RARE EARTH ELEMENT GEOCHEMISTRY OF THE SEDIMENTS, FERROMANGANESE NODULES AND CRUSTS FROM THE INDIAN OCEAN

SUBMITTED TO THE GOA UNIVERSITY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

BY

BEJUGAM NAGENDER NATH NATIONAL INSTITUTE OF OCEANOGR

RESEARCH GUIDE I „ R.R. NAIR \ 111

DEPUTY DIRECTOR AND HEA 0 A ..30"" GEOLOGICAL OCEANOGRAPHY DIVISION- 5-5 1 .46

N f / RAd

NATIONAL INSTITUTE OF OCEANOGRAPHY DONA PAULA, GOA - 403 004, INDIA.

OCTOBER, 1993 "THESE ELEMENTS (REE) PERPLEX US IN OUR RESEARCHES, BAFFLE

US IN OUR SPECULATIONS, AND HAUNT US IN OUR VERY DREAMS.

THEY STRETCH LIKE AN UNKNOWN SEA BEFORE US - MOCKING,

MYSTIFYING AND MURMURING STRANGE REVELATIONS AND

POSSIBILITIES"

SIR WILLIAM CROOKES, 1887

(as quoted in Elderfield, 1988)

CERTIFICATE

Mr. B. Nagender Nath has been working under my guidance since 1991.

The Ph.D. thesis entitled "Rare earth element geochemistry of the sediments,

ferromanganese nodules and crusts from the Indian Ocean", submitted by him

contains the results of his original investigation of the subject. This is to

certify that the thesis has not been the basis for the award of any other

research degree or diploma of other University. ____.---,_ --- 0 ".„..--,.... ., o.

(

:At \ / ( R. R. NAIR ) \\N \s \ ." ..„_. __- -*/ Research Guide ,,, G_ 0 puty Director and Head, Geb j eanography Division National Institute of Oceanography Dona Paula, GOA - 403 004. CONTENTS CONTENTS

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ACKNOWLEDGEMENTS LIST OF FIGURES iv-xi

LIST OF TABLES xii-xiii CHAPTER 1 1-23 INTRODUCTION

1.1. Occurrence and abundance 2 1.2. Historical aspects 3 1.3. General chemistry of REE 5 1.31. Electronic configuration 5 1.32. Ionic radii and lanthanide contraction 6 1.33. Oxidation states 7 1.4. Mineralogy of the rare-earth elements 8 1.41. REE bearing mineral groups 8 1.42. Co-ordination numbers 10 1.43. Isomorphic / elemental substitution 11 1.5. Presentation of REE data 11

1.6. Distribution of REE in marine environment 13 1.61. REEs in oceans / seawater 13 1.62. REE in sediments 15

1.7. Objectives of the work 17 1.8. Details of the samples studied 20

TABLES 23 CHAPTER 2 24-47 RARE EARTH ELEMENTS IN THE NEARSHORE SEDIMENTS SEDIMENTS OF INDIA 2.1. Introduction 24 2.2. Study area 27 2.21. Geology of the Hinterland 28 2.22. Index of geoaccumulation (I geo) / Degree of pollution in sediments 29

2.3. Sample selection criteria 30

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2.4. Materials and methods 31 2.5. Results and discussion 32 2.51. Filtering effect of Estuaries on the geochemistry of marine sediments 32 2.52. Cerium anomalies 33 2.53. Diagenetic effects 33 2.54. REE fractionation and its origin 34 2.55. REE fractionation related to changes in mineralogy 36 2.56. Fractionation related to physico-chemical condition of the Environment 37 2.57. Control of adsorption / coagulation and complexation processes on the fractionation 38 2.58. Significance of HREE depletion in the nearshore sediments compared to shales 39 2.59. How do these REE values reflect the composition of the average upper continental crust? 40

2.6. Conclusions 42

TABLES 44-47 CHAPTER 3 48-61 RARE EARTH ELEMENTS IN THE DEEP-SEA SEDIMENTS OF CENTRAL INDIAN BASIN

3.1. Introduction 48

3.2. Details of the study area 49 3.21. Tectonic setting 49 3.22. Sample description 50

3.3. Materials and methods 51 3.31. REE determination 51

3.4. Results 52 3.41. REE in terrigenous clays 53 3.42. REE in siliceous clays (nodule free) 53 3.43. REE in siliceous oozes / clays associated with manganese nodules 54 3.44. REE in calcareous oozes / clays 54 3.45. REE in pelagic / red clay sediments 55 3.46. Fractionation indices 55 56 3.47. LaeXOBSS Page No.

3.5. Discussion 56 3.51. Lithological control of REE patterns 56 3.52. Relation between REE patterns and diagenetic processes 57 3.53. Ce fractionation related to bottom water hydrography 58 3.6. Conclusion 60

TABLE 61

CHAPTER 4 62-106 RARE EARTH ELEMENTS IN THE FERROMANGANESE DEPOSITS OF THE INDIAN OCEAN

4.1. Introduction 62

4.2. General features of the study area 65

4.3. Material and methods 67

4.4. Results and discussion 70 4.41. Top-bottom fractionation or REE zonation 70 4.42. Interelement relations and their implications 73 4.43. Cerium anomaly variations 78 4.431. Variation of cerium in oxide and nucleating material 78 4.432. Cerium in nodule nuclei and their implications 80 4.433. Cerium in aluminosilicate phase (2M HCI leach) 82 CERIUM IN BULK NODULES 4.434. Mineralogical control 83 4.435. Diagenetic effects on Ce anomalies 85 4.436. Cerium relationship with Fe and P and their implications 86 4.437. Cerium anomaly variation in the light of nodule forming processes 87 4.44. Gadolinium-terbium anomalies 89 4.45. HREE enrichment in sample 320 A 91 4.5. Summary 93

TABLES 94-106

CHAPTER 5 107-135 RARE EARTH ELEMENTS IN HYDROTHERMAL CRUSTS

5.1. Introduction 107 5.2. Description of the study areas and samples 112 5.21. Hydrothermal Crusts 113 5.211. Southeastern seamount, East Pacific Rise (EPR) 113 5.212. Valu Fa Ridge, Lau Basin 114 5.213. Galapagos Spreading Center 115 5.214. Indian Ocean Triple Junction 116

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5.22. Hydrogenous crusts 117 5.221. Central Pacific 117 5.222. Central Indian Basin 118

5.3. Analytical methods 118 5.4. Results and discussion 118 5.41. REE contents 118 5.42. REE patterns 120 5.43. HREE enrichment 121 5.44. Cerium anomalies 122 5.45. Significance of Eu anomalies in hydrothermal crusts 123 5.46. REE and other geochemical constraints on the hydrothermal mineralization in the Indian Ocean area 124 5.47. REE - major element associations - discrepancy in the sorption experiment results 129

5.5. Summary 131 TABLES 132-135 CHAPTER 6 136-139

COMPARISON OF THE REE UPTAKE BEHAVIOUR BY VARIOUS AQUEOUS SEDIMENTARY PHASES IN THE INDIAN OCEAN

6.1 Enrichment Factors 136 6.2 Cerium as an additional resource from manganese nodules 138

CHAPTER 7 140-149 SUMMARY AND CONCLUSIONS

REFERENCES 150-173 LIST OF AUTHORS PUBLICATIONS 174-176 Bebirateb In

mu Mauro parents tutlo gam, me life nub g000 eburation

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to mu Kate &nth gabn Elamobar for tlis tlelp During mg tligtler eburation ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS

The work presented in this thesis is carried out under the guidance of Shri. R.R. Nair. I am indebted to him for his guidance and sustained encouragement throughout the course of this work. My sincere thanks are also to him as the Head of the Geological Oceanography Division and also to the

Director, NIO for providing facilities and allowing to me pursue this topic.

I record a deep-sense of gratitude to Late Padmashri Dr. H.N. Siddiquie for insisting me to undertake a geochemistry topic.

Thoughful and constructive comments by my chemical oceanography division colleague Dr. M. Dileep Kumar are very much appreciated.

Much of the data used in this work has been generated during my stay in Germany on a DAAD fellowship. And for a successful stay at Germany, the credit goes to Dr. Walter L. Pluger, Privat Dozent, my German supervisor. He was generous enough to provide facilities which are very expensive at Europe.

Prof. G. Friedrich has kindly provided a placement in his institute. During my

stay, Neef, Frau Wiechowsky, Frau Siebel and Deissman have run their

instruments for hundreds of my samples. Graduate students Thomas Bohm,

Christopher Mamet, Ralf Buffler, Sabina Lange, Sven Petersen have spent many

hours either in the laboratory helping me with my analyses or drawing my figures. Many thanks to them. Dr. I. Roelandts, University of Liege, Belgium

and Dr. H. Kunzendorf of Riso National Laboratory, Roskilde, Denmark have

generated high quality REE data in their laboratories. My profound thanks to II both of them for showing keen interest in the topics suggested by me. Dr. P.

Herzig has been a source of encouragement for me who always has a scientific atmosphere around him. I thank DAAD for providing the fellowship and CSIR for a deputation.

My sincere thanks to Shri. N.P.C. Reddy and Prof. Y.L. Dora for my Kerala coast sampling and geologists, geophysicists, marine surveyors, electronic and mechanical engineers of the PMN Project for the deep-sea sampling. Thanks are also due to Dr. U. von Stackelberg (BGR, Hannover), E. Fouquet (IFREMER), Dr. K.P. Becker (Aachen), Prof. G. Friedrich who have provided me the ferromanganese crusts from the Pacific.

Constant encouragement and help rendered by my colleagues Mr. M. Sudhakar, Mr. M. Shyam Prasad, Dr. V. Purnachandra Rao, Shri. K.A. Kamesh Raju, Dr. B. Chakraborty and Dr. V.N. Kodagali is gratefully acknowledged. My roommates Dr. R. Mukhopadhyay and Mr. N.H. Khadge always had a word of appreciation for the scientific progress in our room. Dr. V. Balaram, Scientist at

NGRI has kindly allowed me to use their ICP-MS facility.

Much of sampling and laboratory work has been carried out under the financial support from the Department of Ocean Development, Government of

India under their programme 'Polymetallic Nodule Surveys'.

I could not have brought out the thesis in this shape but for the untiring help rendered by my colleague Shri. S. Jai Shankar. His skill with the computer was most useful and beneficial to me. Mrs. Alison Sudhakar typed some parts of this thesis and my thanks to her. iii

Back at home (Warangal), my parents, brother, maternal uncles (Late

Yada Damodar, Vaikuntam and Krishna Murthy), cousins (Madan, Shyam and Srinivas) and my childhood friend Kedareshwar have always taken interest in my adventures with sea and boosted my morale.

I must make a mention of my professional rivals who kept me on heels and helped me to strive for quality.

Last, but not least I acknowledge my loving wife Lata who has taken the burden of running the home during my long hours of absence from home

(mostly shuttling in between Vasco and Panjim or between Central Indian Basin and Goa) and my dearest daughters Pallavi and Purnima for their love. LIST OF FIGURES iv LIST OF FIGURES

Figure 1.1. The periodic table. The rare earth elements are shown in hatched boxes (After Henderson, 1984).

Figure 1.2a. To correct for the above shown saw-tooth variation, the shale values are normalized to the chondrite REE values. The resultant figure shows a smooth pattern compared to the bottom figure.

Figure 1.2b. The natural saw-tooth variation of REE abundances when plotted against their atomic number. NASC - North American Shale Compositite (After Henderson, 1984).

Figure 1.3. Effective ionic radii for REE and their common substituting ions (CN = 8). Ce and Eu have two oxidation states and all other REE exist in trivalent oxidation state.

Figure 1.4. Idealized co-ordination polyhedra around REE (After Cesbron, 1989).

Figure 1.5. The locations of the samples used in this work. 1) Dot close to Indian coast represent the area of near-shore sediments, 2) a box in the Central Indian Basin is the location for deep-sea sediments, manganese nodules and hydrogenetic crusts, 3) open circle denotes the location for hydrogenous manganese nodules of Western Indian Ocean, 4) plus-marks indicate the locations of hydrothermal ferromanganese crusts. The plus-mark south of Central Indian Basin box represents the Rodriguez triple junction, the plus-marks close to American coast represent East Pacific Rise and Galapagos Rise respectively. The plus-mark in the SW Pacific denotes the back-arc spreading center Lau Basin.

Figure 2.1. Study area in the southwestern coast of India showing the sampling locations. Out of the 250 samples collected, 43 representing three sub-environments referred in the text (forced fluvial, brackish and shelf) were analysed for REE.

Figure 2.2. Map showing the geology of the hinterland for the rivers debouching into the Vembanad lake. R1, R2, R3, R4, R5, R6, R7 and R8 represent rivers Bharathapuzha, Chalakudi, Periyar, Muvattupuzha, Meenachil, Manimala, Pamba and Achankovil respectively.

Figure 2.3. Bar diagrams showing the elemental distribution in the lake/lagoon/estuarine sediments compared to the sediments from the inner continental shelf region. Averages for all the elements except sodium and chromium are higher for lagoonal sediments compared to shelf sediments indicating a filtering effect of estuaries. The horizontal line marked for each element represent the total average for all the sediments put together. V

Figure 2.4. The magnitude of cerium anomalies for all the sediments considered here. Ce anomalies have been calculated using the formula of Elderfield and Greaves (1981). Each line represents the cerium anomaly value for each of the sediment studied here. Notice the range of Ce anomalies do not exceed -14. 0.07 in any case. This shows that the Ce is not fractionated compared to its immediate neighbours La and Nd and also indicate that most of the Ce is in the trivalent state.

Figure 2.5. A plot of a) La versus Mn and b) the LREE enrichment factor ( Lan / Lun ) versus Mn. Note the scatter of the data without statistically clear trend indicating that the REE are little affected by diagenesis in these sediments.

Figure 2.6. The range and averages of shale normalized (3-shale average) REE patterns of the sediments studied. The patterns are remarkably similar showing significant LREE enrichment, only with differences in the magnitude of LREE enrichment.

Figure 2.7. Variation of LREE enrichment (La n / Lun x 10) compared to the kaolinite content (%) and the salinity (psu) in different sub-environments.

Figure 2.8. Shale-normalized REE patterns of charnockites formed by retrograde metamorphism. The charnockites are the dominant source rock in the region. The REE data are from Allen et al (1985) and the samples are from southern India (Salem) which is a northern extension of the source area in the present study. Note a significant LREE enrichment over HREE which is similar to the patterns observed for the sediments under study (Figure 2.6).

Figure 2.9a. REE removal versus salinity in mixing experiments conducted by Hoyle et al (1984). Note that 65% - 95% of all the REE are removed at a salinity of about 10 psu.

Figure 2.9b. Fe removal in mixing experiments using the same end members as for the REE experiments. Note that a substantial iron loss by flocculation with the major removal occuring over the same 0 - 10 psu range.

Figure 2.10a. A plot of La (ppm) versus Fe (%) shows a positive relation for the sediments from brackish environment and the shelf. Comparitively, the scatter is more for the fluvial sediments.

Figure 2.10b. Sm (ppm) representing middle REE versus Fe (%) show a similar relation as noticed in Figure 2.10 A.

Figure 2.10c. The plot between Lu (ppm) which represents HREE and Fe (%) show a scatter for all the types of sediments indicating that the HREE are complexed in the saline waters. vi

Figure 2.11. Plot of Al203 - La showing the distribution of cratonic shales, suspended river loads, average upper continental crust compositions and our nearshore sediments. Shales are averages from various stratigraphic successions that range in age from Early Archaean to Cenozoic (Condie, 1991 and references therein). Suspended river load

analyses - . AmM - Amazon; Mek - Mekong; Gan - Ganges are taken from Condie (1991). NASC - North American shale composite - Gromet et al (1984); PAAS - Post- Archaen Australian shale average, UC (upper continental crust) and ACT (average Archaean upper continental crust) from Taylor and Mclennan (1985). EPC (average Early Proterozoic upper continental crust) and AC (average Archaean upper continental crust) from Condie (1991).AII REE data are by INAA (Instrumental Neutron Activation). Almost all nearshore sediments of the present study show La enrichment compared to shales. In fact, the suspended load values reported by Condie (1991) (diamonds) also fall in the extreme higher limits of shales and definitely away from all the estimates of upper crustal composition values. This shows that La (or LREE) in nearshore sediments and suspended loads is genuinely higher than the shales as well as the crustal composition and not due to an analytical artifact as suggested by Condie (1991). Figure 2.12. A Zr-Yb diagram. Zr contents of suspended loads as well as our nearshore sediments are calculated from Hf data using a Zr / Hf ratio of 39. Our values fall away from the field of the shales and upper crustal values with significantly low Zr values but almost equal Yb concentrations. Figure 2.13. A plot of thorium (ppm) versus scandium (ppm). Notice that all the samples studied here have Th/Sc ratios distinctly less than the upper crustal value of 1.0 (Taylor and McLennan, 1985).

Figure 2.14. Major element distributions in the sediments of the present study plotted in the triangular diagram Fe 203T - K2O - Al203. Open circles represent the lake samples and dots represent the shelf sediments. The sediments fall distinctly away from the shale represented by North American Shale Composite (NASC). Shelf sediments fall in the field of residual clays. Regions for shales and residual clays are from Gromet et al (1984) and Reimer (1985), respectively.

Figure 2.15. The REE patterns of one set of values (average of continental shelf sediments from this study) after normalizing with 3 different shale averages used commonly in literature. The three shale averages employed are NASC (represented by circles), PAAS (squares) and 3-shale average (triangles). Note the similarity in patterns except few kinks at Sm for NASC, and at Yb-Lu for both NASC and PAAS normalized patterns (probably small analytical uncertainities).

Figure 3.1. Map showing the sample locations in relation to the physiographic settings and sediment types. vii

Figure 3.2. Shale normalized REE patterns of various types of sediments. Each sediment type showed a characteristic pattern with a bearing on its lithology, minor variations in depositional settings, diagenesis and source area. CaCO, contents shown for the calcareous sediments are from Nath et al (1989).

Figure 3.3. Average cerium anomaly values (calculated with the formula given by Elderfield and Greaves, 1981) for each type of sediment. Terrigenous sediments and pelagic clays have no Ce fractionation. Calcareous sediments are showing strong negative Ce anomalies and siliceous oozes / clays are showing positive anomalies.

Figure 3.4. Variations in behaviour of REE across the series are indicated by the average ratios of Lan / Ybn and similarly by Lan / Gd, and Gd / Ybn for each sediment type. Note these ratios are different for each sediment type. In addition to the previous figure, Ce anomalies are also

denoted here-using an expression . Ce/Ce* = (Cesamo. / Ces,„) / Ce*, with Ce* being obtained by linear interpolation between shale-normalized La and Nd values. Ce / Ce* <1 are considered to have a negative Ce anomaly. Figure 3.5. Ce anomaly values of various types of sediments are plotted against a backdrop of a) bottom water temperatures (4300 m water depth) and b) dissolved-oxygen concentrations (in ml/l) data (data and base map from Warren (1982). Various symbols are used to indicate the sediment types. 1) Thick open circles - terrigenous sediments, 2) thin open circles - siliceous clays with a terrigenous component and recovered from nodule barren area, 3) inverted triangles - siliceous clays / oozes overlain by manganese nodules, 4) triangles - calcareous oozes / clays and 5) filled circles - pelagic / red clays . Note a tongue of cold, oxygenated water at the central eastern margin of the basin (AABW) adjacent to the 90°E ridge. Apparently no relation is seen between the Ce anomaly values and bottom water redox conditions.

Figure 4.1. Figure showing the sampling locations with general physiographic settings.

Figure 4.2. Figure showing the X-ray diffractograms before and after leaching the manganese nodules with 2M HCI. Figure 4.3. Shale normalized REE patterns of tops and bottoms of two oriented nodules 0-1 and 0-2.

Figure 4.4. Diagram showing the relationship between Fe (%), Ce and other REE (ppm) in the oriented nodules. B and T represent bottoms and tops respectively.

Figure 4.5. Positive relation between Co (ppm) and Ce (ppm) in the nodules.

Figure 4.6. Figure depicting the enrichment of REE and depletion of Cu (ppm) in tops. Vice versa in bottoms. VIII

Figure 4.7. Shale normalized REE patterns of ferromanganese encrustations. Note the strong positive cerium anomalies.

Figure 4.8. Shale normalized REE patterns of manganese nodules from a seamount environment. The sample from seamount top (142-0) shows the maximum REE concentration.

Figure 4.9. Shale normalized patterns of three different varieties of nodules. Sample 75C is a typical hydrogenetic nodule from red clay region shows the biggest postive cerium anomaly. Sample 284A is a representative of diagenetic nodule with a low positive anomaly. Smectite containing nodule 320A is enriched in HREE.

Figure 4.10. Shale normalized patterns of nodules recovered from Western Indian Ocean. All of them have identical patterns.

Figure 4.11. Geochemical characterisation scheme of Kunzendorf et al (1988). Fields I, II, Ill and IV represent deep-sea sediments, ferromanganese deposits (crusts, micronodules and nodules), marine basalts (and• related rocks) and seawater respectively. The data points represent the analyses conducted in this study.

Figure 4.12. Megascopic features of broken surfaces of nodules showing the various types of nuclei. The corresponding shale normalized REE patterns are shown adjacent to the photos.

Figure 4.12A. Sample MAR-8 is an elongated, ellipsoidal, smooth textured nodule composed of elongated, disseminated light brown palagonitic nucleating material. XRD revealed good peaks of phillipsite, plagioclase, quartz and smectite which are typical low temperature alteration products of basalt (Honnorez, 1981) in the nucleus. Small peaks of 6-MnO2 are observed in the oxide layers. A clear visual demarcation is noticed between oxide and nucleus.

Figure 4.12B. Nodule SS-5/360B is small sized, rough textured diagenetic nodule. Visual observation on breaking the nodule has shown that the oxide and nucleus to be distinct. But on physical separation and grinding, nucleus was found to be penetrated and mineralised by the oxide material. No significant mineralogical differences were found between oxide and nucleus, except the intensification of quartz peak in the nucleus. The peak heights of todorokite and 6-Mn0 2 remained more or less unchanged.

Figure 4.12C. Third nodule FA2/73D used for nucleus studies is also small sized, rough textured nodule containing a thin layer of oxide which is firmly embedded on nucleus. Apart from quartz increase in the nucleus, moderate intensification of todorokite and slight depletion of 6-Mn0 2 were also observed. ix

Figure 4.12D. Nodule FA5-203B has three distinct layers. Layer 1: The outer oxide layer is rough textured with an older nodule as innermost nucleating material. Layer 2: A brown leached, altered material is sandwiched between the older and younger nodules. Layer 3: The nucleus of smaller, older nodule is fibrous and palagonitic in nature. For the REE anaryses, these three layers were separated. The contents of quartz and plagioclase increase gradually towards the interior. Todorokite is high in layer 2 followed by layer 1 and negligible in layer 3. 6-Mn0 2 is absent in layer 3 with almost equal intensities in outer layers.

Figure 4.12E. Nodule (MAR 16) has a representative of biogenetic nucleating material. This triangular shaped, smooth textured nodule has a shark tooth as its nucleus. Only the tip of the shark tooth was exposed and entire tooth was invisible before the nodule was broke open. The oxide material is massive and the tooth is stained black inside the cavity, probably by iron-oxides. As observed recently (Toyoda and Tokonami, 1990), the enamel seems to be much less permeable to diagenetic exchange. The oxide material is composed of small peaks of 6-Mn0 2 and the tooth material was just sufficient for REE analysis.

