related to. the inclusion in the sediments of basaltic detrital and other volcanic material. 3E4

NON-CRESTAL SEDI ENTS (see iyble 3. 5d ) The interelement associations in these sediments are broadly similar to those for all Indian Ocean surface sediments (see Table 3.2.5a). Iron and manganese are positively correlated together and trace elements such as Ni, Cu, Zn, Li and As are associated with them, possibly indicating a similar source for these elements. The positive correlation of Fe and Al, with such trace metals as Ni, Cu,-Zn and Li may indicate the incorporation of these elements into clay minerals, as a result of the weathering of basaltic material. There is also a positive correlation of Ba with Cr, Zn and Mg in non-crestal sediments. Such an association with Cr and Mg may indicate the presence of these elements in detrital mineral phases.

3.2.5c. Factor Analysis Factor analysis is a multivariate, statistical technique used to describe the total variance of a complex data set in terms of a few, statistically independent factors, which are composed of groups of variables.. The R-mode factor analysis programme used in this study and the procedures involved in factor analysis are described in Appendix A.4. A recent example of the application of factor analysis to aeochemical data from marine sediments is provided by Heath and Dymond's (1977) study. of.Nazca Plate. sediments. Using what the authors call Q-mode, but is in fact probably R mode factor analysis, they were able to describe metal enrichment in Nazca Plate sediments in terms of four factors or inputs - a hydrothermal factor, a -hydrogenous/detrital factor; a biogenic factor and a diagenetic factor, and were able to show how these varied regionally. R-mode factor analysis has been applied to the metal concentrations of the carbonate-free fraction of Indian Ocean surface sediments, according to the procedures described in Appendix A.4.

Such analysis has yielded three factors which together account for 96% of the variance within the data set. From the rotated factor loadings matrix, which is displayed in Fig. 3.2.4a, it is possible to determine which variables, i.e. elements are important in each of the factors. From the factor 'scores, i.e. the sample concentrations re-expressed in terms of the factors Zthemselves a linear transformation of the original variables) it is possible to determine the importance of each factor in each sample. The factor scores have been plotted for the three factors on a ternary diagram in Fig. 3.2.5b, and .on scatterr plots for each pair of factors in Figs. 3.2.5c 3.`2.5d and 3.2.5e. Fig

Rotated Factor Loadings Matrix resulting from R-Mode Factor Analysis on data from the carbonate-free fraction o. sediments from the Central Indian Ocean Ridge.

Factor I - Basaltic detritus

Factor 2 - Biogenic/Clay mineral

- 'do!canic,/Hydrothermal

Fe Al Mn . Ni Co Cr Cd F'b Zn Li Ma Ba Ti Factor 1, which accounts for 37.5% of the total variance is composed of Al, Ti, Ni, Cr, Zn and Mg and lesser amounts of Fie, Cd, Pb and Li. This is probably a volcanic (basaltic) detrital factor an.is of major significance in fracture zone sediments. It is also of minor significance in those crestal and non-crestal sediments from the northern-central part of the sample area along Traverses 3 and 4, and to a lesser extent along Traverse 5. It probably reflects the incorporation of the products of basaltic alteration and erosional breakdown, in the form of Ni, Cr, Ti, Mg rich detrital minerals such as silicates, in the sediments.

Factor 2, which accounts for 35.9% of the total variance is composed of Al, Fe and lesser amounts of Ni and Cu and opposed amounts of Ba, Cr, Cd and lesser amounts of Co, Pb, Zn and Li. The nature of this factor is not immediately clear. However, it is similar, with the exception of the absence of Ca, to the biogenic factor of DSDP basal sediments (see Section 2.2.4c). It is therefore probable that the opposed Ba, Cr, Cd, etc represent the biogenic component (Ca would associate with them if included), while a clay mineral (non-carbonate) component is represented by the Fe, Al and associated trace metals. This factor is of most significance in fracture zone sediments, where clay minerals are an important component and the proportions of CaCO3 are lower. It is of general equal significance in all . remaining crestal and non-crestal sediments, where some metals (Al,. Fe, Ni, Cu) in this factor may be in clay minerals and others (Ba, Cd, Pb, Co, etc) may be in the biogenic carbonate material.

Factor 3, which accounts for 22.3% of the total variance is composed of Fe, Mn, Cu and As with lesser amounts of Ni, Co and Zn. This is the hydrothermal/volcanic factor and probably represents the removal of these metals from the underlying basalts by leaching by circulating hydrothermal fluids followed by their precipitation as authigenic Fe. and Mn oxyhydroxides and Fe silicates in the sediments, together with the incorporation of some basaltic detritus (i.e. the Al). This factor is of major significance in all crestal sediments and is of minor local significance in some sediments from the Argo fracture zone and in occasional non•-crestal sediments at the eastern end of the Argo fracture zone and on the western flank of the Central Indian Ocean Ridge towards .che Seychelles Bank and over the Saya da Malha bank in the central pa.rt of the sample area.. 317

Distribution of factor scores for Factor i.(Basaltic Detritus), Factor 2 (Biogenic/Clay Mineral) and Factor 3. (Volcanic/ Hydrothermal) of Fracture Zone, Crestal and N6n-Crestal, Sediments from the Central Indian Ocean Ridae.

FACTOR 2 — Clay mineral/Biogenic

FACTOR 9 FACTOR 3

—Basaltic detritus — Volcanic/HydrotIQrmal

LEGEND

Fracture Zone Sediments

Crestal - Sēdiments

,Non-Crestal Sediments

(Factor scores have been calculated from the data of the crbonate-Free fraction of the sediments). Factor 2 — Biogenic

Fig 3.2.5c. 3.2.5d. and a e • 3.2.5e. ° •o 0 • o ® e 0 e a e • 0 • 0 0 a • 0 Distribution of Factor scores for Factor 1 • . p 0 • 0 (Basaltic Detritus), • 0 . • •0 0 (O 0 Factor 2 (Biogenic/Clay • • 0® 0 Mineral) and Factor.3 • (Volcanic/Hydrothermal) 0 0 0 •0 se of Fracture Zone, Crestal 0 ••@ ° • • and Non-Crestal Sediments • • from the Central Indian Ocean Ridge Factor 1 — Basaltic Detritus Factor 3 —Volcanic/Hydrothermal

0 0 0 0® (Factor scores have been 0 e e O calculated from the data of the carbonate-free ee00% ® fraction of the sediments)

• •eo o • 0 • • • •• .• • to 0 • • • • • •• • ® 0 • • • • e • °• • • •• 0 • • • • LEGEND 0 e • Fracture Zone Sediment-

O Crestal Sediments Factor 1 • Non-Crestal Sediments Fc.ctor 3

Factor 2 A study of Fig. 3.2.5b shows that el-though the composition of the various groups of surface sediments is not as strikingly different as the groups of Nazca Plate sediments reported by Heath-;and Dymond (1977) the three groups of sediments are distinctive. Fracture zone sediments show the importance of factors 1 and 2, while the crestal sediments show the dominance of factor 3, and to a lesser extent factor 2. The wider spread of non-crestal sediments is accountable for by the general equal importance of all three factors, although the slight dominance of factors 1 and 3 in sub-groups of the samples is shown by the slight clustering in this plot. The scatter plots of pairs of factors emphasises the differences between the three groups of sediments (see Fig. 3.2.5c, 3.2.5d and 3.2.5e).

The grouping of Fe and Mn in the same factor is similar to the findings of Heath and Dymond (1977) for Nazca Plate sediments, 'and probably indicates that in surface sediments Fe and Mn are in part supplied by the same source.

3.2.6 Discussion and Summary

It is possible at this stage to draw some preliminary conclusions regarding the nature of the three groups of Indian Ocean sediments and the possible sources of the elements that they contain.

The crestal sediments are enriched in Fe, Mn, Ba, As, Ni, Cu, Zn, Cr, Mg and Pb and in common with other active ridge sediments, are generally depleted in Al, Ti, SiO 2 and Li. Atypically for active•ridge sediments, •local enrichments in Al and Mg do occur. Heath and Dymond's (1977) study of•Nazca"Plate sediments showed that some Al could be of hydrothermal origin, while Bischoff and Rosenbauer (1977) have suggested that Mg may also be of hydrothermal origin. Horowitz (1974a) howe*.per., has suggested that Al and Mg may be enriched in N. Atlantic active ridge. sediments. due to the presence of, the products of basaltic weathering. In view of the generally equal accumulation rates of Al, Mg and-Ti in ores-U.1, as compared to non-crestal sediments 'it seems probable that the locally elevated •concentrations•of Al- and Mg are due to the incorporation of basaltic alteration products. Tha occ•culation rates of Mn, Ba, Cu, Cr. Zn, Pb and As are higher in crestal compared to non-crestal sediments, as well as beinc higher than accumulation rates for Pacific pelagic sediments. This would suggest `:ha4 these •me.a.ls in crestal sediments are enriched by an additional source, as compared 30

to non-crestal and pelagic sediments. The locally high accumulation rates of Fe in crestal with respect to non-crestal sediments suggests the localised addition of this metal in the crestal sediments also. The strong positive correlation of Fe with Mn and both with certain trace elements, e.g. Cu, Cr, Li, As and the lack of correlations of Fe with Al and of other elements with Al, suggests the incorporation of these elements (Fe, Mn, Cu, Cr, Li, As, etc) in precipitated Fe oxyhydroxides in these sediments. Furthermore, these data also suggest that both Fe and Mn may be supplied by the same source in crestal sediments or occur in different coprecipitated phases, and that their origin is independent of basaltic detritus and volcanic alteration products. The geochemistry of the crestal sediments is dominated by factor 3, the hydrothermal/volcanicy Fe and Mn factor, while the basaltic detrital factor (factor 1) and the clay mineral/biogenic factor. (factor 2) are of minimal significance in these sediments.

Non-crestal sediments have generally lower than average concentrations of Fe, Mn, Ba, As, Ni, Cu, Cr, etc and equal or above average concentrations of Al, Ti, SiO2, Mg and Li. The concentrations ofAl and Ti are generally greater in non-crestal as compared to crestal sediments, although local variations do occur. The accumulation rates of Fe, Mn and associated trace elements are generally "less,than those reported for sediments from the Marie Celeste Fracture Zone by McArthur and Elderfield (1977), less than the accumulation rates of crestal and fracture zone sediments, and generally less than those of Pacific pelagic sediments. This probably indicates that in general the non-crestal sediments do not contain a hydrothermal ^omponent and that the metals have accumulated in them by - hydrogenous precipitation from seawater and by the incorporation in the sediments of weathered volcanic and basaltic material. The positive correlations of Fe with Mn, indicating coprecipitation of Fe and Mn phases, and of Mn with certain trace metals (Ni, Cu and Zn) and of Fe with Al, Li, Ni and Zn suggests that in non-crestal sediments, Mn and associated trace elements may coprecipitate with,authigenic (Fe and) Mn oxides from seawater, while Fe and associated elements are added in the products of basaltic weathering. The geochemistry of the non-crestal sediments is in general not dominated by any single factor. This reflects the roughly e quaff interplay of metal additions from basaltic weathering (factor 1), clay mineral/biogenic concentration (factor 2) and precipitation of authagenic 321

Fe/Mn oxides (factor 3) from seawater, with or without local hydrothermal additions. Factors do, however, show local dominance as in the case of factor 1 in some non-crestal sediments from the centre of the sample area, and factor 2 in non-crestal sediments at the eastern end of the Argo fracture zone and to the west of the ridge-crest in non-crestal sediments from the vicinity of the Seychelles and Saya da Malha Banks.

Fracture zone sediments are generally more distinctive than crestal and non-crestal sediments. They are generally depleted in Ca, Mn, Ba and associated trace elements (Co, Cu, Pb, Cd, Zn) but are enriched in Fe, Al, SiO2, Mg, Ni, Cr and to a lesser extent Ti, Cu, Cd and Zn. The accumulation rates of Fe, Ni, Al, Ti, Mg and Cr are 5t.ex:At24 Than ih. crestal .and non- crestal sediments, and are comparable to metal accumulation rates recorded from sediments from the Marie 'Celeste fracture zone (McArthur and Elderfield, 1977). The accumulation rates of Mn and associated trace elements, however, are lower than those from the Marie Celeste fracture zone and from Pacific pelagic sediments. There is nc correlation between Fe and Mn in fracture zone sediments, no positive correlations of trace elements with Mn, except Cu,and strong positive correlations of Fe with Al, Ti, As, Ni and Zn. The above data may indicate that there is no hydrothermal Mn component in the fracture zone sediments and that Mn in these sediments results from hydrogenous precipitation from sea water. McArthur and Elderfield (1977) have suggested an absence, or low rate of supply of hydrothermal Mn in the fracture zones. The data also point to a possible separate source of Fe in fracture zone sediments, which together with the correlations of Fe with Al, Mg and the high accumulation rates of these metals, may probably be due to incorporation of basaltic detrital material and authigenic clay minerals in these sediments. Such observations are supported by the dominance of factor 1, the basaltic detrital factor in fracture zone sediments, together with factor 2, the clay mineral/biogenic factor, A.h ich is of lower significance in some fracture zone sediments and which together may reflect the local removal of Fe and other elements from basalts exposed along the fracture zone by hydrothermal leaching.

Manganese and associated metals appear to be precipitated from seawater throughout the area as authigenic Mn oxide:, while Fe is relatively uniformly distributed in all non-crestal sediments by the incorporation of basaltic weathering products. Both Fe and Mn and associated trace elements are probably enriched in crestal sediments by hydrothermal leaching of these metals from the underlying basalts and co-precipitation as Fe and Mn oxyhydroxides and possibly Fe silicates. Such processes probably account for . the non-separation of Mn from most of the Fe in crestal and non-crestal sediments. This is similar to the findings of Heath and Dymond (1977) for Nazca Plate sediments. In fracture zone sediments the Fe and Mn are separated, with the low concentrations of Mn probably being precipitated from seawater, while the enriched Fe values may result from inclusion of basaltic detritus and alteration products in the sediments, and perhaps in part by localised hydrothermal. leaching from the underlying basalts.

Another source of enrichment, although of minor importance, in Indian Ocean surface sediments may be the biogenic concentration of such elements as Pb, Cd, Co, Li and Ba. Barium may also be supplied by hydrothermal activity in crestal sediments, while Li may be linked to continental detrital material.

At this stage further discussions of the sources and modes of enrichment of elements in the sediments examined in this work, will be deferred until after discussion of the geochemical partition patterns of the elements described in the next section. 3.3. PARTITION GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE,

3.3.1 Introduction.

Marine sediments are composed of a number of mineralogical components some of which were initially recognised by Murray and Renard (1891), and which have become more well-known in recent years (Chester and Hughes, 1967). A study of the bulk chemical composition of sediments gives an overall picture of the relations of elements to one another in the sediment as a whole. However, it gives no indication of how the various elements are partitioned between the various component's of the sediments. A knowledge of the partition of elements is important in understanding the complex geochemistry of marine sediments and may give an indication of the processes by which elements are incorporated into the sediments.

In order to obtain a clear picture of the partition of elements in some sediments, it would appear best to separate the various minerals physically and then analyse them separately. However, the physical separation of the various mineralogical components of marine sediments by such processes as heavy mineral analysis, magnetic separation or hand picking individual grains under the microscope is extremely time consuming and may not be wholly successful in cases of very fine grained sediments, e.g. clays. To overcome this problem, chemical separation using particular reagents to dissolve particular mineral components, has been widely adopted. The techniques are relatively straight forward but the results are more difficult to interpret than the chemical analyses of individual mineral components.

Various workers have used a number of reagents to dealwith specific problems of chemical partition. Hirst and Nicholls (1958) used 25% (V/V) acetic acid (HAC) and complexing organic reagents to differentiate detrital and non-detrital fractions of carbonate rocks. Goldberg and Arrhenius (1958) used 5% EDTA (ethylene diamine tetra-acetic acid) to examine the partition of trace elements in Pacific pelagic clays. Chester (1965c) used a similar method to that of Hirst and Nicholls (1958). in a. study of reef and non-reef • limestones. Lynn and Bonahi (1963) used a one molar hydroxylamine so)ution 324

to examine the partitiM of Mn in marine sediments. However, it was not until 1967 that Chester and Hughes (1967) developed a technique to study the partition of trace elements in pelagic sediments . This technique, although modified in some respects, has been widely applied since. They - used a series of chemical attacks involving the use of various acids, reducing agents and combined acid/reducing agents in an effort to determine the partition of Fe, Mn, Ni and V between ferromanganese minerals, carbonates (excluding dolomite) and adsorbed elements on the surfaces of other minerals. They were able to show that in sediments of low carbonate content (i.e. pelagic clays) a 25% (V/V) acetic acid (HAC) solution will remove the carbonate minerals and trace elements from adsorbed sites as well as those contained in soluble Fe oxide minerals. Furthermore, that a combined acid/reducing agent (AIR agent), hydroxylamine hydrochloride in 35% (V/V) HAC, will remove all the above plus the reducible FeMn oxide minerals and their associated trace elements. Cronan and Garrett (1973) added a third leaching procedure using hot 50% (V/V) hydrochloric acid (HC1) which removed all but the most resistent silicates and alumino silicates and those insoluble Fe oxides (e.g. well crystalline Fe2 3)"

Further work (Chester and Hughes, 1969; Cronan and Garrett, 1973) on a variety of sediments has shown that Fe and Mn are removed essentially by different leaches. Manganese, with its associated trace elements is almost completely removed (98%, Cronan and Garrett, 1973) by the mixed A/R agent. While the Fe with its associated trace elements will be removed (nearly 85%, Cronan and Garrett, 1973) by the hot HC1 leach. Later work on surface and basal sediments has supported these findings (Sayles and Bischoff, 1973; Horowitz, 1974a, b; Cronan, 1976). Horowitz (1D74a) has shown by X-ray diffraction analysis of the partial attack residues that the combined A/R agent may partially attack some clay minerals in North Atlantic sediments.

No partition analysis hitherto has been reported on Recent Indian Ocean surface sediments. In this work, samples of the three major sediments types (fracture zone, crestal and non-crestal sediments) were subjected to chemical partition studies in order to investigate element partition patterns in these sediments associated with the ridge crest and flanks and fracture zones. The techniques used are those of Chester and Hughes (1967, 1969) as modified by Cronan (1976); 3 .3. 2 Results and Discussion

The sediments studied were those collected by Itihe R.V. 'Valdivia' from Traverse 3 (see Fig. 3.2.1a). The sediments were analysed according to the techniques described in Appendix A.1. The results are reported on a carbonate-free basis (CFB) with the exception of Ca. Table 3.3.2a contains the average composition of the three types of sediments - crestal, fracture zone and non-crestal sediments, together with the percentage of the bulk composition of each element removed by each partial attack. Only significant variations which are not explicable in terms of the analytical precision of the determination methods are discussed below.

IRON (see Fig. 3.3.2a) The partifim of Fe is generally similar in crestal and non-crestal sediments and different from that of fracture zone sediments. A feature of all the sediments is the negligible amounts of Fe removed by the HAC leach which supports the suggestion of Cronan (1976) that the HAC attack does not dissolve the Fe oxides and associated trace elements to the extent as originally suggested by. Chester and Hughes (1967). In crestal and non-crestal sediments between 29 and 33% of the total Fe is soluble in the A/R agent leach, while between 39 and 47% of the total Fe is soluble in the HCl. leach. In basal and surface sediments from other areas the majority of the Fe (up to 98%) has been reported as being associated with the HC1 soluble minerals (Dymond et al, 1973; Sayles and Bischoff, 1973; Horowitz, 1974a; Cronan, 1976). The higher proportions of Fe in the A/R agent leach in crestal and non-crestal sediments may indicate that in these sediments Fe is in an acid-reducible form. In this context, the results of some recent experiments (Hoorby, person. commun. 1978) are of significance.. Synthetically prepared samples of Mn02, FcOOH and a mixed Feign precipitate, together with naturally occurring goethite were subjected to A/R agent and HCl. leach procedures as described in Appendix A..1. It was found that the A/R agent dissolved 100% of the Mn02 precipitate, 99% of the Fe0OH precipitate, 99% of the mixed FeMn precipitate and only 2% of the goethite.. The HC1 leach completely dissolved (i.e. 100%) the Mn02, the FeOOH and the mixed Feign precipitates, together with 98% of the ioethite. X-ray diffraction investigations of the FeOOH _recipitete showed it to be essentially amorphous. These data-.suggest that in crestal and non-crestal sediments the acid reducible Fe may be present in the form of an amorphous Fe oxyhvdroxide 326

3.3.21,. and 3.3.2c.

5 Distribution of Fe, Mn and Ni wt. in thATartial Chemical Leaches of Crestal, Fracture Zone and 4 Non-Crestal Sediments from the Central Indian Ocean Rdige

(All data are expressed on a C.F.B.)

CS fzs ncs

25 PPm x100 20 LEGEND

15 HAC Lea.:.11

10 A/R Agent Leach only

HCI Leach only

CS fzs ncs Insoluble Residue

cs Crestal Sediments

fzs Fracture Zone Sediments

ncs Non-Crestal Sediments

ppm

x100

cs fzs ncs and/or possibly a mixed Fein oxide. Horowitz (1974a) has shown that in North Atlantic sediments the AIR agent may partially attack the clay minerals present, releasing such elements as Fe, A1jetc. However, only small amounts of Al (see below) are leached by the A 3 agent in these Indian Ocean sediments. In fracture zone sediments, the Fe is predominantly held in the HC1 leach (66%) suggesting that in these sediments, Fe is associated with such minerals as Fe smectites (Sayles and Bischoff, 1973) and oxides, e.g. goethite (Buser and Grutter, 1956). In all the sediments Fe is present in the HC1 insoluble phases. The absolute amount is small in crestal and non-crestal sediments, but in fracture zone sediments, 2.1% acid-insoluble Fe occurs, which represents 27% of the total Fe present in the HC1 insoluble phases of fracture zone sediments. This suggests the presence of Fe-bearing detrital - minerals in these sediments.

MANGANESE (see Fig. 3.3.2b) The partition of Mn is broadly similar in all the sediments and is markedly different from that of Fe, reflecting the well-established partition of these elements into generally different mineral phases (Chester and Hughes, 1967; Cronan and Garrett, 1973). Very small proportions of Min (up to 7% of the total) are removed by the HAC leach. This may reflect the leaching of Mn from Mn oxides or its removal from adsorbed sites on the surfaces of clay minerals (Chester and Hughes, 1967) or from the breakdown of Mn coatings on carbonate organisms in carbonate-rich sediments (Copeland, 1970). The majority of the Mn (60 - 84%. of the total) is concentrated in the A/R agent leach of all the sediments. This reflects the concentration of Mn in Mn oxide minerals in the form of coatings and micronodules and is in, agreement with previous results (Bischoff and Sayles, 1973; Cronan and Garrett, 1973; Horowitz, 1974a). A lower proportion of Mn (60% of the total) in the A/R agent leach of fracture zone sediments, together with the lower absolute concentrations, possibly indicated a lower rate of supply of the Mn to these sediments or that the physiochemical conditions in the fracture zones were not ccnducive to Mn oxide precipitation. The majority.of the Mn may have remained in solution and been precipitated outside the fracture zones where the Eh and pH would probably be higher. The proportion of Mn (18 and 14% of the total) in the HC1 leach of fracture zone and non-crestal sediments Tespectivel_y suggests that these lower concentrations of Mn may be held in the lattices of clay minerals in the sediments (Arrhenius, 1963; Horowitz, 1974a. The presence of Mn in the HC1 insoluble mineral (highest in fracture zone sediments - 21%) suggests that HC1 insoluble detrital material is present in these sediments and may be enriched in Mn. Sediments from the Reykjanes Ridge show a similar concentration of Mn in the HC1 insoluble phases (Horowitz, 1974a).

NICKEL (see Fig. 3.3.2c) The partition of Ni is generally similar in crestal and non-crestal sediments, and markedly different from that in fracture zone sediments. In crestal and non-crestal sediments 60% of the total Ni is removed by the HAC leach, little is associated with the A/R agent soluble minerals (FeMn. oxides) and with the HCl insoluble phases, while between 29 and 20% Ni in crestal and non-crestal-sediments respectively is associated with the HC1 soluble minerals. Previous work on metal-rich sediments from other areas has shown that Ni is predominantly associated with the FeMn oxide minerals soluble in A/R agent leach (Horowitz, 1974a, b; Cronan, 1976). However, Chester and Hughes (1967, 1969) and Chester et al (1976) have shown that Ni can also in part be related to the HCI soluble minerals of pelagic and near-basement metal-rich sediments respectively. The high values of Ni in the HAC leach of carbonate-rich crestal and non-crestal sediments may indicate that in part Ni is incorporated in the biogenic carbonate material of these sediments, while minor amounts may also be removed from adsorbed sites on the surfaces of clay minerals. In fracture zone sediments, the •.najority of the Ni (63%) is associated with the HC1 soluble minerals, and roughly equal amounts are. associated with the HAC and A/R agent leaches, and the HC1 insoluble residue (14, 10 and 13%) respectively). The. high concentrations of Ni in the HC1 leech suggests that it has been removed from the lattice of clay minerals and/or Fe oxides which are soluble in this reagent. Nickel has also been shown to be associated with HC1 soluble minerals it Sediments from near Iceland (Horowitz, 1974a) - although not to the sane extent as observed here in Indian Ocean fracture' zone sediments. The small proportion of Ni (13%) in the Hdl insoluble phases probably reflects the presence of detrital minerals in these sediments. If Ni is associated with biogenic carlJonūte material, the lower proportions in the HAC leach of these sedimer LF may be due to the dissolution of CaCO below 3 the lysoeline and the censequ?nt rt;.oval of Ni iron the lattice of the carbonate minerals. COBALT (see rig. 3.3.2d) The partition of Co shows some similarities to that of Ni and is generally similar in the three groups of sediments. The majority of the Co is removed from all the ediments by the HAC leach. This is highest for crestal and non-crestal sediments (60% and 52% of the total) where it probably reflects incorporation of Co in biogenic material. In fracture zone sediments less Co (35%) is soluble in the HAC leach, and although some of it may be removed from biogenic material some may be removed from adsorbed sites on the surfaces of clay minerals. In crestal and non-cresta] sediments additional amounts of Co may also be leached from Mn oxide minerals, along with Ni and Mn, by the HAC leach (Chester and Hughes, 1967). Up to 14% of the total Co is associated with the Mn oxide minerals, soluble in the A/R agent. This contrasts strongly with previously reported data for Co which links it with Ni and Mn in FeMn oxides (Cronan, 1976). Small proportions of Co soluble in the A/R agent leach of non- crestal sediments may, however, be removed from such minerals. Chester (1965a) has suggested that when Co is in the monovalent state in seawater as CoCl} and Co (OH){ it can substitute for H2 in the OH groups in the lattice of clay minerals. Lattice held Co would be removed by the HCI leach and would account for the proportion of Co (up to 23% of the total) in this leach. Such Co may be removed from Fe oxides and silicate minerals which are soluble in the HCI leach. An association of Co with HCl. soluble minerals is in keeping with the findings of Chester et al (1976) for North Atlantic near-basement sediments. A feature of the partition of Co common to fracture zone, crestal and non-crestal sediments is the amount of Co in the HC1 insoluble phases, 31, 27 and 29% respectively. This may reflect the incorporation into the sediments of Co-rich detrital minerals of a basaltic provenance.

CHROMIUM (see Fig. 3.3.2e) The partition of Cr in fracture-zone sediments is different from that in crestal and non-crestal sediments. In fracture zone sediments small proportions. of Cr are associated with the HAC and A/R agent soluble mineral phases (4 and 7% respectively). This probably reflect the removal of small amounts of this metal from biogenic carbonate material and/or removal from adsorbed sites and/or removal from amorphous Fe oxyhydroxide material soluble in the A/R agent. The majority 330.

Fig3.3.2d. ' .3.2e. and 3.3.2f.

Distribution of Co, Cr and Cu in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge Cs

(All data are.expressed on a C.F.B.) Cr Legend as for Fig 3.3.2a.