Figure 4.13. Relationship between Mn/Fe and Ce anomalies. The nuclei data are joined by a line with their respective oxide data. WIO and CIB represent Western Indian Ocean and Central Indian Basin respectively.

Figure 4.14. Relationship between Fe and Ce anomalieS. The nuclei data are joined by a line with their respective oxide data.

Figure 4.15. Relationship. between P and Ce anomalies. The nuclei data are joined by a line with their respective oxide data.

Figure 4.16. Relationship between Ca and Ce anomalies. The nuclei data are joined by a line with their respective oxide data.

Figure 4.17. The shale normalized patterns of 2M HCI leachates of the manganese nodules and their bulk counterparts.

Figure 4.18. The ratio patterns of 2M HCI leach over bulk concentrations.

Figure 4.19. Shale normalized REE patterns of the residues left out after leaching the nodules with 2M HCI. The station numbers are mentioned for each pattern.

Figure 4.20. Geographic distribution of Ce anomalies of the manganese nodules studied here.

Figure 4.21. Relationship between Ca and P. The nuclei data are joined by a line with their respective oxide data.

Figure 4.22. Relationship between Ca and Fe. The nuclei data are joined by a line with their respective oxide data. X

Figure 4.23. Relationship between Fe and P. The nuclei data are joined by a line with their respective oxide data.

Figure 4.24. Gadolinium-terbium anomalies observed in the present study compared to the anomalies observed by De Baar et al (1985b) in the seawater.

Figure' 5.1. Bathymetric map at 12°15'N on the East Pacific Rise showing the location of the studied samples. This map was prepared during the Clipperton cruise (1981) using a multibeam echo sounder (SEA BEAM) (from Fouquet et al., 1988).

Figure 5.2. Bathymetric map of the Lau Basin and the vicinity (from von Stackelberg and von Rad, 1990). Solid triangles = DSDP sites and ODP sites. Black star - massive sulfide deposit. NLSR = Northern Lau spreading ridge. VFR = Valu Fa ridge.

Figure 5.3. Location map of 'Cauliflower garden' hydrothermal field (from Herzig'et al, 1988). Seabeam morphology, contour intervals 10 m. Cross-hatched area delineates the hydrothermal field of location C (Cauliflower garden). Full triangles indicate other hydrothermal products outside this field. At locations marked by full dots faunal elements related to hydrothermal vents were observed. The vertically hatched areas are covered by ponded lavas.

Figure 5.4. Simplified map of the Indian Ocean ridge system indicating the study area north of the Rodriguez triple junction (RTJ). CR = Carlsberg ridge, CIR = Central Indian Ridge, SWIR = Southwest Indian Ridge, SEIR = Southeast Indian Ridge (from Pluger et al, 1990).

Figure 5.5. Shale-normalized REE patterns of hydrothermal ferromanganese crusts from Galapagos spreading center (Garimas K2 - K10), East Pacific Rise (EPR - A to C), Lau Basin (LB - 80 to 116 GA). All of them display a HREE enriched pattern similar to seawater. Ce is either not detected in some cases, or when detected show a negative anomaly. Some of the samples (# Garimas - K2, K3, Lau Basin - 93 KD) show positive Eu anomalies which is a typical feature of hydrothermal plume solutions.

Figure 5.6. Shale-normalized REE patterns of hydrogenous Central Indian Basin crusts (CIB - 1 to CIB - 9) and mixed hydrothermal- hydrogenous crusts from Rodriguez triple junction (GEMINO - K6 to K17). The patterns are same with convex - bowed up shape IREE enrichment except Ce anomalies. Ce anomalies in hydrogenetic crusts are typically positive. The hydrothermal ( or mixed ) crusts from ridge area display a mirror-image pattern as far as Ce is concerned. Ce anomalies' are all characteristically negative which indicate a hydrothermal origin. xi

Figure 5.7. La versus Ce concentrations (adopted from Toth, 1980). Hydrothermal crust La / Ce ratios are similar to seawater, approaching an apparent lower limit of La / Ce -0.25. The GEMINO crusts (the field encircled with small dots) fall close to the line describing hydrothermal deposits.

Figure 5.8. Correlation between FREE and Cu+Ni+Co concentrations in hydrothermal deposits and metalliferous sedimentary deposits (adopted from Clauer et al, 1984). Data characterized by smaller open and filled circles, are from Bonnot-Cortois (1981), corresponding to hydrothermal and metalliferous material from FAMOUS, Galapagos, Cyprus, Leg 54, Bauer deep," Marquesas zone, N. Pacific (Clauer et al, 1984 and references therein). GEMINO crusts (dosed triangles) fall in the field of metalliferous deposits.

Figure 5.9. Co/Zn versus Co+Ni+Cu concentrations (adopted from Toth, 1980). A good correlation of these parameters is shown for hydrothermal and hydrogenous ferromanganese crusts. The low values for hydrothermal crusts indicate low concentrations of seawater derived Co, Ni, and 'Cu, but relatively high hydrothermally derived Zn contents. GEMINO values fall more closer to hydrothermal field. #K14 and #K8 are from Galapagos spreading center.

Figure 5.10. Si versus Al concentrations (adopted from Toth, 1980). Si / Al ratios of nodules are more similar to marine sediments. Ferromanganese crusts and Fe-rich crusts show enrichments in Si. Average marine sediment value is from Turekian and Wedepohl, 1961. GEMINO values fall in a zone in between the two lines.

Figure 5.11a. Ternary diagram of Fe versus Mn versus (Cu+Ni+Cu)*10 (adopted from Toth, 1980). Illustrated are 1) low trace-metal content of hydrothermal crusts, 2) the similar Fe enriched trace- metal depleted compositions of ferromanganese crusts and EPR metalliferous sediments. "Hydrogenous" and "hydrothermal" fields are from Bonatti et al (1972). EPR metalliferous sediment data are from Corliss and Dymond (1975). GEMINO samples fall just at the base of EPR metalliferous data and in hydrothermal field.

Figure 5.11b. Ternary diagram of Fe versus Mn versus Si x 2. The trend of increasing Si with Fe content is shown. Bauer basin smectite data are from Eklund, 1974. GEMINO crusts fall in the EPR metalliferous sediment field. The open squares in the hydrothermal Fe-rich crusts / oxides are from Lau Basin. The filled squares and Mn-rich crusts from Galapagos, EPR and Lau Basins.

Figure 6.1. Figure showing the comparison of REE enrichment factors (REE.mo. / REEupper crust) of the sedimentary phases considered for the study. LIST OF TABLES

xii LIST OF TABLES

Page No.

Table 1.1: Abundance of the naturally occurring REE (ppm) (from Spedding, 1981). 23 Table 1.2: Electronic configuration of the rare earths (from Topp, 1965). 23 Table 2.1: Geology of the catchment area and the characteristic minerals in different rivers debouching into Vembanad lake (from Mallik et al, 1987). 44

Table 2.2: Index of geoaccumulation (l g.) of trace metals in sediments under study. 45

Table 2.3: Elemental concentrations and the light enrichment factors in the study area. 46 Table 2.4: Average compositions of Vembanad lake and adjoining shelf samples compared to the averages of crustal compositions. 47 Table 3.1: Rare earth element concentrations (in ppm), Ce anomalies and other fractionation indices for the deep-sea sediments of the Central Indian Basin. 61

Table 4.1: . Description of locations and samples. 94-96

Table 4.2: Details of nodule nuclei. 97 Table 4.3: REE contents in U.S.G.S manganese nodule reference samples Nod-A-1 and Nod-P-1 (in ppm). Comparison of present results obtained using ICP-MS and 1CP-AES with previously reported data. 98

Table 4.4: Major and minor element data of manganese nodules and crusts (analysis performed on dried samples). 99

Table 4.5: REE concentrations in ferromanganese nodules and crusts (in ppm). 99 Table 4.6: Compositions of ferromanganese nodules reported by Elderfield et al, (1981b). 100

Table 4.7: Correlation coefficient matrix for complete data versus major elements in manganese nodules and crusts. 101

Table 4.8: Correlation coefficient matrix for rare earth elements in manganese nodules and crusts. 102 xiii

Page No.

Table 4.9: Correlation coefficient matrix for complete data versus major elements and important elemental ratios in manganese nodules and crusts. 102 Table 4.10: Important elemental ratios in nodules and crusts used for genetic interpretations. 103 Table 4.11: Details of the crusts and nodules. 104 Table 4.12: Major elements and REE concentrations in the seperated oxide and nucleating material. 105

Table 4.13: REE concentrations in bulk nodules and their corresponding acid leachates (in ppm). 106 Table 5.1: Details of the samples studied. 132

Table 5.2: Chemistry of hydrogenous and hydrothermal crusts. 133-135 STATtMENT

As required under the University ordinance 19.8, I state that the pres00

thesis entitled "Rare earth element geochemistry of the 04in-tents, ferromanganese nodules and crusts from the Indian Ocean" is my original contribution. To the best of my knowledge, the present study is the first comprehensive study of its kind from the area mentioned.

The literature • concerning the thesis has been cited. Due acknowledgements have been made wherever facilities have been availed of.

. Noy2 AA, D4, "7/`• ( B. NA ENDER NATI) CHAPTER 1

INTRODUCTION 1

INTRODUCTION

The rare-earth elements (REE) include the following fifteen chemically similar metals starting from lanthanum to lutetium. The atomic number (z) varies from 57 - 71:

1. Lanthanum (La, z = 57) 2. Cerium (Ce, z = 58) 3. Praesodymium (Pr, z = 59) 4. Neodymium (Nd, z = 60) 5. Prometheum (Pm, z = 61) 6. Samarium (Sm, z = 62)

7. Europium (Eu, z = 63) 8. Gadolinium (Gd, z = 64)

9. Terbium (Tb, z = 65) 10. Dysprosium (Dy, z = 66) 11. Holmium (Ho, z = 67)

12. Erbium (Er, z = 68)

13. Thulium (Tm, z = 69) 14. Ytterbium (Yb, z = 70)

15. Lutetium _ (Lu, z = 71)

They belong to the Group II IA in the periodic table (Fig. 1.1). All of them except prometheum (Pm) exist in nature. Pm is never found .in earth's crust, since it has no stable isotopes and is produced only by nuclear reactions.

But it can be quantitatively obtained from the fission products formed in nuclear Group IA VIII B 2 He IIA 111B IVB VB VIB VIIB 3 4 6 8 10 Li Be B C N 0 F Ne

13 14 15 16 17 18 11 12 Group Na Mg Al S i P S CI .Ar IIIA I VA VA VIA VITA r---VIII IB II B 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Z n Ga Ge As Se Br Kr

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb MO Tc Ru Rh Pd Ag Cd In Sn Sb Te Xe

55 56 72 73 74 75 76 77 78 79 81 82 83 84 85 86 - Cs Bo Hf To w Re Os I r Pt Au 8 H TI Pb Bi Po At Rn

87 88 89 Fr Ra Ac 5 $96WAM A TYMOrZAWA 90 91 92 93 94 95 96 97 98 99 100 102 103 Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw

Fig. 1.1. The periodic table. The rare earth elements are shown in hatched boxes (After Henderson, 1984). 2 reactions (Spedding, 1981). The long series of REE are collectively known as the lanthanide series because they directly follow the element lanthanum.

Yttrium (Y, z = 39) which is not a lanthanide, but belongs to Group IIIA is usually grouped with the REE because of its chemical similarity. When yttrium is included as member of REE group, the sixteen elements La - Lu and Y are collectively termed as "lanthanons". The rare-earth elements have a certain common features in the structure of their atoms which are the fundamental reason for their chemical similarity (Spedding, 1981).

An interest in the geochemistry of the rare earth elements has grown stupendously in the last few years, since the observed degree of REE fractionation, in a rock or mineral, can indicate its genesis (Henderson, 1984).

1.1. OCCURRENCE AND ABUNDANCE

The name rare earths is a misnomer, because they are neither rare nor earths (Spedding, 1982). The early Greeks have termed the substances which could not be changed by heating as the "earths". When these elements were discovered in the early part of the 19th century, they were found to resemble the common earths, the oxides of Mg, Ca and Al. As these so-called earths were found in then very rare minerals, they were termed as "rare earths" (Spedding, 1982) although the later observations have shown that they are not so rare. For example, cerium is more abundant than tin in the Earth's crust.

Even the scarcest rare earths are more abundant than the platinum-group elements. They are much more abundant than Au (4 ppb), Ag (70 ppb) and U (ppm) in the crust (Willer, 1989). Neodymium is more abundant than lead.

Lutetium, the scarcest REE, said to be more abundant than mercury or iodine 3

(Spedding, 1981). The REE are estimated to form about 0.02% of the earth's upper crust by weight. They occur in high concentrations in a considerable number of minerals. These elements are found as mixtures in all massive rock formations ranging in concentration from ten to a few hundred parts per million

(ppm) by weight. Although, the REE contents vary with different rock formations, in general, it has been observed that the more basic (or alkaline) rocks contain lesser amounts than do the acidic rocks.

The average REE contents found in chondritic meteorites, three common rock types and in the earth's crust together with their ranking are presented in Table 1.1 (from Spedding, 1981). The composition of chondritic meteorites is considered to represent their abundance in the cosmos. The elements with even atomic numbers are more abundant than the odd-numbered ones. Therefore, when the elemental abundance in either solar system or the earth's crust are plotted against the individual REE display a saw-tooth variation or a rhythmic alteration (Fig. 1.2).

1.2. HISTORICAL ASPECTS

The first REE mineral "cerite" was discovered by Cronstedt in 1751

(Moller et al, 1989). But various sources of literature recognize that the history of rare earths began in 1787, when Lt. Carl Alex Arrhenius observed an unknown black mineral in a quarry located at Ytterby, near Stockholm

(Henderson, 1984). The quarry was worked for exploiting the feldspars used in the manufacture of porcelain (Cesbron, 1989). In 1794, Johann Gadolin, a

Finnish chemist at the Uppsala University extracted a new earth in an impure form which he believed to be a new element but was, in fact, a mixture of rare earth oxides. He has named the "element" as 'ytterbia' after the village Ytterby. z A bundance , 0 0'01 Henderson, 1984). Fig. against theiratomicnumber.NASC-NorthAmerican ShaleCompositite(After smooth patterncompared tothebottomfigure. values arenormalizedto thechondriteREEvalues.Theresultantfigure showsa 100 0•1 100 10 10 1

1.2. La CePrNdPmSmEuGdTbDyHo ErTmYbLu 111 b) Thenaturalsaw-toothvariation a) Tocorrectfortheabove shownsaw-toothvariation,theshale 1111111,11111 of REE abundanceswhenplotted 0 Chondrites x NASC 4

In 1797, A.K. Ekeberg, again examined it and shortened the name to yttria (Cesbron, 1989; Spedding, 1981). In 1800, M.H. Klaproth named this first rare- earth mineral as gadolinite in Gadolin's honour. From the same mineral, a new earth known as ceria was extracted later in 1803 and was named 'ceria'. After • Sir Humphry Davy has demonstrated that these earths were in fact compounds of oxygen and metallic elements, a number of chemists have verified the existence of these oxides in a wide variety of rare minerals (Spedding, 1981).

Carl Gustaf Mosander, a Swedish chemist during the period from 1839 to 1843, has obtained a number of rare-earth metals from their oxides in impure forms. He found a, new group of elements which were different to many other groups of elements known till then. He has demonstrated that this new group of elements have formed the same classes of compounds with almost the same properties with only slight differences observed in their solubilities.

During the period 1843 to 1939, after the instrument spectroscope was invented, Mosander's oxides were resolved by many workers into a number of other oxides. Because of the ambiguity in resolving the chemical fractionation of the mixed rare-earth salts, the rare-earth elements have found no place in the early periodic table proposed by Russian chemist D.I. Mendeleyev. Only when, the British physicist H.E.J. Moseley has found a direct relationship between the X-ray frequencies and the atomic number of the elements, it was possible to assign unambiguous atomic number to these elements. Moseley could clearly find a place for fourteen rare-earth elements in the periodic table.

Prometheum, the element with atomic number 61 was separated only in 1945, from atomic fission products produced in a nuclear reactor (Spedding, 1981). 5

The demand for rare-earths has increased dramatically after Welsbach has invented their use in gas mantle and lighter flint in 1885. This resulted in a world-wide search for rare-earth minerals.

1.3. GENERAL CHEMISTRY OF REE

1.31. Electronic configuration

The difficulties encountered in separating / simplifying the original yttria

and ceria were because of the marked chemical similarity in the characters of individual REEs. The chemical similarity is with regard to the electronic

configuration of the atoms and ions of the individual elements (Moller, 1968).

Electrons fill the various shells and sub-shells of the atoms, in the order of their stability. When the fifth period of the periodic table ends (with the

element xenon), the 4s, 4p, and 4d subshells are filled, as are the 5s and 5p

subshells. With the beginning of the sixth period, two 6s electrons are added

for the first two elements of the period, and a third electron goes into a 5d

orbital as the element lanthanum is reached. For atoms of elements following lanthanum in atomic number, the energy of 4f orbitals is below that of the 5d.

So, when the next electron is added, it goes into 4f subshell, which upto this

point has remained vacant. This process continues until each of the seven 4f

orbitals is doubly occupied before filling up the 5d orbitals. This shell has a capacity to hold 14 electrons, so the addition of next 13 electrons corresponds

to the filling of this inner shell. The elements in which this occurs are the

14-lanthanide elements from cerium through lutetium. The electronic

configuration of the rare-earths is given in Table 1.2 (adopted from Topp, 1965). 6

The filling up of inner 4f shell makes lanthanides a coherent group of elements. The outer s, p and sometimes d electrons are involved in chemical bonding with other atoms determining the chemical behaviour of each element. The ground state of a particular atom may vary from 4fx 5d' 6s 2 to 4r" 6s2. This is more of a physical difference and has less of chemical importance (Willer, 1968). The electrons may be lost only from 4f orbitals to various oxidation states, but the electrons in these orbitals are well shielded from participating in chemical interactions. Although lanthanum atom contains no 4f electrons, the properties of lanthanum and its compounds vary little from the other lanthanide elements.

1.32. Ionic Radii and lanthanide contraction

REE are characterised by a steady decrease in ionic radius with an increase in atomic number. This is due to an increase in the positive charge in the nucleus which pulls the various subshells, especially the 5s and 5p

subshells closer the nucleus (Spedding, 1981). In effect, the size of the lanthanide atoms decrease with the increase in atomic number. This feature is

popularly known as_lanthanide contraction". According to Shannon (1976), the effective ionic radii, for an oxidation state of three and a co-ordination number

of 8 with respect to oxygen are La - 1.160 A > Ce - 1.143 A > Pr - 1.126 A > Nd - 1.109 A > Pm - 1.093 A > Sm - 1.079 A > Eu - 1.066 A > Gd - 1.053 A > Tb - 1.04 A > Dy - 1.027 A > Ho - 1.015 A > Er - 1.004 A > Tm - 0.994 A > Yb - 0.985 A > Lu - 0.977 A.

This successive reduction in ionic radius does not occur elsewhere in the

periodic table, because at no other point are 14 electrons successively added to

electronic shells, none of which plays a part in chemical bonding (Topp, 1965). 7 The lanthanide contraction is responsible for 1) small variations in basicity within a given oxidation state that permit separations by fractional means and 2)

parallel decrease in• ease, of oxidation of the metals with increasing atomic

number (Miler, 1968). Certain effects of the lanthanide contraction reflect/continue beyond the lanthanide series, thus halfnium resembles zirconium much more closely than zirconium resembles titanium.

1.33. Oxidation states

REE exhibit a nearly constant valency of three in their geochemistry.

Although, the regular oxidation state is 3+ in nearly all the mineral species,

oxidation states +2 may be shown by Eu and Yb, and +4 by Ce and Tb. The

multiple oxidation states of these elements are partly due to the enhanced

stability of half-filled (Eu 2+ and Tb4+) and completely filled (Yb 2+) 4f sub- shells. Ce4+ has an electronic configuration similar to the noble gas xenon (Henderson, 1984). In minerals, Eu2+ is known to be embedded in various mineral

structures (Cesbron, 1989). For example, it can substitute Ca 2+ in plagioclases found under reducing conditions.

Between the lanthanide elements which can exist in tetravalent state, while Tb4+ has not been recorded in any mineral or aqueous medium, but Ce 4+ is common in terrestrial as well as aquatic environments. In fact, oxidation of

Ce3+ into Ce4+ in the seawater and its incorporation in the Mn oxides /

hydroxides has been used as an explanation for the impoverishment of Ce in

sedimentary apatites of marine origin (Fleischer and Altschuler, 1969). Hence,

geochemically only the cations Ce4+ and Eu2+ represent other important oxidation

states. These differences in valencies lead to the anomalies in their distribution

compared to the other strictly trivalent REE. Reduction of Eu is noticed mostly 8 in magmatic processes. A decrease in its oxidation state increases its ionic radius and that results fractionation from the rest of the series. Anomalies of Eu have been reported in many igneous and sedimentary rocks (Henderson, 1984). Reduction of Eu within the ocean basins seems to be confined to hydrothermal

systems (Michard et al, 1983). Cerium also exhibits anomalies compared , to its immediate neighbours La and Pr/Nd. Ce anomalies have been found mostly in the oceanic/sedimentary phases. They are rare among the igneous rocks except during the recycling of oceanic lithosphere.

Like many other elements in seawater (eg., Mn, Fe, Cu, As, Sb and Se),

Ce is also affected by its multiple oxidation states (De Baar et al, 1985a). Seawater is typically depleted in Ce when compared to ferromanganese nodules which often exhibit Ce enrichment. Both these characters are attributed to the involvement of Ce in oxidation-reduction reactions and to a change in the oxidation state. The advantage of Ce and Eu variation compared to other elements having two oxidation states, is the possibility of calculating an anomaly by comparing with their strictly trivalent neighbours.

Consequently, it is possible to identify the oxidation-reduction reactions of Ce and Eu from other processes affecting their oceanic distributions (De Baar et al,

1985a).

1.4. MINERALOGY OF THE RARE-EARTH ELEMENTS

1.41. REE bearing mineral groups

Clark (1984) defined three groups of minerals according to their REE contents,

1) minerals with very low amounts of REE which include many rock- 9

forming minerals, 2) minerals containing minor contents of REE but not as an essential constituents. Around two hundred minerals are known to contain more than 0.1% weight percent of REE (Herrmann, 1970). They enter as

sub-ordinate constituents in variable amounts and act as substitutes for one or more of the common rock-forming elements (Goldschmidt, 1954),

3) minerals with major concentrations of REE, especially the trivalent

elements as essential constituents, eg: monazite.

The REE occur in all major groups of minerals such as

1. Halides - usually found in granitic pegmatites, alkali granites and syenites. At times, fluorite can contain considerable amounts of REE, e.g., cerian flourite of Norwegian pegmatite.

2. Carbonates -More than forty minerals are noted with carbonate anionic

complexes either associated with F ions or with silicate, e.g., bastaenite in various carbonities.

3. Oxides -REE are associated with Ti and Ta-Nb in oxide minerals. These minerals occur as accessory phases of granites and nepheline syenites. Some of them are resistant and found later as detrital material in placer

deposits (e.g., samarskite).