Gs fzs ncs

40-►

35-

30-

254 ppm 20 x10 15

CS fzs ncs of the Cr in fracture zone sediments is divided almost equally between the HC1 soluble phases and insoluble minerals (42% and 47% respectively). Sediments from the Reykjanes and Iceland-Faroes Ridges show a similar pattern of partition of Cr (Horowitz] 1974a). Bertine (1974) has suggested that Cr may be incorporated in sediments by the inclusion in them of unaltered basaltic fragments, which would be insoluble in HC1 and also by Cr becoming enriched in montmorillonites (soluble in HC1; Arrhenius, 1963) which result from the weathering and chemical alteration of basalts. Chester and Messiha-Hanna (1970), Horowitz (1974a) and Chester et al (1976) attribute the presence of Cr in the HC1 insoluble phases of North Atlantic surface and basal sediments to the inclusion of basaltic detrital debris. In crestal and non-crestal sediments, no Cr is associated with the HCl soluble phases and only moderate amounts (16-28%) are held in the HC1 insoluble residue suggesting that there is less Cr-rich detrital material in non-crestal and particularly in crestal than in fracture zone sediments. However, Cr is associated with the HAC soluble minerals (32-33%) indicating that in these carbonate-rich non-crestal and crestal sediments Cr may in part be incorporated into biogenic carbonate material and may also in part be held as adsorbed ions, and be leached from the surface of any reducible Fe oxide material present in the sediments. The majority of the Cr, 39-. 51%, particularly in crestal sediments is removed by the A/R agent. This may indicate that Cr, with other trace metals, may be associated with an amorphous Fe oxide which is soluble in the A/R agent. Bostrt;m and Peterson (1969) found in East Pacific Rise metalliferous sediments that Cris correlated with Mn, both of which are enriched in the A/R agent leach of crestal sediments in this area.

COPPER (see Fig. 3.3.2f) - The partition of Cu in fracture zone sediments is different from that of crestal and non-crestal sediments. •In crestal and non-crestal sediments, 45-47% of the Cu is removed by the HAC leach, which reflects its incorporation into biogenic carbonate material, its removal from adsorbed sites or its leaching from reducible • Fe oxide minerals. Little•.Cu is associated with the HC1 soluble phases (7-12%) and- 22-24% of the total Cu is contained in.. the HC1 insoluble residue, .indicating that although little Cu is associated with Fe oxides - and silicates of the HCl leach certain proportions of Cu mai- be related to basaltic detrital fragments- Between. 14 and 249i of the total Cu i held in the A/R agent soluble phases of the crestal and non-crestal sediments, indicating the incorporation of. Cu into reducible Mn. oxides and Fe oxide minerals. This is in common with findings from Pacific and North Atlantic sediments (Chester and Messiha-Hanna, 1970; Horowitk, 1974a; Chester et al,.1976; Cronan, 1976). In fracture zone sediments about 20% of the total Cu is HAC soluble indicating incorporation into biogenic carbonate material or removal from adsorbed sites and only small amounts of Cu (9%) probably associated with basaltic detritus, are in the HCL insoluble phases. Twenty-two percent of the total Cu is soluble in the A/R agent leach indicating its incorporation in Mn oxides or reducible Fe oxide minerals. The majority of the Cu in fracture zone sediments (56%) is held in the HC1 leach and is predominantly associated with the Fe oxides and to a lesser extent the authigenic clay minerals.

CADMIUM The partition patterns of Cd in _tha . orf ace. sediments vary within the analytical precision of the method used due to the very low levels of Cd recorded in these sediments. They are therefore not discussed further.

LEAD (see Fig. 3.3.2g) Lead is partitioned in a similar manner in all the sediments. In fracture zone, cresfal and non-crestal sediments the majority of the Pb (52%, 71% and 66% respectively) is removed by the HAC leach. The association of Pb with the calcareous biogenic component of marine sediments has been reported previously (Cronan and Garrett, 1973; Greenslate et al, 1973; Bostr8m at al, 1974). The lower absolute concentrations and - proportion: of Pb associated with the HAC soluble carbonate minerals of .fracture zone sediments may result from the loss of Pb from these sediments, due to the partial dissolution of the carbonate material below the lysocline in the fracture zone. In all the sediments Pb is virtually absent from the lICl leach, and only small amounts of Pb are associated with the HC1 insoluble residue, although this is highest in fracture zone sediments (up tc 25% of the total..). It has been shown that in metalliferous sediments, Pb may be derived by the inclusion in the sediments of basaltic detritus (Piper, 1973; Bertine, 1974). The inclusion of unaltered basalt fragments in fracture zone sediments may account for the Pb in the HC1 insoluble residue: TABLE 3.3.2a. Average Partial Compositions of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge.

FE MN NI CO CR 1 2 1 2

A 1250 4 319 7 168 6o 78 60 143 32 CRESTAL B 8180 29 3740 84 16 6 nil 0 227 51 SEDIMENTS C 13500 47 nil 0 82 29. 16 12 nil 0 (VA-69F) D 5780 20 415 9 15 5 35 27 73 16 E 28700 4470 280 130 443

FRACTURE A 509 (1 120 3 77. 14 23 35 21 4 ZONE B 4560 6 2340 60 55 10 7 11 -40 7. SEDIMENTS C 49300 66 713 18 34o 63 15 23 233 42 (VA-73P) D 20900 27 887 21 70 13 20 31 266 47 E 75200 4o6o 542 66 56o

NON- A 1710 7 265 6 145 6o 87 52 153 33 CRESTAL B 7390 32 3110. 71 28 11 23 14 174 29 SEDIMENTS C 8910 39 589 14 50 20 9 5 nil 0 (VA-45,46, D 4990 22 3;7 9 23 9 48 29 131 28 50,55,60, E 23000 4360 246 167 458 80) All data except Ca are CFB. 1 Average Concentration in ppm 2 Average Concentration expressed as percentage of bulk composition A... HAC Leach B A/R Agent leach only (i.e. A/R agent leach - HAC leach) C HC1 leach only (i.e. HC1 leach - A/R agent leach) D Insoluble Residue (i.e. Bulk - HCl leach) E Bulk Composition TABLE 3.3.2a (cont.)

CU CD PB 1 2 1 2 1 1 2

A 162 45 23 55 88 71 85 42 14 42 CRESTAL B 69 19 nil 0 33 19 21 10 4 11 SEDIMENTS C 42 12 4 10 nil 0 29 15 nil 0 (VA-69P) D 87 24 15 35 12 10 67 33 17 48 E 360 43 124 203 35

FRACTURE A 60 20 3 47 37 52 27 15 •4 16 ZONE B 67 22 .:1 2 14 19 28 15 2 8

SEDIMENTS C 172 56 '_ i8 8 11 75 42 9 39 (VA-73P) D 27 9 2 34 18 25 50 28 8 37 E 326 7 71 179 23

NON- A 102 47 16 50 77 66 112 49 13 38 CRESTAL B 52 24 <1 3 21 18 34 15 3 9 SEDIMENTS C 15 7 2 •6 6 5 13 6 1 3 (vA-45,46, D 48 22 13 41 13 11 69 ' 30 17 50 50,55,60v E 218 32 117 228 34- 80) BA CA AL MG SA 2

A 1420 18 324700 94 1540 7 17500 57 nil 0 CRESTAL B 700 9 nil 0 2710 12 2070 7 1210 42 SEDIMENTS C 3850 49. nil 0 11700 52 2470 8 893 21 ( VA-69P) D 1880. 29 23000 6 637o 29 866o 28 1140 40 E 785o 346700 22300 30700 3240

FRACTURE A 400 14 214300 83 1220 1 4620 nil 0 ZONE B 100 3 nil 0 2820 3 40 4:1 383 9 SEDIMENTS C 1560 58 nil 0 36500 41 49960 61 532 12" (VA -73P) D 700 25 45500 17 49000 55 2718o 33 340o 79 E 275o 259800 89500 8.1800 4310

NON- A 2860 27 316700 89 2250 8 14100 45 nil 0 CRESTAL B 180 4:1 nil 0 3980 15 400 1 1050 32 SEDIMENTS C 4680 46 5900 2 10200 40 4370 14 1040 31 (vA-45,46, D 2700 26 32000 9 9780 37 12730 40 1230. 37 50,55,60, E 10300 354600 26200 31600 3220 8o) In all the sediments, minor proportions of Pb (18-19%) are soluble in the AIR agent leach. This probably represents the association of Pb with reducible Mn oxides, as has been reported for ::fetal-rich sediments from elsewhere (Horowitz, 1974a; Cronan, 1976). Ta,absolute concentration of Pb associated with Mn oxides of the AIR agent leach of Indian Ocean sediments is lowest in fracture zone sediments.

ZINC (see Fig. 3.3.2h) Zinc is partitioned differently in fracture zone sediments as compared to crestal and non-crestal sediments. In crestal and non-crestal sediments the majority of the Zn (42-49% of the total) is soluble in the HAC leach, minor amounts are soluble in the A/R agent leach (10-15%) and the HC1 leach (up to 15%) while the remainder is held in the HC1 insoluble residue (30-33%). The-Zn in the HAC soluble phases may be incorporated into biogenic material or may bo removed from adsorbed sites on the surfaces of other minerals, where it may be concentrated in a similar manner to Co. In fracture zone sediments, the majority of the Zn (42%) is associated with the HCl. soluble minerals, equal amounts of Zn are removed by the HAC and A/R agent leaches (15%) and the remainder (28%) is held in the HC1 insoluble residue. The high proportion of Zn (42%) in the HC1 soluble phases of these sediments shows an association, which is common to Cu also, of Zn with Fe oxides and Fe silicates (e.g. smectites). Two features common to fracture zone, crestal and non-crestal sediments are the low proportions of Zn (15, 10 and 15% of the total respectively) associated with the A/R agent soluble Mn oxide minerals, and the higher proportions of Zn (28%, 33% and 30% of the total respectively) associated with the detrital minerals of the HCl insoluble residue. Cronan (1976) has reported that Zn in minor proportions may be associated with reducible Mn oxides in Pacific Ocean DSDP sediments, but is predominantly held in the HC; soluble phases. The Zn in the HC1 insoluble residues suggests. that Zn, like Pb, may also be added to all the sediments by the inclusion of unaltered basaltic detrital fragments from-the breakdown of tholeiitic material.

LITHIUM (see Fig. 3.3.2i) The partition patterns of Li are different in fracture zone -edimerts - as compared to crestal- and non-crestal sediments. 337 120

105

90 Fig 3ti3.2g. 75 3..2.h. and 3.3.2i ppm 60

45 Distribution of Pb, Zn and Li in the Partial Chemical 30 Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean 15 Ridge

CS ncs

(All data are expressed on

24 a C.F.B.)

21 n Legend as for Fig 3.3.2a. 18

PPm 12 x10

CS fzs ncs

35 Li

PPm 20

15 In fracture zone sediments Li exhibits a partition pattern similar to that reported for sediments from the R.eykjanes and Iceland-Faroes Ridges (Horowitz, 1974a). It is chiefly held in the HCl. sc4uble and insoluble mineral phases (39 and 37% respectively). A negligible amount is associated with the A/R agent leach while minor amounts (16%). are removed by the, HAC leach probably from adsorbed sites on the surfaces of clay minerals. The partition of Li between the HC1 insoluble residue and the HC1 soluble phases suggests that the conclusion of Horowitz (1971 a) may be correct, i.e. that Li is of detrital origin. Detrital in this sense must mean the inclusion of fragments of unaltered basaltic detritus containing Li, since the fracture zone sediments in the sample area are remote from a continental detrital source. In crestal and non-crestal sediments Li again shows a strong affinity with the detrital component of the sediments (48-50iū in the HC1 insoluble residue). This may be due to the inclusion of unaltered basaltic detritus. However, in the north of the sample area additional continental detritus containing Li may be added to the sediments from the erosion of the continental granitic rocks of the Seychelles Bank (Laker, 1963). Negligible amounts of Li are associated with the A/R agent and HCl leaches. The remainder of the Li (38-42%) is associated with the HAC soluble minerals. This Li may be removed from adsorbed sites and may in part be incorporated into the biogenic carbonate material of these sediments.

BARIUM (see Fig. 3.3.2j) The partition patterns of Ba are generally similar in all the sediments. Negligible amounts of Bc_ are associated with the A/R agent leach, suggesting that Ba is unsuitable for substitution in the lattice of ;educible FeMn oxides. The majority of the Ba in fracture zone, crestal and non-crestal sediments (58%, 49% and 46% of the total, respectively) is held in the HC1 soluble minerals. This may reflect its removal from the lattices of authigeni clay minerals or the removal from authigenic barytes which may be enriched in the sediments, particularly in the fracture zone sediments. Up to 27% of the total Ba is held in the HAC soluble phases and this may reflect the incorporation of Ba into biogenic carbonate material as well as removal from adsorbed sites on the surfaces of clay minerals. The proportion of Ba removed by the HAC leach is lowest in fracture zone sediments. 339

Fig 3.3.2j. 3)3.2k.and 3.3.21.

Distribution of Ba, Ca and Al in the Partial Chemical Leaches of Crestal, Fracture Zone and Non-Crestal Sediments from the Central Indian Ocean Ridge.

All -data except Ca are expressed on a Ca data on a T.S.B.) •

Legend as for Fig 3.3.2a.

AI Between 24 and 26% of the total Ba is held in the HC1 insoluble fraction of all the sediments. This may reflect the incorporation in the sediments of unaltered basaltic detrital material, containing Ba. Cronan (1976) has reported that for Pacific Ocean bakal non-carbonate sediments, Ba is predominantly associated with the acid-insoluble residue. In Indian Ocean sediments Ba is predominantly associated with the HC1 soluble phases, possibly as authigenic barytes, the Ba in which:k may be supplied from biogenic sources in these carbonate-rich sediments.

CALCIUM (see Fig. 3.3.2k) The partition of Ca is similar in all groups of sediments. The bulk of the Ca (83-94% of the total) is removed by the HAC leach, from the large quantities of biogenic carbonate material present in all the sediments. No Ca was associated with the A/R agent leach in any of the sediments and only 2% of the Ca was soluble in the HCl leach of non-crestal sediments. This would suggest that in all the sediments amounts of phosphatic fish debris, which contain Ca, which has been reported as a common constituent. of pelagic sediments, are absent (Dymcnd et al, 1973; Cronan, 1976). In the non-crestal sediments, the small proportions of HC1 soluble Ca are probably removed from authigenic clay minerals. Up to 17% of the Ca occurs in the HC1 insoluble residue of the sediments.: This proportion is highest in fracture zone sediments and it probably reflects:. the presence of Ca-bearing detrital minerals, such as Ca-plagioclase feldspars_ from the erosion of basaltic material. The presence of such basaltic detrital phases has been inferred in sediments from the Reykjanes and Iceland Faroes Ridges (Horowitz, 1974a).

ALUMINIUM (see Fig. 3.3.21) The partition of Al is similar throughout all the sediments. Up to 8% Al is removed by the HAC leach, while up to 13% Al is removed by the A/R agent leach. This Al is probably removed from colloidal AlOOH and possibly •also from the surfaces. of authigenic clay minerals present in the sediments. This pattern is best seen in crestal and non-crestal sediments: while only negligible amounts of Al occur in the HAC and -A/R agent leaches of fracture-zone sediments. The majority .nf the. Al is partitioned between the Hal soluble. (47,-52%) •and insoluble (29-55%) phases. In fracture zone sediments the majority of the- Al i 55%) is i1 the insoluble residue and lesst r amounts (4I%) in t:+e HCt •leach, while the reverse is the case for crestal and nor-crestal sediments., This pattern suggests that Al, like Fe, occurs in authigenic and detrital clay minerals and other detrital components of the sediments.

MAGNESIUM (see Fig. 3.3.2m) The partition of Mg in fracture zone sediments is different to that in both crestal and non-crestal sediments. A feature common to all the sediments is the negligible amounts of Mg removed by the A/R agent leach, suggesting its absence from such minerals as reducible Fe and Mn oxides. In fracture zone sediments, minor amounts of Mg (6%) are held in the HAC leach, while the majority of the Mg is partitioned between the HC1 soluble (61%) and HC1 insoluble (33%) phases. This is similar to the pattern reported for North Atlantic basal sediments (Horowitz, 1974a) and probably reflects the incorporation in the sediments of the products of\ basalti .c alteration e.g. clay minerals and from the erosional breakdown of basalt and inclusion of unaltered basaltic fragments containing Mg-rich detrital minerals and Fe Mg silicates. In crestal and non-crestal sediments, minor amounts of Mg (up to 14%) are: associated with the HC1 soluble phases, while between 28% (crestal) and 40% (non-crestal) of the Mg is in the HC1 insoluble residue, thus indicating the incorporation in these sediments of unaltered, Mg-rich, basaltic detrital fragments. The majority of the Mg in both crestal (57%) and non-crestal sediments (45%) is held in the HAC leach. In these carbonate-rich sediments, the majority of this Mg is probably removed from high Mg calcite which is a common constituent of Recent carbonate-rich sediments from the Indian Ocean (Wiseman, 1965); although some may also be removed from ion exchange sites on the surfaces of clay minerals and from quantities of sea salt present in these Recent sediments.

TITANIUM (see Fig. 3.3.2n). The partition of Ti in fracture zone sediments is different to that in crestal and non-crestal sediments. However, a feature common to all sediment types is the absence of Ti ffom the HAC leach. In fracture zone sediments, the majority of the Ti (79%) is held in the HC1: insoluble residue while negligible amounts are contained in the A/R agent and HCl leaches. This suggests that in these sediments, Ti is contained in unaltered basaltic fragments. 342

1----

6· ) Fig 3\3.2m. and '\ 3.3.2n. wt. • 4- 7. ~ ~ Distribution of Mg and Ti' in the Partial Chemical Leaches of Crestal Fracture Zone and Non-Crestal Sediments from the Central IndiJn Ocean 1.: ..:.··...... :;..:::·:.;: .. ;::;.·::.:.: ..;..... ::,: ...... :.:..:;.::;.::.: ..:.:'::.::::::'.: ... Ridge. :·i.:~·:::::.:~; ~::::.!::::::::~: ::~ ....:...... ______:~::~~:i:: •. :::·~~ __~~~~--~~;:.::;::~·.~::::.:·.- .:·....~::.:~:.::~.:::.~~:::j.· ~.:~:::~::.::.f:.: .. 1m.M};~~ o cs fzs ncs

;,. -- .. -

.. -'" -,.."" :1.' .r --,~ .. --- 6';~ __ ..-o: .... .t.

All data expressed on'a C.F.B.

Legend as for Fig 3.3.2a .

• 0· -- L -______.. -.--'~ .. _... _.- ...... :..-... -. ",--- - -'" . "-

N.B. Ti was not determ;ned in the HAC leach.

-. In crestal and non-crestal sediments the majority of the Ti (40% and 37% of the total) is held in the HCl i°-soluble residue, probably in a similar fashion to the partition of Ti in these phase in fracture zone sediments. However, unlike fracture .zone sediments, larger proportions of Ti are held in the A/R agent. leach (42 and 32%) and in the HC1 leach (31 and 31%) of crestal and non-crestal sediments. The association of Ti with the A/R agent leach has not been reported from any other area. It may reflect an association of Ti with reducible FeMn oxides or the leaching of Ti from the detrital, Ti-rich phases present in the sediments. The Ti in the PC1 leach is probably contained in clay minerals formed by the chemical alteration of basalt and as such supports the conclusions of Emelyanov (1974) who suggested that Ti will be concentrated in areas of deposition of volcanogenic sediments associated with the submarine. weathering of basaltic material.

3.. 3.3 Summary

The partition patterns of Mn, Pb, Ba, Ca, Co and Al are similar in all fracture zone, crestal and non-crestal sediments. Calcium, cobalt and lead are predominantly associated with the HAC leach in all the sediments reflecting their removal from biogenic carbonate material. Manganese is predominantly associated with the AIR agent leach in all sediments (although the proportion is lowest in fracture zone sediments) which reflects its incorporation in authigenic reducible Mn oxides. Aluminium and barium are predominantly associated with the HC1 soluble and insoluble phases. This reflects the association of both these elements with basaltic detrital material and inclusion in the lattice of clay minerals, and the presence of aū•thigenic barytes. The partition patterns cf the other elements in fracture zone sediments are different compared to those of crestal and non-rrestal sediments.

The partition patterns of crestal and•non-crestal sediments are similar to one another and may reflect the view that active ridge, i.e. crestal, sediments, may only be different from non-crestal sediments in the concentrations of the metals they contain and not in the modes of entrainment (Horowitz, 1974a). In cr:'stal and non-crestal sediments high • proportions of such metals as Ni, Cu, Cd. Zn. Mc to a lesser extent Li and Cr. as well as Pb, Ca. and Co referred to above. are held in the HAC leach. This probably reflects the removal of •hese metals from the 344

biogenic carbonate material s as well as from adsorbed sites on the surfaces of clay minerals, the leaching of trace metals from reducible Fe and Mn oxides not soluble in the HAC leach, and the case of Mg, from sea salt. The remainder of these elements are generally distributed between the A/R agent leach indicating association with authigenic Mn oxides; the HC1 leach indicating association with Fe oxides, silicates and clay minerals, from the chemical alteration of basalt; and the HC1 insoluble residue indicating association with basaltic and possibly continental detrital fragments. Titanium in crestal and non-crestal sediments is divided between the A/R agent and HCl. leaches, but with larger amounts in the HCl. insoluble residue, indicating a major association with basaltic detrital fragments and a lesser association with basalt alteration products. Iron is unusual in crestal and non-crestal sediments in that a large proportion is soluble in the A/R agent, which probably reflects the presence of a reducible, amorphous Fe oxyhydroxide in these sediments.

In fracture zone sediments, there is a marked increase in the proportions of such elements as Fe, Ni, Mg, Cu, Zn, Ti, Cr and Li as well as Ba and Al in the HCl soluble and insoluble phases. This indicates the association of these metals with Fe oxides, silicates and clay minerals from the chemical alteration of basaltic material; authigenic barytes, possibly enriched from hydrothermal sources; and inclusion in the a sediments of basaltic detrital material. Moderate proportions of certain metals (e.g. Pb, Ca, Mg, Co, etc) are associated with the HAC leach of these sediments but it is lower than the proportions of these metals in the HAC leach of non-crestal and crestal sediments. Lower absolute values and/or lower proportions of certain metals (e.g. Fe, Cr, Co and Li) are associated with the A/R agent leach of fracture zone sediments, which together with lower proportions of Mn in the A/R agent leach suggests a lower proportion of authigt.nic Mn oxides in fracture zones as compared with crestal and non-crestal sediments. The higher proportions of metals in the HC1 soluble and insoluble phases is similar to the findings of Horowitz (1974a)" for sediments from the Reykjanes Ridge. In comparison to basal metal-rich sediments from the Pacific Ocean (Cronan and Garrett, 1973; Cronan, 1976) all the groups of sediments have - lower proportions of all metals associated with the A/R agent leach, suggesting metal contributions by biogenic means (in crestal and non-crestal sediments) and basaltic alteration processes (in fracture zone sediments) are of more importance than- concentration of trace elements with authigenic Mn oxides in Indian Ocean than in Pacific Ocean ridge sediments.

• Lower proportions of such metals.as Co, Ni, Pb, Li, Ba, Mg and to a lesser extent Zn, Cu and Cr occur in the HAC leach of fracture zone as compared to crestal and non-crestal sediments. Certain proportions of these metals in the three groups of sediments are probably related to their removal from adsorbed sites on the surface of clay minerals, although this value may not be high. Horowitz (1974a) has shown for North Atlantic sediments that the adsorbed proportions of Cu, Pb, Zn and Li probably amount to not more than 10% of the total soluble in the HAC leach. There is therefore a large proportion of the HAC soluble fraction of these elements which is probably related to biogenic carbonate material in all the groups of sediments. The lower proportions in the HAC leach of ` fracture zone as compared to crestal. and non-crestal sediments can probably be accounted for in terms of the loss of these metals from the biogenic carbonate material due to partial dissolution of CaCO3 below the lysocline in the fracture zone.

3.3.4 Geochemistry of the Carbonate Phase of Indian Ocean Recent Sediments.

3.3.4a Introduction

The geochemical partition data have shown that proportions of certain metals, such as Ni, Cu, Co, Cr, Pb, Cd, Zn, Li and Mg are soluble in the HAC leach of fracture zone, crestal and non-crestal sediments from the Central.. Indian Ocean Ridge. It has been noted previously (Chester and Hughes, 1.967) that metals soluble in this acid will be associated with the carbonate minerals, excluding dolomite; the adsorbed sites on the surfaces of clay minerals and Fe/Mn oxides; and an Fe oxide mineral. Mn and associated trace metals may also be removed by the HAC leach from Mn oxide coatings on carbonate-rich sediments (Copeland, 1970). The low proportions of Fe removed by the HAC leach (see Section 3.3.2) suggest that the. Fe oxide mineral is not as soluble as reported by Chester and Hughes (1967) and consequently the possibility of it being a major contributor to the HAC soluble trace metals can be discounted. Examination of the sediments using optical and scanning-electron microscopes (see Section 3.1) has not revealed the presence of Mn oxide coatings and thus such coatings cannot. contribute to the metals soluble in acetic acid. 346

In order to obtain an accurate picture of the geochemistry of the carbonate material it is necessary to differentiate between that proportion of the metal in the HAC leach which is associated with the carbonate minerals and that proportion which is-adsorbed onto the surface of other mineral phases.

3.3.4b Removal of Adsorbed Trace Metals.

In order to determine the proportions of adsorbed ions in the sediments the following procedure was employed. Samples of a fracture zone sediment (?3P) and a non-crestal sediment (55GK) were mechanically shaken with - de-ionised water (D.I.W.) at room temperature for periods of five, fifteen and thirty minutes and one, two,four and twenty four hours to see if this would remove the adsorbed trace metals, but leave the carbonate material unaffected. The samples were then vacuum filtered and the filtrates analysed for the metal concerned according to the procedure for the HAC. partial attack described in Appendix A.1.

The method proved unsuccessful in removing adsorbed ions. Only a small proportion of the total Ca (0.24 Wt/ to 0.3 Wt%) was removed after shaking for twenty-four hours. This proportion was probably derived from interstitial water evaporates and/or sea salt. This indicates that the D,,T.W. was not a strong enough reagent to remove the adsorbed trace metals, or that there were very small or negligible proportions of adsorbed trace metals in these sediments to be removed.

3.3.4c Geochemistry of the Carbonate Material

As the D.I.W. failed to remove any adsorbed ions, it would appear that the MAC soluble trace elements are held in the carbonate material. It is therefore necessary to have an accurate picture of the geochemistry of the carbonate material in the sediments. The sediments are foraminiferal oozes, and all the calcareous material is in the form of the tests of dead foraminifera. Occasionally coccoliths have also been observed (see Section 3.1) on the surface and interiors of certain forams.

To study the composition of the carbonate,-examples of fracture. zone, crestal and non-crestal sediments were washed with D.I.W. After drying, 347

TABLE 3.3.4a Metal Content of Foraminifera and other Biogenic Components (ppm) .

1 2 3 4 5 6 7 9 10

Al 400. 70 • 170 9800 18400 8900 1400 - 140 1000 Mg 1400 2100 2150 24000 23300 32800 7800 - 11000 10000 Ba 110 170 400 - - 140 700 17 10 Ti nil nil nil 650 610 1900 400 15 - 10 Fe 100 120 150 8200 6700 4450 2000 300 300 100 Mn 39 40 39 145 165 270 120 8 6 100 - - 83 9 4 Ni 13 15 9 Cr 3 12 10 - 54 - 10 Cu 8 17 8 69 32. 71 340 25 . 6 19 Pb 3 11 7 .29 32 32 160 10 2 1.0 Zn 4 7 4 3700 110

All data are T.S.B.

1. Forams from fracture zone sediments (6) ) This study (see over for 2. ) Forams from crestal sediments (3) details of foram. species) 3. Forams from non-crestal sediments (14) ) 4. Globorotalia menardii (Parker, Jones and Brady) (19) 5. Orbulina universa (d'Orbigny )(10) ) Belyayev (1973) 6. Globigerinoides conglobatus (Brady)(8) ) 7, Pacific Ocean Zooplankton (mainly Copepods)(17), BostrBm et al (1974) 8. Foraminifera, Arrhenius (1963) 9. Radiolaria (6), Oldnall (1975) __ 10.Foraminifera, Oldnall.(1975) 348

TABLE 3.3.4a (cont.)

Details of Foraminifera species analysed from each :sediment group:- .

FRACTURE ZONE SEDIMENTS:-

Globigerina bulloides bulloides (d'Orbigny); G. eggeri eggeri Mumbler); Globigerinita gluttinata (Egger); Globigerinoides quadrilobatus immaturus (Le Roy); Globorotalia tumida; G. pumilio; Globorotaloides hexagona (Natland); Turborotalita humilis (d'Orbigny).