4. Phosphates -Though they constitute a less important class than carbonates, they include two major REE ores, xenotime and especially monazite. These are the most widespread of all rare-earth minerals

occurring in - .granites and granitoides, alkaline rocks, carbonatites,

metamorphic and sedimentary rocks (placer deposits). Apatites

frequently contain a few hundred to several thousand ppm REE. In

apatites, REE partially replace Ca. 10

5. Silicates -Among silicates rare-earths occur in silico-carbonates, silico-phosphates, rare- earth silicates, and boro-silicates, e.g.,

hellandite. Most of the minerals are found in pegmatities of nepheline-syenites, granites and alkali granites.

1.42. Co-ordination numbers

The REE occupy a wide variety of co-ordination polyhedra in minerals

(Henderson, 1984). In mineral structures, co-ordination numbers (CN) from 6 to 12 have been observed. Normally, inclusion/agreement of REE in a lattice site of the mineral is governed by the ionic size. The heavy REE with small ionic radii are six fold' co-ordinated, light REE are 10-12-fold co-ordinated, and elements with intermediate size (middle REE) are accommodated in polyhedron with seven to nine oxygens. Among the REE, normally the co-ordination is greater. Some examples of co-ordination numbers found in various minerals are:

6-fold in loveringite (Ca, Ce, Ti oxide) 7-fold in sphene 8-fold in zircon

9-fold in monazite 10-fold in lanthanite (La, Ce carbonate)

11-fold is rare and found in bastanaesite and allanite 12-fold in perovskite.

The size of REE ions (with CN = 8) are shown in Fig. 1.3 (Cesbron, 1989) together with the ionic sizes of other ions that are commonly either replace REE or replaced by REE ions in the mineral lattices. A° K+ 1.5

Ba2 +,0 2- 1.4

1.3 Pb 2+ • Sr2÷ A 2+ 1.2 Na+ • • Ca2+ • 1.1 • • 3÷ • • Th4+ • • 1.0 • •114+ • • • • 4+ Mn 2 + Fe 2 + 0.9 Sc 3 + Zr 4 +

58 60 62 64 66 68 70 I 1 1 1 1 1 1 1 1 I I I I I I La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 1.3. Effective ionic radii for REE and their common substituting ions (CN = 8). Ce and Eu have two oxidation states and all other REE exist in trivalent oxidation state.

a CN=6 b CN=7 CN=8

d CN= 8 e CN=I0 f CN= 12

SOURCE: CESBRON, 1989

g CN=I2

Fig. 1.4. Idealized co-ordination polyhedra around REE (After Cesbron, 1989). 11 1.43. Isomorphic / Elemental substitution

The wide range of co-ordinations allows substitution for REE between the sites as well as with various other elements. In achieving the isomorphic substitution, an appropriate electrostatic charge balance has to be maintained (Cesbron, 1989). Substitution takes place in a mineral where the substituted cation is also large (Henderson, 1984). The most common replacement of REE is by calcium, followed.by Th, U, Na, Fe, Si, Pb, Ba, Zr, K, Mn, Sc etc.

All these substitution depend on ionic radius except in the case of Zr 4 which has a relatively small radius (Fig. 1.3). The ionic radii of the REE compared to those elements listed above are shown in Fig. 1.3 (adapted from Cesbron,

1989). Examples of the important substitutions are:

1) Cat` would replace Pr3+ and Nd3+. Light REE effectively substitute for Ca in the structure of epidote-allanite series, 2) REE are preferentially replaced by Ca in monazites, 3) Ce, La, Nd replaces Ca in titanites,

4) Actinides replace REE in some monazites.

1.5. PRESENTATION OF REE DATA

Plotting the abundances of REE against their atomic number, produces a zig-zag / saw-tooth pattern (Fig. 1.2) which does not facilitate the comparison among various geological samples. This is because the rare-earths in natural geological materials follow the Oddo-Harkin's rule, which states that even atomic numbered elements (z) have a greater cosmic abundance than odd-numbered (z) elements. This is due to the increased stability for nuclei with even numbers of protons and neutrons. Many schemes have been tried 12 in the past to remove this effect. For example, in one scheme, two plots were made separately showing the distribution of even-z and odd-z elements. Normalization to one element (eg., La or Yb) was also popular for some time (Taylor, 1972). This procedure depends heavily on the reliability / precision of the analytical data of that particular element. To overcome these, Coryell et al

(1963) have proposed a method, wherein the sample data is compared to the chondrite or sedimentary rock patterns i.e., dividing the abundances element by element by corresponding meteoritic or sedimentary rock abundance (e.g. Lasa,„pe / Lachonate. This has distinct advantages like

1) it removes the saw-tooth variations among REE,

2) it enables direct comparison both of relative patterns and concentrations.

Now, it is a common practice in geochemical studies to normalize the REE patterns, obtained for a geochemical system, to either chondrite or average shale values. The chondrite values are taken to represent cosmic abundances of the rare-earths, while shale values are taken to represent crustal abundances (Gromet et al, 1984). Any chemical differentiation that has taken place,in the system of interest, can thus be separated from global abundance variations due to nucleosynthesis effects (Jonasson et al, 1985). The zig-zag pattern, for which the normalization is intended to correct, is not expected to be connected in any way with the crystal chemical or thermodynamic properties of REE (Jonasson et al, 1985).

Three shale averages 1) NASC (North American shale composite

Gromet et al, 1984), 2) PAAS (Post-Archaean Australian shales - Taylor and 13

McLennan, 1985) and 3) 3-shale average (3SA - Haskin and Haskin, 1966) are widely used to normalize the REE data of sedimentary / marine samples.

1.6. DISTRIBUTION OF REE IN MARINE ENVIRONMENT

1.61. REEs in oceans/seawater

The major features of the REE in the oceans are well characterised. In general, the dissolved REEs in seawater are HREE-enriched compared to average continental shales, possess negative eNd values and Ce anomalies, and sometimes have small negative Eu anomalies (Goldberg et al, 1963; Hogdahl et al, 1968; Elderfield and Greaves, 1982; De Baar et al, 1983, 1985a; Klinkhammer et al, 1983; Taylor and McLennan, 1988; Elderfield, 1988;

Brookins, 1989; Piepgras and Jacobsen, 1992). The oceans are heterogenous on both small and large scales with respect to REE concentrations because the residence times of REE are shorter than the mixing time of the oceans (1000 yrs). REE studies in the North Atlantic Ocean (Elderfield and Greaves, 1982;

De Baar et al, 1983), the Pacific Ocean (De Baar et al, 1985) and the Indian Ocean (German and Elderfield, 1990) have shown that the REE accumulate in the deep water of the oceans. Regarding the increase in the REE concentrations with depth, two schools of thought exist (Piepgras and Jacobsen, 1992). 1) It is believed that the REEs in the upper water column are reported to be removed by adsorption onto downward settling particles coupled with an upward flux produced produced during sediment diagenesis into bottom waters across the sediment-water interface (Elder-field and Greaves, 1982). 2) The recent data for REE in porewaters from two deep-sea cores indicate that oxic marine sediments do not support a benthic flux of REE to the deep ocean

(Piepgras et al, 1986). Instead, the second school of thought suggests that the 14

REE removal by adsorption on downward-settling particles in near-surface waters is followed by dissolution of REEs from these particles upon settling into deep waters (De Baar et al, 1985).

The negative Ce anomaly in seawater is the most commonly reported feature (eg., Brookins, 1989 for a review). The Ce-anomaly probably reflects both preferential incorporation of Ce 41 into manganese nodules, many of which have large positive Ce anomalies (eg., Piper, 1974b), and onto Fe-Mn oxides on sediment particles. The only exception to the generally found negative Ce anomaly is reported by De Baar et al (1983). They found positive Ce anomalies in the Sargasso Sea. The samples from the same locations have been re-analysed by Sholkovitz and Schneider (1991) recently and the positive

Ce anomalies found by De Baar et al (1983) have been attributed to analytical artifacts.

The shale-normalized REE patterns for seawater show a pronounced HREE enrichment. Adsorption of the REEs by organic surfaces and a variety of inorganic surfaces in seawater produces the HREE enrichments in solution (Byrne and Kim, 1990). The adsorption of the lanthanides onto biogenic and / or hydrogenetic particle surfaces is in response to their polarizing power, large valence, and affinity for calcitic matter (Turner and Whitfield, 1979; Brookins, 1989). Thus, the LREE are said to be more readily fixed than the heavy REE due to differences in polarizability (i.e., Lu has a greater polarizing power than

La), thus the heavy lanthanides are more enriched in seawater relative to the LREE. In addition, complexation of REEs by carbonate ions in solution alongwith the carboxylate and other functional groups on surfaces can produce 15 the REE abundance patterns in seawater which are quite similar to appearance to shale-normalized REE abundances in seawater (Byrne and Kim, 1990).

Three major sources could supply REE to the modern oceans (Derry and

Jacobsen, 1990): the dissolved load from rivers (eg., Goldstein and Jacobsen, 1988a); hydrothermal alteration of the oceanic crust (Michard and Albarede, 1986) and sediments undergoing diagenesis. The diagenetic REE fluxes are not accurately known, but on a global scale it is probably small relative to above mentioned two sources (Elderfield and Sholkovitz, 1987; Sholkovitz et al, 1989). Further, atmospheric transport of REE is not known.

1.62. REE in sediments

After the pioneering work of Minami (1935) on REE in sedimentary

environment (Paleozoic and Mesozoic European and Japanese shales), several fundamental studies by Haskin and Gehl (1962), Balashov et al (1964), Spirn

(1965), Wildeman and Haskin (1965), Haskin et al (1966), Piper (1974 a) have established that the REE contents of most shales are very similar in being

enriched in the LREE relative to the HREE, when normalized to chondrites

(Fleet, 1984). They have further found that the REE contents of most

sediments and sedimentary rocks are similar in the relative abundance of the

individual elements although they differ in absolute concentrations.

Shale-normalized terrigenous input patterns from land to sea display no significant Ce anomalies (eg., Haskin et al, 1966; Sholkovitz, 1990). Ronov et

al (1967) have studied a number of Russian platform clays, sands and carbonates. Their carbonates exhibited lower Ce anomaly values, and both

carbonates and sands displayed distinctly lower La contents, implying that a 16 significant proportion of REE's is contributed by detrital material and, in particular, clays.

Ce anomalies and La abundances in sediments tend to overlap between different environments partly due to the mineralogical variations between various studies (Murray et al, 1991a). Noncarbonate terrigenous sediments have Ce anomalies close to shale and are the main carriers of La and other REE, pelagic sediments display a range of values with both positive and negative Ce anomalies (Nath et al, 1992b) and elevated La abundances. Metalliferous sediments are characterized by largest negative Ce anomalies found in oceanic sediments (eg., Barrett and Jarvis, 1988; Olivarez and Owen, 1989) and exhibit a range of La abundances.

The similarity in REE contents of sediments and shales have led to the idea that the REE abundances could be taken to represent the REE composition of upper crust. The sedimentary processes such as weathering, sedimentary sorting and diagenesis which might have homogenized the REE in sediment from its source to its site have received special attention during the past two decades. The following points have emerged from these studies

(some of them are as listed in Olivarez, 1989):

1) Weathering and sedimentation proceses tend to homogenize the REEs with respect to their variability in igneous source rocks (Taylor and

McLennan, 1985),

the REE content of widely distributed shales and loess deposits (Taylor et al, 1983) are very similar and reflect the average composition of the upper crust, 17

3) the REEs are generally immobile during weathering and transport

processes (e.g., Chaudhuri and Cullers, 1979),

4) the bulk of the REEs are transported in the clay mineral fraction (e.g., Fleet, 1984),

5) the chemical behaviour of REE during the weathering process depends on several factors including Eh, pH, organic and inorganic ligands of the

soils (Cantrell and Byrne, 1987), exchange sites in clays (Aagard, 1974; Roaldset, 1974), mineralogical distribution of REE in the parent material (Braun et al, 1990), 6) the REE are fractionated during weathering process, the weathered .

residual products being enriched in LREE and depleted in HREE (Nesbitt, 1979; Duddy, 1980),

7) although no changes in REE patterns are attributed to diagenetic reactions (eg., Chaudhuri and Cullers, 1979), REE fluxes from sediments

into the overlying water are reported (Elderfield and Sholkovitz, 1987).

1.7. OBJECTIVES OF THE WORK

The chemical coherence and relatively predictable behaviour of the REEs have made them one of the most extensively used geochemical tracers

(Gosselin et al, 1992). They have been used in diverse fields to evaluate cosmochemical processes, igneous processes, evolution of ore deposits, mantle and continental crust evolution, processes controlling elemental fractionation between the continents and the oceans and hydrothermal processes (eg:

Henderson, 1984; Lipin and McKay, 1989), to decipher the primary depositional environment of the precursor sediment (Shimizu and Masuda, 1977; Murray et al, 1990), to assess geological processes such as Precambrian crustal sources 18 (eg: Taylor and McLennan, 1985) and the role of subducted sediment in the genesis of convergent margin magmas (e.g: Ben Othman et al, 1989). In addition, the Ce anomaly in sediments has been proposed as a tracer of large scale marine anoxia in ancient oceans (e.g., Holland, 1984) and this has been used also in studying the DSDP / ODP cores (Wright et al, 1987; Liu et al, 1988; Wang et al, 1986; Liu and Schmitt, 1990). REEs may be an anthropogenic indicator (Olmez et al, 1990) and the determination of the natural man-made inputs -to . the marine system (Murray et al, 1991a). Recently,

Sholkovitz (1992) has studied the chemical evolution of river water chemistry based on REE contents.

REEs alongwith certain other trace metals give information about the various mechanisms, by which they have been incorporated into the sediments such as scavenging onto the particles, co- precipitation, ion pairing and redox reactions (Elderfield, 1988). Many trace elements can be disassociated or get associated during their transport from the source to the depositional sites due to biogeochemical processes in seawater (Elderfield, 1988). But in case of the

REEs, only subtle variations may take place across the series. And this coherence makes them useful geochemical tracers. However, interpreting the REE signatures in aqueous sedimentary phases is complicated by processes that fractionate the REEs including complex formation, ion exchange adsorption / desorption, and colloid transport. To use the REEs as natural tracers we need to understand the extent to which these processes are dominant in different geochemical environments (Gosselin et al, 1992). Considering such utility and requirement of REE research, a comprehensive research project has been undertaken. The basic purpose of this research is to study the relative uptake behaviour of REEs by various aqueous sedimentary phases in the 19

Indian Ocean region. To achieve this goal, I have conducted detailed geochemical analyses on different sedimentary phases such as near shore sediments, deep-sea sediments, manganese nodules and ferromanganese crusts.

Emphasis was placed on the measurements of REE in these sedimentary phases. In addition to compare and contrast the uptake behaviour, the objective of this work was many fold and is detailed point-wise down below:

1) to estimate the relative abundance of REEs in nearshore sediments of freshwater close to river confluence, in brackish water and the adjoining inner shelf

2) fractionation Within the REE series during estuarine mixing 3) fractionation of REE associated with mineral sorting during sediment

transport

4) the nature of sediment REE fluxes into the sea

5) to study the distribution of REE in deep-sea sediments as a function of variations in lithology and bottom water conditions within an ocean basin

6) to document the behaviour of REE throughout,a single ocean basin,with relatively well-constrained sedimentary and oceanographic sources

7) to study the viability of applications of REE fractionations and Ce anomalies in marine sediments for paleotectonic, stratigraphic

reconstructions 8) to determine the top-bottom variations of REE in oriented nodules and

the role of diagenetic contribution of these elements to manganese

nodules

9) variations of REE in suites of manganese nodules from different environments such as shallow basins, deeper abyssal plains, seamount 20 top, and different sedimentary environments

10) the role of bottom water redox conditions on the REE geochemistry of nodules

11) the factors controlling the Ce anomaly variations in the ferromanganese deposits

12) influence of type of nuclei and the aluminosilicate on the magnitude of Ce anomalies

13) to study the end-member REE compositons of pure manganese oxides,

pure iron oxides of hydrothermal origin on one hand and purely hydrogenetic ferromanganese crusts on the other

14) to study the continuum between hydrogenous and hydrothermal end members of Fe-Mn oxides using REE as an evidence, and 15) to use REE in the Fe-Mn crusts as a tracer to study the hydrothermal mineralization.

While most Of these aspects are unknown for the Indian Ocean some of the studies have not been attempted so far for any part of the world oceans. The REE studies on the Indian Ocean sediments are sparse (less than about one hundred samples - Tlig and Steinberg, 1982; Ben Othman et al, 1989; Liu and Schmitt, 1990; Owen and Zimmerman, 1991; Nath et al, 1992b) compared to those on the Pacific sediments (close to a thousand analyses - Elderfield et al, 1981a; Glasby et al, 1987; Toyoda et al, 1990; Kyte et al, 1993 etc).

1.8. DETAILS OF THE SAMPLES STUDIED

In all one hundred and fifty . three REE analyses have been conducted on the various types of aqueous sedimentary phases such as fresh water sediments, brackish water, inner shelf, deep-sea terrigenous sediments, 21 calcareous sediments, red clays, siliceous clays / oozes, hydrogenetic manganese nodule deposits, diagenetic manganese nodules, nodule nuclei, 2M

HCI leachates of nodules, hydrothermal Mn oxide, Fe oxide precipitates, hydrothermal Fe-Mn crusts and hydrogenetic ferromanganese encrustations. A break-up of sample number for each type is presented below and the locations are shown in Fig. 1.4. In addition to REE, major elements and mineralogy have been studied wherever necessary. Detailed descriptions of the samples and physiographic setting of each area are presented in the individual chapters. Sample Details No. Location I. Near shore sediments 43 Fluvial 12 Vembanad lake, SW coast of India Brackish .... 15 - do -

Continental shelf .... 16 Off Cochin, SW coast of India II. Deep-sea sediments 30

Terrigenous clays... 7 Central Indian Basin Siliceous oozes / clays ... 9 - do -

Calcareous oozes ...4 - do - Pelagic / red clays ... 10 - do -

III. Manganese nodules 50

- -- Bulk nodules 27 Central and Western Indian Ocean Physically seperated oxides ... 5 - do - Physically separated nuclei ... 5 - do -

2M HCI leaches 9 - do -

2M HCI leach residues 4 - do - 160E 160W 120

Fig. 1.5. The locations of the samples used in this work. 1) Dot close to Indian coast represent the area of near-shore sediments, 2) a box in the Central Indian Basin is the location for deep-sea sediments, manganese nodules and hydrogenetic crusts, 3) open circle denotes the location for hydrogenous manganese nodules of Western Indian Ocean, 4) plus-marks indicate the locations of hydrothermal ferromanganese crusts. The plus-mark south of Central Indian Basin box represents the Rodriguez triple junction, the plus-marks close to American coast represent East Pacific Rise and Galapagos Rise respectively. The plus-mark in the SW Pacific denotes the back-arc spreading center Lau Basin.

22 Sample Details No. Location

IV. Ferromanganese crusts 30

Hydrogenetic Central Indian Basin & crusts .... 10 Central Pacific Hydrothermal Indian Ocean triple crusts .... 20 junction, East Pacific Rise, Galapagos Rise, Lau Basin

TOTAL 153 23

Table 1.1 : Abundance of the naturally occurring REE (ppm) (from Spedding, 1981). Earth's crust ay. of 20 comp. of N. American Kilauea Element Rank Abundance chondritic 40 North Precambrian basalts meteorites American granites shales

La 35 18 0.3 39 49 10.50 Ce 29 46 0.84 76 97 35 Pr 45 5.5 0.12 10.3 11 3.9 Nd 32 24 0.58 37 42 17.8 Sm 42 6.5 0.21 7 7.2 4.2 Eu 57 1.1 0.074 2 1.25 1.31 Gd 43 6.4 0.32 6.1 5.8 4.7 Tb 59 0.9 0.049 1.3 0.94 0.61 Dy 50 4.5 0.31 - 3 Ho 56 1.2 0.073 1.4 1.22 0.64 Er 54 2.5 0.21 4 3.2 1.69 Tm 65 0.2 0.023 0.58 0.53 0.21 Yb 53 2.7 0.17 3.4 3.5 1.11 Lu 60 0.8 0.031 0.6 0.52 0.2

Table 1.2: Electronic configuration of the rare earths (from Topp, 1965).

Element z 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 6s La 57 2 2 6 2 6 10 2 6 10 2 6 1 2 Ce 58 2 2 6 2 6 10 2 6 10 1 2 6 1 2 Pr 59 2 2 6 2 6 10 2 6 10 3 2 6 2 Nd 60 2 2 6 2 6 10 2 6 10 4 2 6 2 Pm 61 2 2 6 2 6 10 2 6 10 5 2 6 2 Sm 62 2 2 6 2 6 10 2 6 10 6 2 6 2 Eu 63 2 2 6 2 6 10 2 6 10 7 2 6 2 Gd 64 2 2 6 2 6 10 2 6 10 7 2 6 1 2 Tb 65 2 2 6 2 6 10 2 6 10 9 2 6 2 Dy 66 2 2 6 2 6 10 2 6 10 10 2 6 2 Ho 67 2 2 6 2 6 10 2 6 10 11 2 6 2 Er 68 2 2 6 2 6 10 2 6 10 12 2 6 2 Tm 69 2 2 6 2 6 10 2 6 10 13 2 6 2 Yb 70 2 2 6 2 6 10 2 6 10 14 2 6 2 Lu 71 2 2 6 2 6 10 2 6 10 14 2 6 1 2 CHAPTER 2

RARE EARTH ELEMENTS IN THE NEARSHORE SEDIMENTS OF INDIA 24

RARE EARTH ELEMENTS IN THE NEARSHORE SEDIMENTS OF INDIA

2.1. INTRODUCTION

The REE abundance patterns of any sediment/sedimentary rock reflect the net result of fractionation during chemical weathering, physical mixing (erosion), transportation and deposition, and chemical interaction between water and particulate matter during all these processes as well as during diagenesis

(eg: Marsh, 1991). REE in marine sediments have been used to assess the geological processes such as Precambrian crustal sources (eg: Taylor and

McLennan, 1985), -plate tectonic setting (eg: Bhatia, 1985), weathering, diagenesis etc (Nesbitt, 1979; Duddy, 1980; Cullers et al, 1987; 1988; Middleburg et al, 1988). Several key observations have been made during the last two to three decades regarding the REE geochemistry of sediments (listed out in Olivarez, 1989). A few relevant observations are

1) weathering and sedimentation processes tend to homogenize the REE's

with respect to their variability in igneous source rocks (Taylor and McLennan, 1985),

2) the REE content of widely-distributed shales and loess deposits (Taylor et al, 1983) are very similar and reflect the average composition of the upper crust,

3) the REE's are generally immobile during weathering and transport processes (eg: Chaudhuri and Cullers, 1979) and

4) the bulk of the REE's are transported in the clay mineral fraction (see Fleet, 1984). 25 The processes influencing REE's in estuarine sediments are poorly known, though it is an important part of the geochemical cycling of elements in the oceans. Most of the information published has been on the behaviour of dissolved REE in estuaries and rivers (Martin et al, 1976; Keasler and

Loveland, 1982; Hoyle et al, 1984; Stordal and Wasserburg, 1986; Goldstein and Jacobsen, 1988 a,b; Sholkovitz and Elderfield, 1988; Elderfield et al, 1990

and references therein). Mainly, three points have been established from these studies. Large-scale removal of REE from solution in the low-salinity zone of was observed in most of the estuaries. While Martin et al (1976) and Goldstein

and Jacobsen (1988b) have found preferential removal of light REEs (LREE), Hoyle et al (1984) have observed a preferential removal of heavy REE (HREE). On the other hand, Sholkovitz and Elderfield (1988) have noticed nearly complete removal of all REEs.

Secondly, some of the recent studies (Keasler and Loveland, 1982; Hoyle et al, 1984; Goldstein and Jacobsen, 1988; Sholkovitz and Elderfield, 1988) have shown that the shale-normalized REE patterns of riverwaters are not flat as assumed previously but exhibited a significant HREE enrichment. And lastly, a number of factors such as adsorption and co-precipitation with ferric oxides and hydroxides, the source rock control, pH of the environment, stabilization and coagulation by organic colloids have been proposed to be responsible in controlling REE contents in rivers and estuaries.

If the LREE are removed leaving the effective dissolved flux to the oceans somewhat enriched in HREE as suggested by Goldstein and Jacobsen

(1988) and Elderfield et al (1990) is true, it may result in the incorporation of 26

LREE into the estuarine sediments and therefore makes them an important geochemical tracer.

Four aspects are discussed in the present chapter 1) the relative abundance of REEs in sediments of fresh water close to, river confluence, in brackish water and in shelf, 2) fractionation within the REE series during estuarine mixing, 3) REE fractionation during mineral sorting and 4) the nature of REE input through the bedload into the adjoining oceanic environment. This chapter compares the behaviour of REE in surface sediments of Vembanad lake, a tropical estuary with that of the sediments in the adjoining inner continental shelf of Kerala state, southwest coast of India. Vembanad lake has several features, such as its large size and different depositional environments, which make it ideal to study the behaviour of REE in estuarine regions. The construction of a bund mid-way of the lake cuts-off the tidal connection to the southern part of the lake during the non-monsoon season and leads to a dynamic set of biogeochemical processes.

The LREEs in estuarine particulates and continental shelf sediments are found to be enriched compared to shales and the upper continental crust

(Sholkovitz, 1988, 1990; Goldstein and Jacobsen, 1988; Nesbitt, .1-990) and this has been attributed solely to incomplete dissolution of HREEs during sediment decomposition (analytical artifact) (Condie, 1991). Here, a non-destructive (instrumental neutron activation analysis) method is employed to study the

REEs in nearshore sediments, in attempt to resolve this hypothesis. Thus, this study aims to throw light on the nature and composition of sediment REE fluxes into the sea as well as the processes that cause their fractionation. The 27 sub-sections and their titles dealt in the latter parts of the discussion follows that of Condie (1991), to enable us to compare our data to his.

Basu et al (1982) considered the Holocene fluvial sands close to the source rocks to belong to the first leg of the sedimentary cycle. The lack of knowledge of the behaviour of REE during the various segments of the sedimentary cycle prevents one to understand the changes during sediment lithogenesis (Basu et al, 1982). So, we have chosen two more segments of the sedimentary cycle and designated the estuarine/lagoonal (transitional) environment where the fluvial sediments debouche as the second and the continental shelf as the third leg.

2.2. STUDY AREA

The study area comprises of Vembanad lake and the adjoining shelf

(Fig. 2.1). The Vembanad lake is the largest brackish water system of

southwest coast of India, situated between latitudes 9°28' and 10°10'N and longitudes 76°13' and 76°31'E. The length of the lake is about 90km and extends from Alleppey in the south to Azhikode in the north (Fig. 2.1). It is

bordered in the west by the Arabian Sea which is separated by a barrier beach.

The width of the lake varies from a 100 m to about 8 km (Kurup, 1982). The

depth at the Cochin navigation channel varies from 8 to 12 m while in other

parts of the lake it is about 1 - 5 m. There are two openings to the Arabian Sea, one at Cochin which forms the main entrance at the Cochin harbour and

the other at Azhikode. The entire lake system is subjected to regular tidal

influence and has all the characteristics of a tropical estuary (Qasim et al,

1969). Tides in this area are of mixed semi-diurnal type with substantial differences in range and time. The main source of fresh water for the lake is 9, 9 30 30' 76 °05 E 76 30'

Fig. 2.1. Study area in the southwestern coast of India showing the sampling locations. Out of the 250 samples collected, 43 representing three sub-environments referred in the text (forced fluvial, brackish and shelf) were analysed for REE. 28 the large river Pamba in the south, but four other small rivers (Achankoil, Manimala, Meenachil and Moovattupuzha) also empty into the lake. During the southwest monsoon, the lake receives an average rainfall of 3300 mm and is virtually converted into a fresh water basin (Pillai, 1978).

A 1447 m long bund was constructed across the lake at Thannirrnukham during 1975 for preventing the penetration of salt water into the southern areas,

so as to enhance paddy cultivation. The water flow through the bund is

regulated by spill ways and the bund is kept open during June to December

and closed during the rest of the periods. Therefore, the environment south of

Tannirmukham bund is termed as 'forced fluvial'.

2.21. Geology of the Hinterland

The hinterland is composed of Archean crystalline rocks, Tertiary sediments, and laterite cappings on crystallines and sediments of Sub-Recent to

Recent age (Fig. 2.2, Mallik et al, 1987). Among the crystallines, charnockite, khondalite, granite gneiss and granite are dominant and at places traversed by

basic rocks. While charnockite is widespread in the hill ranges of the Western ghats, hornblende and biotite gneisses formed by retrograde metamorphism and

migmatisation, occur locally. Khondalite rocks are exposed only in the south. Laterite, which is a product of residual chemical weathering of both crystalline rocks and Tertiary sediments forms flat-topped hills and ridges between the foothills of the Western ghats and the Arabian Sea. Geology of the catchment area and characteristic minerals for the rivers debouching into the lake are listed in Table 2.1 (from Mallik et al, 1987). Percentage values in the third

column of the table are the average abundances of respective minerals.

75E II ■■ I E .....•W, ...... ■ ,.. 1111.4;Fr. --.... dr N N IME 11•■■■■• ■ 110.■....0... 1.. ih.■.-1..•■■101111/, N .n.. • • • • MillEMEMINTIME.b allai NEEL • • a /I, / ez I „, swam, AMC r4E7".010.- am.,iww,./b-- malillimett-i.---;-"'4-1=7"-.44111144111)---- n__- -- Iv: 1 111 V • EVA .., .... ,, .. -- '''''''''ZI,I=IIIL''- .116. T!!•11 ■■ •telij i;! • •, PONNANI woossei ihrm.-_■+— -..... 1E111MEMM•m...111.181••::===7.:`, ..■ ••■01111 voimmor.:_.-zdnum memmsa...-miumoramil_ A immum•laZI.111EIMINAMI IMEMMEMEMOL•illikliaiMMIP.Z.).- MEMEEMEMMINIL.MESIMIlt.. 01=1=1=1=1=150 Km. IIIMEIMMEMILAE■109111911■••• MEMEEMEE11101.171.1MIED illaRomi. ,Iiiiiimm...... R MIIMEMMEIMMEMBIsammawle__Awriew orrA 6 ■-Amw. ••••••••••.,arirut II lam ••••••16. EIM■INEEPr-- MEMEMEEMMIM _,POV IMMIOMEMMEM - A • 11•1111E•ME. IIMEIMEMEMMIa."11111111111111.1111111111■ lile IMINMEMMOMMailiM0111■T 1••••••• mar=” 2.• r .11 Ma 4 •••• ■••11Mr...... W...■ AN, ■ ■ , AMIIMIS , IIIIIII•••• s4. •111- 11.- I IMINIA WOMEN ■■■■■ RECENT Soil Alluvium PARUR iri-IliSt" •s!..04 lame, ■ PLEISTOCENE Laterite 141"1, `11-411- .1 •• 0 x xx xl -10 10— IX XXXI MIOCENE (Mon a Warkala Beds M■E COCHIN Ls-,!!•1 1:.• 6 ti■— ■ Dharwar, Motavolcanics ■ .411111 A r1.1■111111■ 1 'I rit LOWER PRE- Metasediments lr 11 M11 1111INI h, 1141 MAMIE& Jilt 1'4; CAMBRIAN Lima ■I■■ IlIr lEILW•Ell; sm. Khondalitos ' M1 II ■EMLIEkNIMM6Mor-Ai11104IIIJII LAIIIL aar MB 11 EaVaMEMEMMI..... • MINMEMMEN N IL Ell■M‘IEEEM. Charnockites illanti in ARCHEAN . Cordierite gneiss, VEMBANAD E!!! 1 Hornblende,Biotite- LAKE • I INTRUSIVES El 4..11 gneiss & other uncla- EMMA:nal 11■INV ALLEPPEY WU LEM VW ,....,...... ssified Crystallines IMMIMINILIE 411731111111rVEll .....i....,...... S 111I'M MIME WAEME■MNY" Wall BEIM. VIENEM•-..... NON - DATED Granite IMIN.11M•■•L "IMIIIII LL=Mil ■ IEEE vn o doppawrs.- clio.::•mimist - vimo,ir • MEL SW willaTIMI W.'''.. ...'1 Mr.011M1 • / PRE - Basic Intrusive IMIMIL:wle 4.7LEIVEEMEM116•"%... CAMBRIAN (Dolerltes) IMIIIMIME "MEM ELN. —, E IMME11/ imm....-■ NIUE IL. AM..7.1111 Ibb.-W...LI/ S IMIMM, • APINP-•■--,...iii... -IN•r MIL I •14.1.■ —dm IMMIN•X ‘111. -,Na.f.„■"1...,,,,f ,,,..... _r,..... A MIMEOlow X X.. X X A 0 XXX X 9 1 0 75 E 76

Fig. 2.2. Map showing the geology of the hinterland for the rivers debouching into the Vembanad lake. R1, R2, R3, R4, R5, R6, R7 and R8 represent rivers Bharathapuzha, Chalakudi, Periyar, Muvattupuzha, Meenachil, Manimala, Pamba and Achankovil respectively. 29 2.22. Index of geoaccumulation (19.) / Degree of pollution in sediments

Aquatic sediments and especially estuarine, lagoonal and inner shelf sediments are both carriers and potential sources of contaminants. The study area is one such region with numerous industrial establishments (viz., chemical, fertilizer, metallurgical, insecticides etc.) located in the northern half of the lake. The effluent discharges from these industries and also from city sewage works are identifiable sources of pollution to these backwaters (Nair et al, 1990 and references therein). Incidence of fish mortality from industrial pollution in the upper reaches of the backwaters is also reported (Sarala Devi et al, 1979 and references therein). To check whether the sediments are contaminated, and if so, and to know the degree of pollution, we have determined 'Index of geoaccumulation' (1 9.0). The concept of 'Index of geoaccumulation' was introduced by Mueller (1979), and it is used to measure the metal pollution quantitatively in aquatic sediments. It is calculated as:

= log2 Cr/1.5 x Br, where Cr, is the measured concentration of the element 'n' in the pelitic sediment fraction and Bo is the geochemical background value in fossil argillaceous sediment ('average shale'). Factor 1.5 is to account for the possible variations of the background data due to lithogenic effects. Igo° consists of seven grades, wherein the highest grade (6) reflects 100-fold enrichment above background values (Table 2.2). My observations indicate that five elements (Cr, Zn, Co, Hg and Se) could be recognised as posing significant hazards to the marine environment that appear in the 'blacklists' of various international conventions (Preston, 1989; Foerstner et al, 1990). The average I wo values and grades of all the five elements in the three depositional 30 environments under_ study are listed in Table 2.2. The sediments are moderately to strongly polluted with respect to mercury particularly in the

'forced fluvial' zone; moderately polluted in the case of zinc and unpolluted to moderately polluted as far as Cr, Co and Se are concerned. Whether the Hg levels observed are hazardous or not depends on the bio-availability and the remobilization from these sediments (eg: Campbell et al, 1986). REEs have also been found to be an anthropogenic indicator (Olmez et al, 1990) and understanding the natural processes controlling REE distribution will be immense significance in recognizing the man-made inputs to marine system (Murray et al, 1991a).

2.3. SAMPLE SELECTION CRITERIA

We have analysed the size fraction < 4 [tm of the surface sediments from both the lagoonal and shelf environments. The upper few centimeters of estuarine and shelf sediments seem to be better representatives or integrators of terrigenous REE input than are river suspended particles (Sholkovitz, 1988). The reasons to analyse only sediment fractions less than 4 [tm size are the following: Firstly, particles < 5 [tm escape the estuaries and are effectively transported to the open Ocean (Mayer, 1982 and references therein; Nair and

Hashimi, 1986). So, this size selection may actually represent a true input of

REE from the upper continental crust into the Oceans. Secondly, three major factors associated with sedimentary sorting namely grain size contrasts, general mineralogy and heavy mineral fractionation affect the REE patterns in sediments

(Mclennan, 1989). By selecting the finer size, the effects arising out of first and third factors are reduced. Moreover, Cullers et al (1979, 1987) have found that the majority of REE reside in clay size fractions. Further, inclusion of quartz 31 may decrease the total REE (due to dilution), while monazite and garnet may increase the HREE content.

Since our objective is to study the REE input to marine environment, one would argue that estuarine and shelf suspended particles are the representatives of continental supply (eg., Goldstein and Jacobsen, 1988). But, the surface sediments of these environments are selected since the composition of estuarine particles are complex and variable in time and space and, also because, it is difficult to collect suspended particles which represent the true assemblage of river loads (Sholkovitz, 1990).

2.4. MATERIALS AND METHODS

Sediments were collected by Van Veen grab during several cruises on research boats R.V. Saqitta and R.V. Nautilus during December, 1980 - January, 1981. About 250 surface sediments were collected for studies on sedimentology, mineralogy and geochemistry (Fig. 2.1). Out of these, 20 samples were analysed for major elements and 43 samples were studied for trace and rare earth elements. The fraction < 4µm was separated from the bulk sediments based on settling velocity techniques. Adsorbed salts in these samples were carefully washed off using distilled water and the samples were then dried at 70°C. Analytical procedures for REE, Fe, Sc, Hf and Th determinations have been described in previous publications (eg., Kunzendorf et al, 1985). In brief about 400 mg of dried sediment was irradiated at the Danish research reactor DR3 for four hours (thermal neutron flux: 2 x 10 13 n cm-2s-1).

Measurements of gamma line intensities were carried out in three steps following cooling periods of 1 week, 10 days and 3-4 weeks. Analytical results were checked against international reference materials. Other major elements 32 like Al and Mn were determined using conventional X-ray, fluorescence spectrometer.

2.5. RESULTS AND DISCUSSION

2.51. Filtering effect of Estuaries on the geochemistry of marine sediments

Estuaries/lagoons are said to act as filters between land and the Oceans (Kennedy, 1984). The filtering effects from the viewpoint of physical, chemical, biological and geological processes has been documented (Kennedy, 1984).

This filtering effect has been found to govern the sediment texture distribution in the adjoining shelf of our study area (Nair and Hashimi, 1986). The inner shelf sediments from the regions with relatively fewer estuaries are characterized by coarser texture (average grain size 0.35 mm) than those collected from the areas having large number of estuaries (average grain size 0.03 mm in Nair and Hashimi, 1986). To check whether this filtering effect is reflected in the sediment chemistry, composition of lagoonal/lake sediments are compared to that of shelf sediments. With few exceptions (Na, Cr), mean concentrations for many elements (major, minor and REE) of lagoon/lake sediments are greater than those of adjoining shelf sediments (Fig. 2.3). Only

Na is enriched in shelf sediments. Average Zn, As and La concentrations are more or less similar in both the depositional environments (Fig. 2.3). This data follows the observation made earlier (Salomons and Foerstner, 1984 and references therein) that estuaries and lagoons are efficient traps for riverborne material. For example, a large amount of metals transported by Europen rivers

Rhine and Meuse in the direction of North Sea does not enter the marine environment (Salomons and Foerstner, 1984 and references therein).

AVERAGE CONCENTRATIONS OF SELECTED SAMPLES

lake environment range [7:1 shelf environment

• except Fe and Na - all in ppm •

.111■111.■•■■■

7.18 8.90 0.68 _0.30 Fe A Na

•

... 17.5 29.6 21.2 24.8 256 213 81.8 84.9 0.38 0.48 4.25 4.31 246 368 Co S c Cr Rb Sb Cs Ba

Fig. 2.3. Bar diagrams showing the elemental distribution in the lake/lagoon/estuarine sediments compared to the sediments from the inner continental shelf region. Averages for all the elements except sodium and chromium are higher for lagoonal sediments compared to shelf sediments indicating a filtering effect of estuaries. The horizontal line marked for each element represent the total average for all the sediments put together. 56.2 64.0 119 136 250 280 La Ce FREE 33

2.52. Cerium Anomalies

Cerium behaves differently compared to other REE, due to its oxidation to relatively insoluble Ce(IV) in some aqueous environments. The Ce anomaly assesses relative behaviour of Ce with respect to its immediate-neighbours and provides insight into the role of redox conditions and speciation.

The fractionation of Ce is represented by calculating -the cerium

anomalies (Ceanom).Cee„om are calculated with the formula (log 3 Ce / 2 La + Nd) proposed by Elderfield and Greaves (1981). The values thus obtained are presented in Fig. 2.4. values are close to zero and display no significant Ce fractionation (Fig. 2.4). This implies that the significant proportion of LREEs is contributed by detrital material and also suggest that Ce is in trivalent oxidation state in these nearshore sediments.

2.53. Diagenetic Effects

The REE behaviour during diagenesis is less well understood than that during weathering (Mclennan, 1989). As no porewater REE data are available for this region, the possible effects of diagenetic processes on the REE patterns are assessed by plotting La and Lu against Mn (Fig. 2.5). Mn is used as an index of diagenetic impact, since it has been recognized that the manganese in sediments mobilize in the presence of decomposing organic matter, and it accumulates towards the sediment-water interface (e.g., Sawlan, 1982 and references therein). Since we deal here with surface sediments, a positive relation between REE with Mn may indicate any contribution or mobility associated with diagenetic processes. As seen from the plots, both LREE

(represented by La) and HREE (denoted by Lu) are poorly related with Mn CEREUMI ANOMALIES following Elderfield & Greaves 1981

SHELF I LAKE

0.06— i. 0.04

0.02— 0 0.02

0.04

0.06

[log 3 Ce/2 La oNd]

Fig. 2.4. The magnitude of cerium anomalies for all the sediments considered here. Ce anomalies have been calculated using the formula of Elderfield and Greaves (1981). Each line represents the cerium anomaly value for each of the sediment studied here. Notice the range of Ce anomalies do not exceed -1+ 0.07 in any case. This shows that the Ce is not fractionated compared to its immediate neighbours La and Nd and also indicate that most of the Ce is in the trivalent state. 100

80 — • • A • • • 60 — ♦ t A A • La (ppm) • • 40 —

20 —

i I 0 1 I 0.03 0.06 0.09 0.12 0.15 Mn (%) 5

4

3 — • • La (N)/Lu (N) A A• • • 2 — • • • i 4 A

1 •

0 I I I 0.03 0 06 0. 09 0. I 12 0.15 Mn (%) Fig. 2.5. A plot of a) La versus Mn and b) the LREE enrichment factor ( La n / Lu n ) versus Mn. Note the scatter of the data without statistically clear trend indicating that the REE are little affected by diagenesis in these sediments. 34 revealing that diagenetic reactions have an insignificant role in the fractionation and mobility of REE in these nearshore sediments. This is in agreement with the suggestion of Chaudhuri and Cullers (1979), based on their studies on siliciclastic sediment cores from the Gulf Coast, that essentially no changes in

REE patterns could be attributed to diagenetic reactions; and also with Sholkovitz (1988) who has shown the absence of diagnetic signals in REE distribution in shelf and slope sediments of the Northwest Atlantic Ocean.

2.54. REE Fractionation and its origin

The total concentrations of REE, Sc, Hf, Th are presented for all the samples in Table 2.3. Data for Al and Mn are also included. The shale-normalized patterns of lake and shelf sediments are shown in Fig. 2.6. The prominent feature for all these sediments is the consistent and significant enrichment of light REE (LREE) over heavy REE (HREE) relative to shales.

The magnitude of the LREE enrichment is equal to the ratio of (Lasampk, / Lase) / (Lusampie / Lush.) (= Lan / Lun). These values are consistently higher than unity and vary between 1.55 to 2.94 (Table 2.3). The LREE enrichment in these modern sediments is in agreement with the data on sediments from shelf and slope regions off North America, the Amazon river, the East China Sea, the

Yellow Sea (Sholkovitz, 1988; 1990); suspended loads of major rivers Amazon,

Missisippi, Indus, Grate Whale, Ohio (Goldstein and Jacobsen, 1988); Holocene sand samples from first-order streams (Basu et al, 1982), fine-grained sediments of Zaire fan, S. Atlantic (Van Weering and Klaver, 1985) and Amazon fan muds (Nesbitt et al, 1990).

Although the LREE enrichment values (La n / Lun) encompass a small range, the averages for three sub-environments under study (i.e., shelf, mixed average lake • 3.0 total average •

'average shelf ■ YYY VVVVVVVVVVVVVVVVVVVVVVVV vvvvvvvvvv. VVVVVVVVV VVVVVVVVVVV. VV VVV V VVVVVVV VVVVVVVVVVV,V VV VVVVVVVVVVVVVVVVVVVVV . V V V V VVVVVVVVVVVVVVVVVVVVV . VV V V VVVVV VVVVV VVVVVV VVVVVVVVVVVV range VV VVVV V V V V VVVVVVVVVVVVVVVV% VVVVVVVVVVVV VVVVVVVVVVVVVV VVV V • VVVVV VVvVVV VVVVV vV. VVV VVVVVV VVVVVVVVVVV VVVVVV vv vvvvvvvvvvvvvv VVVVV VVVVVVV VVVVV VVVVVVVVV. VVVV 1 VV VVVVVVVVV y VVV vvvvvv. VVVVV vvvvv VVVVVV vvvvvvvvvv. VVVVVV , VVVVVV 'VVVVVV VVV VVVVVV V VVVVVV VVVVV.... VVvVVVVVVVV VVVVVVVVVVVVVVVVVVVVVVVV • VVVVVV VVVVVV . VVVVVVVV VVVVVV . VVVVVVVVVVVVVVVVV• VVVV VV VVVVV vVVVV VVVVV VV VVVVV VV VVVVV V .VVVVVVVVVVVVVVVVVVVV VVVV "VVVVVVVVVVVVVVVVVVVV V VVVVVVVVVVVVVVVVVVVVV vvvVV,VVVVVVVV VVVVVVV '.VVVVVVVVVVVVVVVVVV VVVVVVV VVvVVVVVVVV VVVVVV V VVVVVV ''VVVVVVVVVV VVVVV VVVVVV - VVVVVVVVVVVVVVVVVV - vvvVVVVV VVVVVVV "VVVV VVVVVV

0.4

La Ce Nd Sm Eu Tb Yb Lu

Fig. 2.6. The range and averages of shale normalized (3-shale average) REE patterns of the sediments studied. The patterns are remarkably similar showing significant LREE enrichment, only with differences in the magnitude of LREE enrichment. 35 and forced fluvial) reduce gradually from the source area (Fig. 2.7). The forced fluvial sediments which are close to the river confluence have the highest average Lan / Lun value (avg. 2.41) followed by mixed/brackish water sediments (avg. 2.24) and diminishing further in shelf environment (avg. 2.0). This is

interesting since it is generally believed that clastic sediments tend to retain the relative abundance of REEs present in their source region and transferred

nearly in bulk from source to sediment (eg: Taylor and Mclennan, 1985).