CRESTAL SEDIMENTS:

Globigerinoides quadrilobatus trilobus; Globorotalia tumida; G. pumilio; G. fimbriata; G. cultrata menardil: Hastigerina siphonifera;H. pelagica.

NON-CRESTAL SEDIMENTS:-

Orbul,ina universa; Pulleniatina obliquiloculata; Hastigerina pelagica; Globigerina praec;alida; Globigerinoides conglobatus; G. quadrilobatus immaturus; Sphaeroidinella dehiscens; Globorotalia pumilio; G. fimbriata; G. cultrata menardii. 349

groups of the most abundant species were hand picked from the samples using a binocular microscope and were chemically analysed using the total digestion technique described in Appendix A.1. The 1process is extremely time consuming and hence groups of abundant species in each sample type were selected rather than individual species. The groups of species analysed from each sample type are given in Table 3.3.4a.

All the data discussed in this section relating to foraminifera are expressed on an uncorrected, total sediment basis (T.S.B.). They are uncorrected for CaCO3, because the material under discussion is virtually pure CaCO3 with included trace metals and hence it would be meaningless to express the data on a carbonate-free basis.

Table 3.3.4a contains the average composition of the groups of foraminifera in the three main types of sediments — fracture mine, crestal and non-crestal. Also contained in Table 3.3.4a are analyses of three species of foraminifera (forams) recovered from the northern Indian Ocean, together with analyses of zooplankton, forams and radiolaria from other areas. All the data shown in Table 3.3.4a are expressed on a T.S.B. The analyses of Co and Cd and Li in the sediments are below, and equal to, rezpectively, the detection limits of the determination method. Therefore their variations within the three groups of sediments cannot be considered further. No Ti values are recorded because no. Ti was detected. The composition of the three species of forams from the. northern Indian. Ocean given by Belyayev (1973) (see Table 3.3.4a, Columns 4, 5 and 6) are most interesting. Examples of these three species were included In the analysis of the groups of foraminifera taken from sediments analysed in this study, yet the trace metal values recorded by Belyayev (1973) are very much higher than those recorded for those in this study (Columns 1, 2 and 3, Table 3,3.4a). The high Al, Ti, Mg and Fe concentrations in the forams, analysed by Belyayev (1973) are greater than the total amount of Al, Mg and Fe soluble in the HAC leach of the sediments (see Table 3.3.4b). This suggests that alumino-silicate and/or oxide' material from the chambers of the foraminifera has been included in the analysis and Belyayev's (197:i).data do not represent a true picture of the geochemistry of the carbonate material of the three species of forams. The compositions of the forams from the sediments are in general all within the ranges of metal contents of the other planktonic organisms reported in Table 3.3.4a except for the radiolaria (Column 9, Table 3.3.4a). 350

TABLE 3.3.4b. Chemical Composition of Foraminifera Species in Central Indian Ocean Ridge Sediments.

Al .Mg Ba Mn Ni Cr Pb

A 70 2100 170 120 40 15 12 17 11 7 B 190 2190 180 160 40 23 20 22 12 12 CRESTAL 94 100 67 6o 76 9P} SEDIMENTS C 37 96 75 59 D 3 55 17 3 7 40 32 34 65 25

A 400 1400 110 100 39 13 3. 8 3 4 FRACTURE B 410 1540 133 170 40 28 8 22 13 ' 10 ZONE C 96 91 83 59 98 48 38 38 24 40 SEDIMENTS D 1 5 12 41 3 7 1 8 13 6

A 170 2150 400 150 39 9 10 8 7 . 4 NON- B 375 2350 480 290 44 15 , 16 10 8 11 CRESTAL C 45 91 83 52 89 62 63" 8o 88 ' 38 SEDIMENTS D 4 41 23 4 5 37 21 36 6o 18

* All data are TSB

A. Chemical Composition of Foraminifera in ppm* B. Chemical Composition of the HAC leach in ppm* C. A expressed as a percentage of B D. A expressed as a percentage of the TSB bulk composition 351

In Table 3.3.4b the compositions of the groups of forams extracted from the sediments, together with the composition of the HAC leach for each group of sediments are presented. All the data\are expressed on an uncorrected T.S.B. Table 3.3.4b also contains values of the composition of the forams expressed as percentages of the composition of the HAC leach and of the bulk composition. No data for Ti, Co, Cd and Li are reported for the reasons given above. These data allow a comparison to be made between the composition of forams from the different sediment types analysed. From Table 3.3.4b it would appear that the proportions of Mg, Ba, Mn and also Fe in the carbonate material are similar in all groups of sediments, while that of Al is greater in carbonate material of fracture.zone as compared to crestal and non-crestal sediments. The proportion of Pb, Ni, Cr, Cu and to a lesser extent Zn in the carbonate material is lower in fracture zone as compared to crestal and non-crestal sediments. These latter data would suggest that dissolution of CaCO3 below the lysocline in fracture zone sediments has resulted in the selective removal of Pb, Ni, Cr and Cu and released them to the bottom waters. These metals may then become incorporated into non-carbonate authigenic mineral phases, such as clay minerals which are probably forming in fracture zone sediments. Such a conclusion may be supported by the higher proportions of Al in the carbonate material of fracture zone sediments. This Al is probably related to colloidal AlOOH and clay minerals in the chambers of the forams and is not contained in the lattice of carbonate minerals. S.E.M. photographs (see Section 3.1) indicate a greater frequency of occurrence of clay minerals in the chambers of forams in fracture zone as compared to crestal and non-crestal sediments. The similar proportions of Mg, Mn, Ba and Fe in the carbonate phases of all the sediments, indicate that these metals may not be associated with the lattice of carbonate minerals, but with mineral phases that are unaffected by dissolution below the lysocline. The Mg in the HAC leach may be related to the presence of sea salt in the sediments, while the Mn, Fe and Ba could be present in ion exchange sites on the surfaces of clay minerals and reducible FeMn oxides (Chester and Hughes, 1967), as well as probably being contained in small particles of FeMn oxides in the 'chambers of the forams. The presence of such FeMr oxides, has been reported on the tests of foraminifera in cores from the Caribbean Sea, by Ericson and Wollin (1913). However, such phases were not visible on the S.E.M. photographs TABLE 3.3.4c. Inter Element Associations in Foraminifera Species from Fracture Zone, Crestal and Non-Crestal Sediments from the Central Indian Ocean Ridge.

n = 23

Carb Carb Fe -.51 Fe Al. .76 Al Mn .42 - .48 Mn Ni .36 -.91 -.80 .71 Ni Cr .75 .62 -.38 Cr Cu .41 .50 ;71 .73 .62 .39 Cu P►, - .70 .78 .51 .98 .84 Pb Zn - - -.78 .58 .45 .97 -53

Mg - .79 .76 .60 -.79 .84 .85 .79 Mg taken in this study. (see Section 3.1). Ericson and Wollin (1973) also suggest that Mn may be held in the lattice of carbonate minerals in the tests of forams. If this is the case in Indian Ocean forams, it must be unaffected by dissolution below the lysocline in the\racture zone. sediments. This could then indicate the preferential incorporation of Mn in robust species of foraminifera together with other trace metals, which are resistant to dissolution.

Correlation coefficients calculated according to the procedures described in Appendix A.4, for the T.S.S. data from the analysis of the forams, (see Table 3.3.4c) confirm the association of Ni, Cr and Cu with carbonate material, as well as showing a positive correlation of Mn with carbonate material, which may support the opinion of Ericson and Wollin (1973) that Mn may be incorporated into the lattice of carbonate minerals. The associations of Cu, Pb and Mg with Al suggests, as is shown by the data presented above, that particularly in fracture zone sediments, these metals could be held as adsorbates or in the lattice of clay minerals. The association of certain trace metals with Fe (Cr, Cu, Pb) and with Mn (Ni, Cu, Pb) may indicate that these trace elements could be coprecipitated with any MnFe oxide grains present.

In summary, the composition of the carbonate material (see Table 3.3.4b) suggests that nearly all the Pb (up to 94%) and relatively high proportions of Ni, Cr, Zn and Cu (38-80%) in the HAC leach of crestal and non-crestal sediments are incorporated into the biogenic carbonate. material itself. This would support the previous data which indicate that calcareous organisms concentrate trace elements in their tests, (Pb - Horowitz, 1974a; Croaan, 1976; Ni, Cr and Cu - Turekian and Imbrie,, 1966; Fujita, 1971; Bostr8m et al, 1974; Oldnall, 1975; and Mn - Ericson and Wollin, 1973). The data for those metals which are less concentrated in the calcareous foraminifera, e.g. Zn and to a lesser extent Ni, could support the view that in marine sediments they are in part held as adsorbates (Chester, 1969; Chester and Hughes, 1969; Horowitz, 1974a). Aluminium in the HAC leach is associated with AlOOH and partially formed clay minerals. Magnesium in the HAC leach is associated with sea salt, while Fe and Mn may be associated with reducible FeMn oxide minerals.

The variable concentration of elements in the tests of calcareous forams may in part explain the differences in partition patterns between carbonate-rich sediments and those pelagic sediments in which carbonate material is low or absent. 354

3.4 DISCUSSION OF THE GEOCHEMISTRY OF RECENT SEDIMENTS FROM THE CENTRAL INDIAN OCEAN RIDGE.

3.4.1. Geochemical Comparisons Between Groups of Sediments.

3.4.1a. Introduction

It is useful, having discussed the comparisons between the three groups of sediments in the sample area, to draw comparisons between these groups of sediments and other Indian Ocean sediments and with surface metal-rich sediments from other areas. In order to do this a - number of averages have been calculated. All the data used,.with the, exception of Ca and CaCO3, are expressed on a carbonate-free basis (CFB). The results are displayed in Table 3.4.1a for Indian Ocean sediments and in Table 3.4.Ib for metal-rich sediments from the Indian, Pacific and Atlantic Oceans. Comparisons have already been drawn between metal accumulation rates for Pacific and Indian Ocean surface sediments (see Section 3.2.4). No further reference will therefore be made to them here. Likewise comparisons between the geochemical partition patterns of Indian Ocean sediments and those from the Pacific and Atlantic Oceans have been discussed in Section 3.3.2 for particular metals.

The comparisons discussed below are shown graphically in ternary diagrams plotted for Indian Ocean sediments (Figs. 3.4.1a, b and c) and for surface metal-rich sediments from the Indian, Pacific and Atlantic Oceans (Figs. 3.4.1d, a and f).

3.4.1b. Comparisons of Fracture Zone, Crestal and Non-Crestal Sediments with other Indian Ocean Surface Sediments (see Table 3.4.1a, Figs 3.4.1a, b and c).

The high concentrations of Mn, Ni and Zn in sediments from the Central Indian Basin (Column 7, Table 3.4.1a) are caused by the presence of abundant Mn micronodules. The compositions of sediments from locations outside the sample area (Columns 4-10, Table 3.4.1a) are similar to those of non-cristal sediments (Column 3, Table 3.4.1a) (and also to those of pelagic sediments from the Pacific and Atlantic Oceans (Columns 6 and 7, Table 3.4.1.b) sugaesting that the metal concentrations contained in them are explicable in terms of normal oceanic sedimentation processes. TABLE; 3.4.1a. Composition of Sediments from Locations in the Indian Ocean

2 3 4 5 7 8 9 10 11

( CnCO3 65.61 85.47 75.73 15.93 9.49 29.53 1.97 1.42 77.51 34.22 76.96 ( en* 26.88 34.36 30.56 6.78 4.18 12.20 1.17 0.95 31.41 14.13 31.07 ( Al. 8.12 2.38 2.70 6.45 7.23 3.89 4.90 6.38 4.80•. 5.71. 3.18 wt% ( Ti 0.39 0.18 0.18 0.75 - - 0.28 0.22 0.41 0.20 ( Mg 7.31 2.35 2.29 - - - 2.59 - 2.81 (ro 6.86 4.31 2.24 . 6.04 4.18 3.86 3.71 3.77 3.51 4.19 3.19

( nn 3650 6690 6990 - - - 800 - 6580 ( Mn 3960 7730 3840 4940 1540 1580 1446o 6440 2760 2240 4750 ( Ni 481 178 133 180 115 95 775 244 110 183' 179 ( Co 68 118. 77 92 44 70 85 90 8o 80' 85 ( Cr 533 539 375 180 - 70 - 270 215 429 ppm ( Cn 306 319 223 178 120 95 741 286 130 180 254 ( Cd 7 13 1.0 9 . - - 6 - 11 17 11 ( Ph 71 92 71 64 50 60 69 78 87 79 76 Zn 168 161 130 234 155 130 213 133 136 172.''"'~ 141 ( Li 24 31 27 - - - 31 28 ( As 7 20 7 ------10

All data CFB, except * which are TSB TABM 3.4.1a cont.

1. Fracture Zone Sediments (Average) ) 2. Crestal. Sediments (Average) ) This study 3. Non-Crestal Sediments (Average) ) 4. Madagascar Basin ) 5. Mozambique Basin ) Unpublished Data, Moorby, 1976 ) (supplied to author - person. communic, 1976) 6. Crozet Basin 7. Control Indian Basin ) 8. Wharton Basin ) 9. NW Indian. Ocean Sediments (Average) Unpubl. Data (Colley, 1977, person. communic). 10.:Indian Ocean Sediments, Bender and Schultz,(1969); BostrSm et al (1969); Horowitz(1970) 11.Average Surface Sedimenta, Central Indian Ocean Ridge - This study. -357 Fig. 3.4.1a, 3.4.1b and 3.4.1c Distribution of Ca, A1, Fe and Mn in the Carbonate-Free Fraction of Indian Ocean Sediments

Al ■ Fracture Zone Sediments1 Al Central Indian Ocean Ridge•. ® Crestal Sediments, Central Indian Ocean Ridge.` Non-Crestal Sediments, Central Indian Ocean Ridg IJ W Indian Ocean Sediment

O Wharton Basin Sediments

Mozambique Basin Sediments

3 Crozet Basin Sediments

Madagascar Basin Sediment

Fe Mn

Ca Mn The fracture zone sediments are generally enriched in Al, Mg, Fe, Ni, Cr and Cu with respect to sediments from other areas in the Indian Ocean. Partition analysis has shown that in the fracture zone sediments the enriched metals are probably contained in detrital basaltic fragments together with acid soluble clay minerals (e.g. Fe smectites) and Fe • oxides formed by the chemical alteration of the basalt. The fracture zone sediments are depleted in Mn, Co, Pb and Zn with respect to some sediments from outside the sample area, e.g. Madagascar and Wharton Basin. Partition analysis has shown that Pb, Co and possibly Zn, may be linked with authigenic Mn oxides by coprecipitation. The lower values of these metals in fracture zone sediments suggests that the physiochemical conditions in the fracture zone (i.e. low Eh and pH) were not suitable to allow the precipitation of abundant authigenic Mn oxides.

The crestal sediments of the Central Indian Ocean Ridge are generally enriched in Fe, Mn, Ni, Co, Cr, Cu and Pb and depleted in Al, Ti with respect to sediments from other locations in the Indian Ocean. This pattern of enrichment is typical of active ridge sediments from other areas (Bostrdm et al, 1966; 1969; Piper, 1973; Horowitz, 1974a), although the Fe enrichment in crestal sediments with respect to other Indian Ocean sediments is not as marked as in fracture zone sediments. This enrichment pattern, together with data from metal accumulation rates, correlation coefficients, factor analysis and partition studies suggest an additional source of Fe and Mn and associated trace elements to those supplied by normal oceanic sedimentation processes in crestal as compared to non-crestal sediments and those sediments from the basins outside the study area. The process of metal addition is probably that of hydrothermal leaching of the metals from the underlying basalt and their precipitation along the ridge axis. The localised nature of this process is evident from the variable composition of the crestal sediments in the area, and is similar to that reported previously from other active ridges (Cronan, 1972; Horowitz, 1974a; McArthur and Elderfield, 1977).

3.4.1c. Comparisons of Fracture Zone and Crestal Sediments from the Indian Ocean with Surface Sediments from the Pacific and Atlantic Oceans (see Table 3.4.1b, Figs, 3.4.1d, e and f).

Both crestal and fracture Zane sediments are generally more enriched TABLE 3.4.1b. Composition of Surface Sediments.from the Indian, Pacific and Atlantic Oceans.

1 3 4 5

( CaCO * 65.61 85.47 11.50 11.50 0.70 0.70 3 ( Ca* 26.88 34.36 5.00 5.00 0.66 0.66. ( Al 8.12 2.38 5.30 5.88 0.50 7.82 9.34. Wt% ( Ti 0.39 0.18 0.50 1.52 0.02 0.45 (Mg 7.31 2.35 4.19 3.00 ( Fe 6.86 4.31 9.62 10.30 18.00 5.74 5.06

( Ba 3650. 6690 ( Mn 3960 7730 5400 1200 60000 4400 4800. ( Ni 481 178 214 39 430 110 210. ( Co 68 118 40 100 ( Cr 533 539 128 110 55 83 100 ppm ( Cu 306 319 399 92 730 120 320 ( Cd 7 13 ( Pb 71 92 349 63 152 47 70 ( Zn 168 161 228 140 380 125 160 ( Li 24 31 36 12 ( As 7. 20

All data CFB except * which are TSB

Average Fracture Zone Sediment ) 1. This study 2. Average Crestal Sediment ). 3, Median Valley Sediment, Mid-Atlantic Ridge, 45°N, Horowitz (1974a) 4. Reykjanes Ridge Crestal Sediment, Horowitz (1974a) 5. East Pacific Rise Surface Sediment, Bostrdim and Peterson (1969), Horowitz (1970) 6. Surface Atlantic Sediment, Wedepohl (1960), BostrLm et al (1969), Chester et al (1976). 7. Pacific Pelagic Sediment, Chester (1965b), Cronan (1969) 360 Fig. 3.4.1d, 3.4.1e and 3.4.11 Distribution of Ca, Al, Fe and Mn in the Carbonate-Free Fraction of • Surface Sediments from the Indian, Pacific and Atlantic Oceans

Al O Crestal Sediments, Central Indian Ocean Ridge y Fracture Zone Sediments, Central Indian Ocean Ridge

East Padific Rise Sediments:_

❑ Reykjanes Ridge Sediments

O •Mid-Atlantic Ridge, 45°N, Median Valley Sediments

Fe Mn

Ca in Fe, Mn and associated trace elements and depleted in Al, Ti, than pelagic sediments from the Pacific and Atlantic Oceans. This, taken with other data, supports the conclusion that in beth crestal and fracture zone sediments, sources in addition to thoso of normal oceanic sedimentation are required to account for the concentrations of Fe, Mn and associated trace metals. In general, however, the Central Indian Ocean Ridge sediments are depleted in Fe, Mn, Cu, Pb and Zn in comparison with East Pacific Rise sediments while being enriched in Ti, Al and Cr. In comparison with sediments from the Mid-Atlantic Ridge, the Central Indian Ocean Ridge sediments are enriched in all elements except Fe and Al and Ti. The intermediate composition of Central Indian Ocean Ridge sediments with the exception of Fe between those from the East Pacific Rise and the Mid-Atlantic Ridge has been recognised previously (Bostram and Peterson, 1966; Bostrlim et al, 1969, 1972; Cronan, 19724 Horowitz, 1970, 1974a, b). This is probably due to the inteiDlay of two independent processes. The first of these is variations in intensity of ridge activity and spreading rate between the three major oceans (Heirtzler, et al, 1968). Since it is considered that it is volcanic processes which cause certain metal enrichments in active ridge sediments (Bostrom et al, 1966, 1969; Cronan, 1972; `Horowitz, 1974a), differences in intensity of activity and/or in the nature of hydrothermal ridge additions will produce different metal concentrations in the sediments associated with the three ridges. The intensity of ridge activity has been shown to be highest in the Pacific Ocean (Heirtzler .et al, 1968). The intermediate composition of Central Indian Ocean Ridge sediments for metals such as Fe, Mn and associated trace metals could reflect the intermediate activity of the Central Indian Ocean Ridge in comparison with East Pacific Rise and the Mid-Atlantic Ridge (Heirtzler et al, 1968). The higher values of Fe, Cu, Pb and Zn in Mid-Atlantic Ridge Median Valley sediments (Column 3, Table 3.4.1b) as compared to Central Indian Ocean Ridge sediments is probably due to the proximity of these sediments to a hydrothermal vent (Horowitz, 1974a). The second process is variation in detrital input between the three oceans, causing dilution of the ridge crest metal additions. This has been shown to be highest in the Atlantic Ocean (Ku et al, 1968). The elevated values of Ti, Al and Cr in Central Indian Ocean Ridge sediments over sediments from the East Pacific Rise and lower values of these metals in Central Indian Ocean Ridge sediments in comparison to Mid-Atlantic Ridge sediments suggests that the detrital input into the Central Indian Ocean 362

Ridge sediments is intermediate between that of sediments on the East Pacific Rise and Mid-Atlantic Ridge. The low values of Fe along the crest of the Central Indian Ocean Ridge and its higher concentrations in fracture zone sediments suggests,that Fe may be added to the sediments to a greater degree along the fracture zones than on the ridge crest. Variability in the addition of Fe along the ridge axis may account for its overall depletion with respect to Mid-Atlantic Ridge and East Pacific Rise sediments, since in some areas the concentrations of Fe in crestal sediments are higher than those reported in Mid-Atlantic Ridge and Reykjanes Ridge sediments by Horowitz (1974a). (Such local variability in metal additions along the Central Indian Ocean Ridge and other ridges has already been referred to in Section 3.4.1b).

3.4.2 Sources of Metals in Recent Sediments from the Central Indian Ocean Ridge.

The source or sources of metals in active ridge sediments is still a matter of some controversy. However, it is now generally recognised that the enrichment of a particular metal may result from several processes rather than a single one (Cronan, 1976; Heath and Dymond, 1977). The general view is that processes related to ridge crest volcanism, i.e. hydrothermal activity, are the main source of the majority of the metals in active ridge sediments. However, opinions differ as to the exact nature of these processes. It has been suggested that metal enrichment can be caused by the introduction of deep seated, hydrothermal fluids, possibly of carbonatite composition (Bostrtim et al, 1966, 1969, 1975) or that the enriched metals are leached from the basaltic crust by the circulation of heated seawater through cracks and fissures, i.e. by hydrothermal circulation (Corliss, 1971; Dymond et al, 1973; Piper,.1973; Cronan, 1976; Heath and Dymond, 1977). The metals precipitate from the higher temperature, acidic hydrothermal fluids as they mix with cooler more alkaline bottom waters, near the spreading centre. The residues of the hydrothermally leached basalts which may be included in the sediments, are probably composed of, among other phases, clay minerals and may act as a further source of elements (Gorbunova, 1974; Hajash, 1975; Bischoff and Dickson, 1975). Metals may also be added in the products of the s3ow weathering of volcanic pyroc3.astics and cold basalt by seawater {Sayler and Bischoff, 1973: Berti.nes 19711; Horowitz and Cronan, 1976) a5 well as 363

by the incorporation into the sediments of basaltic detrital fragments from the mechanical breakdown of the basement (Bertine, 1974; Horowitz, 1974a). Hydrogeneous precipitation from seawater h.s been suggested as. a further source of metals in the lattices of, and adsorbed onto the surfaces of, such authigenic minerals as clays and FeMn oxides (Dymond et al, 1973; Cronan, 1976; Heath and Dymond, 1977). Certain trace metals may also be held in the calcareous material of carbonate-rich sediments (Turekian and Imbrie, 1966; Belyayev, 1973; Bostram et al, 1974; Oldnall,' 1975), however, the contribution from this source is probably of minor importance in comparison to the proportion of metals supplied by hydrothermal sources.

Geochemical partition studies of active ridge sediments are relevant to any discussions on the proportions of metals supplied by different sources. However, before turning to a discussion of the sources of particular metals it is necessary to draw attention to a number of general features of recent sediments from the Central Indian Ocean Ridge.

As described, sediments from the Central Indian Ocean Ridge are divisible into three groups - fracture zone,, crestal and non-crestal sediments. The higher accumulation rates and absolute concentrations of such metals as Fe, Mn, Mg, Ba, As, Ni, Cu, Cr and Zn in fracture zone and/or crestal sediments indicates that metal enrichment is occurring from sources other than those of normal oceanic sedimentation,.i.e.hydro geneous precipitation from seawater, slow weathering of basaltic material, detrital input and biogenic concentration. It would also appear from local variations in metal concentrations and accumulation rates along the axis of the active ridge segments that such an additional process of. metal enrichment may be localised within the sample area. The additional source of most of the metals is probably related to hydrothermal processes.

In all the sediments Ca is associated with biogenic carbonate material. This is exemplified by its removal in the HAC leach, i.e. dissolution of the carbonate material, which in all the sediments is in the form of the tests of dead foraminifera. Minor amounts of Ca are supplied to the •, sediments by the inclusirn of Ca-rich detrital minerals due to the breakdown of basaltic material. The majority of the Ba in all the sediments is in the HC1 soluble phases which suggests that it is in the form of barytes and/or in the lattice of clay minerals (Arrhenius, 1963). In not%crestal and fracture zone sediments the Ba in these phases is probably supplied from biogenic sources, as well as precipitated from seawater. However, the higher concentrations and increased accumulation rate in crestal sediments, ,r may indicate that the Ba in authigenic barytes is supplied by hydrothermal sources as suggested by Heath and Dymond (1977)1 for East Pacific Rise sediments.

Titanium in all the sediments is associated with the acid insoluble residue, i.e. the detrital phases. It is probably enriched in the sediments due to the inclusion of unaltered basaltic detrital fragments, containing such minerals as anatase and ilmenite. This is particularly the case in fracture zone sediments where there is an increased proportion of detrital components.

Aluminium in all the sediments is associated with the HC1 soluble and insoluble phases. Itis enriched and has the highest accumulation rate in fracture zone sediments, while it is, in general, depleted and has low rates of accumulation in crestal sediments. The Al is predominantly contained in•clay minerals. Aluminium is probably also contained in unaltered basaltic detrital fragments in fracture zone sediments. Heath and Dymond (1977) have suggested that some Al in Nazca Plate sediments may be of hydrothermal origin. Although Al concentrations are above average in some crestal sediments, this increase is accompanied by high Ti and Mg, which probably indicates that Al is added to the sediments a, a result of the alteration of basaltic material.

A common feature of crestal and non-crestal sediments is the association of Fe and Mn, in the same factor (see Factor Analysis, 3.2.7) and in part in the same partial chemical leach, i.e. the A/R agent leach (see Partition Chemistry,, 3_3). This is similar to the findings of Heath and Dymond (1977) for Nazca Plate sediments.

In non-crestal sediments, the concentrations of Mn and associated trace elements (Co, Cr, Cu, Pb) can probably be accounted for in terms of normal" au thigenic precipitationration of Mn oxides from seawater. - The rates of accumulation of these metals. in-these sediments are similar to those of Pacific pelagic sediments, The partition analysis indicates that Fe and its associated trace elements (Cu, Ni, Zn, Li, Mg) are in part in the form of an amorphous Fe oxyhydroxide, while the\correlation coefficients, factor analysis and partition analysis suggest that Fe is also associated with such metals as Al, Mg, Ni, etc in clay minerals. These data probably indicate that Fe and associated trace metals are removed from basalt by slow weathering by seawater, and that they are in part precipitated with Al, into clay minerals and partly coprecipitated with trace metals to form an Fe oxyhydroxide.

In crestal sediments, as has already been stated, the increased concentrations of Fe and Mn and associated trace elements, and increased rates of accumulation of these metals, indicate that they may be supplied by hydrothermal processes. While this is true, there is probably a non- hydrothermal component of Mn and associated trace elements (Co, Cr, Cu and Pb) in these sediments as well, which is precipitated from seawater, in a similar manner to that suggested by Cronan (1976) for Pacific basal -metal-rich sediments. However, the partition data, together with the other evidence suggest that in crestal sediments Fe and Mn and their associated trace elements are probably removed from the underlying basalts by hydrothermal leaching and are precipitated as Mn oxides, Fe oxyhydroxides and possibly Fe silicates. BostrLm et al (1972) have suggested- that more basalt would have to be leached hydrothermally to supply the Mn in active ridge sediments than is required to supply all the Fe, if Fe is added by this process. However, if the authigenic precipitation of Mn from seawater, which is relatively uniform throughout the oceans (Elderfield 1976) is deducted from the total Mn in crestal sediments, the remaining Mn can probably be supplied by leaching of the quantity of basalt required to supply the Fe. Such a conclusion is supported by experimental studies of seawater/basalt interactions (Gorbitnova, 1974; Bischoff and Dickson, 1975; Hajash, 1975; Wolery and Sleep, 1976). The partition of a proportion of the Fe in the HC1 soluble phases, together with other data indicates that some of the Fe in crestal sediments is in a non-reducible Fe oxide and possibly silicate form. It is also probable that certain metals e.g. Al, Ti, Cr, Li,' etc. which. are associated with Fe may be added to crestal sediments in the form of clay' minerals. as well as in minor amounts of unaltered basaltic fragments. It seems probable that the As in the crestal sediments is coprecipitated with Fe (Calvert and Price, 1977). Its 366

higher accumulation rate in crestal sediments suggests that it may be supplied by leaching from the underlying basalts, while in non-crestal sediments its lower concentrations and rates of accumulation could be accounted for by simple authigenic precipitation from seawater.