Similar fractionation of LREE from HREE is characteristic of the host rocks/sediments present in this area. The rivers debouching into the lake

traverse the Archaean granites; gneisses and charnockites formed by retrograde metamorphism in the hinterland followed by laterites and lateritic

soils in the vicinity of lake. Numerous analyses of granites for REE (eg. Basu

et al, 1982), gneisses and charnockites formed during retrograde metamorphism

(Allen et al, 1985) and lateritic profiles (Braun et al, 1990) have indicated

significant LREE enrichment (Fig. 2.8). Therefore, the LREE enriched patterns

observed for these sediments are most probably derived from parent rocks. In addition, preferential adsorption of LREE's relative to HREEs by clay minerals such as illite, kaolinite, montmorillonite, and chlorite (eg. Cullers et al, 1979) could be an important mechanism. Preferential sorting of trace/heavy minerals such as zircon, sphene, monazite, garnet, allanite, hornblende, feldspar and mica (eg. Frost and Winston, 1987) produce REE fractionation in some cases,

but which may not hold good here, since our samples represent only the clay fraction ( < 4 [an). 80

La / Lu Kaolin( to Fq Salinity

60

40

2

0 Forced Fluvial Brackish Shelf Fig. 2.7. Variation of LREE enrichment (La n / Lun x 10) compared to the kaolinite content (%) and the salinity (psu) in different sub-environments. Sa mp le / Sha le 0.1 i0 from Allenetal(1985)andthesamplesare southernIndia(Salem)whichisanorthern Fig. HREE whichissimilartothepatternsobservedfor sedimentsunderstudy(Fig.2.6). extension ofthesourceareainpresentstudy. NoteasignificantLREEenrichmentover metamorphism. Thecharnockitesarethedominant sourcerockintheregion.TheREEdataare La Ce 2.8. Shale-normalized REEpatternsofcharnockites formedbyretrograde Nd SmEu REE Tb Yb Lu 36

2.55. REE fractionation related to changes in Mineralogy

What is important in the present context is the variation of the degree of

fractionation from the source to shelf. What could be the controlling factors of

this variation?

As mentioned above, the highest fractionation is observed in the

sediments south of Tannirmukham bund, where the tidal incursion is minimum,

and proximal to the river confluence areas. On the other hand, the La/Lu

A values decrease in the mixed zone and are lowest in inner-shelf region (Fig. 2.7). This corresponds to the variation in kaolinite content in these sediments. A characteristic decrease in kaolinite concentration from 73% at the head to

59% at the mouth of the Vembanad lake and only 50% in the shelf region has been reported (Reddy et al, 1992 - Fig. 2.7). This is accompanied by a

concomitant increase in montmorillonite from 10 to 23%. These clay-mineral

changes from the source in this region has been related to the size-sorting of

clay minerals with coarser kaolinite depositing closer to the source than the finer montmorillonite (Reddy et al, 1992). Kaolinite and illite are large sized

compared to that of montmorillonite (Gibbs, 1977). Accordingly, during

transportation of sediments, physical size seggregation leads to the accumulation of the larger sized particles of kaolinite and illite close to the

source areas.

The rivers supplying sediment to Vembanad lake traverse lateritic terrains

during their last stages before joining the lakes. Lateritisation takes place

during intense chemical weathering under humid, tropical climate. Under these

conditions, the alkali and alkaline earth elements are leached off and the clay mineral that ultimately produced is kaolinite (Weaver, 1989 and references 37 therein). Mobilization and fractionation of REE occur during chemical weathering of crystalline rocks (Nesbitt, 1979: Duddy, 1980). The clay grade materials near the top of the weathering profile are usually enriched in LREE caused by preferential removal of HREE by soil waters to lower horizons (Nesbitt et al, 1990). The variation in patterns as well as the LREE enrichment factors appear therefore to result from mineral and grain size sorting. The REE patterns of sediments/suspended material have been said to suffer little alteration during their transportation (Nesbitt et al, 1990), but our work shows that differential sorting during transportation leads to slightly varying REE patterns.

2.56. Fractionation related to physico-chemical conditions of the Environment

REE patterns of sediments can also be affected by physico-chemical environments (eg. pH, Eh, salinity etc.) during the process of erosion, transportation and deposition (e.g., Zhao et al, 1992). The majority of clay minerals in the sediments are secondary minerals formed during sedimentary recycling, and their trace element features should be controlled by the physico- chemical conditions of the environment during sedimentation (Kolmer and

Wirsching, 1986). Since the study area has both fresh water input as well as tidal influence, the physico-chemical environment could play an important role in the chemistry of the sediments. Salinity is the most fluctuating parameter in the lake, which is significantly influenced by the fresh water discharges from the adjoining rivers, tidal action from the sea and the rate of the mixing process

(Kurup, 1982). During the sampling period (December-January, 1980-1981), the salinity was almost zero, with a maximum of 1 psu (practical salinity units) in the southern part of the lake (south of Tannirmukham). The salinity increased 38 gradually from 5 psu at the north of the bund to about 22 psu at the sea connection (Fig. 2.7; values from Kurup, 1982). Kaolinite is unstable in marine waters and flocculates under low saline conditions (Whitehouse et al, 1960) compared to smectite and illite. So, the latter two minerals tend to accumulate away from the nearshore zone. These processes also must have helped in the accumulation of LREE enriched kaolinite in the lake (fresh water to brackish water conditions) sediments compared to the shelf (purely marine environment).

2.57. Control of adsorption/coagulation and complexation processes on the fractionation

It has been established that the removal of iron from river water during estuarine mixing is caused by destabilization of mixed iron oxide - organic matter colloids, the major iron phase in rivers (Sholkovitz, 1976; Boyle et al,

1977) by salinity. Experiments conducted by Elderfield et al (1990) and Hoyle et al (1984) have shown that the total REE, like that of Fe undergo rapid coagulation during estuarine mixing. This results from salt-induced coagulation of freshwater Fe - humic colloids when exposed to seawater. Tracer experiments using water from Luce River (Hoyle et al, 1984) have revealed that the major part of the REE removal has occurred before salinity has reached 10 psu. There was also a substantial iron loss by flocculation (Fig. 2.9) with the major removal occurring over the same 0 - 10 psu salinity range. The similarity in the patterns of removal of the REE and Fe (Fig. 2.9) shows that their behaviours are similar. In this context, the sediment data of present study are examined by plotting iron versus REE (Fig. 2.10). La and Sm representing light and middle REE respectively show good relations as far as the brackish water and shelf sediments are concerned. The data is much scattered for the from Hoyl'e et al. (1984) 100 Er ▪ Yb Dy • Gd 80 • Eu • Sm

(%) • Nd • Ce • La 60 REMOVAL 40

REE 7100— • • Fe 20— <—I 75.. 0 • 50_ •/ I I 1 1 cc • 4 8 12 16 20 z 25_ • O , SALINITY (%o) cc 0 I 10 20 30 SALINITY(%.)

Fig. 2.9. A) REE removal versus salinity in mixing experiments conducted by Hoyle et al (1984). Note that 65% - 95% of all the REE are removed at a salinity of about 10 psu. B) Fe removal in mixing experiments using the same end members as for the REE experiments. Note that a substantial iron loss by flocculation with the major removal occuring over the same 0 - 10 psu range. 100 A shelf environment • brackish environment 80 — • "forced fluvial" • •• • A O. AA • A • • • 60 —

La (ppm) 40

20—

0 1 3 6 9 12 15 Fe (%) Fig. 2.10 A) A plot of La (ppm) versus Fe (%) shows a positive relation for the sediments from brackish environment and the shelf. Comparatively, the scatter is more for the fluvial sediments. 20 A shelf environment • brackish environment 16 • "forced fluvial"

• 12 — Aok • • A A LP, • A• A A Sm (ppm) A 8

3 6 9 12 15 Fe (%) Fig. 2. 10 B) Sm (ppm) representing middle REE versus Fe (%) show a similar relation as noticed in Fig. 2.10 A. 1.0 shelf environment • brackish environment 0.8—O8- A "forced fluvial"

0.6— A Lu (ppm) A A • A •• • A A • 0.4 4A •

0.2

I I 3 6 9 12 15 Fe (%)

Fig. 2.10 C) The plot between Lu (ppm) which represents HREE and Fe (%) show a scatter for all the types of sediments indicating that the HREE are complexed in the saline waters. 39 fresh water sediments. This confirms the above results that the REE as well as Fe are flocculated and removed together during the estuarine mixing. The

fresh water region has salinities less than 1 psu which does, not induce any coagulation of iron-rich colloids. On the other hand, a plot of Fe against Lu

representing HREE shows a poor relation (Fig. 2.10). The variations in the

relations in Fig. 2.10 in the brackish and shelf sediments could be due to the

greater complexing capacity of HREE in the seawater. HREE enrichment in seawater has often been attributed to the greater complexing capacity (Turner

et al, 1981; Cantrell and Byrne, 1987) and also to the greater stability of HREE

complexes in seawater (eg: Goldberg et al, 1963). Hence, LREE are more

prone to be removed from seawater (eg: Nath et al, 1992 a). In this case, the _ . Fe and REE have an identical removal by salt coagulation, but on exposure to

the seawater, the HREE from the sediments are lost to the seawater by complexation. Thus, a poor relation is noticed between Fe and Lu for all the

types of the sediments.

2.58. Significance of HREE depletion in the nearshore sediments compared to shales

The depletion of HREEs in suspended loads of rivers and in modern shallow marine and abyssal sediments (compared to shales) has been found by

Goldstein and Jacobsen (1988) and Sholkovitz (1988). This lead them to

suspect that the REE patterns of shales may not be as representative of their

cratonic sources as are those of suspended loads in rivers.

Condie (1991) has attributed all the results reporting LREE enrichment

so far to the incomplete heavy mineral dissolution during open-beaker

dissolution (Sholkovitz, 1988, 1990), teflon bomb decomposition (Goldstein and 40

Jacobsen, 1988) or fusion with Pb silicate and subsequent analyses by Secondary Ion Mass spectrometry (SIMS - Nesbitt et al, 1990). Accordingly, the incomplete decomposition of zircon or other heavy minerals rich in HREEs leaves the solution to be rich in LREEs. To avoid this problem, Condie (1991) has suggested the use of a non-destructive method such as instrumental neutron activation analysis (INAA). The samples of the present study have been analysed by INAA but still show a significant LREE enrichment. Thus the problem of HREE depletion in nearshore marine sediments is not just due to incomplete heavy mineral dissolution during chemical analysis.

2.59. How do these REE values reflect the composition of the average upper continental crust?

Four estimates of the average composition of upper continental crust (Condie, 1991) are shown in Figs. 2.11 and 2.12. These include the now widely cited values of Taylor and Mclennan (1985) as well as two new averages for the Archaen (AC) and the Early Proterozoic continental crust (EPC) from published data, geological maps, U/Pb zircon ages and Sm/Nd isotopic ratios

(Condie, 1991 - Table 2.4). These four estimates have been suggested to bracket the range of variability in upper crustal composition. They all fall within the veil of shale averages (Fig. 2.11 and 2.12), but the composition of samples from the present study and also the suspended loads reported from other areas

(analysed by INAA) by Condie (1991) fall away from the upper continental crustal composition (Fig. 2.11). Additional support comes from the Th/Sc and La/Th ratios being particularly sensitive to average provenance composition because Th is highly incompatible whereas Sc is relatively compatible, and both elements are said to be transferred quantitatively into terrigenous sediment during sedimentation (Taylor and Mclennan, 1985). Samples from this study 35

- 0 SHALES 0 30 0 SUSPENDED MATERIAL

25 c:11 ArnM g 9 0 ec,,,0 o en 00 x ° O 20 A 00 0 0 0° I-Ipms° 435) ACT :6:iblASCO 00, 0 EB 0 < 15 AC El 0 u8" 0 EPC 0 SEDIMENTS FROM PRESENT STUDY 0 0 * LAGOONAL-FRESH WATER 10 X LA GOONAL -BRACKISH o INN ER SHELF

t I . I 5 I 1 0 20 30 40 50 60 70 80 La (ppm) Fig. 2.11. Plot of Al20?.- La showing the distribution of cratonic shales, suspended river loads, average upper continental crust compositions and our nearshore sediments. Shales are averages from various stratigraphic successions that range in age from Early Archaean to Cenozoic (Condie, 1991 and references therein). Suspended river load analyses - AmM - Amazon; Mek - Mekong; Gan - Ganges are taken from Condie (1991). NASC - North American shale composite - Gromet et al (1984); PAAS - Post-Archaen Australian shale average, UC (upper continental crust) and ACT (average Archaean upper continental crust) from Taylor and Mclennan (1985). EPC (average Early Proterozoic upper continental crust) and AC (average Archaean upper continental crust) from Condie (1991).All REE data are by INAA (Instrumental Neutron Activation). Almost all nearshore sediments of the present study show La enrichment compared to shales. In fact, the suspended load values reported by Condie (1991) (diamonds) also fall in the extreme higher limits of shales and definitely away from all the estimates of upper crustal composition values. This shows that La (or LREE) in nearshore sediments and suspended loads is genuinely higher than the shales as well as the crustal composition and not due to an analytical artifact as suggested by Condie (1991).

400

350

Ga n 0 300 • 0 0 _ 000 0 0 .Arnm EP 200 0 A POOAA SA NASC - 0 o g B3C O 0 0 .000 . 0 • AC #0o • ® 0 00 roArnG Me K 150 0 ACT 0 0 ♦ E9 * 461 * 100 - 0 * *a. x 4 i x it • _ • x ...x x x xx • x • • 50 1.5 2 2.5 3 3.5 4 4.5 Yb ( ppm)

Fig. 2.12. A Zr-Yb diagram. Zr contents of suspended loads as well as our nearshore sediments are calculated from Hf data using a Zr / Hf ratio of 39. Our values fall away from the field of the shales and upper crustal values with significantly low Zr values but almost equal Yb concentrations. 41 have lower ratios than the upper crustal value of 1.0 (Taylor and Mclennan, 1985 - Fig. 2.13; Table 2.4). Similarly, the ratio La/Th in both the lake and shelf

sediments are greater than 3.7 (averaging about 4) (Table 2.4) which is higher than the upper crustal value of 2.8 (Taylor and Mclennan, 1985). Further

support comes from the major element data. A triangular plot with Fe 2O3, Al203 2O at each apex shows that the samples from the present study fall and K

distinctly away from the North American Shale Composite (NASC) and also out of the shale field (Fig. 2.14). This plot further depicts that these sediments lie

close to the field of residual clays. These results immediately raise the question (Sholkovitz, 1988; Goldstein and Jacobsen, 1988), that whether the

rivers, as the main source of REE and other elements, deliver the continental particles with a composition similar to the shales and in turn represent the

upper continental crust or not.

Three shale averages 1) NASC (North American Shale Composite Gromet et al, 1984), 2) PAAS (Post-Archaean Australian shales - Taylor and

Mclennan, 1985) and 3 shale average (3SA - originally put forth by Haskin and Haskin, 1966, revised by Piper, 1974a and re-revised by Sholkovitz, 1988) are

widely used to normalise the REE data of sedimentary/marine samples. These shale values were used since they are supposed to exhibit chondrite normalized

REE patterns similar to those of the average upper continental crust. Among

these, 3SA, which is an average of European shale (ES), NASC and Russian

platform shale (Haskin and Haskin, 1966), has been said to strongly reflect the high REE content of .Russian platform shale (Condie, 1991). A single sample

data (average values of our continental shelf sediments) is plotted after

normalizing with all the three shale averages discussed above (Fig. 2.15). The

patterns are remarkably similar, all showing LREE enrichment (except few kinks

1

2 1i1 'Jill i ► ► ► 1 ► 1 1 ►► 2 10 1 1 10 1 10 Sc ( PPM )

Fig. 2.13. A plot of thorium (ppm) versus scandium (ppm). Notice that all the samples studied here have Th/Sc ratios distinctly less than the upper crustal value of 1.0 (Taylor and McLennan, 1985). A 1 2 03 Fe2 0 3 4.-- CHLORITE-4-

Fig. 2.14. Major element distributions in the sediments of the present study plotted in the triangular diagram Fe203T - 1c0 - Al203. Open; circles represent the lake samples and dots represent the shelf sediments. The sediments fall distinctly away from the shale represented by North American Shale Composite (NASC). Shelf sediments fall in the field of residual clays. Regions for shales and residual clays are from Gromet et al (1984) and Reimer (1985), respectively.

REE-PATTERNS

• NASC (Gromet et al. 1984) ❑ PAAS (Sholkovitz 1988) 3.0 - A Shale composite (Sholkovitz 1988) • 2.0- • • •••• •

0 \A -A

1.0 - • • 0.8- • 0.6— I I I I La Ce Nd Sm Eu Tb Yb Lu Fig. 2.15. The REE patterns of one set of values (average of continental shelf sediments from this study) after normalizing with 3 different shale averages used commonly in literature. The three shale averages employed are NASC (represented by circles), PAAS (squares) and 3-shale average (triangles). Note the similarity in patterns except few kinks at Sm for NASC, and at Yb-Lu for both NASC and PAAS normalized patterns (probably small analytical uncertainities). 42 observed at Sm for NASC and Yb-Lu for both NASC and PAAS which are within the limits of analytical uncertainities). Therefore, it is concluded that all the three values remove effectively the odd-even distribution of the elements and serve their purpose. The problem of whether they actually represent the

average terrigenous input into the ocean requires additional data input from

present day continental shelves both carbonate and clastic. If the majority of the

continental sediments entering into the ocean have LREE enriched patterns, it

is interesting to resolve the mechanism which results in an unfractionated

pattern for shales and also the flat shale normalized patterns observed in some of the terrigenous deep-sea sediments (Nath et al, 1992).

2.6. CONCLUSIONS

1) All the sediments studied for REE representing three tropical coastal sub-environments of southwest India namely a) forced fluvial, b) brackish water and 3) inner-shelf (marine) show a significant LREE enrichment, La > Lu by a factor of 2 on a shale-normalized basis.

2) The REE abundances and the LREE fractionation indices have been

found to decrease from the southern end of the lake (site of river-lake confluence) to the shelf region analogous to the distribution of clay

mineral kaolinite and also the salinity of the overlying waters. This is

different to earlier observations on sedimentary rocks / ancient sediments, that REE contents of clastic sediments tend to be constant during the transport from source to sediment. 43

3) The significant LREE enrichment observed in these nearshore sediments as well as in other river particulates entering the oceans, compared to the composition of shales and estimates for the upper continental crust,

is not attributable to incomplete dissolution methods as suggested by

Condie (1991), since all of them were analysed by a non-destructive

technique, INAA. This in turn suggests that caution is to be exercised in assuming the shales to generally resemble the continental input to the oceans. 44 TABLE 2.1: Geology of the catchment area and the characteristic minerals in different rivers debouching into Vembanad lake (from Mallik et al, 1987). River Geology of catchment area Characteristic minerals

Muvattupuzha Charnockites, various types of Hornblende (19.1%), gneisses, Pleistocene, hypersthene (15.5%) recent sediments

Minachil Charnockites, khondalites and Hornblende (15.6%), recent sediments hypersthene (7.2%), garnet (6.1 %)

Manimala Charnockites, laterites, Quilon Sillimanite (18.1%), and Warkalai beds hornblende (10.3%)

Pamba Charnockites, laterites, Quilon Opaque (60.2%) and Warkalai beds hypersthene (16.7%)

Achankovil Charnockites, laterites, Quilon Opaque (70%), zircon and Warkalai beds (9.9%)

Periyar Charnockites, various types of Opaque (69.7%), zircon gneisses, Pleistocene, Recent (12.9%) sediments Table 2.2 : Index of Geoaccumulation (I geo) of trace metals in sediments under Study.

Metal examples Sediment Pollution intensity Accumulation Shelf Brackish forced (19.9) class fluvial

Very strongly polluted >5 6

Strong to very strong >4 - 5 5

Strongly polluted >3 - 4 4 Hg(3.12) Hg(3.8)

Moderately to strongly >2 - 3 3 Hg(2.52)

Moderately polluted >1 - 2 2 Zn(1.55) Zn(1.48) Zn(1.62) Se(1.26) Unpolluted to moderately >0 - 1 1 Cr(0.92) Cr(0.74) Cr(0.64)

Practically unpolluted <0 0 46

Table 2.3: Elemental concentrations and the light enrichment factors in the study area.

S. No. Al 20, Mn La Ce Nd Sm Eu Tb Yb Lu Sc Hf Th Lan / Lu„ in ppm LAKE

Forced fluvial 182 72 150 58 11.1 2.36 1.25 2.71 0.49 27.5 2.55 16.0 2.18 180 25.22 852 69 143 60 10.4 2.26 1.39 2.86 0.49 27.1 2.56 15.6 2.08 175 23.39 558 64 133 82 9.5 2.05 1.73 2.59 0.38 26.6 2.70 17.3 2.47 174 70 145 85 10.2 2.15 1.05 2.89 0.41 26.8 2.72 15.2 2.51 173 70 124 10.3 2.42 1.30 2.42 0.41 26.3 2.63 16.7 2.54 167 22.79 465 69 144 67 10.7 2.33 1.27 2.34 0.39 24.4 2.64 16.5 2.65 162 24.59 697 75 174 68 11.4 2.73 1.40 3.23 0.55 28.1 3.14 17.9 2.03 157 23.55 774 78 172 73 11.7 2.69 1.64 2.61 0.44 26.4 2.65 17.8 2.63 155 80 179 71 12.1 2.73 1.61 2.70 0.44 28.0 2.90 18.9 2.71 154 76 171 53 11.8 2.70 1.75 2.60 0.52 26.9 3.01 18.4 2.17 153 24.97 1084 68 146 50 10.3 2.15 1.24 2.84 0.40 26.5 2.73 17.2 2.52 151 25.07 852 70 153 67 10.5 2.29 1.44 2.55 0.44 26.0 2.55 17.6 2.38 Brackish 146 22.43 387 58 121 49 9.5 2.01 1.10 2.34 0.36 23.7 2.41 14.7 2.36 143 78 174 86 11.8 2.49 1.29 3.04 0.45 27.1 2.49 18.3 2.57 141 73 161 71 11.1 2.30 1.40 3.21 0.37 25.7 2.22 17.9 2.94 139 66 144 55 10.4 2.35 1.24 2.73 0.41 25.6 2.75 1,7.9 2.39 137 22.85 581 58 125 65 9.2 2.12 1.12 2.31 0.37 24.4 2.20 15.8 2.34 135 22.49 379 57 123 58 9.6 2.11 1.18 2.12 0.43 24.2 2.28 15.1 2.00 130 22.34 387 58 116 43 9.4 2.06 1.06 2.34 0.43 23.3 2.21 14.4 2.02 126 53 110 52 9.3 1.95 1.08 2.31 0.37 21.8 2.27 13.8 2.15 120 21.26 325 54 115 63 9.1 2.09 1.07 2.40 0.44 22.8 2.32 13.9 1.84 117 21.85 387 47 103 8.1 1.84 0.85 2.23 0.31 19.9 1.97 12.2 2.28 115 50 107 43 8.9 1.93 1.06 2.33 0.35 21.9 2.01 13.6 2.12 113 21.91 387 51 111 58 8.4 2.00 1.02 2.26 0.38 22.1 2.18 13.7 2.00 106 - • 50 109 34 8.8 1.95 1.11 2.31 0.32 21.6 2.23 13.6 2.34 103 59 118 68 10.1 2.09 1.11 2.58 0.41 22.5 2.35 13.9 2.15 101 22.62 256 56 115 53 9.7 2.08 1.23 2.73 0.40 22.8 2.85 14.4 2.10 SHELF

6 52 109 45 8.9 1.92 1.50 2.42 0.40 22.7 2.13 13.1 1.93 13 50 100 8.8 1.84 1.20 1.75 0.36 21.7 1.95 12.4 2.09 15 19.90 256 51 105 8.9 1.99 1.41 2.54 0.36 21.3 2.25 13.2 2.05 17 20.56 318 53 107 54 9.6 1.94 1.00 2.48 0.42 21.8 2.16 13.3 1.89 19 58 117 57 10.8 2.19 0.91 2.70 0.47 24.0 2.38 14.4 1.84 21 57 120 42 9.8 2.23 1.38 2.27 0.34 24.0 2.17 14.0 2.47 28 59 119 70 10.0 2.03 1.22 2.75 0.44 23.9 2.53 14.5 2.01 35 55 113 9.4 2.09 1.10 2.29 0.38 22.8 2.25 12.1 2.12 37 58 118 61 10.1 2.19 1.35 2.30 0.37 23.7 2.30 14.9 2.33 41 21.65 256 58 123 10.5 2.22 1.01 2.46 0.47 24.8 2.49 14.1 1.86 43 57 140 73 10.0 2.15 1.28 2.62 0.55 24.1 2.53 13.1 1.55 50 21.13 256 60 119 49 10.3 2.19 1.19 2.87 0.46 23.4 2.36 14.8 1.93 59 61 151 71 10.6 2.21 1.25 2.60 0.43 23.7 2.47 14.9 2.11 61 60 121 57 10.0 2.13 1.24 2.23 0.41 23.7 2.32 14.0 2.17 63 20.95 194 56 110 74 9.8 2.03 1.18 2.60 0.41 22.8 2.23 12.6 2.02 65 54 134 78 9.5 2.05 1.24 2.62 0.42 22.6 2.10 13.2 1.94 47 Table 2.4 : Average compositions of Vembanad lake and adjoining shelf samples compared to the averages of crustal compositions.

Element AC EPC UC NSS Lake Shelf

Al203 (%) 14.9 1510 15.2 23.2 20.8 Zr 148 188 190 98* 89* La 29 38.3 30 64 56 Ce 57.5 76.2 64 137 119 Sm 4.0 6.2 4.5 10.1 9.8 Eu 0.96 1.3 0.88 2.23 2.09 Gd 3.14 4.93 3.8 Tb 0.46 0.72 0.64 1.26 1.22 Yb 1.25 2.25 2.2 2.58 2.34 Lu 0.21 0.36 0.32 0.41 0.42 La/Th 2.8 4.0 4.1 Th / Sc 1.0 0.64 0.62

Except Al203, all the concentrations are in ppm. AC: Archean continental crust (Condie, 1991) EPC: Early Proterozoic continental crust (Condie, 1991) UC: Average upper continental crust (Taylor and McLennan, 1985) NSS: Nearshore sediments of the present study. The values represent the averages.

Zr contents in nearshore sediments are calcuted from Hf concentrations following Condie (1991). CHAPTER 3

RARE EARTH ELEMENTS IN THE DEEP-SEA SEDIMENTS OF CENTRAL INDIAN BASIN 48 RARE EARTH ELEMENTS IN THE DEEP-SEA SEDIMENTS OF CENTRAL INDIAN BASIN

3.1. INTRODUCTION

Although many studies have been conducted on the rare-earth element behaviour in the oceanic water column (eg., Elderfield and Greaves, 1982; De Baar et al, 1985; Piepgras and Jacobsen, 1992), relatively little is known on the regional variations in REE compositon of fine-grained sediment deposited in an ocean basin (Murray et al, 1991a). A better resolution of the parameters controlling the distribution of REE in sediments across ocean basins is much needed since the variations in REE contents in marine sediments are used to assess geological processes such as Precambrian crustal sources (eg., Taylor and McLennan, 1985) and the role of subducted sediment in the genesis of convergent margin magmas (eg., Ben Othman et al, 1989). Previous studies have also investigated REE sources to sediments of the East Pacific Rise (eg.,

Ruhlin and Owen, 1986), and other ridge-crest sediments (Piper and Graef, 1974; Kunzendorf et al, 1988; Barrett and Jarvis, 1988; Olivarez and Owen,

1989), modern Pacific sediment (eg., Toyoda et al, 1990) and also to the normal marine sediment (eg., Sholkovitz, 1990). Tlig and Steinberg (1982) measured REE's in different size fractions of sediments from the Indian Ocean and concluded that REE patterns of coarser fractions exhibit a negative Ce anomaly related to biogenic components such as foraminifera and diatoms whereas fine fractions show a positive Ce anomaly related to submarine alteration of volcanic material.

The distribution of REE, in particular cerium anomalies, in marine sediments and sedimentary rocks have been used as indicators of the 49 depositional environment (Murray et al, 1990), widespread marine anoxia (Wang et al, 1986; Hu et al, 1988; Liu et al, 1988; Liu and Schmitt, 1990) and also have been related to the Antarctic Bottom Water (AABW) circulation effects (Glasby et al, 1987) and to surface productivity variations (Toyoda et al, 1990). The REE signatures are not found to be related to the sediment lithology and are not masked by diagenetic processes (Murray et al, 1990). Based on the

REE studies in seawater, the validity of such applications has been questioned (De Baar et al, 1988; German and Elderfield, 1990a). Here we study the distribution of REE as a function of variations in lithology and bottom water conditions in modern sediments obtained from a single ocean basin (Central

Indian Basin). By documenting REE behaviour throughout a single ocean basin with relatively well-constrained sedimentary and oceanographic sources, we are able to test the various postulated theories on the REE composition of the sediments.

3.2. DETAILS OF THE STUDY AREA

3.21. Tectonic setting

The Central Indian Basin has an area of 5.6 x 106 sq.km bounded by a mid-Indian ridge in the west, 90° ridge on the east, Chagos plateau in the northwest and in the south by Southeast Indian Ridge (SEIR) and the Rodriguez triple junction. The basin seems to have a complex evolutionary pattern from the existence of aseismic ridges and fracture zones. The complex evolutionary pattern is created by the evolution of the triple junction, the slower spreading Central Indian Ridge and the southeast Indian ridge (Raju and

Ramprasad, 1989) at a medium rate. In general, the bottom topography in the western parts of the basin is rugged than that in the east ( Raju and

50

Ramprasad, 1989). A number of small (about 109) and large (19) seamounts are reported to be present in this basin (Mukhopadhyay and Khadge, 1990).

3.22. Sample description

Thirty deep-sea surface sediments from the Central Indian Basin

collected either with Pettersson grab or with boomerang grab were chosen for the detailed geochemical analyses. A small stainless tube was fitted inside a boomerang grab which is used for collecting manganese nodules. The sediments from this basin were studied for mineralogy (Rao, 1987; Rao and

Nath, 1988), porewater chemistry (Nath and Mudholkar, 1989), geochemistry (Nath et al, 1989) and sedimentation rates (Banakar et al, 1991). This basin has a variety of sub-environments representing distinguishable sediment types

(Fig. 3.1). The sediments used in this work can be divided into five types 1) terrigenous sediments from the northernmost part of the basin (area 1) with land derived illite, kaolinite + chlorite making up 90% of the clay minerals; 2)

siliceous sediments from the northern central part of the basin (area 2) which are not overlain by nodules and affected by terrigenous sedimentation (Rao and

Nath, 1988; Nath et al, 1989); 3) siliceous sediments from the southern central

part of the basin (area 3) with abundant nodule coverage (Sudhakar, 1989a); 4) calcareous sediments from shallow areas (area 4) including one sample in area 1 (#2494) and 5) pelagic / red clays from the southernmost part of the basin.

Illite, kaolinite + chlorite contents decreage from north to south (proximity to the

continental source - Rao and Nath, 1988). Montmorillonite content is highest in the pelagic clays, compared to the terrigenous clays and siliceous sediments,

suggesting it being derived from the weathering of proximal ridge basalts (Rao

and Nath, 1988). An average accumulation rate of 2 mm/ka has been reported

INDIA

Maldive Is.

MSS, 4••••••• .111110111•1111111L mwernimm• •••

ammo■ammummo Orwrammu ■ma■■ m • mhaimurAimunnThwi mownramooriorome moranommiodusefr:dr • INNI•••••11111t. MI111111,011 111111•411111 MINIM "4/ • " MIN► 51111 11m0111111111111•111111M ■1111•111111■ 1111111 ■11•11■111

SILICEOUS SEDI

120 / 124 121 127 •

Jae ° 150G PELAGIC- =ROWN-CLAY

r.

Fig. 3.1. Map showing the sample locations in relation to the physiographic settings and sediment types. 51 for area 3 in Fig. 3.1 (Banakar et al, 1991). Samples 2501, 2513, 231, 210, 156 and 157 are of Late-Quaternary age (erosion affected) and the remaining (Fig. 3.1) are of Recent age (S.M. Gupta, personal communication, 1991).

3.3. MATERIALS AND METHODS

3.31. REE Determination

The method used in this work for the extraction of rare- earth elements is based on digestion of the sample in a hydrofluoric acid-nitric acid mixture, dissolution of the sample residue in hydrochloric acid and ion-exchange separaton from matrix 'elements (e.g., Al, Ca, Fe...) on Dowex 50WX8 cation- exchange resin and later analysis by sequential Inductively coupled argon plasma spectrometry (ICP). The details are given in Roelandts (1990). Ion-exchange has long been recognised as a convenient and extremely efficient separation method for REE separation. The cation-exchange resin Dowex 50WX8 (100-200 mesh, hydrogen form) was washed before removing the fine particles. The analyses were performed at University of Liege, Sart Tilman, Belgium.

The procedure outlined in Roelandts and Michel (1986) which has been followed is described below. One gram of powdered sample was treated with a mixture of 38-40% hydrofluoric acid and 5 ml of concentrated nitric acid in a platinum dish. After complete evaporation to dryness on a water-bath, 5 ml of concentrated nitric acid was added and the mixture was evaporated to dryness. The nitric acid addition and evaporation was repeated thrice. The residue was then transferred to a 100-m1 beaker with water, treated with concentrated 52

hydrochloric acid and again evaporated to dryness. The final residue was taken up in 30 ml of 1.75 M hydrochloric acid.

The 1.75 M HCI attack solution was poured, through a filter, on to a preequilibrated 50WX8 resin column. After completion, the column was washed with 100 ml of 2M HCI. The effluent which contains all the major elements was discarded. The REE retained on the column were then quantitatively eluted

with 75m1 of 8M nitric acid. A flow rate of approximately 1m1/min was

maintained throughout the cation exchange procedure.

The REE eluate was slowly evaporated to dryness on a hot plate and the residue was 'dissolved in 5% (V/V) nitric acid. The solution was quantitatively transferred into a 25-m1 volumetric flask and made upto volume

with 5% nitric acid. This solution was transferred to a polypropylene bottle for storage until the samples were analysed for REE.

Calibration of the instrument was made using single-element solutions of REE (in 5% nitric acid). All measurements were made in triplicate. The relative precision for the ranges of concentration encountered here are within 1-2%. The accuracy of the analyses was checked by analysing simultaneously the international sediment standards such as MAG-1 (marine sediment) and

SCo-1 (cody shale). The accuracy was better than +5% for all the elements.

3.4. RESULTS

Rare-earth element concentrations, the ratios indicating the REE

fractionation, cerium anomaly values and Lau,. percentages of all the samples,

including the averages for each sediment type, are presented in Table 3.1. 53

The shale-normalized patterns for all the samples studied are presented in Fig. 3.2. Each sediment type depicts a characteristic shale normalized pattern (Fig.

3.2).

3.41. REE in terrigenous clays

The terrigenous sediments show flat patterns with very little fractionation. The calculated Ce anomalies (following Elderfield and Greaves, 1981) ranged from -0.07 to 0.09 with an average of -0.005 (Fig. 3.3). These sediments (Area 1) belong to the southern end of the Bengal Fan and thus are derived from terrigenous sedimentation from the Indian sub-continent. Since the area is prone to continental influence (Rao & Nath, 1988; Nath et al, 1989), the time of sediment exposure to seawater may be minimal, thereby leaving Ce 3+ a lesser chance for oxidation and precipitation.

3.42. REE in siliceous clays (nodule free)

The REE patterns for siliceous clays (area 2) differ from those for terrigenous sediments (Fig. 3.2). This area is also affected by terrigenous sedimentation although to a lesser extent. Since siliceous organisms have extremely low REE concentrations (Piper, 1974; Elderfield et al, 1981), the REE could have been present in the clay minerals, authigenic minerals (such as zeolites, Fe-rich montmorillonites) or adsorbed onto Fe-Mn oxyhydroxides. The

Ce anomalies are all positive ranging from 0.05 to 0.17 with an average of 0.1 (Fig. 3.3).

2 - Terrigenous sediments ( TYPE- I )

I "" 2532,2535 2501 4kb 2483,2513 0.4 2537 2 - (nodule free region) (TYPE-2)

1 -

0.5 2 - Siliceous clays (overlain by nodules) ( TYPE - 3)

0.4 .7: ,,, (TYPE -4) 1 7 Calcareous sediments %.;an o 0/A=8 0, Ca o :620/ oNo S121 o 0 ° • o S124 N o_o by :87% o\00 s2412974 0.1 - 0\

0.05 ( TYPE-5) 6- Pelagic/red clays o- 0 O -0210 164,144 0 148 132 I _ gr'191 1306 150,231 0.6 0 1 MORB

0.07 =

La Cc Nd r*.n Cdiu y O Yb Lu Fig. 3.2. Shale normalized REE patterns of various types of sediments. sediment type showed a characteristic pattern with a bearing on its litholog y, minor variations in depositional settings, diagenesis and source area. CaCO 3 contents shown for the calcareous sediments are from Nath et al (1989). Cerium anomalies in CIB sediments

Sediment type

Calcareous

Terrigenous

Siliceous nod free

Siliceous with nods

Pelagic clays

1 —0.4 —0.2 0 0.2 Cerium anomaly Fig. 3.3. Average cerium anomaly values (calculated with the formula given by Elderfield and Greaves, 1981) for each type of sediment. Terrigenous sediments and pelagic clays have no Ce fractionation. Calcareous sediments are showing strong negative Ce anomalies and siliceous oozes / clays are showing positive anomalies. 54

3.43. REE in siliceous oozes / clays associated with manganese nodules

The siliceous clays/oozes from the area 3 have similar REE contents as the sediments in area 2. The REE patterns, however, are different and resemble those in manganese nodules with slightly convex pattern (Fig. 3.2). The significant difference between the sediments of area 3 and those from the other areas is the presence of a well developed positive Ce anomaly. The Ce anomaly values range from 0.13 to 0.23 with an average of 0.17 (Fig. 3.3).

The manganese nodules from this area have lower positive Ce anomalies compared to nodules overlying the pelagic clays to the south (Nath et al, 1990,

Nath et al, 1992a).

3.44. REE in calcareous oozes / clays

The calcareous ooze/clay samples are not confined to one geographic

domain (Fig. 3.1) but are grouped on the basis of their similar lithological

character. This group of samples has extremely low REE contents (Table 3.1). All of them possess significant negative Ce anomalies, ranging from -0.08 to

-0.53 with an average of -0.3 (Fig. 3.3). The magnitude of Ce anomalies is inversely proportional to their CaCO 3 contents (Fig. 3.2). The sediment (#127) proximal to the Mid-Indian Ocean Ridge has an identifal REE pattern to that of

other Indian Ocean ridge crest sediments (Kunzendorf et al, 1988).. Positive

europium anomalies are observed only in these sediments (Fig. 3.2). This is interesting since positive Eu anomalies are reported only in areas affected by

either eolian or hydrothermal sedimentation or created by extremely reducing

conditions (McLennan, 1989; Murray et al, 1991b). Since both these conditions

are not reported from this arsa, the positive Eu anomalies observed here are 55 probably due to the inclusion of alkali feldspars. A slight increase in the primary or detrital feldspar component could also lead to positive Eu anomalies in bulk sediments (Murray et al, 1991b and references therein).

3.45. REE in pelagic / red clay sediments

The red clay sediments (area 5) have two fold enriched REE contents than that of siliceous and terrigenous sediments (Table 3.1). In addition, they all have light REE depleted patterns (Fig. 3.2) with an average depletion

(Lan/Yb,) factor of 0.42. The patterns resemble that of typical mid-oceanic ridge basalts (MORB data from McLennan, 1989 - Fig. 3.2).

3.46. Fractionation indices

The variation in the degree of fractionation across the REE series are determined by the average ratios of Lan / Ybn [.(Lasample / Lassa) / (Ybsarno. /

Ylas „)], Lan / Gd, [.(Lasamp,. / Lash,„) / (Gd ^/mae Gcl„,,, e)] and Gd, / Yb,

[.(Gdsample / Gcls„,J (Thsapie / Ybsnj] for each sediment type (Fig. 3.4). These indices denote the fractionation among the light, middle and heavy REE. These fractionation indices / ratios are characteristically different for each sediment type (Fig. 3.4). La, / Ylan gradually decrease in the order of calcareous sediments (1.03) > terrigenous sediments (0.88) > nodule free siliceous sediments (0.69) > siliceous sediment associated with nodules (0.62) and the least for pelagic/red clays (0.57). Lan / Gd, follows the same trend. Gd, / Yb, is highest for calcareous sediments but almost similar for pelagic clays and terrigenous sediments on one hand and for both the types of siliceous sediments on the other (Fig. 3.4).

0.5 I" 1.5 0.6 0.8 I 0.4 0.6 0.8 Ce/Ce * Lon /Ybn Lan /Gdn Gdn lYbn

• • Calcareous Terrigenous Siliceous ( Nod. Free

Siliceous with Nod. ciPolagic clays

Fig. 3.4. Variations in behaviour of REE across the series are indicated by the average ratios of Lan / Ybn and similarly by Lan / Gdn and Gdn / Ybn for each sediment type. Note these ratios are different for each sediment type. In addition to the previous figure, Ce anomalies are also denoted here using an expression Ce/Ce* = (Ce ample / Ces ha,e)/Ce*, with Ce* being obtained by linear interpolation between shale-normalized La and Nd values. Ce/Ce* <1 are considered to have a negative Ce anomaly. 56

3.47. La...,

The excess lanthanum, to what can be supplied by the continental material to these sediments, is calculated as

La„,, = - [Alsamo. x (LaNksc / AltiAsc)], where Law, denotes Laex and NASC is North American shale composite. NASC values are from Gromet et al (1984). The Laex ranges between 23 and

49% with an average of 35% for the terrigenous sediments and is lowest compared to other sediments (Table 3.1). Laex values are much higher (average 58%) for the pelagic/red clays than those of terrigenous sediments

(Table 3.1) indicating a higher degree of REE scavenging in areas away from higher sedimentation. Low Law, values and also the absence of Ce fractionation in terrigenous sediments indicate that these sediments are dominated by included terrigenous particulate matter with Ce / Ce* close to 1 (Condie, 1991; Murray et al, 1991a).

3.5. DISCUSSION

3.51. Lithological control of REE patterns

The chemical trends presented above are consistent with the earlier studies 1) terrigenous sediments show flat shale normalized patterns (Piper, 1974); 2) calcareous sediments have a negative Ce anomaly (Palmer, 1985);

3) siliceous sediments devoid of manganese nodules and affected by terrigenous sedimentation (Rao and Nath, 1988; Nath et al, 1989) have low Ce anomalies (Fig. 3.3); 4) siliceous sediments overlain by Ce poor nodules have well developed positive Ce anomalies (Elderfield et al, 1981) and 5) pelagic clays show many fold REE enrichment over shales (Thompson et al, 1984) and 57

LREE depletion in them (Fig. 3.2) is consistent with the presence of weathered products from mid-oceanic ridge basalts (Rao and Nath, 1988; Nath et al,

1989). These variations within an abyssal basin indicate lithologically dependent Ce fractionation and REE patterns. This does not agree with the suggestion, based on the cherts and shales of Lower Jurassic to middle Cretaceous age (Murray et al, 1990), that the sediment REE is controlled by the depositional environment alone and not related to the lithology and diagenetic changes. However, the sediment types shown here do not necessarily exhibit similar REE patterns globally. For example, pelagic red clays in our samples show LREE depleted (including Ce in Fig. 3.2) patterns.

In contrast, Pacific pelagic red clays exhibit both positive Ce anomalies (Thomson et al, 1984; Toyoda et al, 1990) and negative Ce anomalies (Glasby et al, 1987; Piper, 1988). These differences probably may reflect the proximity to the source areas and varying adsorptive capacity of the individual sediments (Murray et al, 1991a).

3.52. Relation between REE patterns and diagenetic processes

Our data suggests that in addition to lithology, the effect of diagenesis on the REE patterns could also be significant. Though, no signs of post-depositional changes in REE contents are found in the sediments buried to depths of upto 5 km from the Gulf of Mexico (Chaudhuri and Cullers, 1979), and also the Precambrian sediments which have undergone a degree of

metamorphism, as well as diagenesis (Wildeman and Condie, 1973; Wildeman and Haskin, 1973), diagenetic changes in Eh are most likely to affect Ce and Eu because of their ability to exist in different oxidation states under geological

conditions (Fleet, 1984). Dominance of oxic diagenetic processes revealed by 58 porewater nutrient studies (Nath & Mudholkar, 1991, diagenetic smectite formation (Rao and Nath, 1988), todorokite rich and Ce poor diagenetic nodules

(Nath et al, 1990, Nath et al, 1992b) were reported from sediments of area 3.

All these factors suggest that the sediments in area 3 are diagenetically affected. During the diagenetic formation of smectite, most of the Fe available at the sediment-seawater interface is taken up by sediments, leading to the formation of Mn-rich-nodules (Lyle et al, 1977; Rao and Nath, 1988). This in turn would lead to more iron-oxide sites in sediments available for Ce adsorption. This is even reflected by the stronger positive Ce anomalies in sediments than the overlying nodules. Therefore, the positive Ce anomalies and nodule-like REE patterns of sediments from area 3 are possibly a result of diagenetic processes. Such a diagenetic overprinting would lead to different REE fractionations (German and Elderfield, 1990) and may constrain the usage of sediment/sedimentary rocks for tectonic and stratigraphic reconstructions

based solely on depositional environments (see Murray et al, 1990). The extent of REE fractionation during diagenesis also has implications on recent

interpretations that the REE (Ce anomaly in particular) composition of biogenic sedimentary phases can be used as a paleoredox indicator (Wang et al, 1986; Wright et al, 1987).