Only in fracture zone sediments is the fractionation of Fe and Mn, which has been recognised previously (Krauskopf, 1956, 1957; Cronan, 1976) observed. The. Fe and Al, Mg, Ti, Cr, Cu, Ni, Zn, Li are contained in the HCL soluble and HCL insoluble phases. The data indicate that these metals are supplied by the breakdown and chemical alteration of basalts exposed along the fracture zone. Unaltered basaltic detrital fragments are - probably included in fracture zone sediments, and some of the, metals may also result from local hydrothermal leaching processes occurring along the fracture zone. Work on fracture zones from the equatorial Atlantic Ocean (Bonatti et al, 1976a, b, c) has indicated that these features may be sites of hydrothermal circulation and leaching of metals. Mineralisation has been reported from the Vityaz fracture zone in the north of the sample area in the Indian Ocean (Baturin and Rozanova, 1975; Rozanova and Baturin, 1975), in the form of FeNiTi and CuFe sulphides associated with ultramafic rock bodies. The breakdown and alteration of such ultramafic rocks, which have been reported as being present at other sites in the Indian Ocean fracture zones (Engel and Fisher, 1975) as well as of the underlying basalts in the fracture zone may account for the increased values of Ni, Cr and Cu in fracture zone sediments. However, at present there is insufficient evidence to suggest that the fracture zones of the Indian Ocean are major sites of hydrothermal activity. The low values of Mn in fracture zone sediments are probably attributable to a depositional environment not conducive to Mn02 precipitation.

Biogenic concentration, although being of minor importance in comparison to hydrothermal enrichment processes, does account for proportions of such metals as Ni, Co, Cr, Cu, Cd, Pb, Zn, Li, Ba, Mn and Mg in the carbonate rich sediments. Proportions of these metals are taken up into the tests of the foraminifera during their life processes (Bostrbm et al, 1974; Oldnall, 1975), when the species die off, their tests settle onto'the ocean bottom and are incorporated into the sediments. Analysis of the carbo:late material confirms that certain of the metals may be held in the lattice of the carbonate minerals, while proportions of these metals may a2so be associated with authigenic clay minerals and FeMn oxides which are contained 367

in the chambers of the dead forams. An association of metals with mineral phases in the foram chambers is particularly noted in fracture zone sediments and probably results from the partial_ dissolutipn of the carbonate material below the lysocline, the release of the carbonate associated trace elements and their incorporation into authigenic minerals, forming in the foram chambers. The analysis of the carbonate material shows that Cu, Ni and Cr and Pb are incorporated into the biogenic material of the carbonate sediments. This is different to the finding of Horowitz (1970) who found negative correlation of Pb with CaCO3 in Indian Ocean sediments.

3.4.3 Conclusions.

1. The carbonate-rich crestal sediments, in general, have metal concentrations above those for pelagic sediments and are enriched in, and show high accumulation rates of, such metals as Fe, Mn, Ba, As, Ni, Cu, Zn, Cr, Mg and Pb, while being generally depleted in Al, Ti, SiO2 and Li. Such a pattern is typical of active ridge sediments from other mid-ocean ridges, although local variations do. occur. There is .a strong association of Fe and Mn in these sediments and their geochemistry is dominated by the volcanic/hydrothermal factor (i.e. the Fe, Mn, Cu, As factor) of the factor analysis.

2. The crestal sediments show patterns of enrichment and depletion similar to those of other metal-rich, active ridge sediments, and are generally of intermediate composition between active ridge sediments from the Mid-Atlantic Ridge and East Pacific Rise. This probably reflects the interplay but independence of ridge crest derived metal additions and detrital sedimentation.

3. The non-crestal sediments have in general below average concentrations of such metals as Fe, Mn, Ba, As, Ni, Cu, Cr, etc, equal or above average concentrations of Al, Ti, SiO2' Mg, Li and low accumulation rates, which are equal to those of normal Pacific pelagic sediments for Fe, Mn, and associated trace metals. The geochemistry of these sediments is not dominated by any single factor, but reflects the interplay of all three factors (volcanic,/hydrothermal, biogenic/clay mineral and basaltic detrital) in their composition. 368

4. The partition patterns in crestal and non-crestal sediments are similar and reflect the view that active ridge crestal sediments are only different from non-crestal and pelagic sediments in the concentrations of the metals they contain and not in the modes of entrainment. High proportions of such metals as Ca, Ni, Cu, Cd, Pb, Zn, Mg, Co, Li, Cr, Mn and Ba are held in the HAC leach of crestal and non-crestal sediments and in general reflect their removal from the. large amounts of carbonate material present in these sediments. The remainder of these metals are divided between the A/R agent leach (Mn oxides), the HC1 leach (Fe oxides, clay minerals) and the HC1 insoluble residue (detrital minerals). Iron is removed in both the A/R agent and HC1 leaches, Al in the HC1 leach and insoluble residue, and Ti predominantly in the HC1 insoluble residue.

5. Fracture zone sediments are distinct from crestal and non- crestal sediments in having lower concentrations of Ca, Mn, Ba and associated trace metals and being enriched in, and having high accumulation rates of Fe, Al, Mg, Ti, Ni, Cr and Ca. There is no positive correlation of Fe and Mn in these sediments which may indicate that Fe and Mn are supplied to fracture zone sediments by independent sources. The geochemistry of the fracture zone sediments is dominated by the basaltic detrital factor, to a lesser extent the clay mineral/biogenic factor and in a very localised, more limited extent, the volcanic/hydrotherma factor.

6. The partition patterns of fracture zone sediments are distinct from those of crestal and non-crestal sediments. Although proportions of such metals as Co, Pb, Mg, Cd, Co etc are held in the HAC leach, i.e. mainly in the carbonate material, this is less than in crestal and non- crestal sediments. The major proportions of such elements as Fe, Ni, Mg, Cu, Zn, Ti, Cr, Li, Al and Ba are held in the HC1 soluble and insoluble phases (i.e. the Fe oxides, silicates, clay minerals and detrital phases), only minor amounts of these metals are removed by the A/R agent leach.

7. The data suggest that in the non-crestal sediments the metal concentrations can be accounted for in terms of normal oceanic sedimentation by such processes as authigenic precipitation of MnFe oxides (Mn, Fe, As Co, Cr. Cu. Pb;, precipitation of authigenic clay minerals (Al, re, Mg. and trace metals), basaltic and continental (7) detrital input (Li, Cr, Ti, Mg) and biogenic concentration (Ba, Mg, Cu, Cr, Co, Ni, Pb, Cd, Zn, Li, Ca). 3.9

8. In crestal sediments, although there may be authigenic Mn oxides (and associated trace elements) precipitated from seawater, the data suggest that the Fe, the excess Mn and associated .trace metals (As, Ba, Ni, Cr, Cu, Co, Pb, Zn) are in part leache.from the underlying basalts by hydrothermal activity and are precipitated as FeMn oxyhydroxides. Inclusion of the hydrothermally leached residues, as clay minerals, and unaltered basaltic detrital fragments may account for the Al, Mg, Ti in crestal sediments. Biogenic concentration is of importance for only certain metals, e.g. Ni, Co, Cu, Cr, Pb, in crestal sediments.

9. In fracture zone sediments, the separation of Fe and Mn may result from their supply by independent processes. The Fe, together with such metals as Al, Mg, Ti, Cr, Cu, Ni, Zn and Li are added to these sediments by the inclusion of basaltic and ultramafic alteration products as leached residues and unaltered fragments. Localised hydrothermal leaching of Fe and associated metals may also be of some importance in some fracture zone sediments, although there is no evidence to suggest that extensive hydrothermal activity has occurred in Indian Ocean fracture zones studied. There is in general no hydrothermal Mn component in fracture zone sediments. The limited precipitation of the Mn oxides from seawater, which may be locally enriched by the Mn leached from the basalts of the fracture zone, may be due to lower Eh and pH than on the ridge crest. Biogenic concentration is of some importance for some metals in fracture zone sediments.

10. A feature of the Central Indian Ocean Ridge sediments is the large proportions of certain metals held in the HAC leach, i.e. mainly in the carbonate material, of the sediments. Analysis of the carbonate material has shown that certain metals substitute in the lattice of the carbonate minerals. This process is most extensive in crestal and non-crestal sediments, but also occurs in fracture zone sediments. A reason for the lower proportions of carbonate lattice held trace elements in the HAC leach of fracture zone sediments, is that the partial dissolution of the carbonate material below the lysocline in the fracture zone may selectively release some carbonate lattice held trace elements to the bottom waters. Analysis of the biogenic component of fracture zone sediments indicates that some of these released trace metals may be taken up into authigenic FeMn oxides and clay minerals which. are forming in the chambers of the undissolved. foram species in these sediments. Further amounts are, however, probably still entrained in the carbonate lattice due to the preservation of particular- species at depths in the fracture zone. 370

Secthsn

4. GENERAL DISCUSSION : THE GEOCHEMISTRY OF INDIAN OCEAN METAL-RICH SEDIMENTS.

4.1 INTRODUCTION

4.2 GEOCHEMICAL COMPARISONS BETWEEN INDIAN OCEAN SURFACE- AND BASAL SEDIMENTS

4.5 GENERAL SUMMARY : VARIATIONS IN METAL ENRICHMENT PROCESSES WITH TIME IN INDIAN OCEAN SEDIMENTS 371

4. GENERAL DISCUSSION : THE GEOCHEMISTRY OF INDIAN OCEAN METAL-RICH SEDIMENTS.

4.1 Introduction.

Having discussed the geochemical investigations•of Indian Ocean basal sediments and surface sediments separately, it is useful, by way of summary, to consider geochemical comparisons between these groups of sediments. Furthermore, with the aid of geochemical partition studies, it is useful to sumriarise the variations in processes of metal enrichment, in terms of each element, during the geological evolution of the Indian Ocean. In order to carry out these comparisons a number of averages have been calculated. All the data used, with the exception of Ca and CaCO3, are expressed on a carbonate-free basis (CFB) to facilitate comparisons with preiriously published data. The results are displayed in Tables 4.2.1 and 4.2.2. The comparisons discussed below in Section 4.2 are shown graphically on ternary diagrams in Figs. 4.2.1a, b and c and 4.2.2a, b and c.

4.2 Geochemical Comparisons between Indian Ocean Surface and Basal Sediments.

4.2.1 Comparisons of Basal Metal-Rich with Surface Metal-Rich sediments from the Indian Ocean (see Table 4.2.1 and Fig. 4.2.1a, b, c).

Basal metal-rich sediments (Column 1, Table 4.2.1) are enriched in Al, Ti,Fe, Ni, Mn.and As and depleted in Mg, Ba, Cr and Ca with respect to crestal sediments from the Central Indian Ocean Ridge (Column 6, Table 4.2.1). The enrichment in Mg, Ba, Cr and Ca in crestal sediments from the Central Indian Ocean Ridge may in part be due to the incorporation of these metals in biogenic carbonate material universally present in these sediments. The metal accumulation rates in basal metal-rich sediments, crestal and fracture zone sediments are broadly similar, although the rates of accumulation of Fe, Mn, Ba, Ni, Cu and Cr are higher in basal metal-rich sediments. The accumulation rates of Zn, As and Al are generally higher in basal metal-rich than in crestal metal-rich sediments. Cronan et al (1974) have suggested that concentrations of Fe in DSDP Leg 24 basal sediments may be taken as an indice.tion of ridge activity, the mechanism by which this metal is probably supplied (Corliss, 1971; Piper, 1973; Cronan, 1976). On this basis it would appear from the Fe concentrations in basal metal-rich sediments, together..

372

TABLE 4.2.1 Composition of Metal-Rich Sediments from the Indian Ocean.

( CaCO3* 36.67 60.81 21.98 1.70 65.61 85.47 76.96 ( Ca* 15.00 24.50 9.32 1.27 26.88 34.36 31.07 ( Al 5.47 6.68 6.38 7.01 8.12 2.38 3.18 wt% ( Ti 0.83 0.62 0.83 0.67 0.39 .0.18 0.20 ( Mg 1.92 2.02 3.10 1.92 7.31 2.35 2.81 ( Fe 9.25 6.16 6.40 6.05 6.86 4.31 3.19

( Ba 2010 4350 880 1170 3650- 6690 6580 ( Mn 15220 3470 2310 2620 3960 7730 4750 ( Ni 203 123 92 130 481 178 179 ( Co 61 71 47 50 68 118 85 ( Cr 193 219 145 149 533 539 429 ppm ( Cu 175 135 98 119 306 319 254 (Cd 4 2 3 3 7 13 11 ( Pb 84 82 59 59 71 92 76 ( Zn 199 178 150 148 168 161 141 ( Li 39 72 55 46 24 31 28 ( As 42 16 6 9 7 20 10

All data are CFB, except * which are TSB

1. Metal-Rich DSDP Sediments (32) ) 2. DSDP Basal Carbonates (NMR) (7) ) 3. DSDP Basal Clayey-carbonates (NMR) (46) ) 4. DSDP Basal Clays (NMR) (69) ) This study 5. Fracture Zone Sediments, CIOR (7) ) 6. Crestal Sediments, CIOR (16) ) 7. Average Surface Sediment CIOR (69) ) 373 Fig. 4.2.1a., 4.2.1b and 4.2.1c. Distribution of Ca, Al, Fe and Mn in the Carbonate- A Free Fraction of Recent and Basal Metal-Rich Sediments from --- the Indian Ocean. A A A A 0 A 0 o A A e0 A A

Ca Al

Al Crestal Sediments

0 Fracture Zone Sediments

Metal-Rich Basal Carbonates

0 Metal-Rich Basal Clayey Carbonates Metal-Rich Basal Clays.

1=e 374

with the accumulation rates, on average, that on ''a number of occasions in the geological past, ridge activity causing metal additions may have been up to 2.25 times greater than now9along parts of the Mid-Indian Ocean Ridge System.

In comparison with fracture zone sediments (Column 5, Table 4.2.1), the basal metal-rich sediments are enriched in Ti, Fe, Mn and As and depleted in Al, Mg, Ni, Cr and Ca. The enrichment in fracture zone sediments of such metals as Al, Mg, Ni, Cr, Cu and Co, together with the higher accumulation rates of Al, Mg and Ti may indicate that they are concentrated in basaltic detrital fragments formed from the breakdown of the basalt. Furthermore, they may also be contained in clay minerals which are residues formed from the hydrothermal leaching of basaltic material exposed along such features (Bonatti, 1975; Bonatti et al, 1976, a, b and c). The enrichment and high accumulation rates in basal metal-rich sediments of Fe, Mn, As, Ni and Cu etc. suggests that these metals may be precipitated from hydrothermal fluids after being leached from the basalts. In the fracture zones, these influences may not operate to the same degree. Under the possibly restricted, deep water conditions in the fracture zones, unsuitable physio-chemical conditions (i.e. low Eh and ph) might not permit the precipitation of Mn (Fe) oxides and. their associated trace metals.

4.2.2 Comparisons of DSDP Indian Ocean Metal-Rich Sediments with Metal- Rich Surface Sediments from the East Pacific Rise and the Mid-Atlantic Ridge (Table 4.2.2, Figs. 4.2.2a, b and c).

The comparison between surface and DSDP metal-rich sediments is a valid one since Cronan (1974) and Horowitz (1974a) have shown these sediments to be chemically similar in the Pacific and Atlantic Oceans respectively.

The Indian Ocean DSDP metal-rich sediments are enriched in all metals except Al and Cu when compa-ed to normal Pacific pelagic sediments (Column 3, Table 4.2.2) and all metals except Al when compared to surface Atlantic sediments (Column 6, Table 4.2.2). The lower Al value in the Indian Ocean sediments is probably due to the larger amounts of detrital phases in the normal pelagic sediments, while the Cu value is probably caused by greater amounts of Cu being held in the lattice, and adsorbed onto the surface, of clay minerals (Chester and Hughes, 1967, 1969). This is in keeping with the chPracteristics of metal-rich as compared to normal pelagic sediments (Bostr8m et al, 1966; Crgnan, 1974).

375

TABLE 4.2.2 Chemical Composition of Indian Ocean DSDP Metal-Rich Sediments and Metal-Rich Surface Sediments from the East Pacific Rise and Mid-Atlantic Ridge.

1 2 3 4 5

( CaCO3* 36.67 - 0.70 - 11.50

( Ca* 15.00, 0.66 - 5.00 ( Al 5.47 0.50 9.34 5.30 5.88 7.82 wt% ( Ti 0.83 0.02 - 0.5 1.52 0.45 ( Mg 1.92 - - 4.19 3.00 ( Fe 9.25 18.00 5.06 9.62 10.30 5.74

( Ba 2010 - - - ( Mn 15220 6000o 4800 5400 1200 440o ( Ni 203 430 210 214 .39 110 ( Co 61 - 100 - 40 ( Cr 193 55 100. 128 110 83 ppm ( Cu 175 730 320 399 92 120 ( Cd 4 - ( Pb 84 152 7o 349 63 47 ( Zn 199 380 160 228 140 125 ( Li 39 - - 36 12 ( As 42 - - 204

All data are CFB, except * which are TSB

1. Indian Ocean Metal-Rich Sediments (32) (This study) 2. East Pacific Rise Sediments.(Bostrdm & Peterson, 1969; Horowitz, 1970) 3. Pacific Pelagic Sediments (Chester,1965a; Cronan, 1969) 4. Mid-Atlantic Ridge, 45°N, Median Valley Sediments (Cronan, 1972; (Horowitz, 1974a)

5. Reykja.nes Ridge Sediments (Horowitz, 1974a)

6. Surface Atlantic Sediments (Wedepohl_ ; 1(160; Bostr8m et al, 1969, Chester et al, 1976). 376 Fig. 4.2.2a., 4.2.2b & 4.2.2c. Distribution of Ca, Al, Fe and Mn in the Carbonate-Free Fraction of Indian Ocean Metal- Rich Basal Sediments and Metal-- Rich Surface Sediments from the. East Pacific Rise and Mid- Atlantic Ridge.

Ca Al

Al p Indian Ocean Metal-Rich Basa= Carbonates

O Indian Ocean Metal-Rich Basa* Clayey-Carbonates t Indian Ocean Metal-Rich Basal Clays O East Pacific Rise Surface Sediments ® Reykjanes Ridge Surface Sediments • Mid-Atlantic Ridge, 45°N, Median Valley Sediments

A o 0 Ap 0 ® @ Ads. 0

Ca 377

As compared with metal-rich sediments from the East Pacific Rise (Column 2, Table 4.2.2), the Indian Ocean DSDP metal-rich sediments are depleted in all metals except Al, Ti and Cr. This.istin keeping with the findings for basal sediments from- the Pacific and Īndian oceans which reflect the greater ridge crest hydrothermal and lesser detrital

influences in the Pacific Ocean. ■

When compared to Reykjanes Ridge sediments (Column 5, Table 4.2.2) a similar pattern emerges to that when basal sediments from the Indian and Atlantic Oceans are compared, i.e. higher Fe, Mn, Ni, Cu, Cr, Zn and Li, and lower Al, Ti and Mg in Indian Ocean DSDP metal-rich sediments. This - is a reflection of the greater degree of variations in ridge activity in the Indian Ocean as compared to the Atlantic Ocean. The higher Mg in Reykjanes Ridge sediments p_obably reflects the larger amounts of detrital material (Ku et al, 1968) in Atlantic surface sediments as compared to Indian Ocean basal metal-rich sediments.

When compared to surface sediments from the Median Valley at 45°N, Mid-Atlantic Ridge (Column 4, Table 4.2.2) the pattern found in comparison to other basal and surface sediments from the Indian and Atlantic Oceans is not seen. Here the concentrations of Mg, Fe, Cu, Pb, Zn and As are higher than in Indian Ocean DSDP metal-rich sediments, but the Al, Ti, and Cr values are lower than in Indian Ocean DSDP metal-rich sediments. The sediments from the Median Valley of the Mid-Atlantic Ridge are sampled in close proximity to a volcanic-hydrothermal vent (Cronan, 1972; Horowitz, 1974a), and the enriched values can be accounted for in terms of localised hydrothermal addition to the sediments (Cronan, 1972; Horowitz, 1974a).

The intermediate composition of Indian Ocean DSDP metal-rich sediments as compared to surface metal-rich sediments from the Pacific and Atlantic Oceans is explicable in terms of the interplay of the same two independent processes (described in Section 3.4.1d). The compositional variations between the groups of surface and DSDP metal-rich sediments from the Atlantic and Pacific, and Indian Oceans respectively are well-illustrated in Figs. 4.2.2a, b and c. 378

4.3 General Summary : Variations in Metal Enrichment Processes with Time in Indian Ocean Sediments.

Geochemical partition studies, together with other supportive evidence, provide a means of indicating the processes of metal enrichment in marine sediments and of estimating the proportions of the total concentrations of particular elements supplied by different processes. This is particularly important, in order to resolve the proportions of elements supplied by ridge crest (hydrothermal) activity. Variations in metal enrichment processes with time are now discussed for each element in question. .

It is apparent, from the geochemical partition data, that the processes of enrichment, and possible sources, of certain elements have remained relatively constant during the course of the geological evolution of the\ Indian Ocean.

In the carbonate-rich sediments, particularly the Recent surface sediments- Ca is associated with biogenic carbonate material. In certain sediments, e.g. fracture zone sediments and basal sediments (DSDP Sites 213, 239) minor amounts of Ca are associated with Ca-rich detrital minerals as a result of the breakdown of basaltic material. Also where Ca concentrations are low, minor amounts are contained in phosphatic fish debris as in the basal sediments at DSDP Sites250A and 212 of Mid-Late Cretaceous age. Generally speaking, however, the Ca contents are a reflection of the presence or absence of a carbonate sedimentation regime.

In sediments of all ages, titanium is enriched due to the inclusion of unaltered basaltic detrital fragments containing such minerals as anatase and ilmenite. The slow weathering of basaltic materia] may also add Ti in clay minerals to the sediments. In crestal and non-crestal Recent surface sediments, small amounts of Ti may be precipitated with authigenic Mn oxides. The concentration of Ti appears to have been unaffected by variations in ridge derived metal additions during the course of time.

Aluminium in all the sediments is included in clay and detrital:minerals. There appears to be no evidence to support the observation of Heath and Dymond (1977) for Nazca Plate sediments that some Al may be of hydrothermal origin in Indian Ocean sediments. The concentrations and rates of accumulation of Al, like those of Ti, are unassociated with variations. in ridge activity (i.e. spreading rates). 379

Magnesium, shows a very similar pattern of enrichment to that of Al, being associated with the clay and detrital minerals. There is no evidence to support the observation of Bischoff and ROsenbauer (1977) for Pacific sediments that some. Mg may be of hydrothermal Origin in any of the Indian Ocean sediments studied here. However, there is evidence, particularly in crestal and non-crestal Recent surface sediments, that where large amounts of carbonate material occur, Mg may be held in the lattice of high-Mg calcite. This only occurs to any marked degree in the Recent surface sediments, while very minor amounts of Mg may be associated with proportions of sea salt in all the Indian Ocean sediments studied here.

Lithium in all the sediments is related to clay and detrital mineral phases. Horowitz (1974a) has suggested for N-Atlantic sediments that detrital Li may be of a continental origin. Continental detrital input seems unlikely into these Indian Ocean sediments, except in the form of airborne dust, due to their great distance from a continental source. However, in the case of non-crestal Recent surface sediments in the north of the sample area only, some detrital Li may be incorporated from the erosion of the granitic rocks of the Seychelles Bank, as well as its incorporation by the slow weathering of basaltic material. In Recent, crestal and non-crestal surface sediments from the Central Indian Ocean Ridge, large proportions of Li are associated with the biogenic carbonate material of these foraminiferal oozes.

The geochemical partition data for Cd, although not fully comprehensive, show that in general in all the Indian Ocean sediments studied here, Cd is incorporated into the sediments by the normal processes of oceanic sedimentation, such as the slow weathering of basaltic material, authigenic precipitation from sea water and the inclusion of basaltic detrital fragments. Only in the carbonate-rich surface sediments from the Central Indian Ocean Ridge and basal carbonates from DSDP Sites 251A, 220, 213, 245 and 250A is Cd significantly related to the biogenic carbonate component of the sediments. In no sediments studied here are the concentrations of Cd affected by variations in ridge crest activity, i.e. spreading rates.

The geochemical partition data indicate that the processes of enrichment of the remaining elements have been effected, strongly in some cases,. by the variations in ridge crest activity as a result of variations in seafloor spreading rates. 380

Barium in all the sediments generally occurs in the form of the HCL soluble barytes, where it is supplied from biogenic sources, as in Recent crestal and non-crestal surface sediments and in basalt sediments from DSDP Site 256, and may also be associated with the lattice of clay minerals and authigenically'precipitated from seawater. In basal sediments from DSDP Sites 245 (Early Palaeocene), 215, 236 (Mid-Palaeocene) and 251A (Early Miocene) the higher absolute concentrations and accumulation rate of Ba suggest a ridge derived hydrothermal source for this Ba. The same is also true, but to a lesser degree for Recent crestal surface sediments. The hydrothermal addition of Ba to these sediments is related, and due, to the increased ridge activity as a result of the increased rates of seafloor spreading during these periods. Hydrothermal addition of Ba has been suggested for East Pacific Rise sediments by Arrhenius and Bonatti (1965) and Heath and Dymond (1977).

Iron in all the sediments is contained in the HCL soluble clay mineral phases.. However, additional amounts of Fe contained in the HCL leach of certain sediments (crestal surface sediments, DSDP Sites 251A(Early Miocene 215, 236 (Mid-Palaeocene), 213, 245 (Early Palaeocene) and 239, 216 (Late Cretaceous)) may be due to the presence of Fe oxyhydroxides and silicates which result from the hydrothermal leaching of this element from the underlying basalts. These sediments also have higher accumulation rates of Fe. Such a hydrothermal addition of Fe to these sediments is related to the increased ridge activity, caused by increased rates of seafloor spreading during these periods. Such effects are most marked in basal metal-rich sediments of Palaeocene age when seafloor spreading rates were at their highest (Schlich, 1974) due to the northward movement of the Indian continental plate (see Section 1.2). The effect is less marked in Recent surface sediments from the Central Indian Ocean Ridge where only local enrichments occur. In Recent surface fracture zone sediments increased concentrations of Fe are associated with the HCL soluble and I-ICL insoluble phases and reflect the inclusion in the sediments of the alteration products (clay minerals and unaltered detrital fragments) of basaltic and possibly ultramafic rocks exposed along the fracture zones, with anly local hydrothermal additions.

The patterns of enrichment of nickel, copper and zinc are generallj, similar to those of Fe. Their entrainment is associated with the slow weathering of basaltic material and the authicenic precipitation nf Mn oxides from seawater. In basal sediments of Palaeocene (DSDP Sites 215, 236, 381

213, 245) and Late Cretaceous (DSDP Sites.239) age increased proportions of Ni, Cu and Zn in the A/R agent and HCL leaches reflects, in part, the removal of these metals by hydrothermal leaching from\the basaltic rocks and their precipitation with Fe and Mn`oxyhydroxides and Fe silicates. Such processes are.probably related to the increased ridge activity during these periods. In Recent fracture zone surface sediments from the Central Indian Ocean Ridge the increased concentrations of Ni, Cu and Zn in the HCL soluble and insoluble phases in clay mineral and unaltered detrital fragments probably reflects the alteration of basaltic and ultramafic rocks exposed in these features. In Recent crestal and non- crestal surface sediments large proportions of these metals are linked - to the biogenic component of these foraminiferal oozes. The same is true, but to a lesser extent, for the fracture zone sediments due to the dissolution of the biogenic carbonate and release of the carbonate lattice held trace metals (Ni, Cu, Zn) below,the lysocline.

Manganese in all the sediments is authigenically precipitated from seawater as Mn oxides, which are soluble in the A/R agent leach. In two groups of sediments additional proportions and concentrations of Mn occur in the A/R agent leach. In Recent crestal surface sediments, local enrichments of Mn and increased rates of Mn accumulation are related to its removal from the ocean floor basalts with the Fe by hydrothermal leaching. The An is precipitated as Mn oxides and mixed FeMn oxides in the crestal region of the Central Indian Ocean Ridge. In basal metal-rich sediments from DSDP Sites 251A (Early Eocene), 215, 236 (Mid-Palaeocene) and 213,245 (Early Palaeocene) the increased concentrations of Mn are too high to be accounted for solely by hydrothermal leaching of the underlying basalts. It may therefore be necessary to suggest a deeper seated volcanic (i.e. lower crustal) source for This increased Mn. Such hydrothermal and deep seated additions of Mn are related to the increased ridge activity, i.e. seafioor spreading rates recorned during this period. Manganese may also be linked to biogenic material (DSDP Sites 221, 249) and removed by the slow weathering of basaltic material (DSDP Sites 224, 21.2) as well as authigenically precipitated from seawater.

The pattern of enrichment of cobalt is generally similar to that of Mn in that it tends to be coprecipitated with authigenic Mn oxides from seawater, although its enrichment has been generally unaffected by variations in ridge-crest activity. In certain basal sediments it is entrained due to slow, weathering of basaltic material by seawater (DSDP Sites 221, 224, 220, 211, 239, 256, 257, 212, 249) while in the Recent surface sediments from the Central Indian Ocean Ridge it is linked to the biogenic component of these foraminiferal oozes.

The enrichment of lead in the Indian Ocean sediments studied here has, to a minor extent, been affected by variations in ridge crest activity (spreading rates) in that it is enriched in the A/R agent leaches of sediments of Palaeocene age (DSDP Sites 213, 236, 215, 245) and Recent surface crestal sediments, where it is precipitated with authigenic Mn oxides and has probably been supplied by the hydrothermal leaching of the underlying basalts. Generally speaking the major link is between this element and the biogenic component of the sediments, where it is incorporated into the lattice of the carbonate minerals. Minor amounts are related to unaltered basaltic fragments included in the sediments as in Recent fracture zone surface sediments and certain basal sediments, e.g. DSDP Sites 251A, 224 and '249.

The enrichment of chromium in the sediments is principally related to the inclusion of unaltered basaltic detrital fragments. In Recent fracture zone sediments, the high concentrations of Cr in the HCL soluble mineral. phases may also result from the removal of this metal from ultramafic as well as basaltic rocks by slow weathering processes. There appears to be no relationship between the enrichment of Cr and increased ridge crest hydrothermal activity and seafloor spreading rates. In Recent non-crestal surface sediments minor amounts of Cr are related to the biogenic component of these carbonate, foraminiferal oozes.

No partition data are available for arsenic. However, on the basis of accumulation rates and correlation data some As is coprecipitated with Fe perhaps after its release from crustal rocks, but also probably from normal seawater. In Recent crestal surface sediments additional amounts of As may be supplied by the hydrothermal leaching of basaltic material. In certain basal sediments (DSDP Sites 245, 213, 215) the concentrations and rates of accumulation of. As are very high indeed, and the correlation of Mn with As may indicate that it is supplied to these Sediments from v=olcanic sources and by authigenic precipitation from seawater. The enrichment of As appears to be sensitive to variations in ridge crest activity ano seafloor spreading rates. 383

In summary, it would appear that the patterns and processes of enrichment of Ca, Ti, Al, Mg, Li, Cd and Cr have remained relatively ti constant and have not been directly affected by variations in ridge crest activity and seafloor spreading rates, during the geological evolution of the Indian Ocean. However, it would appear that the concentrations of such elements as Fe, Ni, Cu, Zn, Mn, As, Pb and to a lesser extent Co include a hydrothermal, ridge crest derived component. Furthermore, the input of this component into the Indian Ocean sediments studied here varied during the course of the geological evolution of the Indian Ocean and has been of greatest extent during the Palaeocene and Early Miocene periods, to a lesser extent during the Late Cretaceous and has been of only local signifidance in Recent times. The rate of input of this component (increasing the concentrations of certain elements) can be related in general terms to the rates of seafloor spreading. 384

BIBLIOGRAPHY

ALLAN, J.E. (1961) Report of 3rd Australian Spectroscopy Conference, reviewed by Durie,R.A., Nature, 192, Pp. 927-929.

ANGINO, E.E., BILLINGS, G.K. (1972), Atomic Absorption Spectroscopy in geology (Methods in Geochemistry and Geophysics 7). Amsterdam, Elsevier.

ANDELSECK, C.G., Jr., BERGER, W.H. (1975) On the Dissolution of Plank- tonic foraminifera and associated microfossils during settling and on the seafloor, Pp. 70-81. IN Dissolution of Deep Sea Carbonates, Spec. Publ., No. 1 , Cushman _ Foundation for Foraminiferal Research, U.S.A. •

ARRHENIUS, G. (1963) Pelagic Sediments IN The Sea, 2, Pp.-655-727, Hill, M.M. (Ed). Wiley Interscience.

and BONATTI, E. (1965) Neptunism and Vulcanism in the Oceans, IN Progresses in Oceanorg aphy, 3, Pp. 7-26, Sears, M. (Ed). Pergammon, London.

ASTON, S.R., CHESTER, R., GRIFFITHS, A and RILEY, J.P. (1972a) Distribution - of Cadmium in North Atlantic Deep Sea Sediments, Nature, 239, P. 393.

BRUTY D., CHESTER, R. and RILEY, J.P. (1972b) The Distribution,- of Mercury in North Atlantic Deep Sea Sediments, Nature, (Phys. Sci), 237, P.125.

CHESTER, R. and JOHNSON, L.R. (1972c) Uptake of Cobalt from seawater by Aeolian Dust, Nature (Phys. Sci), 235, Pp. 380- 381.

ATWATER, T. and MENARD, H.W. (1970) Magnetic Lineations in the north-east Pacific, Earth. Planet. Sci. Letts,2, Pp. 445-450.

AVRAHAM, Z.B. and BUNCE, E.T. (1977) Geophysical Study of the Chagos- Laccadive Ridge, Indian Ocean. J. Geophys. ~..Res., 82, Pp. 1295-1305. BAKER, B.H. (1963) Geology and mineral resources of the Seychelles Archipelago. Geol. Surv. Kenya Mem.,2. Min. Comm. and ,Industry, Kenya.

BARANOV, V.I., KUZMINA, L.A. (1958) The rate of silt deposition in the Indian Ocean. Geochemistry, 2., Pp. 131-140.

BARRETT, D.M. (1977) Agulhas Plateau off southern Africa: A geophysical - study-. Bull. Geol. Soc. Atm, 88. Pp. 7't.-9-763. BATURIN, G.N. and ROZANOVA, T.V., (1975) Ore Mineralisation in the Rift Zone of the Indian Ocean. IN Rift Zones of the World Ocean, Pp. 431-441. Tetoruk-Schneider, R. (Ed.). J. Wiley & Sons, New York and IPST, Jgrusalem.

BE, A.W.H. (1965) The influence of depth on shell growth of Globigerinoides saccalifer (Brady). Micropalaeo., 11, Pp. 81-97.

BELYAYEV, N.V. (1973) Characteristics of the Chemical Composition of the tests of Planktonic Foraminifera. Oceanol., 13, Pp. 247-250.

BENDER, M., BROECKER, W., GORNITZ, V., MIDDEL, U., KAY, R., BISCAYE, P. and SHINE-SOON-SUN (1971) Geochemistry of three cores from the East Pacific Rise. Earth. Planet. Sci. Letts., 12, Pp. 425-433.

KU, T. and BROECKER, W. (1970) Accumulation rates of Mn in Pelagic Sediments and Nodules. Earth. Planet. Sci. Letts., 8, Pp. 143-148.

and SCHULTZ, C. (1969) The Distribution of trace metals in cores from a traverse across the Indian Ocean. Geochim. Cosmochim.'Acta., 33, Pp. 292-297. BERGER, W.H. (1967) Foraminiferal Ooze Solution at Depths. Science, 156, Pp. 383-385.

(1970) Planktonic Foraminifera : Selective Solution and the Lysocline. Mar. Geol., 8, Pp. 111-138.

(1971) Factors affecting the Pelagic Fossil Record. Geol. Soc. Am. Abs. with Programs., 3, Pp. 503-504.

(1974) Deep Sea Sedimentation.IN The Geology of the Continental Margin., Pp. 213-241, Burk, C.A., Drake, C.L. (Ed), Springer-Verlag. New York.

and WINTERER, E.L. (1974) Plate stratigraphy and the fluctuating carbonate line. Spec. Pubis. Int. Ass. Sediment., 1, Pp. 11-48.

(1976) Sedimentation of Deep Sea Carbonate : Maps and Models of Variations and Fluctuations. IN Marine. Plankton and Sediments., Riedel, N.R., Saito, T. (Ed)., Micropalaeontology Press.

ADELSECK, C.G., Jr., and MAYER, L.A. (1976) Distribution of Carbonate in Surface Sediments of the Pacific Ocean. J. Geophys. Res., 81, Pp. 2617-2627.

BERGH, H.W. (1971) Sea Floor Spreading in the Southwest Indian 0cean. J. Geophys. Res., 76, Pp. 6276-6282. 386

BERTINE, K.K. (1974) Origin of Lau Basin Rise Sediments. Geochim. Cosmochim. Acta, 38, Pp. 629-640. t and GOLDBERG, E.D. (1972) Trace Elements in Clams, Mussels and Shrimp. Limnol. Oceanog., 17, Pp. 877-884.

BIGNELL, R.D. (1975) Timing, distribution and origin of submarine mineralisation in the Red Sea. Trans. Inst. Min. Metall. (B)., 84, Pn. 1-6.

CRONAN, D.S. and TOOMS, J.W. (1976) Metal Dispersion in the Red Sea as an aid to marine gec,chemical exploration. Trans. Inst. Min. Metall. (B), 85, Pp. B274-B278.

BILLINGS, G.K. (1963) Major and trace element relationships within co.;- existing minerals of the Enchanted Rock Batholith, Llano Uplift, Texas. Thesis, Rice University, Houston, Texas. Unpub. 74pp.

(1965) Light scattering in trace element analysis by Atomic Absorption. Atomic. Absorp. Newsletter, 4, Pp. 357-261. ..

BISCHOFF, J.L. (1969) Red Sea Geothermal brine deposits : their mineralogy chemistry and genesis. IN: Hot Brines and Recent Heavy Metal Deposits in the Red Sea, Degens, E.T. (Ed)., Springer- Ver?_ag, New York.

and DICKSON, F.W. (1975) Seatwater-Basalt Interaction at 200~C and 500 bars : Implications for Origin of Sea Floor Heavy-Metal Deposits and Regulation of Seawater Chemistry. Earth.Planet. Sci. Letts., 25, Pp. 385-397.

and ROSENBAUER, R.J. (1977) Recent Metalliferous Sediment in the North Pacific Manganese Nodule Area. Earth,Planet. Sci. Letts., 33, Pp. 379-388.

BLOW, R.A. and HAMILTON, N. (1975) Palaeomagnetic evidence from DSDP Cores of northward drift of India. Nature, 257, Pp.570-572.

BONATTI, E. (1975) Metallogenesis at oceanic spreading centres. Ann. Rev. Earth. Planet. Sci., 3, Pp. &o1-4)1.

and JOENSUU, O. (1966) Deep Sea Iron Deposit from the South Pacific. Science, 154, Pp. 643-645.

FISIiER, D.E., JOENSUU, O. and RYDELL, H.S. (1971) Post depositional mobility of sortie transition elements, phosphorus uranium and thorium in deep sea sediments. Geochim. Cosmochim. Acta., 35, Pp. 189-201.

FISHER, D.E., JOENSUU, I., RYDELL. H.S. and BEYTH, M. (1972) Iron-Manganese-Barium Dep'-sit from the Northern Afar Rift. (Ethiopia) Econ. Geol., 67, Pp. 717-730.

387

BONATTI, E., GUERSTEIN-HONNOREZ, B.M. and HONNOREZ,,,J. (1976a) Copper- Iron Sulphide Mineralisations from th Equatorial Mid- Atlantic Ridge. Econ. Geol., 71, Pp. '1515-1525.

and HONNOREZ, J. (1976b) Sections of the Earth's Crust in the Equatorial Atlantic (in press - authors Manuscript, 15pp)- HONNOREZ-GUERSTEIN, M.B. HONNOREZ, J. and STERN, C. (1976c) Hydrothermal Pyrite Concretions from the Romanche Trench (Equatorial Atlantic) : Metallogenesis in Oceanic Fracture Zones. Earth. Planet. Sci. Letts, 32, Pp. 1-10.

BOSTROM, K. (1967) The Problem of Excess Manganese in Pelagic Sediments. IN : Researches in Geochemistry, 2, Pp. 421-452, Abelson, P.H. (Ed). J Wiley and Sons, New York.

and PETERSON, M.N.A. (1966) Precipitations from Hydrothermal Exhalations on the East Pacific Rise. Econ. Geo1.,61, Pp. 1258-1265.

(1969) The Origin of Aluminium-Poor Ferromanganoan Sediments in Areas of high Heat Flow on the East Pacific Rise. Mar. Geol., 7, Pp. 427-447.

PETERSON, M.N.A., JOENSUU, O. and FISHER, D.E. (1969) Aluminium-Poor Ferromanganoan Sediments on Active Ocean Ridges. J. Geophys. Res., 74, Pp. 3261-3270.

and VALDES, S. (1969) Arsenic in Ocean Floors. Lithos, 2, Pp. 351-360.

and FISHER, D.E. (1971) Volcanogenic Uranium, Vanadium and Iron in Indian Ocean Sediments. Earth. Planet. Sci. Letts., 11, Pp. 95-98.

JOENSUU, O., VALDES, S and RIERCA, M. (1972) Geochemical History of the. South Atlantic Ocean Sediments since Late Cretaceous. Mar. Geol., 12, Pp. 85-121.

JOENSUU, O.. MOORE. C., BOSTROM, B., DALZIEL, M. and MOROWITZ, A. (1973a) Geochemistry of Barium in pelagic sediments. Lithos, 6, Pp. 159-172.

KRAEMAR, T. and GARTNER, S. (1973b) Provenance and Accumula- tion Rates of Opaline Silica, Al, Ti, Fe, Mn, Cu, Ni and Co in Pacific Pelagic Sediments. Chem. Geol., 11, Pp. 123-148.

JOENSUU, 0. and BROHM, I. (1974) Plankton : Its Chemical Composition and its significance as a source of pelagic sediments. Chem. Geo1.,14, Pp. 255-271. 388

BOSTROM, K., JOENSUU, 0. VALDES, S., CHARM, W. and GLACCUM, R. (1976) Geochemistry and Origin of East Pacific Sediments, sampled during DSDP Leg 34. IN : Initial Reports of the DSDP, 34, Pp. 559-574, Yeats, R.S., et al (d). U.S. Govt. Print. Office, Washington, D.C.

BOWIN, C. (1973) Origin of the Ninety East Ridge from Studies near the Equator. J. Geophys. Res., 78, Pp. 6029-6043.

BROECKER, W.S. and BROECKER, S. (1974) Carbonate Dissolution on the Western Flank of the East Pacific Rise. IN : Studies in PalaeoOceanography, Pp. 44-57, Hay, W.W.(Ed)., Soc. Econ. Palaeo. Miner. Spec. Paper, No. 20.

BRUTY, D., CHESTER, R., ROYLE, L. and ELDERFIELD, H. (1972) Distribution of Zn in North Atlantic Deep Sea Sediments. Nature, (thys. Sci), 237, Pp. 86-87.

CHESTER, R. and ASTON, S. (1973) Trace Elements in Ancient Atlantic Deep Sea Sediments. Nature (Phys. Sci), 245, PP- 73-74. BURKLE, L.H., VENKATARATHNAM, K. and BOOTH, J.D. (1974) Sediment Transport by Antarctic Bottom Water in the Western Indian Ocean. Trans. Am. Geophys. Union., 55, P. 312.

BUSER, W. and GRUCTER, A. (1956) Uber die natur der mangankuollen. Schweiz Mineral. Petrog. Mitt. 36, Pp. 49-62.

CALVERT, S.E. and PRICE, N.B. (1972) Diffusion and Reaction Profiles of Dissolved Manganese in the Pore Waters of Marine Sediments. Earth. Planet. Sci. Letts., 16, Pp. 245-249.

(1977) Shallow water, Continental Margin and Lacustrine Nodules : Distribution and Geochemistry. IN : Marine Manganese Deposits, Pp. 45-86, Glasby, G.P. (Ed)., Elsevier Scient. Publ. Co., Amsterdam.

CHESTER, R. (1965a) Adsorption of Zn and Co On Illite in Sea Water., Nature, 206, Pp. 884-886. (1965b) Elemental Geochemistry of Marine Sediments. IN : Chemical Oceanography, 2, Pp. 23-80, Riley, J. and`Skirrow, G. (Ed)., Academic Press, New York.

(1965c) Geochemical Criteria. for Differentiating Reef from Non-Reef facies in Carbonate Rocks. Bull. Am. Ass. Geol., 49, Pp. 258-276.

ASTON, S.R. and BRUTY, D. (1976) The Trace Element Partition Geochemistry in an ancient Deep Sea Sediment Core from the Bermuda Rise. Mar. Geol.? 21, Pp. 271-288.

and HUGHES. M.J. (1966) The distribution of manganese, iron and nickel in .a north Pacific deep-sea clay core. Deep Sea Res., 13, Pp. 627-634.

389

CHESTER, and HUGHES, M.J. (1967) A Chemical Technique for the Separation of Ferro-Manganese Minerals, Carbonate Minerals and Adsorbed Trace Elements from Pelagic Sediments. Chem. Geol., 2, Pp. 249-262.

(1969) The Trace Element Geochemistry of a North Pacific Pelagic Clay Core. Deep Sea Res., 16, Pp. 639-654.

and MESSIHA-HANNA, R. (1970) Trace Element Partition Patterns in North Atlantic Deep Sea Sediments. Geochim. Cosmochim. Acta., 34, Pp. 1121-1128.

and STONER, J.H. (1974) The Distribution of Zn, Ni, Mn, Cd, Cu and Fe in some surface waters from the world oceans. Mar. Chem, 2, Pp. 17-32.

COOK, H. (1971) Iron and Manganese Rich Sediments Overlying Oceanic Basalt Basement, Equatorial Pacific, Leg 9, DSDP. Geol. Soc. Am. Abs. with Program., 3, Pp. 530-531. COPELAND, R. (1970) Trace Element Distribution in Sediments of the North Atlantic Ridge. Unpubl. Thesis, Massachusetts Institute of Technology, Cambridge, Mass.

CORLISS, J.B. (1971) The Origin of Metal-Bearing Submarine Hydrothermal Solutions. J. Geophys. Res., 76, Pp. 8128-8138.

CRONAN, D.S. (1969) Average Abundances of Mn, Fe, Ni, Co, Cu, Pb, Mo, V, Cr, Ti and P in Pacific Pelagic Clays. Geochim, Cosmochim. Acta.,33, Pp. 1562-1565. (1972) The Mid-Atlantic Ridge near 45oN., XVII,- Al, As, Hg, and Mn in Ferruginous Sediments from the Median Valley. Can. J. Earth. Sci., 9. PP. 319-323.

(1973) Basal Metalliferous Sediments Cored during Leg 16, Deep Sea Drilling Project. IN : Initial Reports of the DSDP, 16, Pp. 601-604, Van Andel, Heath, C.R. (Ed)., U.S. Govt. Print. Office, Washington, D.C.

(1974) Authigenic Minerals in Deep Sea Sediments IN : The Sea,.9, Pp. 491-526, Goldberg, E. (Ed)., John Wiley and Sons Inc., New York.

(1976) Basal Metalliferous Sediments from the eastern Pacific .: Bull. Geol. Soc. Am., 87, Pp. 928-934.

DAMIANI, V.V., KINSMAN, B.J.J., (1974) Sediments from the Gulf of Aden and the western Indian Ocean. IN : Initial Reports of the DSDP, 24, Pp. 1047-1110, Fisher, R.L. et al (Ed)., U.S. Govt. Print. Office, Washington. D.C. 390

CRONAN, D.S. and GARRETT, D.E. (1973) Distribution of Elements in Metalliferous Pacific Sediments collected during the Deep Sea Drilling Project. Nature (Phys. :Sci.), 242, Pp. 88-89.

and TOOMS, J.S. (1969) The Geochemistry of manganese nodules and associated pelagic sediments from the Pacific and Indian Oceans. Deep Sea. Res., 16, Pp. 335-359. VON ANDEL, T.H., HEATH, C.R., DINKELVEN, M.G., BENNETT, R.H., BURRY, D., CHARLESTON, S., KANEPS, A., RODOLFO, K.S. and YEATS, R.S. (1972) Iron Rich Basal Sediments from the Eastern Equatorial Pacific - Leg 9.6, Deep Sea Drilling Project, Science, 175, Pp. 61-63.

CURRAY, J.R. and MOORE, D.G. (1971) Growth of the Bengal Deep Sea Fan and Denudation of the Himalayas. Bull. Geol. Soc. Am., 82, - Pp. 563-572.

DASCH, E.J. DYMOND, J.R., and HEATH, G.R. (1971) Isotopic Analysis of Metalliferous Sediments from the East Pacific Rise. Earth. Planet. Sci. Letts., 13, Pp. 175-180.

DAVID, D.J. (1961) The determination of Molybdenum by Atomic Absorption Spectrophotometry. Anal., 86, Pp. 730-740.

DAVIES, T.A., LUYENDYK, B.P. et al (1974) Initial Reports of the Deep Sea Drilling Project, 26, U.S. Govt. Printing Office, Washington, D.C. 1129 pp. _

DAVIS, J.G. (1973) Statistics and Data Analysis in Geology. 55Opp., John Wiley and Sons Inc., New York.

DEITRICK, R.S., SCLATER, J.G. and THIEDE, J. (1977) The Subsidence of Aseismic Ridges, Earth. Planet. Sci. Letts., 34, Pp. 185-196.

DIETZ, R.S. and HOLDEN, J.C. (1970) Reconstruction of Pangea : Breakup and Dispersion of Continents, Permian to Present. J. Geophys. Res., 75, Pp. 4939-4956. (1971) Pre-Mesozoic Oceanic Crust in the Eastern Indian Ocean (Wharton Basin?) Nature, 229, Pp. 309-312...-

DYMOND, J., CORLISS, J.B., HEATH, G.R., FIELD, C.W., DASCH, E.J. and VEEH, H.H. (1973) Origin of Metalliferous Sediments from the Pacific Ocean. Bull. Geol. Soc. Am., 84, Pp. 3355-3372. and STILLINGER, R. (1976) Chemical Composition and Metal Accumulation Rates of Metalliferous Sediments from Sites 319, 320, and 321. IN : Initial Reports of the DSDP, 34, Pp. 575-588, Yeats, R.S., Hart, S.R. et al (Ed). U.S. Govt. Print. Office, Washington, D.C.

?nd HEATH. G.R. (1977) History of Metalliferous Sedimentation at Deep Sea Drilling Site 319• in the South Eastern Pacific. Geochim. Cosmochim. Acta, 41, Pp. 741-753. 391

DYMOND, J. and VEEH, H.H. (1975) Metal Accumulation Rates in the South East Pacific and the Origin of Metalliferous Sediments. Earth. Planet. Sci. Letts., 28, Pp. 13-22.

EDMOND, J.M (1974) On the dissolution of carbonate and silicate in the deep ocean. Deep. Sea. Res., 21, Pp. 455-480.

ELDERFIELD, H. (1976) Manganese' Fluxes to the Oceans. Mar. Chem, 4, Pp. 103-122.

GASS, I.G., HAMMOND, A. and BEAR, L.M. (1972) The Origin of Ferromanganese Sediments associated with the Troodos Massif, Cyprus, Sediment., 19, Pp. 1-19.

EMELYANOV, E.M. (1974) Titanium in Atlantic Ocean Sediments. Geochem. Intern., 11, Pp. 441-446.

ENGEL, C.G. and FISHER, R.L. (1975) Granitic to ultramafic- rock complexes of the Indian Ocean Ridge System, western Indian Ocean. Bull. Geol. Soc. Am., 86, Pp. 1553-1578. EPSTEIN, M.S. and RAINS, T.C. (1976) Evaluation of Xenon Mercury Arc Lamp. for Background Correction in Atomic Absorption Spectrometry. Anal. Chem., 48, Pp. 528-531.

ERICSON, D.B. and WOLLIN, G. (1973) Precipitation of Manganese Oxide in Deep Sea Sediments IN : Interuniversity Program of Research on Ferromanganese Deposits on the Ocean Floor, Phase I Report, Pp. 99-103., unpubl. N.S.F.

EWING, M., AITKEN, T. and EII:TREIM, S. (1968) Giant Ripples in the Madagascar Basin. Trans. Am. Geophys. Union, 49, P.218.

EWINGS, M., EIFFREIN, S., TRUCKAN, M. and EWING, J.I. (1969) Sediment distribution in the Indian Ocean. Deep Sea. Res., 16, Pp. 231-248.

FISHER, R.L., BUNCE, E.T., et al (1974) Initial Reports of the Deep Sea Drilling Project, 24, U.S. Government Printing Office, Washington, D.C., 1183pp.

JOHNSON, G.L. and HEEZEN, B.C. (1967) Mascarene Plateau, Western Indian Ocean. Bull. Geol. Soc. Am., 7 , Pp. 1247-1266.

FISHER, R.J., SCLATER, J.G. and Mc KENZIE, D.P. (1971) Evolution of the Central Indian Ridge, Western Indian Ocean. Bull. Geol. Soc. Am., 82, Pp. 553-562.

FISHER, R.A. and YATES, F. (1964) Statistical Tables for Biological, Agricultural and Medical Research. Oliver and Boyd Ltd., Edinburgh.

FLEET, A.J. (1977) Geochemistry of Drilled (DSDP) Sediments from the Southern Indian Ocean. Unpubi. Fh.D. Thesis, University of London. 392

FLEET, A.J. HENDERSON, P. and KEMPE, D.R.C. (1976) Rare Earth Element and Related Chemistry of some drilled Southern Indian Ocean Basalts and Volcanogenic Sediments. J. Geophys. Res., 81, Pp. 4257-4268.

and KEMPE, D.R.C. (1974) Preliminary Geochemical Studies of Sediments from DSDP Leg 26, Southern Indian Ocean. IN Initial Reports of the DSDP, 26, Pp. 541-551., Davies, T.A., Luyendyk, B.P. et al (Ed). U.S. Govt. Print. Office, Washington, D.C.

FRANCIS, T.J. and RAITT, R.W. (1967) Seismic Refraction Measurements in the Southern Indian Ocean. J. Geophys. Res., 72, Pp. 3015-3041.

FUJITA, T. (1971) Concentration of Major Chemical Elements in Marine Plankton Geochem. Journ., 4, Pp. 143-156.

GIESKES, J.M., KASTNER, M. and WARNER, T.B. (1974) Geochemical Evidence for Extensive Diagenesis in Hole 245 Of DSDP. Trans. Am. Geophys. Union., 55, P.313.

GIRDLER, R.W. (1964) Geophysical studies of rift valleys. Phys. Chem. Earth., 5, Pp. 65 - 75. GLASBY, G.P., TOOMS, J.S. and HOWARTH, R.J. (1974) Goechemistry of manganese concretions from the north-west Indian Ocean. New Zealand. Journ. Sci., 17, Pp. 387-407.

GOLDBERG, E.D. (1954) Marine Chemistry. 1. Chemical scavengers of the sea. Journ. Geol., 62, Pp. 249-265.

and ARRHENIUS, G. (1958) Chemistry of Pelagic Pacific Sediments. Geochim. Cosmochim. Act., 11, Pp. 153-212.

and KOIDE, M. (1963) Rates of sediment accumulation in the Indian Ocean. IN: Earth Science and Meteorics, Pp. 90-102, Geiss, J., Goldberg, E.D. (Ed). North Holland, Amsterdam.

and GRIFFIN, J.J. (1970) The Sediments of the Northern Indian Ocean. Deep. Sea. Res., 17, Pp. 518-537.