3.53. Ce fractionation related to bottom water hydrography

Ce anomaly values are plotted against bottom water temperatures and dissolved oxygen levels (Fig 3.5; data from Warren, 1982). The Antarctic

Bottom Water (AABW) enters this basin via the saddles of 90°E ridge, with less

influence in the western part of the basin (Fig 3.5), as can be seen from

differences in the temperature and dissolved oxygen patterns from east to west. 4 -4.1 m1/I 4.1 -4.2 mI/I 4.2 -4.3 ml/ I 4.3 -4.5 m1/1

Fig. 3.5. Ce anomaly values of various types of sediments are plotted against a backdrop of a) bottom water temperatures (4300 m water depth) and b) dissolved- oxygen concentrations (in m1/1) data (data and base map from Warren (1982). Various symbols are used to indicate the sediment types. 1) Thick open circles - terrigenous sediments, 2) thin open circles - siliceous clays with a terrigenous component and recovered from nodule barren area, 3) inverted triangles - siliceous clays / oozes overlain by manganese nodules, 4) triangles - calcareous oozes / clays and 5) filled circles - pelagic / red clays . Note a tongue of cold, oxygenated water at the central eastern margin of the basin (AABW) adjacent to the 90°E ridge. Apparently no relation is seen between the Ce anomaly values and bottom water redox conditions. 59

The differences do not, however, seem to show any relation with the Ce anomalies in the sediments. The sediments from the AABW influenced eastern region do not have a very different Ce anomalies than the sediments from other regions. For example, siliceous sediments in east (#2528) and west (#153) have identical Ce anomaly value (0.17). This differs from the finding (Glasby et al, 1987) that the pelagic red clays from the Pacific influenced by the AABW, contain higher Ce/La ratios (implying higher redox conditions) than those of siliceous sediments. As shown here, the differences could be related to lithology and also to redox conditions within the sediments (German and

Elderfield, 1990a) and not necessarily to those in the overlying water. Despite the documented rapid response of the seawater Ce fractionation to water column redmi conditions (Sholkovitz et al, 1989; German & Elderfield, 1989,

1990b) our results suggest that the surface sediments may not respond instantaneously to the redox conditions of the overlying water. This could be due to the fact that the sediments comprise of lithogenous, biogenous and hydrogenous phases (Olivarez and Owen, 1991) in much higher abundance than those in the water column. Among these, only biogenic and authigeneic phases are vulnerable for the levels of oxygen in seawater (Piper, 1991).

Further, the calcareous sediments overlain by bottom water with differing characteristics (#127 and #2494) have similar magnitudes of Ce anomalies (Fig.

3.5). In fact, the Ce anomalies in these sediments seem to be controlled by the amount of carbonate content (Fig. 3.5). This negates the observation of Courtois and Hoffert (1977) that Ce anomalies in sediments are independent of calcium carbonate content. 60 3.6. CONCLUSION

These results suggest that the attempts to document secular variations in seawater chemistry in the past through the Ce anomalies in sediments need to consider the depositional environment (Murray et al, 1990), proximity to source areas (Murray et al, 1991a), redox conditions within the sediments (German and Elderfield, 1990; Murray et al, 1991b), together with the lithology and the diagenetic processes. Caution is therefore to be exercised in depending on

REE distributions for determining paleotectonic, stratigraphic and redox reconstructions. TABLE 3.1: Rare earth element concentrations (in ppm), Ce anomalies and other fractionation indices for the deep-sea sediments of the Central Indian Basin.

Sample# La Ce Nd Sm Eu Gd Dy Yb Lu XREE Ce/Ce* La. La/Yb Gd/La Yb/Gd Terrigenous

2537 24.8 43.5 23.6 5.1 1.32 4.8 4.2 1.95 0.25 109.5 0.86 2.7 1.07 1.27 0.74 2513 26.8 50.8 26.2 5.9 1.47 5.4 4.8 2.45 0.32 124.1 0.92 - 0.93 1.31 0.82 2483 26.7 47.4 26.4 5.9 1.5 5.6 5.0 2.49 0.33 121.3 0.92 3.3 0.92 1.35 0.81 2501 38.6 87.1 36.4 8.1 1.77 7.2 6.0 3.05 0.42 188.6 1.08 1.1 1.08 1.20 0.77 2535 32.2 73.2 32.7 7.5 1.76 7.3 6.7 3.69 0.50 165.5 0.75 2.4 0.75 1.46 0.91 2533 32.9 75.2 34.2 8.2 1.72 7.7 7.5 4.52 0.6 172.5 0.62 1.5 0.62 1.51 1.07 2532 28.1 72.9 28.8 7.0 1.52 6.5 6.2 3.64 0.48 155.1 0.66 1.1 0.66 1.48 1.02 Avg 30.0 64.3 29.7 6.8 1.58 6.4 5.8 3.11 0.41 148.0 2.0 0.86

Siliceous - Nodule barren

F101 32.8 81.0 37.6 9.4 2.17 8.9 8.3 4.40 0.60 185.2 1.14 0.64 1.75 0.89 F157 32.9 87.8 33.2 7.5 1.66 7.0 6.8 3.79 0.53 181.2 1.29 0.75 1.38 0.98 F156 33.5 81.8 31.2 7.2 1.48 6.8 6.7 4.05 0.56 173.3 1.21 0.71 1.30 1.07 F150 32.0 81.9 32.7 7.9 1.72 7.5 7.3 4.25 0.60 175.9 1.23 0.64 1.51 1.03 F153 25.4 81.1 28.2 6.8 1.62 6.3 5.9 3.04 0.43 158.8 1.48 0.71 1.60 0.88 Avg 29.6 80.3 31.0 7.4 1.66 7.0 6.7 3.77 0.53

Siliceous - Nodule bearing

2528 21.2 68.1 23.3 5.7 1.28 5.5 5.3 3.10 0.43 133.9 1.49 0.59 1.67 1.01 NR129 18.4 68.0 21.4 5.2 1.25 4.9 4.6 2.42 0.32 126.5 1.68 0.65 1.71 0.90 NR1 24 24.5 72.5 28.5 7.2 1.67 6.9 6.4 3.45 0.47 151.6 1.34 - 0.61 1.82 0.90 NR120 31.3 101.0 38.3 9.8 2.38 8.9 8.0 4.19 0.56 204.4 1.45 3.5 0.64 1.80 0.85 Avg 27.4 80.0 32.9 8.3 2.03 7.9 7.3 3.85 0.52

Calcareous

24 94 10.7 7.4 8.7 1.5 0.60 2.0 1.8 0.81 0.10 33.7 0.36 28.0 1.13 1.19 0.74 S127 9.2 5.2 6.4 1.0 0.48 1.4 1.3 0.58 0.07 25.6 0.30 60.8 1.68 1.0 0.77 5124 18.0 27.1 18.1 3.2 1.16 4.2 3.6 1.50 0.20 77.1 0.73 8.5 1.34 1.50 0.65 S121 20.1 34.8 19.8 4.6 1.26 4.6 4.2 1.90 0.25 91.5 0.8 1.45 1.50 0.75 Avg 14.5 18.6 13.3 2.6 0.88 3.1 2.7 1.20 0.16 57.0 0.56 32.4 1.03

Pelagic / red clays

100 27.9 69.2 32.3 8.0 2.05 7.8 7.1 3.75 0.49 158.6 1.13 3.7 0.64 1.81 0.87 160 35.1 89.5 44.2 11.3 2.82 11.1 10.2 5.42 0.75 210.4 1.13 0.56 2.03 0.88 136 32.8 85.2 42.1 10.7 2.64 10.6 9.8 5.06 0.68 199.6 1.14 3.2 0.56 1.92 0.86 231 32.8 75.3 38.1 9.6 2.33 9.5 8.4 4.49 0.63 181.2 1.05 3.6 0.63 1.88 0.85 150 32.0 81.9 32.7 7.9 1.72 7.5 7.3 4.25 0.60 175.9 1.23 3.5 0.64 1.51 1.03 148 50.4 110.0 62.1 15.5 3.86 15.8 14.4 7.48 1.03 280.6 0.98 5.5 0.58 2.02 0.86 132 55.6 117.0 68.3 17.0 4.23 17.6 16.1 8.50 1.14 305.5 0.93 6.3 0.56 2.04 0.87 144 57.8 114.0 72.2 18.3 4.51 18.7 17.0 8.94 1.22 312.7 0.87 6.9 0.55 2.10 0.87 154 56.8 110.0 73.2 18.2 4.68 19.2 17.4 9.06 1.21 310.0 0.85 7.2 0.54 2.20 0.85 210 96.0 156.0 117.1 29.1 7.43 31.2 28.1 14.86 2.01 481.7 0.73 8.3 0.55 2.10 0.86 Avg 51.8 106.0 63.2 15.8 3.93 16.3 14.8 10.44 1.07 CHAPTER 4

RARE EARTH ELEMENTS IN THE FERROMANGANESE DEPOSITS OF THE INDIAN OCEAN 62

RARE EARTH ELEMENTS IN THE FERROMANGANESE DEPOSITS OF THE INDIAN OCEAN

4.1. INTRODUCTION

The geochemistry of ferromanganese nodules is important for our understanding of the physical and chemical processes of the ocean because these bodies preserve a record of their sources and the subsequent changes.

In addition, the composition of manganese nodules provide insight into the chemistry and elemental mobility in overlying waters and underlying diagenetic fluids of the associated sediments.

Numerous studies of the chemical and mineralogical compositions of ferromanganese nodules have documented highly variable compositions both locally and regionally (see reviews by Glasby, 1977; Cronan, 1980; Halbach et al, 1988; Baturin, 1988). The rare earth elements (REE) in manganese nodules and the associated sediments also display considerable variability on both regional and local scales (Baturin, 1988).

Seawater is believed to be a major contributor of REE in manganese nodules (Goldberg, 1961, Goldberg et al, 1963). In contrast to the contribution of minor metals from .porewater diffusive fluxes to the nodules, REE source is not believed to be the same (Elderfield et al, 1981 a, Piper, 1988). Unless strongly complexed, the soluble Ce 3+ is oxidized to Ce4+ and precipitated from solution as insoluble Ce oxide. This excursion of Ce from trivalent to tetravalent state creates Ce anomalies in marine environment and explains the negative Ce anomalies in seawater (Goldberg, 1961) and the well known positive Ce anomalies in the manganese nodules (eg:Piper, 1974). The positive 63

Ce anomalies in manganese nodules are widely reported from all the Oceans

(Addy, 1979; Elderfield et al, 1981; Tlig, 1982 etc.). Though, negative anomalies in the manganese nodules were found and thought to be due to hydrothermal activity (Elderfield and Greaves, 1981) they are now reported in purely hydrogenetic nodules far removed from the known hydrothermal sites

(Glasby et al, 1987; Calvert et al, 1987 and Piper 1988).

After the substantial contributions from Elderfield and others in 1981 and

1982, there was a gap of three to four years. Renewed interest on the REE geochemistry of ferromanganese nodules, crusts, sediments and micronodules lead to a spurt of publications in the recent years (Aplin, 1984; Ingri and Ponter,

1987; Calvert et al, 1987; Glasby et al, 1987; Kunzendorf et al, 1987, 1989 and

Piper, 1988) mostly in the light of new findings on the REE geochemistry of seawater (Elderfield, 1988; Elderfield and Greaves, 1982; De Baar et al, 1985 a, b, and Brookins, 1989 and references therein). These studies have illustrated that a clear understanding of the uptake and distribution of REE in manganese nodules and associated phases may enable us to constrain models of REE cycling in oceanic environment. This advancement with respect to REE studies of Fe-Mn deposits is more or less confined to the Pacific. The recent review of Baturin (1988) shows the lack of data on REE in Atlantic and Indian

Ocean nodules. While Addy (1979) is the sole contributor for Atlantic studies, only twentytwo nodules have been analysed from the Indian Ocean

(Pachadjanov et al, ,1963 - two analyses; Glasby et al, 1978 - two; Volkov,

1979 - twelve; Tlig, 1982 - five, Ben Othman et al, 1989 - one) until this work is started. Of the twenty two nodules analysed from the Indian Ocean to date, data on fourteen are published in Russian and five in French that are inaccessible for wide readership. Trivalent REE in Indian Ocean nodules 64 according to Tlig (1982) are incorporated partly by occlusion on fine clays or oxyhydroxides and partly by surface to surface transfer as was proposed by

Ehrlich (1968).

In this chapter, I attempt to address the following points which hitherto were not possible.

1. Distribution and behaviour of all 14 naturally occurring REE in manganese nodules.

2. Variations of the REE in suites of ferromanganese nodules from different environments such as shallow basins, deeper abyssal plains, seamount top, and different sedimentary environments

3. Interelement relation of REE with almost all the major elements and their implications.

4. The role of bottom water redox conditions on the REE geochemistry of nodules.

5. Top-bottom variations of REE in oriented nodules and the role of diagenetic contribution of these elements to manganese nodules from the Indian Ocean.

Since manganese nodules act as the major sinks of oceanic cerium, cerium geochemistry is dealt in considerable detail. I wish to emphasize on the factors controlling Ce anomaly variations in manganese nodules. Specifically these are:

1) What is the influence of a) various types of nodule nuclei and b) the

aluminosilicate fraction on the magnitude of Ce anomalies? 2) Which

mineral phase has a major impact? and 4) Do the redox variations of the 65

depositional environment have any control on the Ce anomalies of manganese nodules?

The major and minor element geochemistry data for the Central Indian Basin have been reported by Moorby and Cronan (1981); Cronan and Moorby (1981); Jauhari (1987, 1989); Ahmed and Hussain (1987); Pattan (1988);

Mukhopadhyay and Nagendernath (1988); Sudhakar (1989 a and b); Banakar et al (1989); Valsangkar and Khadge (1989); Nath and Sudhakar (1992). These papers have emphasized the role of hydrogenetic contribution, contribution from oxic and suboxic diagenesis, the thickness of the acoustically transparent layer (ATL), and the local topography on the geochemistry of manganese nodules. Inspite _of . many publications on major and minor element geochemistry, not much of an attention has been paid towards understanding the REE geochemistry of ferromanganese nodules and crusts of the Indian

Ocean. Although sources for REE are common to all the Oceans, significant differences exist between the three major Oceans (Goldstein and O'Nions,

1981). This study assumes importance since the Indian Ocean has a mixture of both end members, a continental source (Atlantic type) and a seawater source (Pacific type).

4.2. GENERAL FEATURES OF THE STUDY AREA

The studied areas of the Indian Ocean include the Central Indian Basin and Mascarene Basin. The Central Indian Basin is bordered by ridges on three sides. The Chagos-Laccadive ridge marks the northwestern limit of the Central Indian Basin. This arcuate ridge extends north-south over nearly 2500 km between 14°N and 9°S. In the south, the Basin is bounded by the southeast Indian Ridge (SEIR) which separates the Central Indian Basin and Crozet 66

Basin. On the east, it is bordered by the N-S trending ninety east ridge. The

90°E ridge extends over nearly 5000 km between 9°N and 31°S. This north-south continuation demarcates between the western and eastern Indian

Ocean. The Central Indian Basin merges in the north into the Bay of Bengal

(Schlich, 1982).

The Mascarene Basin lies between Madagascar Island to the west and the Mascarene Plateau to the east. This Basin corresponds to the northwest extension of the Madagascar Basin. The two basins are separated by a major transform fault, viz., the Mauritius Fracture Zone. To the south, the Mascarene Basin abuts the northeastern flank of the Madagascar Ridge. To the north, the basin extends towards the Farquhar Group, the Amirante Trench and the

Seychelles Bank (Schlich, 1982).

The nodules were recovered from the vicinity of Tromelin Island.

Mauritius, Reunion, Rodrigues, Tromelin and Cargados- Carajos which are collectively called as Mascarene Islands. Tromelin island is the summit of a prominent conical seamount from the abyssal plain (over 4500 m), midway between Madagascar and Cargados Carajos. Tromelin islet is exposed mainly as a mass of sand, rising to nearly 5m and surrounded by coral reefs with few

outcrops of volcanic rocks ranging from quartz tholeiite to quartz trachytic

pumice and from olivine dolerite to glassy andesitic rocks (Upton, 1982). These

volcanic outcrops may be contributing nuclei for this suite of nodules (Nath and

Prasad, 1991). The depths observed in this area are above the carbonate

critical depth in contrast to most of the areas of Central Indian Basin (Kolla

and Kidd, 1982). 67 4.3. MATERIALS AND METHODS

The manganese nodules from the Central Indian Basin were collected during various cruises undertaken by the National Institute of Oceanography (India) as part of a mine site identification. The nodules from the Mascarene and from the Somali-Seychelles Basins were recovered during the Exclusive Economic Zone (EEZ) surveys of the island nations Mauritius and Seychelles respectively.

Twenty seven samples from the Central Indian Basin, five nodules from the Mascarene Basin and one nodule from the Somali- Seychelles Basin have been used in this study. The samples were selected to represent a seamount top, slope, an abyssal hill, siliceous sediment affected by terrigenous influx, a highly productive siliceous environment, red clay and carbonate sedimentary domains. From a total of 27 samples of the Central Indian Basin, three are manganese crusts, tops and bottoms of two oriented nodules, three nodules are from pelagic clay substrates and the remaining are recovered from siliceous ooze/clayey sediments. The description of the samples is presented in Table

4.1 and the sample locations are shown in Fig. 4.1. From this set of samples, five nodules were selected (2 from the Mascarene Basin - #MAR 8 and MAR

16; and three from the Central Indian Basin - #SS-5/360B, FA2/203B and FA2/73D) to determine the REE contents of separated oxide layers and nuclei and their influence on each other. The separation of oxide layers from their nuclei was done physically. Details of the nuclei types are given in Table 4.2 and described in detail in Fig. 4.12.

In addition, nine nodules from the Central Indian Basin were selected and analysed for their bulk contents and the fraction soluble in 2M HCI.

• 50 ° F 70° 90° 130° E

20°N

0°

20°S

50° 70° 90° 1 1 cf 130° Fig. 4.1. Figure showing the sampling locations with general physiographic settings. 68

Sample to reagent ratio was 1:100. The leaching was conducted on a hot sand bath for approximately ten minutes until the residue turned from brownish black to white. After cooling, the residue was separated with vacuum filtration using Millipore 0.42 um membrane filters. Since, the residue weights were very low, only four residues could be analysed. The main intention of the 2M HCI leach is to determine the partitioning of REE in Fe-Mn oxides and aluminosilicate fractions, since Central Indian Basin has significant lithogenous supply from ridge rock weathering (east, west and south) and a terrigenous influence from the north (Rao and Nath, 1988). In the past, various leaching schemes have been employed for the separation of Fe-Mn oxides from geological material (see Piper, 1988 for references). Although 2M HCI is a strong leach, it was preferred over other methods since, Sholkovitz (1989) found major artifacts for

REE studies with many leach solutions except with HCI and HNO 3 having a strength > 0.2M. The samples before the leaching and residues left out were studied for mineralogy by X-ray diffractometry using Mo and Cu targets. Comparison of diffractograms revealed that the manganese minerals such as todorokite and 6-Mn0 2 were totally dissolved and the leach residues (after leaching) have shown only aluminosilicate minerals such as feldspars, clay minerals, quartz and minor amounts of phillipsite (Fig. 4.2).

Major and minor element analyses were conducted on fused glass beads at the RWTH, Aachen, Germany using a Philips X-Ray Spectrometer (PW 1400). Glass beads were prepared with a specpure lithium tetraborate flux, the sample to flux ratio being 1:7. REE analyses on seventeen samples

(#GAR-EN, 649H, P-SET, MAR-1, MAR-3, MAR-22, 198-0, 142-0, 219-0,

320-A, 284-A, 75-C, 134/10, 0-1TOP, 0-1BOT, 0-2TOP, 0-2BOT) were conducted with an ICP-MS at National Geophysical Research Institute,

BEFORE LEACH

T

1

0 SK 23/252 A

0 - QUARTZ RESIDUE PL - PLAGIOCLASE FELDSPAR PL T- TODOROKITE

K- KAOLINITE

to

1 1 1 I 1 1 29 30 7 8 9 10 II le 1 7' 181 19 20 21' 1 222 232 ' 24 15 216 27 28

Degree 2 0

Fig. 4.2. Figure showing the X-ray diffractograms before and after leaching the manganese nodules with 2M HCI. BEFORE LEACH

D

D

SK 23/252A - QUARTZ D- bMnO2 RESIDUE a

t I 1 I i 1 35 38 37 38 39 64 65 66 67 68 _ . Degree 2 e

Fig. 4.2 (contd). Figure showing the X-ray diffractograms before and after leaching the manganese nodules with 2M HCI. T BEFORE LEACH

SK 23 / 259 A

0 - QUARTZ PL- PLAGIOCLASE FELDSPAR RESIDUE K- KAOLINITE KF- POTASH FELDSPAR

PL

k PL PL PL

KF

PL PL PL PL

)1*

vikoviow too

I I I 7 8 9 101 6 117 18 20 21 22 23 2:11 2 1 5 26 277 2 1e 29 30

Degree 2 e

Fig. 4.2 (contd). Figure showing the X-ray diffractograms before and after leaching the manganese nodules with 2M HCI. BEFORE LEACH

0

0

SK 23/ 259A .0- QUARTZ

P L - PLAGIOCLASE FELDSPAR

- bMn02

I I ► 35 36 37 38 39 64 65 66 67 68

Degree 2 e

Fig. 4.2 (contd). Figure showing the X-ray diffractograms before and after leaching the manganese nodules with 2M HCI. 69

Hyderabad, India. For the determination of REE, the samples including two replicates were digested in a mixture of HCI, HCIO 4 and HF in an open digestion system. The resulting dried residue was dissolved in HNO 3. The REE were measured using an Inductively Coupled Plasma- Mass Spectrometer

(ICP-MS) VG PLASMAQUAD. The details of the methodology and instrumental settings are described in detail by Balaram and Saxena (1988). In contrast to IDMS (Isotope Dilution Mass Spectrometry) which can be used only for polyisotopic elements, and ICP, which generally requires prior chromatographic separation from the matrix, the ICP-MS offers very low detection limits for all fourteen REE, simple spectra and relatively fast analytical turnaround (Jarvis,

1988). Considering its low abundance in geological material, indium was used as an internal standard. 100 ng/ml of indium was added to all the samples and standards. 'In was used to monitor the signal drift during analysis. After optimising the 115In signal, the system was operated covering the mass range of m/z 113-176.

The second set of REE analyses made on 1) separated oxide and nuclei and 2) 2M HCI leachates and their bulk samples with an ICP-AES (Inductively coupled Plasma-Atomic Emission Spectrography) after ion exchange separation following (Roelandts and Michel, 1989; Roelandts, 1988 and Roelandts, 1990) at University of Liege, Sart Tilman, Belgium. The details of the method are described in Chapter 3.

Precision and accuracy were monitored by replicate (by ICP- MS) and triplicate (ICP-AES) analyses of USGS manganese nodule Nod-A-1 (Standard made from Atlantic nodules) and Nod-P-1 (Pacific). Measurement precision varied with the type of instrument. Precision was better than +2.5% for all the 70 elements by ICP-AES except for Eu in P-1 (±5%); +6% for ICP-MS except for Ho af8%).

There are no recommended values for the REE of Nod-A-1 and P-1. So, our values are compared with the average calculated from the published reports (Table 4.3; Rankin and Glasby, 1979; Flanagan and Gottfried, 1980; Baedecker, 1980; Fries et al, 1984; Govindaraju, 1986; Date et al, 1987; Ingri and Ponter, 1987; De Carlo, 1990). They are better than ±10 La, Ce, Pr, Nd, Sm, Eu, Dy, Ho, Er, Tm Yb and Lu; +15% for Gd, Tb and also for Eu, Dy, Yb in case of A-1 by ICP-MS.

I have adopted the formula Ce anomaly = log ( 3Ce n / 2Lan + Nd,) given in ( Elderfield and Greaves, 1981) for calculating the Ce anomalies, where the subscript 'n' refers to the shale-normalized REE contents (values from Sholkovitz, 1988).

4.4. RESULTS AND DISCUSSION

The results of the major, minor and rare earth element analyses are presented in Tables 4.4 and 4.5.