GOLDSCHMIDT, V. (1938) Geochemistry , Muir, A. (Ed). Clarendon, Oxford, 703pp• GORBUNOVA, Z.N. (1974) Composition of Clay Minerals in the Hydrothermally Altered and Adjacent Sediments in an area of seafloor in the South Pacific Basin, Oceanol., 14, Pp. 123-127.

GREENSLATE, J.L., FRAZER, J.Z. and ARRHENIUS, G. (1973) Origin and deposition of selected transition elements on the seabed IN : The Origin and Distribution of Manganese Nodules in the Pacific and Prospects for Exploration. Pp. 45-7C, Morgenstein, M. (Ed)., Hawaii Inst. Geophys., Honolulu.

393

GRIFFIN, J.J., WINDOM, H. and GOLDBERG,-E.D. (1968) The distribution of clay minerals in the World Ocean. Deep Sea. Res., 15, Pp. 433-459.

HAJASH, A. (1975) Hydrothermal Processes along th~a,Mid-Ocean Ridges : An Experimental .Investigation. Contrib. Mineral. Petrol., 53, Pp. 205-226. HART, R.A. (1973a) A Model for chemical exchange in the Basalt-Seawater System of Oceanic Layer II. Can. J. Earth. Sci., 10, Pp. 799-816. r

(1973b) Geochemical and Geophysical Implications of the Reaction Between Sea Water and the Oceanic Crust. Nature, 243, Pp. 76-78.

HEATH, C.R. and DYMOND, J. (1977) Genesis and Transformation of Metalliferous Sediments from the East Pacific Rise, Bailer Deep and Central Basin, North West Nazca Plate. Bull. Geol. Soc. Am., 88, Pp. 723-733.

DYMOND, J. and VEEH, H.H. (1977) Metalliferous sediments • from the south-east Pacific : The IDOE Nazca Plate Project. IN : Sedimentation in the South-east Pacific, Lisitzin, A.P. (Ed), Moscow, USSR Acad. Sci. (in press).

HEATH, G. and MOBERLY, R. (1971) Non-Calcareous Pelagic Sediments from the Western Pacific, Leg VII, DSDP. IN : Initial Reports of the DSDP, 7, Pp. 987-990. Winterer E. et al (Ed) U.S. Govt. Print. Office, Washington, D.C.

HEEZEN, B.C. and THARPE, M. (1965) Physiography of the Indian Ocean. Phil. Trans. Royal. Soc. 259A, Pp. 137-149.

HEINRICH, E. Wm. (1966) The Geology of Carbonatites, Rand McNally, Chicago.

HEIRTZLER, J.R., DICKSON, G., HERRON, G., PITMANN, III, W. and LE PICHON, X. (1968) Marine Magnetic Anomalies, Geomagnetic Field Reversals and Motions of the Ocean Floor and Continents. J. Geophys. Res., 72, Pp. 2119-2135.

VEEVERS, J.V., BOLLI, Hans M., CARTER, A.N., COOK, P.J., KRASHENINNIKOV, V.A., McKNIGHT, U.K., PROTO-DECIMA, F., RENZ, G.W., ROBINSON, P.T., ROCHER, Jr. K. and THAYER, P.A. (1973) Age of the Floor of the Eastern Indian Ocean. Science, 180, Pp. 952-954.

HIRST, D.M., and NICHOLLS, G.D. (1958) Techniques in sedimentary geochemistry. 1. Separation of .the detrital and non-detrital fractions of limestones. J. Sedim. Petrol., 28, Pp. 46i-468.

HOROWITZ, A. (1970) The Distribution of Pb, Ag, Sn, Tl, Zn in sediments on Active Ocean Ridges. Mar. Geol., 9, Pr. 241-259.

(1974a) Geochemical investigations of sediments associated with the Mid-Atlantic Ridge. Unpubl. Ph.D. Thesis, University •of London. 394

HOROWITZ, A. (1974b) The Geochemistry of Sediments from the. Northern Reykjanes Ridge and the Iceland-Faroes Ridge. Mar. Geol., 17, Pp. 103-122.

and CRONAN, D.S. (19:76) The Geochemistry of Basal Sediments from the North Atlantic Ocean. Mar. Geol., 20, Pp. 205-228.

HULESMANN, J. (1966) On the routine analysis of carbonates in unconsolidate- sediments. J. Sedim. Petrol., 36, Pp. 622-625.

JENKYNS, H.C. and HARDY, R.G. (1976) Basal Iron-Titanium-rich Sediments from Hole 315A (Line Islands), Central Pacific. IN : Initial Reports of the DSDP, 33, Pp. 833-836., Schlanger S.O. et al (Ed). U.S. Govt. Print. Office, Washington, D.C.

JOHNSON, B.D., POWELL, C.McA. and VEEVERS, J.J. (1976) Spreading history of the eastern Indian Ocean and Greater India's northward flight from Antarctica and Australia. Bull. Geol. Soc. Am., 87, Pp. 1560-1566.

KEMPE, D.R.C. and EASTON, A.J. (1974) Metasomatic Garnets in Calcite (MiCarb) Chalk at Site 251, Southwest Indian Ocean. IN : Initial Reports of the DSDP, 26, Pp. 593-601, Davies, T.A. et al (Ed), U.S. Govt Print. Office, Washington, D.C.

IENNETT, J.P., HOUTZ, R.E., ANDREWS, A.B., EDWARDS, A.R., GOSTIN, V.A., HAJOS, M., HAMPTON, M.A., JENKINS, D.G., MARGOLIS, S.V., OVENSHIRE, A.T., and PERCH-NIELSON, K. (1974) Development of the Circum-Antarctic Current. Science, 186, Pp. 144-147.

and WATKINS, N.D. (1975) Deep-Sea Erosion and Manganese Nodule Development in the Southeast Indian Ocean, Science, 188, Pp. 1011-1013.

(1976) Regional deep-sea dynamic processes recorded by Late Cenozoic Sediments of the southeastern Indian Ocean. Bull. Geol. Soc. Am., 87, Pp. 321-339.

KIDD, P.B. and DAVIES, T.A. (1978) Indian Ocean Sediment Distribution Since the Late Jurassic. Mar. Geol., 26, Pp. 49-70.

KOIRTYOHANN, S.R. and PICKETT, E.E. (1965a) Spectral Interference in Atomic Absorption Spectroscopy. Soc. Appl. Spectry. 4th Nat. Meet., Denver, Colorado.

(1963b) Background Correction in long path Atomic Absorption Spectrometry. Anal. Chem., 37, Pp. 601-603.

(1966a) Spectral Interference in Atomic Absorption Spectrometry. Anal. Chem., 38, Pp. 585-587.

— '1966b) Light scattering by particles in atomic absorption spectrometry. Anal. Chem., 38, Pp. 1087-1088. 395

KOLLA, V., SULLIVAN, L., STRETTER, S.S. and LANGSETH, Jr. M.G. (1976a) Spreading of Antarctic Bottom Water and Its Effects on the Floor of the Indian Ocean, inferred from Bottom Water Potential temperature, turbidity, andlrseaflōor photo- graphy. Mar. Geol., 21, Pp. 171-189.

BE, A.W.H. and BISCAYE, P.E. (1976b) Calcium Carbonate Distribution in surface sediments of the Indian Ocean. J. Geophys. Res., 81, Pp. 2605-2616.

MOORE, D.G.,CURRAY, J.R. (1976c) Recent Bottom-Current Activity in the Deep Western Bay of Bengal. Mar. Geol., 21, Pp. 255-270.

HENDERSON, L. and BISCAYE, P.E. (1976d) Clay Mineralogy and Sedimentation in the Western Indian Ocean. Deep. Sea. 'Res., 23, Pp. 949-961.

SULLIVAN, L. and BISCAY,:, P.E. (1978). Recent Sedimentation in the South East Indian Ocean with special reference to the Effects c,f Antarctic Bottom Water. Circulation. Mar. Geol., 27, Pp. 1-17.

KRAUSKOPF, K.B. (1956) Factors controlling the concentrations of thirteen rare metals in sea water. Geochim. Cosmochim„ Acta., 9, Pp. 1-32b.

(1957) Separation of manganese from iron in sedimentary processes. Geochim. Cosmochim. Acta., 12, Pp. 61-84. (1970) Introduction to Geochemistry, 721pp., McGraw- Hill Book Co., New York.

KRAUSS, J.A. and TAFT, B.A. (1964) Equatorial Undercurrent of the Indian Ocean. Science, 143, Pp. 364-366.

KRISHNAN, M.S. (1960) Geology of India and Burma. 4th edition, 604pp. Higginbothamus, Madras.

KU, T.L. BROECKER, W.S. and OPDYKE, N. (1968) Comparison of Sedimentation Rates Measured by Palaeomagnetic and Ionium Methods of Age Determination. Earth. Planet. Sci. Letts., 4, Pp. 1-16.

KW INA, J. (1975) Tectonic Development and Metallogeny of Madagascar with Reference to the Fracture Pattern of the Indian Ocean. Bull. Geol. Soc. Am., 86, Pp. 582-592.

LAIRD, M.G., COOPER, R.A. and JAGO, J.B. (1977) New Data on the Lower Palaeozoic sequence of northern Victoria Land, Antarctica, and its significance for Australian-Antarctic relations in the Palaeozoic. Nature, 256, Pp. 107-110.

• LANDERGPEN, S. (1g64) On the geochemistry of deep sea sediments.Rept. Swed_ Dee -Sea Expedition, 10(7). Pp. 57-154. 396

LANGSETH, M.G. and TAYLOR, P.T.'(1967) Recent heat flor measurements- in the Indian Ocean. J. Geophys. Res., 72, Pp. 6249-6260.

LARSON, R.L. (1975) Late Jurassic-Sea Floor Spreading in the Eastern Indian Ocean. Geology, 3, Pp. 69-71. '

LAUGHTON, A.S., MATTHEWS, D.H. and FISHER, R.L. (1971) The Structure of the Indian Ocean. IN : The Sea, 4(2), Pp. 543-586., Maxwell, A.E. (Ed.), Wiley Interscience.

LAUGHTON, A.S., McKENZIE, D.P. and SCLATER, J.G. (1972) The Structure and Evolution of the Indian Ocean. IN : Marine Geology • and Geophysics, 8, (24th International Geophysics Conference), Pp. 65-73, Murray, J.W. and Peffetier, B.R. (Ed.)., Montreal.

LEATHERLAND, T., BURTON, J., McCARTNEY, M and CULKIN, F (1973) Concentrations of some Trace Metals in Pelagic'Organisms and Hg in Northeast Atlantic Ocean Water. Deep. Sea. Res., 20, Pp. 679-686.

LE PICHON, X. (1960) The Deep Water Circulation in the Southwest Indian Ocean. J. Geophys. Res., 65, Pp. 4061-4074.

LE PICHON, X. and HEIRTZLER, J.R. (1968) Magnetic Anomalies in the Indian Ocean and Sea Floor Spreading. J. Geophys. Res., 73, Pp. 2101-2117.

LI, Y-H., BISCHOFF, J. and MATHIEU, G. (1969) The Migration of manganese in the Arctic Basin Sediment. Earth. Planet. Sci. Letts., 7, Pp. 265-270.

LUYENDYK, B.P. (1974) Gondwanaland dispersal and the early formation of the Indian Ocean. IN : Initial Reports of the DSDP, 26, Pp. 945-952 Davies, T.A. et al (Ed.), U.S. Govt.Print. Office,:. Washington, D.C.

and DAVIES, T.A. (1974) Results of DSDP Leg 26 and the Geologic History of the Southern Indian Ocean. IN : Initial Reports of the DSDP, 26, Pp. 909-943,.Davies, T.A. et al (Ed.), U.S. Govt. Print. Office, Washington, D.C.

and RENNICK, W. (1977) Tectonic History of aseismic ridges in the eastern Indian Ocean. Bull. Geol. Soc. 88, Pp. 13't7-1356.

LYLE, M.W. and DYMOND, J. (1976) Metal Accumulation Rates in the South East Pacific — errors introduced from assumed bulk densities. Earth. Planet. Sci. Letts.,J, Pp. 164-168.

LYNN, D.C. and BONATTI, E. (1965) Mobility of manganese in diagenesis of deep sea sediments. Mar. Geol.. 3. Pp. 457-474.

MARCHIG. and VALLIER, T.L. (1974) Geochemical Studies of Sediment and Interstitial Water, Sites 248 and 249, Leg 25, Deep Sea Drilling Project. IN : Reports of the DSDP, 2, Pp. 405-415. Simpson, E.S.W., et al (Ed.), U.S. Go:-t. Print. Office, Washington, D.C. • 397

MARKL, R.G. (1974) Evidence for the breakup of eastern Gondwanaland by the Early Cretaceous. Nature, 251, Pp. 196-199. MATTHEWS, D.H., VINE, F.J. and CANN, J.R. (1965) G logy of an area of the Carlsberg Ridge; Indian Ocean. Bull. Geol. Soc. Am., 76, Pp. 675-682.

McARTHUR, J.M. and ELDERFIELD, H. (1977) Metal Accumulation Rates in Sediments from the Mid-Indian Ocean Ridge and Marie Celeste Fracture Zone. Nature, 226, Pp. 437-439. McELHINNY, M.W. (1970) Formation of the Indian Ocean, Nature, 228, P 977-979. and LUCK, G.R. (1970) Palaeomagnetism and Gondwanaland. Science, 168, Pp. 930-932.

McKELVEY, B.C. and FLEET, A.J. (1974) Eocene Basaltic Pyroclastics at Site 253, Ninety East Ridge. IN : Initial. Reports of the DSDP, 26, Pp. 553-565, Davies, T.A. et al (Ed), U.S. Govt. Print. Office, Washington, D.C.

McKENZIE, D.P. and MORGAN, W.J. (1969) The Evolution of the Triple Junction. Nature, 224, Pp. 125-133. and SCLATER, J.G. (1971) The Evolution of the Indian Ocean since the Late Cretaceous. R. Astron. Soc. Geophys. Journ., 24, Pp. 437-528.

(1973) The Evolution of the Indian Ocean Solent. Am., 228, Pp. 62-72.

McMURTRY, G.M. (1974) The Mineralogy and Geochemistry of Sediments from the Nazca Plate. Geol. Soc. Am. Abs. with Program., 6, Pp. 218-219.

McMURTRY, M.C. and BURNETT, W.C. (1974) Hydrothermal Metallogenesis in the Baler Deep of the South Eastern Pacific. Nature, 254, Pp. 42-44. MENARD, H.W. (1967) Seafloor spreading, topography, and the second layer. Science, 157, Pp. 923-924. MOORBY, S.A. (1978) The Geochemistry and Mineralogy of Some Ferro- manganese Oxides and associated deposits from the Indian and Atlantic Oceans. Unpubl. Ph.D. Thesis, University of London.

MORGAN, W.J. (1972) Deep Mantle Convection Plumes and Plate Motions. Bull. Am. Ass. Petrol. Geolog., 56, Pp. 203-213.

MURRAY, J. and RENARD, A.F. (18911. Deep Sea Deposits. Report on the Challenger Expedition 1873-76. Thomson C.1%. and Murray. (Ed.), H.M.S.O., 525pp. 398

NAG, (1975) Numerical Algorithms Group COC Mini Library Manual Mark 4, Unpuhl. Nottingham Algorithms Group, University of Nottingham.

Z NARAIN, H., KAILA, K.L. and VEPMA, R.K. (1968) Coatinental Margins of India. Can. Journ..Earth. Sci., 5,~Pp. 1051-1065.

NORTON, I and MOLNAR, P. (1977). Implications of a revised fit between Australia and Antarctica for the evolution of the Eastern Indian Ocean. Nature, 267, Pp. 338-340.

OLDNALL, R.J. (1975) Possible Sources of Metals in Pelagic Sediments: With special reference to the Bailer Basin. M.Sc. Thesis, University of Hawaii, IN : Contributions to the Geochemistry of Nazca Plate Sediments, South East Pacific, Hawaii, Inst. Geophys., Honolulu.

OPDYKE, N.D. and GLASS, B.P. (1969) The Palaeomagnetism of sediment cores from the Indian Ocean. Deep. Sea, Re5,,'16, Pp. 249-261.

PAYNE, J.R. and CONNOLLY, J.R. (1972) Pleistocene Manganese Pavement Production : Its Relationship to the Origin of Manganese in the Tasman Sea. IN : Ferromanganese Deposits on the Ocean Floor, Pp. 81-92. Horn, D.R. (Ed.), Office -of IDOE, NSF, Washington, D.C.

PETERSON, M.N.A. (1966) Calcite : Rates of dissolution in a vertical profile in the Central Pacific. Science, 154, Pp. 1542-1544.

PETERSON, M. et al (1970) Initial. Reports of the Deep Sea Drilling Proiect, U.S. Govt. Print. Office, Washington, D.C. 501pp.

PIMM, A.C. (1974) Mineralisation and Trace Element Variation in Deep Sea Pelagic Sediments of the Wharton Basin, Indian Ocean. IN_ : Initial Reports of the DSDP, 22, Pp. 469-476, von der Borch C.C. et al (Ed.), U.S. Govt. Print. Office, Washington, D.C.

McGOWRAN, B. and GARTNER, S. (1974) Early Sinking History of the Ninety East Ridge, Northeastern Indian Ocean. Bull. GEO. Soc. Am., 85, Pp. 1219-1224.

PIPER, D.Z. (1973) Origin of Metalliferous Sediments from the East Pacific Rise. Earth. Planet. Sci. Letts, 19, Pp. 75-82.

PRICE, W.J. (1974) Analytical Atomic Absorption Spectrometry. Heyden and Sons Ltd., London, 239pp.

RATEEV, M.A., GORBUNOVA% Z.N., LISITZYN, L.P. and NOSOV, G.L.. (1969) The Distribution of Clay Minerals in the Oceans. Sediment., 13, Pp. 21-43.

RILEY, P. and CHESTER, R. (1971) The Geochemistry of Deep Sea Sediments. IN : Introduction to Marine Chemistry, Pp. 389-423., Riley, P. Chester, R. (Ed.). Academic Press.

RONA, P.A. (1972) Exploration Methods for the Continental Shelf : Geology, Geophysics, Geochemistry. NOAA Tech. Ren, EPL 218 AOML 6, 47pp, NOAA, U.S. Dept. of Commerce, Boulder, Colorado. 399

ROZANOVA, T.V. and BATURIN, G:N. (1971) Hydrothermal Ore Shows on the floor of the Indian Ocean. Oceanol.,;11, Pp. 874-879.

RUBEY, W. (1951) Geologic History of Sea Water. Bull. Geol. Soc. Am., 62, Pp. 1111-1148.

RUDDIMAN, W.F. and HEEZEN, B.C. (1967) Differential solution of Planktonic Foraminifera. Deep. Sea. Res., 14, Pp. 801-808.

SAYLES, F.L. and BISCHOFF, J.L. (1973) Ferromanganoan Sediments in the equatorial East Pacific. Earth. Plan. Sci. Letts., 19, Pp. 330-336.

KU, T.L. and BOWKER, P.C. (1975) Chemistry of Ferromangoan Sediments of the Bailer Deep. Bull. Geol. Soc. Am., 86,` Pp. 1423-1431.

KU, T.T. and BOWKER, P.C. (1976) Elemental Accumulation Rates in the Bailer Deep - A Correction. Authors manuscript, jpp.

SCHLICH, (1974) Sea Floor Spreading History and Deep Sea Drilling Results in the Madagascar and Mascarene Basins, Western Indian Ocean. IN : Initial Reports of the DSDP, 25, Pp. '663- 678, Simpson, E.S.W., et al, (Ed.), U.S. Govt. Print. Office, Washington, D.C.

and PATRIAT, P. (1971) Mise en ēvidence d'anomalies magnetique= axiales sur la branche ouest de la dorsale m4dio-indienne. Compt. Rendus. Acad. Sci. Paris, 272, Pp. 700-703.

SIMPSON, E.S. 1 and VALLIER, T.L. (1974) Regional Aspects of Deep Sea Drilling in the western Indian Ocean, Leg 25, DSDP. IN : Initial Reports of the DSDP, 25, Pp. 743-759. Simpson, E.S.W. et al (Ed.), U.S. Govt. Print. Office, Washington, D.C.

SCLATER, J.G. ABBOTT, D. and THIEDE, J. (1977) Palaeobathymetry and Sediments of the Indian Ocean. IN : Deep Sea Drilling Project in the Indian Ocean, JOIDES, Spec. Publ. 1, Heirtzler, J.R. (Ed.).

and FISHER, R.L. (1974) Evolution of the East Central Indian Basin with Emphasis on the Tectonic Setting of the Ninety East Ridge. Bull. Geol. Soc. Am., 85, Pp. 683-702.

and HARRISON, C.G.A. (1971) Elevation of Mid-Ocean Ridges and the Evolution of the Southwest Indian Ridge. Nature, 230, Pp. 175- 177. LUYENDYK, B.P. and MEINKE, P. (1976) Magnetic Lineations in the southern part of the Central Indian Basin. Bull. Geol. Soc. Am.. 87. Pp. 371-378. VON DER BORCH, C.C., VEEVERS, J.J., HEKINIAN, R.,THOMPSON, R.W.. PīMM, A.C., McGOiiRk . E.. GARTNER. Jr. S. and JOHNSON, D.A. (1974) Regional Synthesis of the Deep Sea Drilling Results froom Leg 22 in the Eastern Indian Ocean. IN : Initial Reports of the DSDP.22, Pp, 815-831, von der Borch, C.C. et al (Ed.), U.S. Govt. Print. Office, Washington, D.C. 400

SHARMA, G.S. (1970) Transequatorial movement of water masses in the Indian Ocean. Journ. Mar. Res., 34, Pp. 143-154.

SIDDIQUE, H.N., VEEPAYYA, M., RAO, Ch.M., MURTY, P.h,.N. and REDDY, C.V.G. (1976) Deep Sea Drilling Project Site 219 on the Laccadive Ridge. Ind. Journ. Mar. Sci., 5, Pp. 18-34.

SIMPSON, E.S.W., SCHLICH, R., et al (1974) Initial Reports of the Deep Sea Drilling Project, 25, U.S. Govt. Print. Office, Washington, D.C., 884pp.

SKYORNYAKOVA, I.S. (1964) Dispersed Iron and Manganese in Pacific Ocean Sediments. Intern. Geol. Rev., 7, Pp. 2161-2174.

SMITH, A.G. and HALLAM, A. (1970) The Fit of the Southern Continents Nature, 225, Pp. 139-144.

STRAKHOV, N.M. (1976) Origin of increased amounts of elements in pelagic sediments of the oceans. Intern. Geol. Rev., 18, Pp. 406- 416.

SWALLOW, J.C. (1964) Equatorial Undercurrent in the Western Indian Ocean. Nature, 204, Pp. 436-437.

SWANSON, S.B. and SCOTT, R.B. (1974) Hydrothermal Products of Leg 34, Basalts : Deep Sea Drilling Project. Geol. Soc. Am. Abs. with Program., 6, P.979.

SYKES. L.R. (1968) Seismological Evidence for Transform Faults, sea floor spreading and continental drift. IN : The history of the Earth's Crust, Phinney, R.A. (Ed.), Princeton University Press, Princeton.

THOMPSON, A.J. and THORESBY, P.A. (1977) Determination of Arsenic in Soil. and Plant Materials by Atomic Absorption Spectrophotometry with Electrothermal Atomisation. Anal., 102, Pp. 9-16.

TOPPING, G. (1969) Concentration of Mn, Co, Cu, Fe and Zn in Northern Indian Ocean and Arabian Sea. Journ. Mar. Res., 27, Pp. 318-326.

TUREKIAN, K.K. and IMBRIE, J. (1966) The Distribution of Trace Elements in Deep Sea Sediments of the Atlantic Ocean. Earth. Planet. Sci. Letts, 1, Pp. 161-168.

and WEDEPOHL, K. (1961) Distribution of Elements in Some Major Units of the Earth's Crust. Bull. Geol. Soc. Am., 72, Pp. 175-192.

UDINTSEV, G.B. (1965) •Results of Upper.Mantle Project studies in-the •Indian Ocean by the research vessel Vityaz. IN : The World" Rift System, Rept. of UKC Symposium, Pp. 148-172, Irvine, T.N.. (Ed.) Can. Geol. Survey, Paper 66-14, Ottawa.

and D Th ITRIEV, L.V. (1975) Ultrabasic Rocks of the Ocean F3oor and the Role of Rift Zone Ultrabasites. IN : Rift Zones of the World Oceans, Pp. 186-206, Teteruk-Schneider, R. (Ed.), J Wiley and Sons, New York, and IPST, Jerusalem. 401

UDINTSEV, G.B., SHIRSHOV, P.P. et al (1975) Geological. - Geophysical Atlas of the Indian Ocean, Pergamon, Oxford, 151pp.

VALUER, T.L. and KIDD, R.B. (1977) Volcanogenic Sediments in the Indian Ocean. IN : Deep Sea Drilling Project in the Indian Ocean, JOIDES, Spec. Publ. 1, Heirtzler, J.R.Td.), in press.

VEEH, H.H. and BOSTROM, K. (1971) Anomalous ~34U/~ 38U on the East Pacific Rise. Earth. Planet. Sci. Letts., 10, Pp. 372-374.

VEEVERS, J.J., HEIRTZLER, J.R. et al (1974) Initial Reports of the Deep Sea Drilling Project, 27,- U.S. Govt. Print. Office, Washington D'.C., 1060pp.

VENKATARATHNAM, V. and BISCAYE, P.E. (1973) Clay Mineralogy and Sediment- ation in the eastern Indian Ocean. Deep. Sea. Res., 20, Pp. 727-738. VON DER BORCH, C.C., NESTEROFF, W.D., GALEHOUSE, J.S. (1971) Iron Rich Sediments cored during Leg 8 of the Deep Sea Drilling Prōject. IN : Initial Reports of the DSDP. 8, Pp. 829-836, Tracey, Jr., J.I. (Ed.), U.S. Govt. Print. Office, Washington, D.C.

and REX, R.W. (1970) Amorphous Iron Oxide Precipitates in Sediments Cored During Leg V, Deep Sea Drilling Project. IN : Initial Reports of the DSDP, I, Pp. 541-544, McManus, D.A. et al (Ed.), U.S. Govt. Print. Office, Washington, D.C.

SCLATER, J.G. et al (1974) Initial. Reports cf the Deep Sea Drilling Project, 22, U.S. Government Printing Office, Washington, D.C. 890pp.

VON GUMBEL, G. (1878) Ueber die im Stillen Ocean auf dem Meeresgrunde Vorkammenden Manganknollen Sitz. Berichted. K. Bayerischen Akademie d. Wissenschaften Munchen Matem Phyrik Klaussen, Pp. 159-209.

VON HERZEN, R. and LANGSETH, M. (1966) Present status of oceanic heat-flow measurements. Phys. Chem. Earth, 6, Pp. 365-407.

WARNER, T.P. and GIESKES, J.M. (1974) Iron-Rich Basal Sediments from the Indian Ocean : Site 245, Deep Sea Drilling Project. IN Initial Reports of the DSDP, 25, Pp. 395-404. Simpson, E.S.W. et al (Ed.), U.S. Govt. Print. Office, Washington, D.C.

WARREN, B.A. (1974) Deep Flow in the Madagascar and Mascarene Basins. Deep. Sea. Res., 21, Pp. 1-21.

WATK_INS, N.D., GUNN, B.N., NOUGIER, J. and BAKSI, A.K. (1974) Kerguelen : Continental Fragment or Oceanic Island. Bull. Geol. Soc. Am., 85, Pp. 201-212.

and iENNETT, J.P. (1977) Erosion of Deep Sea. Sediments in the Southern Ocean between Longitudes 70o and 190oE and Contrasts in manganese nodule development.- Mar. Geol., 23, Pp. 103-111.

WEDEPOHL, K. (1960) SpurreiAnaly`cische Untersachur.gen in Tietse:tonen aus dem Atlantik. Geochim. Cosmcchim. Acta, 18, Pp. 200-231. 402

WEISSEL, J.K. and HAYES, D.E (1971) Asymmetric Sea Floor Spreading south of Australia. Nature, 231, Pp. 518-522.

WELLS, A.K., WELLS, M.K. and HATCH, F.H. (1972) Petrology of the Igneous Rocks, 13th Ed, Murby, London, 551pp. WHITMARSH, R.B.1 WESSER, O.E., ROSS, D.A., et al (1974) Initial Reports of the Deep Sea Drilling Project, 23, U.S. Govt. Print. Office._ Washington, D.C. 118Opp.

WISEMAN, J.D.H. (1965) Calcium and Magnesium Carbonate in some Indian Ocean Sediments. IN : Progress in Oceanography, 3, Pp. 373-383, Sears, M. (Ed.). Pergammon, London.

WOLERY, T.J. and SLEEP, N.H. (1976) Hydrothermal Circulation and Geochemical Flux at Mid-Ocean Ridges. Journ. Geol., 84, - Pp. 249-275.

WYRTKI, K. (1971) Oceanographic Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, D.E. 531PP- ZANDER, A.T. (1977) Factors Influencing Accuracy in Background Corrected Atomic Absorption Spectrometry. Intern. Lab., Jan/Feb., Pp. 15-24. 403

end ices

A.1 SAMPLE PREPARATION AND ANALYTICAL PROCEDURES

A.2 PRECISION AND ACCURACY

A.3 CALCIUM INTERFERENCE - EFFECTS AND CORRECTION PROCEDURE

A.4 DATA HANDLING APPENDIX Al. SAMPLE PREPARATION AND ANALYTICAL PROCEDURES

A1.1 Sample Preparation.

Since none•of the samples used in this study were collected by the author, no core-cutting was involved in the sample preparation. The samples were usually supplied to the author in a wet state in some form of polythene container, i.e. bag, sealed tube, etc. The samples were first of all dried. In the case of the Valdivia sediments this was done by freeze drying using a Chemical Laboratory Instruments Ltd. Model S.B.4 Freeze Drier. In the case of all other samples they were oven dried at between 40 and 60°C, for periods of up to 96 hours. When dry, the samples were disagreggated in an agate pestle and mortar. Smear slides and samples for palaeontological and binocular microscopic studies were prepared at this time for the Recent sediments. Following this all the samples were ground in an agate pestle and mortar until they passed through an 80 mesh sieve (i.e. were < 190p in size). They were then stored in sealed polythene tubes until required for chemical analysis.

A1.2 Analytical Procedures for Analysis by Flame Atomic Absorption Spectrophotometry

A1.2.1 Introduction In the following analytical procedures all the weighing was performed using an Oertling V.20 Electronic Analytical Balance. The principles of Atomic Absorption Spectrophotometry (AAS) and the method of operation of the spectrophotometer are well known, but will be briefly summarised in Section A1.2.5. The analytical procedures are summarised in the following sections and diagrams. 405

A1.2.2 Bulk Chemical Analysis

0.29ms sample weighed out accurately into PTFE* Beakers

V 8-10 mis Analar 40% HF and 6 mis 1:1 mixture of conc. HNO and HC1O4 added

V Samples strongly fumed for 20 mins at 200°C to ensure boiling point of HC101 reached and in order too prevent ppt. of CaF2 Samples cooled.

W 1-2 mis 1{F added. Samples evaporated to dryness.

2 mis HClO added, fumed for 10 mins at 20°C, and evaporated to dryness.

5 mis 6 MHCLadded. When cool the samples transferred to 25 ml graduated flasks, PTFE beakers washed out and made up to volume with DIir+ to give 1 M HC1 soln.

10 + 100 fold dilutions !made where necessary.

Solutions analysed by Atomic Absorption Spectrophotometry with standard solutions in same sample medium, i.e. HC1.

PTFE - Poi .•tetrafluoro-Ethylene + -= De-Ionized Water 406

A1.2.3 Chemical Partition Analysis.

A1.2.3a. Introduction One intention of chemical analyses is to study the relationship of particular trace metals to particular mineral phases. Chemical partition techniques allow us to do this by using different reagents to dissolve particular mineral phases. Although the resultant data are more difficult to interpret than chemical analyses of physically separated mineral phases, they provide a useful indication as to how trace metals are partitioned in marine sediments. The historical background to the development of these partition techniques is described in Section 2. The procedures used here are those of Chester and Hughes (1967, 1969) and Cronan (1976). The reagents and the mineral phases soluble in them are as follows: 1) 25% (V/V) acetic acid - carbonates (excluding dolomite), loosely adsorbed ions from Fe oxides, clay minerals etc., and soluble Fe oxide/hydroxide. 2) Acid-reducing agent (hydroxylamine hydrochloride (HC1)in 35% (V/V) acetic acid - all the above, plus reducible Mn oxides, mixed Fe Mn oxides and reducible Fe oxides. 3) Hot 50% hydrochloric acid - all the above, plus Fe oxide minerals (e.g. crystalline goethite) and all but the most insoluble alumino-silicates (i.e.. clay minerals) and silicates. Reagents 1). and 2) used by Chester and Hughes (1967 , 1969), reagent 3) added by Cronan (1976).

C N.B. The combined acid-reducing agent was prepared by dissolving 25gms of hydroxylamine hydrochloride in 1OOmis of DIW. This was then mi::ed with the 35% (V/V) acetic acid in the ratio of 150 mls hydroxylamine hydrochloride to 35Omis acetic acid]. A1.2.3b Chemical Partition Procedures

ACETIC ACID ACID--REDUCING HYDROCHLORIC LEACH AGENT LEACH ACID LEACH

V V V 1gm of sample weighed out accurately into 1gm of sample weighed 100m1, stoppered, 'Quickfit' conical flask out accurately into 100m1 beaker

10mis 25%(V/V) 50mis mixed acid- 10mis 50% HC1 added acetic acid added reducing agent added

Samples mechanically Samples allowed to Beakers covered with a shaken for four hours stand for four hours watch glass, the samples: and allowed to settle with occasional heated until just boil agitation ing and refluxed\for three hours until solutions were a clear yellowish-orange colour t.

V Samples vacuum filtered through previously weighed out millipore filter papers. 1 1 Filtrates and washings collected in 100m1 conical flasks

Acetic acid destroyed Excess acid-reducing by evaporating solutions agent evaporated and and addition of conc. destroyed by excess, HNO . HNO conc. HNO , efferv- off.3 3 escence asd more HNO added until solutions are clear yellow colour

Leach with 1 M HC1 d Filtrates made up in 25ml graduated flasks together with washings, to give a 1 M HC1 solution

10 + 100 fold dilutions made where necessary

* -ISolutions analysed by Atomic Absorption Spectre- E phdtometry using standard solutions in same.sample medium, i.e. 1 M HC1. I 408

A1.2.3c Analysis of the Leached Residues The residues from the partition attacks were dried in a dessicator over silica gel to constant weight and then the weigpt loss was determined. Re-leaching of the residues with the same reagents was carried out to check the leaching procedures. Only in the case of the acetic acid leach were significant additional amounts of certain trace metals removed by re-leaching and prolonged exposure to the reagent. Some previous workers have used the analysis of the leached residue rather than the leach solution to de+.ermine the relationship of trace metals with particular mineral phases (e.g.Horowitz, 1974a, b; Horowitz and Cronan, 1976). In order to check the composition of the residues they were analysed in the following way. It was considered undesirable to remove the residues from the filter papers by mechanical methods, i.e. scraping, since recovery would be less than 100% due to retention of material in the pores of the filter papers. Ignition in a Muffel furnace involved high temperatures at which it was possible that certain volatile metals, e.g. Hg, As, etc. might be. lost (Moorby, 1977, person. communic). The following procedure was therefore adopted:-

V Weighed cellulose nitrate filter paper plus residue were placed in a= PFTE* beaker

Up to 5 mis of acetone added until filter paper was dissolved

V About 5 mis conc. HNO added, solutions evaporated ildnt brown fumes of NO ceased to be evolved. 2 (This destrys the filter paper and the acetone) .

PTFE beaker treated as normal bulk chemical analysis.

'Blank' filter papers were analysed in each sample batch and the low trace element values were deducted from those measured for the residues plus the filter papers.

*PTFE = PolyTetraFluoro-Ethylene. 409

A1.2.4 De-Ionized Water Washed Samples Washing with DIW was carried out on samples required for palaeontological study (the filtrates not being analysed by AAS) and on some Recent sediments, in an attempt to remove adsorbed ions from the surfaces of minerals (the filtrates were analysed by AAS) (see Section 3.3.4). The following procedure was adopted:-

1 gm of sample weighed out accurately into 100m1 stoppered, 'Quickfit' conical flask

20 mis DIW added 1

Samples mechanically shaken for required periods of time, e.g. four hours and allowed to settle

Vacuum filtered through previously weighed out millipore filter papers

Residues dried in a dessicator over silica get to constant weight and weight loss determined

Filtrate and residue washings collected 25 or 50 ml graduated flasks w Conc. HC1 added to give 1 M HC1 solution made up to volume with DIW

10 + 100 fold dilutions made where necessary

Solution analysed by Atomic Absorption ›Spectrophotometry using standards in same medium' as sample solutions. 410

A1.2.5 Atomic Absorption Spectrophotometry - Method of Operation

A1.2.5a. Basic Principle Atomic Absorption Spectrophotometry is a single.element, rapid method of chemical analysis for both major and trace elements which has been widely applied in geology. Atomic absorption is based on the principle that atoms are capable of absorbing radiant energy at discrete, i.e. resonant, wavelengths in the ground state, i.e. their lowest energy level. When light is passed. through a flame, into which the solution of the element to be determined is sprayed, a proportion of the incident light will be absorbed. This increase in absorbance, when the sample of the element is introduced, is proportional to the concentration of the element in solution. The application and use of AAS in geology has been fully described by Angino and Billings (1972) and Price (1974).

A1.2.5b Configuration. An Atomic Absorption Spectrophotometer consists of the following basic units (see Fig. A1.2.5b). The light source is provided by a hollow cathode lamp of the element being determined. It is neon or argon filled and emits a sharp line spectrum. The sharp line spectrum passes into the flame. This is the system for producing an atomic vapour of the sample. This is done by the premixing of the oxidant (compressed air or nitrous oxide) and the fuel (acetylene) in various proportions depending on the element being determined, with the sample in solution, using a nebuliser/ atomiser assembly. These are burnt or a high velocity head. The incident light beam having passed through the flame passes into the wavelength 0 selector, which comprises the slit, which allows through a beam 2.0A in width and thence to the monochromator to isolate the discrete resonance line. This is connected to the final unit, which is the detector, amplifier and read-out facility which can be a plotter or digital volt- meter.

A1.2.5c Method of Operation. Measurement was made on standard solutions of known concentrations and on solutions of the samples. The samples were compared with a calibration graph prepared from the standard solutions. In the case of the two instruments used in this study - a Perkin Elmer 403 and a Unicam 5P9O - High Velocity Head Monochromator. Sharp Line Spectr Slit

Flame

Wave Length Hollow Cathode Selector Lamp lA Detector, Y Amplifier and Oxidant (e.g. asespa. Fuel (e.g. C2H2) Read out compressed air Facility or N20 Nebuliser/Atomiser / Sample in Assembly Solution

FIG. A1.2.5b. Configuration of an Atomic Absorption Spectrophotometer 412

TABLE A1.2.5c Conditions of Operation of Atomic Absorption Spectrophotometers

ELEMENT LAMP WAVELENGTH FUEL SLIT DETECTION CURRENT (mA) (nM) MIXTURE WIDTH LIMIT (Oxidant SETTING (ppm) /Fuel)

Al 25 309.2 60/40* 4 5.0 Ti 40 365.2 60/ 40* 3 5.0 Fe 1-10 248.7 ug/ml 20 3 0.1 10-100 371.9 55/27 ug/ml Mn 1-10 279.5 ug/ml 20 10-100 55/27 3 0.1 ug/ml Ni 20 232.0 55/27 3 0.1 Co 20 240.7 55/27 3 0.1 Cr 25 357.9 55/27 . 3 0.1 Cu 15 324.7 55/27 4 0.01 Cd 10 228.8 55/27 4 0.02 Pb 10 283.3 55/27 4 0.5 Zn 15 213.8 55/27 4 0.01 Li 15 335.4 55/27 4 0.01 Cal 10 422.7 5/ 12 0.05mm 0.1 Mg1 4 285.2 5/1.22 0.08mm 0.05 Ba1 15 553.5 5/3.4*2 o.o4-0.05 .0.5 mm

* Nitrous/Acetylene, all others Air/Acetylene

1 Determined on Unicam SP90 Spectrophotometer. All other elements determined on Perkin Elmer 403 Spectrophotometer.

2 Litres/min. All others are flow rate meter reading. 413

a direct read-out facility is provided giving values in units of concentrations, i.e.1ug/m1 rather than in units of absorbance. It is very convenient and obviates the need to prep re many calf- bration graphs. In each batch of about 50 samples, up to 20% of the total i.e. 10) were composed of sample duplicates, international standards, control samples and blanks. The determinations were carried out in a 1M HC1 sample and standard medium. In the case of the determination of Ca and Mg, lanthan•,mi chloride was added as a releasing agent. Determinations for these elements were- carried out on dilutions prepared using a diluspence machine; a Hook and Tucker Variable Diluter. The determination of Ba was carried out on 5 ml sample aliquots to which 0.5mls of a 1% K+ solution was added. This was done because under the flame conditions used, which give optimum sensitivity, Ba was ionized in the flame and did not remain in the atomic state. At higher flame temperatures Ba ionizes less but the sensitivity is extremely poor. Therefore K solution was added in order to swamp the flame with ionized species, to prevent ionization of Ba and to push the equilibrium to the right. The conditions of operation of the Atomic Absorption Spectrophotometers are given in Table A1.2.5c. This method of analysis is subject to interference and this is discussed in Appendix A3.

A1.2.5d All the sediments used in this study have in general been analysed for all the above metals. In addition As has been determined on all samples using the method described in A1.3 and Si02 on selected samples by the method described in A1.4. All the analyses have been carried out in the Applied Geochemistry Research Group by the author. 414

A1.3 Analytical Procedures for Analysis by Flameless Atomic Absorption Spectrophotometry

\1/4 A1.3.1 Introduction Arsenic is determined using the volatile hydride method. Although one is measuring changes in absorbance as in flame AAS, the method of sample introduction and volatilization is different. This method has been adequately described by Thompson and Thoresby (1977) and since the author used the apparatus constructed by them, only a brief summary of the method will be given here.

A1.3.2 Sample Preparation. The samples were prepared by the following acid leaching procedure:-

V 0.10gm weighed out accurately into 250m1 wide necked conical flasks

W 20mis conc. HNO + 10mis 15N H2SO4 added

Solutions evaporated to low volume until dense white fumes of SO3 had been evolved

Solutions cooled and diluted to 25mls with DIW

Solutions transferred to 50m1 graduated flasks and made up to volume together with the siliceous residue with DIW to give 3N H2SO4 solutions

Dilutions were made where necessary with 3N H2SO4

!Solutions analysed by volatile. hydride 'method for flameless Atomic Absorption )Spectrophotometry using. standard solutions i n the sa:~me medium as the samples 415

A1.3.3 Volatile Hydride Method - Flameless Atomic Absorption Spectrophotometry Rather than producing the population of groundstate atoms by. volatilization of the sample solution in a flame as in flame AAS, the volatile hydride:method produces the groundstate atoms by the thermal decomposition of the volatile hydride. This method can only be used for those elements on the right of the Periodic Table (As, Sb, Te, Se, Bi, Sn and Pb) which have volatile hydride compounds. The volatile hydride, arsenic hydride (AsH3), is produced by reaction of 1 ml of the acid sample solution with 1 ml of 2% sodium borohydride solution (NaBH4). This powerful reducing agent, highly soluble in acid solution, reduces - the sample solution, releasing AsH3 vapours and hydrogen (H2). A stream of nitrogen carrier gas blows the AsH3 molecules into a heated quartz tube where, thermally unstable, they decompose producing groundstate As atoits. The quartz tube, heated by an electric coil, takes the place of the flame used in flame AAS and it is through this that the incident sharp line spectrum of the As hollow cathode lamp passes. The remainder of the apparatus - monochromator, detector, readout facility, etc. is discussed by Thompson and Thoresby(1977). A slit width of 0.5mm was used and As was determined at a wavelength of 193.7nm. The readout facility for the apparatus was a chart recorder. Measurement was made on standard solutions of known concentration and sample solutions. The samples were compared to calibration graphs prepared from the standard solutions. Each analytical batch of thirty solutions contained one in three sample duplicates, international standards, control samples and blanks.

A1.4 Colorimetric Determination of Silica (SiO2)

A1.4.1 Introduction The sample preparaticn for colorimetric determination of SiO involves 2 a fusion process with sodium hydroxide (NaOH) followed by a colorimetric determination using standards prepared from the BS99 sodafeldspar. The process is complicated and time consuming since only nine samples can be prepared at one time due to the instability of the sample solutions. Silica determinations were therefore carried out only on selected, samples. 416

A1.4.2 Reagents 7.5% Ammonium molybdate solution. 7.5gms of Analar ammonium molybdate were dissolved in 75m1s of DIW. To this i.► s added 1Omis of 50% H2SO4. The solution was made up to 100m1s, mixed thoroughly and stored in a polythene bottle until use. Reducing Solution. 0.5gm of Analar anhydrous sodium sulphite was dissolved in 10mis of DIW and to this was added 0.15gm of 1-amino-2- maphol-4-sulphonic acid. The solution was stirred to ensure complete dissolution. 9 gms of Analar sodium metabisulphite were dissolved in 90mls of DIW. The second solution was added to the first, mixed thoroughly and stored in a polythene bottle for not more than three days.

A1.4.3 Sample Preparation

3gm Analar NaOH, fused by gentle heating in a Nickel J crucible. Allowed to cool and stored in a dessicator.

0.10gms sample weighed out accurately into prefused crucible.

V Heated until fusion occurred and for a further 5 minutes at dull red heat I Crucible cooled, half filled with DIW and allowed to stand until all contents had dissolved

Alkaline solution in crucible carefully added to 600m1 beaker containing 300mls DIW and 20 mis 12M HC1. Ensuring no contact between acid solution and crucible and between alkaline solutions and glass, crucible thoroughly washed out with DIW and washings added to beaker.

V Solution cooled, transferred to 1000m1 graduated flask and made up to volume with DIW

100mis of the solution, transferred to polythene bottle for storage, prior to determination of SiO 2 417

A1.4.4 Method of Determination. In colorimetry the measurement of the dispersion of incident light through the complex or suspension of a compound (in this case Si molybdate) of a sample solution as compared with that of a reference solution (in this case the sample blank) is a measure of the absorbance. The absorbance is directly proportional to the concentration of the element (Si) in solution. The sample and standard solutions were prepared in the following way:-

10mis of solution pipetted into 200m1 graduated flask

2m1s ammonium molybdate solution was added. Solution mixed And maintained at 20-25°C in a water bath for 10 mins.

W 10mis 8% Tartaric acid was added, followed by 2mls of reducing solution

Solution made up to volume with DIW and allowed to stand for one hour before measurement. Solution was a blue colour

'l Si0 determined using a Pye Unlearn Instrument SP600 Spectrophotometer at a wavelength of 650nm. Reagent blank = reference cell.

The standard solutions were prepared from BS99 soda feldspar. Each analytical batch contained nine samples (of which one was a sample duplicate and one an international standard), three standards and a sample blank. The sample solutions were compared to calibration graphs prepared from the standards. This relates the absorbance to the SiO content in1ig/ml. The weight percentage of SiO2 was calculated 2 from the following equation:- Cn X F X 100 wt% sio2 = W

where Cn = Cone Si02, }ig/ml F = Dilution Factor (20, i..e. 10mis in 200mis). W _ Sample Weight in mg...,

A1.5 Calcium Carbonate Determinations.

A1.5.1 Introduction The calcium and aluminium contents of the samples allow the sediments to be grouped into three classes - carbonates, clayey carbonates and clays. The CaCO3 content of all the sediments have been calculated from the spectrophotometrically determined Ca values using the method of Dymond et al (1976). It is necessary to obtain accurately determined CaCO3 values for use in the carbonate-free correction (see Appendix A4) particularly at the higher CaCO (780%) concentrations 3 (Dymond et al, 1976). The CaCO3 contents of the carbonates have been determined from the CO2 values which were obtained by the acid-dissolution -- process described below.

A1.5.2 Method of Determination of CaCO3 The volumetric method uses a Collins Calcimeter which measures the amount of CO2 liberated when about 0.2gms of sample react with 5 mis 25%HC1•. The equation for the reaction is:- CaCO3 + 2HC1 CaCl2 + H2O + CO2

The calcimeter is similar to the apparatus used by Hflesmann (1966), however in the calcimeter the liberated CO2 displaced a column of water, rather than a column of mercury, as the method described by Hiilcsmann (1966). The column of water is displaced in a calibrated tube, thus allowing the amount of liberated CO2 present to be read off directly in c.c. The sample and water column were allowed to equilibrate to room temperature in a water bath (see Fig. A1.5.2 ) before the reading was taken. The apparatus is calibrated using Analar CaCO3 and the values obtained were corre:ted to . pure CO2. Correction is also made for volume differences in reaction vessels, differences in sample weighvs, variations in room temperature and ūarimetric pressure. 419

Fig. A1.5. . Collins Calcimeter Apparatus and Method of Calculation for wt% CaCO3

—' To reaction vessel

to valve II

Water Bath

Calibrated Tube

To apparatus

100m1 'Quickfit' Conical Flask

0.2gm Sample. Reaction initiated by inverting reaction vessel, which is then placed in water bath to equilibrite to room temperature before reading is taken

Mercury Thermometer File containing 5 ml 25% HC1

0.2gms Analar CaCO3 jr 44cc CO2 Wt% CaCO calculated as follows:- 3 Wt%CaCO3 = Xx44.0 x AxWxTxPxV C s where X = cc of CO2 evolved C = cc of CO 2 obtained from Analar CaCO a particular sample s 2 3 batch 100.08 A = Correction factor for atomic- weight of CO,, in CaCO3 44.08 W = Correction for difference in sample weight, = 0.20 Recorded sample uit T = Correction factor for variation in room temperature,

R.T. at measurement P = Correction for variations in barimetric pressure, F = P initial P at measurement Vo and V = Correction fOr volume of reaction vessel, V Vo 100cc VA VA = actual volume 420

It has been suggested that this method may dissolve amounts of MgCO3 (dolomite). However, tests on dolomite show that it will only be dissolved by 25% HCl if the reaction vessel was heated. Although Mg has shown to be present in these sediments (see Sections 2 and 3) it is probably contained in sea salt in interstitial evaporates, in high-Mg calcite of the foraminifera tests (Wiseman, 1965) and in clay and detrital minerals rather than in a discrete Mg carbonate phase. Since the sediments studied come from an oxidising, open ocean environment it is unlikely that the gas evolved contained any H2S, which might have been evolved when sulphide minerals were attacked by HCl, and is therefore CO2 liberated from the dissolution of the calcium carbonate in the sediments. The 'apparatus was calibrated against pure Analar CaCO at the 3 beginning of each analytical batch. The calibration was checked periodically during the course of the day, about one sample in six being pure CaCO3. About one sample in every ten was a sample duplicate. The method was extremely easy to use, very rapid and it was possible to measure about 30 samples per analyst day plus standards.

A1.5.3 Results The method itself is extremely precise and the measurements compare variably with those data obtained by the use of Dymond et al's (1976) method. The overall precision for several analytical runs is +-1.6%. This compares favourably with the precision recorded in this study for the spectrophotometrically determined Ca and hence CaCO3 values (see Appendix A2). The international standard used was the USGS National Bureau of Standards, Argillaceous Limestone, NBS-lb. The precision on the determinations of this standard made by the author was 11.7% (expressed as a coefficient of variation). The variation between the reported CaCO3 content of NBS-lb and those determined here, i.e. the. accuracy, was - 1.0%o (expressed as a coefficient of variation). 421

APPENDIX A.2 PRECISION AND ACCURACY

A2.1 Introduction It is extremely important in any geochemical sady to firstly ensure that the analytical method is precise, in order to provide meaningful geochemical results and secondly that the results compare favourably with previously well-analysed international standards, which are also included in the sample runs. In this context it is therefore important to define what is meant by such terms as accuracy, precision and reproducibility. In this study the following definitions are recognised.

INTER-BATCH PRECISION This is a measure of the internal consistency of the analytical method for each element determined within an analytical batch, i.e. a set of samples analysed on one occasion. In this study inter-batch precision has been measured by determining the percentage error on duplicate samples and duplicate control samples and has been averaged for each element.

INTRA-BATCH PRECISION or REPRODUCIBILITY The analysis of each sample has been carried out in triplicate (and in some cases in quadruplicate). The intra-batch precision is therefore a measure of the degree to which the concentration of a particular element in each sample can be reproduced from several different analytical batches over a period of time. This is measured by determining the percentage error of replicate analyses of each sample and averaging it for each element. Since this will include determinations of percentage error of some samples within the sample analytical batch, i.e. the inter-batch precision, it can be considered as the orerall analytical precision for each element for the whole study.

ACCURACY A measure of the precision only shows that the data.are internally consistent. The inclusion of previously well-analysed international standards and control samples within each analytical batch, throughout the period of study, allows-the accuracy to be determined by comparison with such data. The accuracy is thus a measure of whether the results rec_o;-ded are absolutely correct with respect to results recorded previously 422

for particular standards. The accuracy of the results depends upon the analytical precision of the determination method.

A2.2 Precision The values for the inter- and intra-batch precision for each element determined in this study are tabulated in Table A2.2, together with the number of determinations used to obtain the figure. The inter- batch precision has been calculated using sample duplicates, duplicate control samples and duplicate international standards. The intra-batch precision has been determined using the replicate analyses of all the samples, control samples and international standards used in this study., The higher figures for intra-batch precision as compared to inter- batch precision are probably a reflection of the variation in error over a period of two and a half years as compared to those obtained within a" single analytical batch. Although it is useful to have such a Table for reference it is misleading to a certain extent because the values are the result of several sources of error. Within the analytical process, three possible sources of error can be suggested:- (i) (a) variation in error due to variations in the operating conditions, the instruments used, and operator error; (i) (b) variation in error due to the concentration of the trace elements, i.e. near the detection limit, the error will be increased due to the small concentrations present; (ii) possible error introduced due to calcium interference correction; and (iii)possible error introduced when the data are corrected to a carbonate-free basis. Not included in the values of analytical precision are those cases where drastic errors would have been introduced by sample spillage, etc. In these cases the samples in question were discarded and reanalysed at a later date. Errors introduced by the weighing procedure are not included in the figures in Table A2.2. Error from this source probably amounts to about a maximum error of - 1%. The geochemical variations discussed above are only those which are not explicable in terms of the analytical precision of the determination method.

423

TABLE A2.2 Inter-Batch and Intra-Batch (Analytical) Precision Values for Analyses carried out during this Study.

ELEMENT INTER-BATCH PRECISION INTRA-BATCH (ANALYTICAL)PRECISION (or PERCENTAGE N0. OF PERCENTAGE NO. OF Component) ERROR ANALYSES ERROR ANALYSES

CaCO3 3.57 141 8.90 820 Ca 2.97 141 5.36 820 AI 5.11 124 2.92 830 Mg 10.02 117 4.20 767 Ti 16.78 117 5.60 767 Ba 28.28 117 18.21 654 Fe 5.75 124 4.23 83b Mn 5.63 124 4.94 830 Ni 17.47 124 14.08 830 Co 32.09 124 25.57 829 Cr 6.86 124 7.17 830 cu 5.79 124 5.35 829 Cd 51.11 124 51.38 827 Pb 24.92 124 17.01 829 Zn 6.79 124 6.49 830 Li 7.49 124 3.66 830 As N.D. 3.47 336

(All determinations made on the carbonate-free corrected values). Percentage Error calculated as follows:- Let X1 and X2 be concentration value for element E in same sample then X1 .} X = XM then X1 - XM = XA x 100 = / 2 X A 1'4 and X2 - XM = XB x 100 = B and then % + '°B = Percentage Error. A NM 424

A2.3 Accuracy Accuracy has been determined using international standards and a well-analysed group of control samples. It is important when selecting such samples to ensure that they correspond as closely as possible to the composition of the samples being analysed, in order to reduce matrix effects which will interfere with the determination procedure (Angino and Billings, 1967). The high concentrations of Ca present in some of the samples (up to 38%) made it extremely difficult to find a satisfactory standard. The principal standard used was a sedimentary tock standard, the USGS National. Bureau of Standards, Argillaceous limestone, NBS-1b. This is an excellent standard for the major elements, Si, Al, Fe, Ti, Mn; Ca, Sr, Mg, Na, K and P, but no published analyses exist for the wide range of trace elements determined in this study (i.e. Ba, Ni, Co, Cr, Cu, Cd, Pb, Zn, Li and As). The analyses of the NBS-1b for these trace elements havebeen compared with other in-group analyses of the material carried out by different analysts. The agreement is satisfactory. However, this method provides no external control of the data, which are only internally consistent. This also applies to the data for the group bulk standard VAS-01 which was used in an effort to obtain some measure of control of the accuracy of the trace element data in the carbonate-rich sediments. The results tabulated for VAS-01 are in good agreement with the results obtained by other analysts. In order to obtain a measure of the accuracy for the trace elements, and also of the elemental concentrations of the low carbonate sediments, a Red Clay Standard was analysed. This is an international standard and hence gives a measure of the accuracy obtained for the data.

The results of these comparisons, together with the percentage variations (expressed as percentage errors) are tabulated in Table A2.3. It can be seen that in all cases the agreement is satisfactory, beating in mind that the accuracy is strongly dependent on the analytical precision of the determination method. TABLE A2.3 Concentrations of Major and Trace Elements in International Standards and Control Samples, Together with Values of Analytical Accuracy Determined for this Study.

CONTROL SAMPLE ARGILLACEOUS LIMESTONE RED CLAY VAS-01 (INTERNAL) NES-1b (USGS) D D MEAN NO OF PERCENTAGE DETERM USGS NO.OF ACCURACY DETERM REPORTED NO.OF N N ACCURACY VALUE DETERM ERROR (IE VALUE REPORTED DETERM % VALUE VALUE DETERM % ELEMENT OBTAINED ACCURACY) VALUE ( CaCO 88.20 0.7 87.7 3 43 92.2 37 4.8 N.D. N.D. - N.D. ( ca 35.58 43 0.6 35.2 36.4 37 3.2 1.49 1.47 24 1.4o ( Al 1.81 3o 4.8 6.1 5.7 20 6.9 9.49 9.48 24 0.2 Wt% ( Mg 2.69 13 5.2 1.8 2.1 27 8.6 1.89 1.87 24 1.1 ( Ti 0.26 29 5.4 0.21 0.27 23 7.8 0.50 0.49 24 2.0 ( 11a 0.061 12 17.8 0.03 N.R. 19 N.D. 0.06 0.045 . 24 25.0 ( Fe 2.51 3o 4.8 5.1 5.o 22 9.8 5.63 5.60 24 0.50

( Mn 4366 30 4.8 14300 1580o 22 9.1 4450 4500 24 1.2 ( Ni 230 30 12.0 120 N.R. 20 N.D. 129 120 24 7.0 ( Co 120 28 13.0 85 N.R. 18 N.D. 65 N.R. 24 N.D. ( Cr 540 3o 6.0 580 N.R. 18 N.D. 104 100 -'2711' 4.0 ( Cu 230 30 6.1 63 N.R. 22 N.D. 105 100 24 5.0 ppm ( Cd 11 25 18.2 8 N.R. 19 N.D. 5 N.R. 24 N.D. ( Fb 120 32 16.5 45 N.R. 20 N.D. 44 33 24 25.0 ( Zn 190 30 4.9 210 N.R. 23 N.D. 134 130 24 3.0 ( Li 32 25 6.4 50 N.R. 9 N.D. 69 66 24 4.4 ( As N.D. N.D. N.D. 21 ' N.D. az. N.R. 20 N.D. N.R. - N.D. na (A11 determinations made on carbonate-free corrected data) N.D. - Not determined, N.R. - Not reported col 426

APPENDIX A3. CALCIUM INTERFERENCE - EFFECTS AND CORRECTION PROCEDURE

A3.1 Nature of'the Interference Compared to other spectral methods, atomic absorption spectrophotometry (AAS) is relatively free from interferences. However, certain interferences do exist. Within a flame, light is absorbed not only by the element being determined, but also by the flame and by other elements which may be present. If the spectral line of the element being determined cannot be resolved from those of other elements, a positive analytical error results because of the addition of the two signals. This is known as spectral interference. A special type of this interference; known as molecular absorption, is what has been encountered in this study. Molecular absorption is particularly prevalent in solutions of h4gh salt content (Angino and Billings, 1967). The interfering elements, of which Ca appears to be the most effective (Billings, 1965; Angino and Billings, 1967), block or absorb light in the flame. Since measurement of absorbance in AAS is done against a background reading, the result of the molecular absorption is to raise the background signal, thus causing an enhancement of, and hence a positive error in, the absorbance measurement. Multi-element, rather than single element, interferences may have an enhancing or a- lowering effect on the measurement of the absorbance. Molecular absorption has been noted since 1961 (Allan, 1961; David, 1961) and together with methods for its correction has been documented more recently (Angino and Billings, 1967; Zander, 1977). Initially the phenomenon of particles of salt in the flame, impeding the passage of light, was termed 'light scattering' (Billings, 1963; 1965). However, later workers have demonstrated that many interferences; termed as 'light scattering' (Koirtyohann and Pickett, 1965a; 1965b; 1966a; 1966b; Angino and Billings, 1967) are actually caused by molecular absorption of sue' species as Sr0 and Ca(OH)2, which absorb resonant light in the flame, as well as the element being determined. The degree of the interference increases with decreasing wave length, with increasing molarity of the major elements, e.g. Ca, Fe, Al, etc. and is dependent on flame parameters. In this study, molecular absorption results from the high concentrations of Ca (up to 38 Wt%) in.the samples. 427

A3.2 Effect of the Interference Billings (1965) has recorded that 18% of the absorption on the Zn resonance line was caused by a 10,000 mg/1 solution\of spec-Pure Ca, which contained no Zn. In this study, molecular absorption by Ca, has been recorded on the resonance lines of Ni (332nm), Co (240.7nm), Cu (324.7nm), Cd (228.8nm), Pb (283.2nm), Zn (213.8nm) and Ba (553.6nm). The effect of the molecular absorption can be seen from Table A3.2 in which are recorded the results of analyses of a surface sediment from the Central Indian Ocean Ridge by AAS for the trace elements in question, with and without the application of the Ca interference correction procedure described in Section A3.3.

A3.3 Method of Correction It has been suggested that for elements such as Ba, Cd and Zn, non-resonance lines near the absorbing line may be used for correcting the effects of molecular absorption (Angino and Billings, 1967). However, this method can be extremely time consuming, and was thus impracticable for the number of replicate determinations carried out in this study, as well as being unsuitable for certain elements. An alternative method is to prepare pure interference standards, without the element being determined, measure these on the resonance line, and , knowing the amount of the interfering element in the sample, subtract its equivalent signal from the total signal. This method can also be used for multi-element interferences, if the combined effect is additive (Billings, 1965). However, this is not always the case and negative molecular absorptions may occur (Koirtyohann and Pickett, 1965b). In this study, the molecular absorption is probably totally attributable to the large concentrations of Ca present and not from the Fe and Al concentrations present. Therefore the method employed was based on a single rather than a multi-element interference effect. The samples were first analysed for Ca, in order to determine the range of Ca values in the samples.Then Ca standard solutions were prepared in the same sample medium from spec-Pure chemicals, covering the range of concentrations of the measured Ca. The standards were then measured on the resonance lines in question and the absorbances caused by the known. concentrations of the calcium were recorded. From a correction graph of the type shown in Fig A3.3, it was possible to determine the absorbance duc. to molecular absorption of a particular trace element at a particular Ca 428

TABLE A3.2 Analysis of a Surface Sediment from th' Central Indian Ocean Ridge by AAS, with and without t& Application of the Ca Interference Correction Procedure.

TRACE WITHOUT WITH ANALYTICAL ELEMENT CORRECTION CORRECTION PRECISION (ppm) (ppm) (Percentage Error)

Ni 1050 280 14.1 co 720 185 25.6 Cu 895 491 5.4 Cd 188 15 51.4 Pb 1100 130 17.0 Zn 586 248 6.5 Ba 21800 7250 18.2

Wt% of Ca in Sample =35.60

(All data expressed on a CFB) Fig. A3-3 Example of Correction Graphs used in the Ca Interference 429 Correction Procedure t n

me Ni (Co) Ele

1 Trace —7(:)-- Zn\(Cu) /m g

p Cd

x 1000 pg/rn Ca

- - - _ - " Fig, .A3.4.3 Diagramatic Representation of the Comparisons Between the Spectra of Hollow Cathode, and Deuterium Lamps

Difference in Intensity

'Intensity

!)20 Lamp

10A' Z3Z04 35004 Wavelength, X, A°. magstsr . 430

concentration. All the samples were corrected against such graphs and the value due to molecular absorption was deducted from the total absorbance before the results were recalculated to 4 carbonate-free basis. The system of correcting the sample values was performed auto- matically using'the computer program AABIN (see Appendix A4.3). The method of correction described above has been applied to all the samples analysed in this study. It does not take account of any interelement molecular absorption effects which might occur in these samples and must therefore only be taken as a first order correction method. However, due to the low concentrations of Fe and Al present in these samples it seems probable that the molecular absorption, in this instance, is a single element interference, caused by the high concentrations of Ca in the samples. A possible source of error may result from the presence of small amounts of such elements as Ni, Zn, Cu, etc in the spec-Pure chemicals used in the preparations of the standard solutions. The effect is unfortunately difficult to assess, due to the fact that the determination of such elements may be subject to the same type of interference effects reported here, if similar analytical procedures are employed. However, the manufacturers of the Spec-Pure CaCO3, Johnson Matthey Chemicals Ltd, report that only detectable amounts of Mg (2 ppm), Si, Cr, Sr (1 ppm) and Al, Fe and Ag (.41 ppm) were found. Analysis for many other elements, including Ni, Co, Cu, Zn, Pb, Cd and Ba, by a wide range of methods including emission spectrophotometry, polarography, AAS, flame emission spectrophotometry, X-ray fluorescence and Instrumental Neutron Activation analysis showed that they were not reportable (i.e. detectable) with respect to their more sensitive emission or absorption lines. The contribution from such impurities is therefore likely to have a minimal effect on the absorption values recorded for these standards. Furthermore, it is unlikely to significantly affect the validity of the correction procedure.

A3.4 Deuterium Background Correction. The deuterium background correction is an alternative correction method to the one employing interference standards described in Section A3.3, for the correction of molecular absorption. It was employed by Koirtvohann and Pickett 1965b) and Billings (1965) who considered it particularly useful for Ba, Cd and Zn. Its merits have been discussed more recently by Angino and Bill .ngs (1967) and Zander (1977 and incll, ied references), but it should be noted that some controversy exists as to its 431

validity in all analytical situations. The method employs a hydrogen (deuterium) lamp as well as the normal hollow cathode lamp. Using the hollow cathode lamp the elemental plus background absorbances are measured. Then by employing a switching mechanism to superimpose the deuterium spectrum along the exact optical path of the hollow cathode lamp's sharp line spectrum the background absorbance is measured at an adjacent wave length. The difference between the two signals is the absorbance due to the element, the background correction having removed the effect of molecular absorption. Koirtyohann and Pickett (1965b) report that by using this method it is possible to correct for absorption by matrix salts for Zn in H2SOJ} and Cd in solutions of Na salts. However, the authors point out that the correction is only valid if the background absorption of the precise wave length of the elemental line is the same as the average absorption of the wave length interval passed by the monochromotor (Koirtyohann and Pickett, 1965b). This may not always be the case and fine structure in the background, perhaps unresolved, may also cause error. For example, adjacent lines to that of the element being determined, e.g. M g in Na salts, Na line 2852.8Ā near Mg 2852.1Ao line, would absorb part of the continuous radiation passed by the monochromotor set to detect Mg, thus causing too large a correction to be made. In order to test the validity of this correction method with respect to the carbonate-rich sediments analysed in this study, a number of tests were carried out employing the deuterium background correction facilities -available to the author using a group of sediments and standard solutions, spiked with known amounts of Ca (3600, 2400, and 1000 pg/ml Ca). The elements investigated were Ni, Cu and Cd. The behaviour of Co is probably similar to that of Ni, Zn to that of Cu and Pb to that of Cd. The results of the tests using the deuterium background and interference standards correction procedures are shown in Figs. A3.4.1 and A3.4.2. No data are available for Cu. This was due to the degradation in the signal to noise ratio to such an extent as to make impossible the measurement of the background absorption. In a recent review of various methods of background correction in AAS, including the deuterium background correction method (or the continuum source corrector) Zander (1977) points to the degradation of the signal to noise ratio, and consequently the. detection limit as being a distinct disadvantage of the deuterium background correction method. Zander (1977) also points out that with elements Which have resonance lines near and above 350nm (e.g. Cu 324.7nm) there is a rapid 432

Fig: A3.11.1 Results of Use of Ca interference Standard Correction Method. and Deuterium Background Correction Method on Ca spiked standards and sample Solutions for Determination of Nickel (Ni) by AAS

020 UNDER corrects

0 -73 0 E 0 0 U y

0 U .0 0 DSO OVER corrects 0 k

0 cS

Q Cl

.-~

.3 pg/ml Ni, Ca interference correction standard method

1:1 correspondence of Methods

Samples

41, Spiked (Ca) Standards 433

decrease in the intensity of the continuum lamps used. This may further contribute to the difficulty of using this correction method for Cu. For this reason, and in view of the results of the test; carried out here, it was decided not to attempt this method of correction for Ba, which is determined at a wavelength of 553.6nm. The results for Cu, using the Ca interference standard correction method, are shown in Table A3.4 and provide an adequate correction for the molecular absorption by Ca. From a study of Figs. A3.4.1 and A3.4.2 it would appear that there is a difference in the amount of absorbance caused by the background absorption measured by the two methods. This may be due to an under correction by the deuterium background correction method or an over correction by the calcium interference standards method. The data on the sample solutions for Ni and Cd are not sufficient to resolve this problem. However, from Table A3.4, it will be seen that the calcium interference standards method eliminates the effect of molecular absorption by Ca, while the indication from Figs. A3.4.1 and A3.4.2 is that the deuterium background correction only partially removes this interference effect. Zander (1977) notes that once the deuterium background correction system is installed and optimised it affords automatic and continuous background corrected results. However, he points out that the system does have a number of disadvantages. His observations are pertinent to the present study and may explain the differences observed in the correction procedures outlined above. Since only one detecting device, i.e. a phototube, is used, it is essential to ensure that the beams from the hollow cathode and deuterium lamps are exactly superimposed (Zander, 1977). This is usually achieved by a mirror and switching mechanism as described by Koirtyohann and Pickett (1965b). This is extremely difficult to achieve in practice unless an unreasonable financial outlay is to be made on the equipment. In extremely expensive systems the error introduced by this is considerably lessened, but on the instrument used for this study, a Perkin Elmer 403 Spectrophotometer, some miscorrection must be expected. Koirtyohann and Pickett (1965b) state it is not necessary to. balance the intensity of the two beams. However, Zander (1977) is of the opinion tl-at the intensities must be. balanced so as to provide a reasonable comparison. This presents a problem. The sharp line spectrum o ) from a hollow cathode lamp is very intense and very narrow (0_1-0.3A in width at a particular wave length (see Fig. A3.4.3). However, the spectrum from a deuterium lamp is a broad beam arc spectrum covering a very wide range of wave lengths, and therefore at the discrete wave length at which.

434

!Fig. x3.4.2 Results of Use of Ca interference stand. rd Correction Method and Deuterium r3ackcround Correction Method on Ca spiked standard and sample solutions for the Determination of Cadmium (Cd) by .SAS

.0S 1)20 Uhf corrects

.07

.06

.05

.04

.03 D20 OVER corrects

.02

.01

.01 .02 .03 .04 .05 .06

rg/ml Cd, Ca interference standard correction method

1:1 correspondence of methods

Samples

4. Spiked (Ca) standards TABLE A3.4 Results of Use of Ca Interference Standard Correction Method and Deuterium Background Correction Method on Ca Spiked Standards and Sample Solution for the Determination of Cu by AAS.

STANDARDS 'SAMPLES

Conc Conc Uncorrected Ca Interf D20 Sample Conc Uncorrected Ca Interf. D`0 Cu Ca Value Stand Corrected No Ca Value Stand Correct corrected value )lg/ml pg/ml pg/ml Corrected Value pg/ml pg/ml Value r g/ml Value ug/ml Pg/ml

0.00 3600 0.08 0.00 23 1750 0.76 0.705 2400 0.06 0.00 24 2550 0.85 0.80 1000 0.03 0.00 25 2100 0.60 0.546 0.01 3600 0.09 0.01 28 2850 0.41 0.34 2400 0.061 0.011 29 2800 0.72 0.65 1000 0.04 0.01 3o 2800 0.62 0.55 0.05 3600 0.135 0.055 2400 0.11 0.05 loo0 0.08 0.05 436

the background is measured the difference in intensities is very large. If this is too large it could lead to incomplete de vection of the background absorption. Increasing the intensity of the 'continuum source emission, produces marked instability in the source, and this adds significantly to the total noise of the system (Zander, 1977). It is therefore necessary to reduce the intensity of the hollow cathode source by lowering the operating current below its optimum level. The .intensities can also be balanced by using a larger slit width. However, this may result in non-linearity of the analytical calibration curve, due to an increase of non-absorbable radiation from the hollow cathode source. It may also increase the probability of spectral interference from matrix atomic lines, which can absorb the radiation from the deuterium source (Epstein and Rains, 1976). A wider slit width should be avoided since\it is in direct conflict with the need to reduce the spectral band width so that interfering lines are eliminated (Zander, 1977). A deterioration in the two deuterium sources used by the author (Perkin Elmer and Actyvian) seems likely and this may account for instability in the intensity of their radiation and may be a possible source of error. In the case of the determination of Cu, this instability may probably have lead to the degradation in the signal to noise ratio to such an extent as to make impossible the measurement of the background absorption.

In summary, it may appear that the deuterium background correction. procedure is unsuitable for Ba due to the decrease in intensity of the continuum lamps experienced at higher wave lengths (553.6nm). Furthermore, that for the carbonate-rich sediments analysed here it is possible to correct for the molecular absorption of Ca on Ni, Cd and Cu using the interference standards correction method. In this study the deuterium background correction produces unsuitable measurements of the background absorption for Ni, Cd and Cu, due to instability of the intensity of the deuterium source, the difficulty in matching the intensities of the deuterium and hollow cathode lamps, the resultant high noise levels and the difficulty of obtaining exact superimposition of the two spectra along the optical path of the spectrophotometer, at a reasonable . financial cost. At this stage. no data are available for Co, Zn and Pb. 437

APPENDIX A4. DATA HANDLING

A4.1 Recalculation of the Element Concentrations Chemical data in marine sediments have in recent years been reported on a treated rather than an untreated basis. The justification for this has been that it represents a more accurate picture of the geochemistry and because it facilitates comparisons with previously published data in the literature. Some authors have suggested normalisation of chemical data in marine sediments to Al+Fe+Mn (Bostr8m et al, 1969), to 4Al+Fe+Mn+Ti+P (Bostr8m et al, 1972) and to A1203 (Piper, 1973). The first two methods are reported to have the advantage of also removing the diluting effects.of opali.ne silica, dried sea salt, organic carbon and weakly chemically bound water, as well as the diluting effects of calcium carbonate. The ratiōnale behind these methods assumes both a constant and slow rate of sedimentation (Bostr8m et al, 1972; and Piper, 1973). In the case of the basal DSDP sediments neither of these criteria are uniformly fulfilled (see relevant DSDP volumes). In the case of the Central Indian Ocean Ridge surface sediments, the dominant phase is CaCO3,together with minor amounts of aluminous and oxide material. The dominance of CaCO3 would make the corrections suggested by Bostr8m and his co-workers (1969, 1972) and Piper (1973) inappropriate for such material. For the reasons stated above therefore, these corrections were considered unsuitable and were not applied to either the surface or DSDP basal sediments. The most common method employed has been to recalculate the chemical compositions to a carbonate-free basis. The rationale on which this is based is that it gives a clearer picture of the sediment geochemistry, since in the oceans on areas like the mid-ocean ridges, calcium carbonate is being deposited at such a rate as to dilute the trace element values. This portion of the rationale is reasonable. However, there are two problems associated with this. The first problem which is conceptual, is that it is assumed that if the carbonate-free correction was to be valid, the carbonate fraction of marine sediments should not contain proportions of the trace elements which one was interested in. The chemical partition results presented in this thesis suggest this may not be the case and that proportions of Ni, Co. Pb, Cd, Cu. Zn and Be may be contained in the lattices of the carbonate minerals. This is further supported by the findings of other workers (e.c. Turekian and Imbr?.ei 1966; Fujita,. 1971; Bertine and Goldberg, 1972;, Aston et al, 1972a; Belyayev, 1973; Leatherland et al, 1973; Oldnall, 1975). The observations reported above 438

may therefore invalidate the correction for certain elements. The second problem is analytical and relates to the errors introduced in the calculation of the correction factor by the 'k.rrors in the determination of the Ca and CaCO3 concentrations. This has been discussed and illustrated in Appendix A2, (A2,2(iii)) and the reader should refer to this section. It is important that it is possible to compare the data generated by this study with previously published data from the literature. For this reason and considering the strong diluting effect which the calcium carbonate does have in about one third of the sample population, the data have been corrected to a carbonate-free basis (CFB). They are also presented on a total sediment basis (TSB), i.e. uncorrected, and the CFB data should mainly be used as a means of comparison with those results which are available in the literature.

A4.2 Calculation of CaCO3 Values. J The attention of the reader has already been drawn to the serious error in the determination of the carbonate-free factor introduced by small errors in the determination of the CaCO3 content (see Appendix A2.2(iii)). It is therefore necessary when determining the concentration of CaCO3 to use a method which introduces the lowest errors at the highest CaCO3 concentrations. The method used in this study is that suggested by Dymond et al (1976), who used the Ca values determined by AAS to calculate the CaCO3 values. Alternative methods using the AAS determined Ca values have assumed that all the Ca is held in the carbonate phases. Chemical partition studies (this study) have shown that this is not the case and that Ca may be held in detrital minerals of the acid insoluble residue. The method of Dymond et al (1976) has the advantage that it takes account of the presence of Ca in the non-carbonate phases and in sea-salt in the sediments, as well az in the carbonate phases. The equation used is:-

CaT -- 0.41S Ca NC (1) wt% CaCO3 = CaC - CaNC 439

Where CaT Wt% of Ca in total sample, determined by AAS CaNC - Wt% of Ca in non CO3 fraction Ca0 = Wt% of Ca in CaCO which is 40.04% \ 3 S = Wt fraction of salt in the sample 0.41. Constant to allow for the wt% of Ca in the salt fraction of the sample, which Dymond et al (1976) report as 1.14% for sea salts.

This constant term is arrived at from Eq.2, Dymond et al (1976). CaNC, the Ca in the non-carbonate fraction is an unknown value. Dymond et al (1976) use a value of 0.731'0.12% for Nazca Plate sediments which they arrived at by measuring the Ca content of the HAC leached residue of twelve samples. It is obviously inappropriate to use such a value, measured on Pacific Ocean sediments, for Indian Ocean sediments. The chemical partition studies conducted as part of this study provide a means for estimating an average value of CaNC for Indian Ocean sediments. An average value of CaNC for Indian Ocean Recent Sediments of 1.02-0.69% Ca arrived at by a similar method to that described by Dymond et al (1976) from 23 samples has been used in Eq.1 for surface sediments. In the case of DSDP sediments there is less Ca in the non-carbonate fraction, and an average value of 0.33-0.10% Ca was used for CaNC in Eq. 1. This value was arrived at from 63 samples measured in a similar way to the method of Dymond et al (1976). S, the proportion of sea salt in the samples was measured by washing one gram of sample with 10 mis of DIW. The solutions were filtered, dried and the weight loss calculated. An average value of 1% sea salt has been used in Eq. 1 for both DSDP and surface Indian Ocean sediments. Dymond et al (1976) have shown what erro:.•s can be expected in the CaCO3 determinations with over- and under-estimation of CaNC (see Fig. 1 and accompanying discussion, Dymond et al;. 1976). The error in the CaCO3 determinations is highest in sediments with less than 10% CaCO3, while in sediments with 90% CaCO3 the error.drops to 1%, This therefore provides an accurate estimation at higher CaCO3 where small variations in this determination can introduce large errors in the calculation of the carbonate-free factor. Although the error in this method is greatest in sediments with less than 10% CaCO (Dymond et al. 1976) it is of less 3 importance since a carbonate-free correction is not necessarily appropriate- for such sediments. 440

This method is however still dependent on the precision of the Ca determinations, which in this survey is - 1.4 - 3.0. Dymond et al (1976) report that their method is the most accurate for determining CaCO3 in high carbonate sediments when compared to other methods which do not rely on the measurement of Ca, e.g. Leco combustion and acid dissolution. The method for determining CaCO3 described in Appendix A1.5 has an overall precision of - 1.68. The CaCO3 values determined by this method are compared with those calculated by Dymond's method (see Fig. A4.3). They show an overall variation of = 1.4% over a range of 0.01 - 6.82, this being lowest at high values of CaCO3. Such a small variation between the two methods would appear to confirm the validity of Dymond et al's (1976) approach, in providing the most accurate estimation of the CaCO3 content of carbonate-rich sediments.

A4.3 Data Processing. The analytical data presented during this study have been processed using. the facilities (CDC 6500/Cyber 173 Computer) of the Imperial College Computer Centre using a. number of programs to carry out a variety of manipulations. All the programs used form part of GEOLB12, the AGRG program library, compiled by Dr R J Howarth. The following notes on the functions of the various programs used are taken from the summaries supplied in the GEOLB12 Manual. This volume is unpublished but available to AGRG members. The AABIN program is a. recent addition to GEOLB12 and is the program which recalculates the analytical data to a carbonate-free basis. By in-putting the sample weights, analytical results (jig/ml), sample volumes, variable value for Dymond's formula, Ca interference correction values, the program performs the Ca interference corrections on the data using a linear regression technique, calculates the CaCO3 values using Dymond's formula and then recalculates the data on total sediment and carbonate- free bases. SCTPLT and TERNRY are elementary plotting programs. SCTPLT is a two variate line printer scatter plotting program which has provision for each point to be allocated a symbol, showing to which class it belongs. It also calculates a correlation coefficient for thetotal data set. It has a log10 transformation option. TERRY is a three variate line printer plotting program producing a'triangular diagram plot. The data are presented in classes and each class is identified by a different symbol. The program has provision for scaling each of the three variables. TERNRY can 441

Fig A4.3

Calcimeter CaCO versus Dymond et al's (1976) CaCO3 Determinations 3

t0 a)

a) a) E •U U

ces C7 U V o\°

Wt. % CaCO Dymond et al's (197,6) Method 3' 442

also output the plot on microfilm for easy inclusion of the results in publica£ions using a more sophisticated program, TPLOTG. MRSTAT and CORSTAT are elementary statistics azd correlation coefficient programs which calculate means, standard deviations, ranges, coefficient of variation, skewness, kurtosis and correlation matrices.

Both have a facility for omitting missing data. MRSTAT has a log10 transform option. CORSTAT is a more complex version of MRSTAT in that it tests skewness and kurtosis and on the basis of these tests whether a log transform or arithmetic is most suitable. Correlations are 10 expressed at the 95% Confidence level from tables of Fisher and Yates (1964).. UFACTOR is an II-Mode factor analysis program which is an adaption of the R-Mode factor analysis program of Davis (1973), but using the MRSTAT/CORSTAT routine to produce the correlation matrix from the data. R-Mode factor analysis is a multivariate statistical technique which is used to describe the variance in a complex data set in terms of a few statistically independent, artificial factors, each of which is composed of a combination of variables. In order that the factors'are not dominated by the most abundant elements, i.e. variables, UFACTOR begins by standardising the data (Davies, 1973, pp. 476-8). This involves subtracting the mean of each variable from each observation and dividing by the standard deviation for each variable. The standardised or trans- formed values now have a mean of zero and a standard deviation of unity. This procedure preserves the inter-sample concentration associations of each metal and the relative intra-sample ratios. Hence the resultant factors are not dominated by the most abundant elements as they would be if the raw, untransformed data were used. UFACTOR also uses a number of routines different to those suggested :,y Davis (1973). The eigenvalues and eigenvectors are extracted using FO2AMF and FOAM' respectively and routine FO1AAF for the matrix inversion in the factor score calculation. These three routines originate from the Nottingham Algorithms Group (NAG, 1975). UFACTOR also uses two routines from the AGRG GEOLB12 program library, OLA1 and OUTB1 for outputting the correlation matrix, the covariance matrix and the factor loading matrix. The routine SCORES, from the GEOLB12 AGRG library, calculates toe factor scores which represent the loading of each factor on each of the cases.