4.41. Top - Bottom Fractionation or REE Zonation

Elderfield et al (1981 b) and Murphy and Dymond (1984) have found significant variation in the REE concentrations of tops and bottoms of Pacific

manganese nodules. In order to see the possibility if the prevalence of this phenomenon in the nodules from the Indian Ocean, tops and bottoms of two oriented nodules were analysed for REE in the present study. 71

The analytical results and the shale normalized REE patterns of these samples are shown in Tables 4.4 and 4.5 and Fig. 4.3 respectively. The shale values of REE, used to normalize, are from Piper (1974b) and Sholkovitz

(1988), which represent a mean value of North American, European and

Russian shales. The Mn/Fe ratios vary from 1.3 to 1.9. Of the two nodules studied, one nodule (0-2) has shown distinct top and bottom zonation similar to that reported initially by Raab (1972) and later by other workers (Calvert et al, 1978; Dymond et al, 1984; Moore, 1984; Friedrich et al, 1988). In contrast, nodule 0-1 shows almost equal Mn/Fe ratios (Table 4.5).

The mineralogy of oxide layers of nodule 0-2 revealed differences between top and bottom. While 6-Mn0 2 dominates todorokite in the top part of the nodule, no significant differences are found between 6-Mn0 2 and todorokite in the bottom of the nodule. A strong quartz peak and higher concentrations of Al and Si (Table 4.4) in the bottom of the nodule than its top indicate the

presence of larger amounts of aluminosilicate debris in the bottom of the

nodules.

Similar to the results found by Elderfield et al (1981 b), a positive co-variation between REE and Fe (Fig. 4.4), Co (Fig. 4.5), and negative relation

with Cu (Fig. 4 6) are observed. The fact that the top enriched in Fe while

bottoms in Mn phase (Cu, Ni and Zn) (this study; Pattan and Mudholkar, 1990;

Banakar, 1990 from the same area) indicates the role of diagenesis. Apart from

Si and Al which are attributed to aluminosilicate phase and Mn, P is the only major element which shows a consistent zonation covarying with Fe. The

association of REE with Fe and P could be due to two competing phases: Fe

oxyhydroxide and P phase from phosphatic fish debris (Elderfield et al, 1981 • • 0-1 TOP 2 0 0 0-1 BOTTOM A A 0-2 TOP 1:1---0 0-2 BOTTOM

Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 4.3. Shale normalized REE patterns of tops and bottoms of two oriented nodules 0-1 and 0-2. O O O O

m O c

rs) 0

O 0 12 14 16 18 20 22 IRON

Fig. 4.4. Diagram showing the relationship between Fe (%), Ce and other REE (ppm) in the oriented nodules. B and T represent bottoms and tops respectively.

a a

0 M M

a a a era a a

rs is a a

Es M El I I 1 1 0 .2 . 4 .6 .8

- Ce anomaly Fig. 4.5. Positive relation between Co (ppm) and Ce anomaly in the nodules. COPPER

Fig. 4.6. Figure depicting the enrichment of REE and depletion of Cu (ppm) in tops. Vice versa in bottoms. 72 b; Calvert and Piper, 1984) or could be incorporation in a single iron (phosphate) oxyhydroxide phase alone (Glasby and Thijssen, 1982 a; Glasby et al, 1987).

The depletion of the REE in the bottoms of nodules may be explained as follows: Enhanced concentrations of REE in porewater compared to seawater and their upward diffusion into overlying sedimentary layers has been observed in anoxic, reducing nearshore sediments (Elderfield and Sholkovitz, 1987; German and Elderfield, 1989; Sholkovitz et al, 1989). There is a lack of porewater REE data in the deep sea, pelagic areas in the Indian Ocean to suggest a contribution of REE through early diagenetic processes. But the similarity in vertical and interoceanic REE distributions to nutrients in seawater (Elderfield and Greaves, 1982; De Baar et al, 1983, 1985a, b; Klinkhammer et

al, 1983) can be used as an evidence for a significant upward diagenetic flux

of REE from the sediments (De Baar et al, 1988). Inspite of these findings, most of the bottoms of the nodules studied so far (this study; Elderfield et al,

1981a; Murphy and Dymond, 1984) show REE depletion. Similarly, many of

the nodule bottoms studied so far (including this work) have less iron contents

(eg: Banakar, 1990; Pattan and Mudholkar, 1990 from the same study area), by implication less iron oxyhydroxide surfaces available for adsorptive scavenging

which leads to a depletion of REE in bottoms.

No REE zonation is observed in 0-1 nodule of this study (Table 4.5) and

Nodule 2 (Table 4.6) of Elderfield et al (1981 a). The similarities in metal

concentrations in top and bottom samples and the mineralogy ( 6-Mn0 2 and

todorokite of roughly equal intensitities) seem to be due to the phenomena of 230 nodule turning over. Based on the studies of 226Rat30Th, Th/ 32Th activity 73 ratios, differences in 230Th and 731Pa and inconsistent transition metal top/bottom variations, Krishnaswami and Cochran (1978) and Moore (1984) have concluded that nodules turn over on a 1-10 thousand year time scale in Pacific. In addition, Banakar (1990) found evidences for the turnover of nodules from the

Central Indian Basin. The possibility of nodule 0-2 turning over is remote since, three fourths of this nodule is buried in the sediment (Table 4.1).

4.42. Interelement relations and their implications

Correlation matrices have been prepared for three different subsets of data and presented in Tables 4.7, 4.8 and 4.9. The relationship between complete nodule (and crust) chemistry data and only major and minor elements have been shown in Table 4.7, the relations among REE in Table 4.8. Table 4.9 shows the correlation between bulk chemical data of nodules (and encrustations) and important elemental ratios which were used for genetic interpretations in the previous works (e.g., Addy, 1979; Elderfield et al, 1981 a, b; Aplin, 1984; Glasby et al, 1987; Kunzendorf et al, 1987; Calvert et al, 1987).

The major associations noticed are Mn, Cu, Ni, Zn, Ba and Mg on one hand and in the second are Fe, Ti, P, Co and Pb and the LREE upto Nd (La,

Ce, Pr, Nd). The third group consists of Si, Al and K. The first and the second elemental groups represent the diagenetic and hydrogenetic component, respectively, and the third group accounts for the aluminosilicate phase. No

REE is associated with group 1 (Table 4.8), except for a statistically insignificant relation between Mn and middle REE (Sm and Eu). Strong negative correlations are displayed between Si, Al and K and REE. The HREE are behaving as a coherent group with noticeable relationship with only IREE 74

(Table 4.9). The three major groups observed here are in accordance with those observed in the Pacific nodules (Calvert and Price, 1977).

Negative correlations between REE and the aluminosilicate phase indicates that the surface transfer of continental detritus or the occlusion of detrital material (Ehrlich, 1968) is an unlikely mechanism for the incorporation of

REE into manganese nodules in the present study. This is supported by Sr and Nd isotopes studied initially by Goldstein and O'Nions (1981) and recently by Ben Othman et al (1989). The 87SrP6Sr ratios for manganese nodules from various Oceans studied (Ben Othman et al, 1989) range between 0.70895 - 0.70918 but the points cluster around the well known seawater value (0.70919).

Also, ENd data are reported to be -8.9 for Indian Ocean nodules and therefore similar to the seawater value (-8) for this region (Ben Othman et al, 1989). Recently, Glasby et al (1987) have calculated from the data of Ludden and

Thomson (1978, 1979) and Glasby and Thijssen (1982 b) and concluded that the lithogenous material derived from weathered basaltic material may contribute only 5% of the total La in nodules. The aluminosilicates are negatively correlated with Cu (Table 4.7), an element which is strongly enriched by oxic diagenesis in contrast to the observation of Dymond et al (1984). This in turn, may mean that the process of aluminosilicate component coupling with oxic diagenesis may not be prevalent in the studied area. In addition, higher aluminosilicate contents also seem to dilute the REE concentrations as is evidenced by nodule 75C (Tables 4.4 and 4.5).

The lack of correlation between the REE with the Mn phase elements, and its inverse relationship with Mn/Fe ratios (Table 4.8) supports the idea that the nodules with marked diagenetic signature (higher Mn/Fe ratios and negative 75 correlation of REE to Cu, Ni, Zn etc) show a depletion of REE (Calvert et al, 1987). Sample 284 A in this suite of samples is taken as a representative of diagenetic nodules (dominating todorokite). This sample shows the lowest concentrations of REE (except for sample 75C). Oxic diagenesis is said to be responsible for 58% of total La in the bulk nodule material studied by Dymond

et al (1984). The prevalence of oxic diagenesis at the sediment water interface in this region (Nath and Mudholkar, 1989) and the statistically insignificant relation of Mn with trivalent REE (Table 4.9) and no relation with La (Table 4.8)

indicate that oxic diagenesis may not be contributing REE into this suite of nodules.

As described above, the Fe bearing phase may represent a hydrogenetic

component. Mainly two schools of thought prevail for the explanation of Fe, P, REE co-variance in the manganese nodules. Based on elemental correlations

and leaching experiments, Elderfield et al (1981 b) suggested that two different

phases are governing the REE distribution in the manganese nodules: (a). phosphatic phase from fish debris and (b). Fe-oxyhydroxide phase. In contrast, Calvert and Price (1970, 1977) and later Glasby and Thijssen (1982a) and

Glasby et al (1987) suggested that Fe and P are present in the form a ferriphosphate phase. None of these workers considered the calcium

abundance in nodules to interpret the REE data. As mentioned above, the REE are associated with Fe and P, may be as an iron phosphate phase or two separate phases. Besides, trivalent REE seems to covary well with Ca (Table 4.9). In ferromanganese nodules, the REE are bound initially in two

phases: a phosphatic phase and a surface layer of phosphate which had previously been adsorbed onto hydrous iron oxide phase. Upon diagenesis, the

REE may be incorporated into recrystallized biogenic apatite (Jonasson et al, 76

1985). The association of REE with Ca, may thus be due to a minor biogenic apatite (calcium phosphate). Liu et al (1988) have attributed the striking peak for Sm enrichment in a manganese nodule observed by Goldberg et al (1963) to apatite co-precipitation with the Mn- Fe oxyhydroxides and subsequent replacement of Ca++ by the middle REE, particularly Sm +3, whose ionic radii is similar to ca++. The REE associated with a biogenic apatitic phase could be minor as there is no mineralogical (XRD) support for the presence of apatite.

The removal of Ti, Co and Pb from seawater via higher valence states (Co3+, PLY", Ti4+) has been suggested initially by Goldberg (1961) and later studies on the surfaces of manganese nodules by X-ray photoelectron spectroscopy (Dillard, 1982; 1984) have shown occurrences of Co(III), Pb(II), Pb(IV) and Ti(IV) thereby explaining the association of these elements.

Calvert et al (1987) have used elemental ratios to describe REE fractionation, with Yb representing the HREE, Sm the middle REE and La the LREE. Normalizing Yb and La with Sm and comparison with the shale ratios for these elements gave an idea about HREE and LREE enrichments/depletions (Table 4.10).

La/Sm ratios for this suite of samples indicate that the nodules from the

Western Indian Ocean (Mascarene and Seychelles Basins) are LREE enriched

compared to the nodules from the Central Indian Basin. The La/Sm ratio varies from 5.0 to 5.2 in crusts and from 1.6 to 5.4 in manganese nodules of Central

Indian Basin (Table 4.10). Among the nodules from Central Indian Basin, only those from the top of the seamount (Sample 142-0) and from the pelagic clay (75C) have high La/Sm ratios (Table 4.10). On the other hand, all the three

nodules from the Western Indian Ocean (Mascarene Basin) show higher La/Sm 77 ratio than the shale ratio. The nodule from the shallowest depth (Table 4.1) shows the highest -La/Sm ratio (Table 4.10). So only the nodules and encrustations from shallower depths (Table 4.1) are showing LREE enrichment in concurrence with the findings by Calvert et at (1987).

The HREE are behaving as a separate entity. HREE are correlated well among themselves (Table 4.8) and related only to middle REE (mainly Sm and Eu) and are not related with any major / minor element (Table 4.9). This means that significant fractionation of REE occur in this group of samples. The shale normalized patterns (Figs. 4.7 to 4.10) depict a convex structure with enriched IREE compared to LREE or HREE (excluding Ce and excepting

Sample 320A which is discussed later). HREE fractionation is further supported by Yb/Sm ratios. No sample has a higher Yb/Sm ratio (except 320A - Table 4.10) than that of shale (0.47; Sholkovitz, 1988). This indicates a HREE depletion in these nodules. This is in contrast to the HREE enrichment in seawater (Goldberg et al, 1963; Hogdahl et al, 1968; Elderfield and Greaves, 1982; De Baar et al, 1983, 1985 a, b; Piepgras and Jacobsen, 1988) compared to the shale REE pattern (Gromet et al, 1984). The HREE enrichment in seawater has been attributed to their greater complexing capacities (Turner et al, 1981; Cantrell and Byrne, 1987) and to the greater stability of HREE

complexes in seawater due to their smaller ionic radii and enhanced ionic interactive potential (Goldberg et al, 1963; Elderfield and Greaves, 1982).

Hence, LREE compared to HREE are more susceptible to removal from

seawater and for subsequent incorporation into manganese nodules. Therefore,

LREE enrichment and HREE depletion are found in these nodules. • PSET O---O GAR-EN A A 64911 Q10

V)

Lai za.

Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 4.7. Shale normalized REE patterns of ferromanganese encrustations. Note the strong positive cerium anomalies. 20 - • • 219-0 0-0 198-0 6—o 142-0

1 lLa Cei Pr ti d Smi EuI d d D1y Ho 1 Er Yba LuI

Fig. 4.8. Shale normalized REE patterns of manganese nodules from a seamount environment. The sample from seamount top (142-0) shows the maximum REE concentration. • • 75C o 0 320A A 284A 20 —

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 4.9. Shale normalized patterns of three different varieties of nodules. Sample 75C is a typical hydrogenetic nodule from red clay region shows the biggest postive cerium anomaly. Sample 284A is a representative of diagenetic nodule with a low positive anomaly. Smectite containing nodule 320A is enriched in HREE. • • MAR 1 20— 0 0 MAR 3 41--A MAR 22 0-0 RVG 134/10/2 L—tj 1 0 " <

V) a."

—1 CL. E V)

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 4.10. Shale normalized patterns of nodules recovered from Western Indian Ocean. All of them have identical patterns. T T ITU FEU T F T VT TM I I I Tit TTT I I I I :deep-sea sediments La Yb p-nodules, crusts

lll:marine basalts

Ms ea water 10

1 on ti iona t Frac

Redox vatic t. Ce from Kunzendorf et al (1988) La

0.1 1 10 Fig. 4.11. Geochemical characterisation scheme of Kunzendorf et al (1988). Fields I, II, Ill and IV represent deep-sea sediments, ferromanganese deposits (crusts, micronodules and nodules), marine basalts (and related rocks) and seawater respectively. The data points represent the nodule analyses conducted in this study. 78

In addition, the determination of the degree of fractionation using the geochemical characterization plot (Fig 4.11) as proposed by Kunzendorf et al

(1988) do not reveal wide variation.

4.43. Cerium anomaly variations

Large positive Ce anomalies have been often reported in manganese nodules from various locations (see Fleet, 1984 for an earlier review and

Brookins, 1989 for a recent review). They are attributed to the oxidation of

Ce(III) to Ce02(0 (Goldberg, 1961), although negative Ce anomalies are also reported in a few cases (Courtois and Clauer, 1980; Elderfield and Greaves, 1981; Glasby et al, 1987; Calvert et al, 1987). Many workers (for eg., Piper,

1974 a; Glasby, 1973; Addy, 1979; Elderfield et al, 1981 a, b; Shaw and Wasserburg, 1985 and others) concur with the idea of Goldberg (1961) that Ce is preferentially taken up by nodules. All the samples studied here except 320A show positive Ce anomalies (Figs. 4.7 to 4.10) between 0.12 (Sample 198-0) and 0.62 (crust PSET) (Table 4.11). The Central Indian Basin nodules show low positive Ce anomalies except the hydrogenetic nodule from red clay (Figs. 4.8 and 4.9). All the nodules from the Western Indian Ocean (Mascarene and Somali-Seychelles Basins) show markedly high Ce anomalies (Fig. 4.10).

The detailed results and discussion in this section are presented in the order of 1) Ce geochemistry of separated oxide and nuclei, 2) cerium in aluminosilicate phase (2M HCI leach) and 3) cerium in the bulk samples.

4.431. Variation of cerium in oxide and nucleating material

Although all the REEs and major elements are analysed in the separated oxides and nuclei, only La, Ce and Nd among the REE and relevant major 79 oxides are presented here. The results of separated oxide and nucleating material along with their relative weight proportions are presented in Tables 4.2 and 4.12. In addition, calculated bulk compositions are also presented (Table 4.12). The shale normalized patterns upto Sm of these three sets of data are presented in Fig. 4.12.

The oxide and nuclei of diagenetic nodules from the Central Indian Basin (#555-360B, FA2/73D and FA5/203B) possess similar concentrations of Mn, Fe

and P in contrast to nodule from Mascarene Basin (#MAR 8) wherein the

concentrations are much less in nuclei (Table 4.12; Figs 4.13 to 4.15). In all the cases, Si and Al (Table 4.12) which represent a lithogenic component are enriched in the nuclei, and reverse is the case of Ca (Table 4.12; Fig 4.16).

Low Ca concentrations are due to the presence of palagonitic material. During the process of palagonitization, Ca among other elements is depleted (eg: Honnorez, 1981). The nucleating material of nodules (MAR 16 and FA5/203B) was not sufficient for major element analysis. Nevertheless, the REE data is

presented (Table 4.12).

Ce concentrations are invariably low in the nuclei. The lowest

concentrations are found in the nuclei of hydrogenetic nodules with massive oxide layers (MAR 16 and MAR 8). Inspite of the diversity of nucleus types

within these hydrogenetic nodules, Ce concentrations in oxide layers are six to

seven fold. On the other hand, the nuclei of diagenetic nodules show a

maximum fall of 30% (Table 4.12). The shark tooth (MAR 16) has highest concentrations of La and Nd when compared to other nuclei. This is consistent

with the earlier observation that ichthyoliths from deep sea areas have higher concentrations of REE (Elderfield and Pagett, 1986). MAR 8 30

01 20

E cr)m 10

Fig. 4.12. Megascopic features of broken surfaces of nodules showing the various types of nuclei. The corresponding shale normalized REE patterns are shown adjacent to the photos.

A) Sample MAR-8 is an elongated, ellipsoidal, smooth textured nodule composed of elongated, disseminated light brown palagonitic nucleating material. XRD revealed good peaks of phillipsite, plagioclase, quartz and smectite which are typical low temperature alteration products of basalt (Honnorez, 1981) in the nucleus. Small peaks of ti-MnO, are observed in the oxide layers. A clear visual demarcation is noticed between oxide and nucleus. B SS5-360B • Oxide o Nucleus o Calculated bulk

B) Nodule SS-5/360B is small sized, rough textured diagenetic nodule. Visual observation on breaking the nodule has shown that the oxide and nucleus to be distinct. But on physical separation and grinding, nucleus was found to be penetrated and mineralised by the oxide material. No significant mineralogical differences were found between oxide and nucleus, except the intensification of quartz peak in the nucleus. The peak heights of todorokite and 6-Mn0 2 remained more or less unchanged. C FA 2 -73D

• Oxide o Nucleus o Calculated bulk

5 al j U) 3 32 CL. E 2 La Ce Nd Sm

C) Third nodule FA2/730 used for nucleus studies is also small sized, rough textured nodule containing a thin layer of oxide which is firmly embedded on nucleus. Apart from quartz increase in the nucleus, moderate intensification of todorokite and slight depletion of 15-Mn0 2 were also observed. FA 5 - 203 B

• Layer 1 o Layer 2 a Layer 3 ■ Calculated bulk

- 7

- 2 6Int 0

D) Nodule FA5-203B has three distinct layers. Layer 1: The outer oxide layer is rough textured with an older nodule as innermost nucleating material. Layer 2: A brown leached, altered material is sandwiched between the older and younger nodules. Layer 3: The nucleus of smaller, older nodule is fibrous and palagonitic in nature. For the REE analyses, these three layers were separated. The contents of quartz and plagioclase increase gradually towards the interior. Todorokite is high in layer 2 followed by layer 1 and negligible in layer 3. 6-MnO 2 is absent in layer 3 with almost equal intensities in outer layers. MAR . 16 o Oxide o Nucleus o Calculated bulk . -20

-10

2 1:a Ce Nd Sm E) Nodule (MAR 16) has a representative of biogenetic nucleating material. This triangular shaped, smooth textured nodule has a shark tooth as its nucleus. Only the tip of the shark tooth was exposed and entire tooth was invisible before the nodule was broke open. The oxide material is massive and the tooth is stained black inside the cavity, probably by iron-oxides. As observed recently (Toyoda and Tokonami, 1990), the enamel seems to be much less permeable to diagenetic exchange. The oxide material is composed of small peaks of b-MnO, and the tooth material was just sufficient for REE analysis. 4 5

Fig. 4.13. Relationship between Mn/Fe and Ce anomalies. The nuclei data are joined by a line with their nyicie data. WIO and CIB represent Western Indian Ocean and Central Indian Basin respectively. Fig. Ce - Anomaly 4.14. Relationship betweenFeand Ceanomalies.The nucleidataarejoined byalinewith their 5 I Fe (%) 10 1 15 20 I 1 I 0.1 0.2 0.3 0.4 P(%) Fig. 4.15. Relationship between P and Ce anomalies. The nuclei data are joined by a line with their , respective oxide data. Fig. 4.16. 80

As far as the Ce anomalies are concerned, nuclei showing distinct separation from its oxide and nuclei without oxide material penetration (Fig.

4.12) have considerably lower Ce anomalies (Samples 73D, MAR 8 and MAR 16) than their oxide counterparts. The behaviour of Ce is different in the nodule with shark tooth as its nucleus. The Ce anomaly of the coating phase is highly positive and that of shark tooth is negative. A good positive Ce anomaly in the oxide coating is compatible with its ferromanganese nature. The negative Ce anomaly of the shark tooth is similar to the characteristic feature of seawater (Goldberg et al, 1963).

4.432. Cerium In nodule nuclei and their implications

From the separate analyses of oxide and nuclei and their weight ratios, the probable bulk concentrations and Ce anomalies are calculated (Fig. 4.12;

Table 4.12). Inspite of the diversity in the type of nucleating material and the variable composition, the total impact on the overall Ce anomalies seems to be negligible in most of the cases except the sample FA2/73D.

As far as the sample with the shark tooth is concerned, the strong negative Ce anomaly has not influenced the bulk Ce anomaly of the nodule. This may be due to the fact that the size of the Ce anomaly is controlled by oxygen fugacity and the complex pore water-solid partitioning behaviour of fish teeth apatite and other REE scavengers such as Fe/Mn oxides (Staudigel et al, 1986). Recently, REE and Nd isotopes in biogenic phosphates and in fish teeth from oceanic and continental environments have been studied (eg.,

Elderfield and Pagett, 1986; Staudigel et al, 1986 and Wright et al, 1987). The REE and Ce anomalies of phosphates are believed to record the signals of paleodepositional environment of the water column (Wright et al, 1987 and Liu 81 et al, 1988), sedimentary environment (McArthur and Walsh, 1984; Elderfield and Pagett, 1986), geochemical/isotopic processes (eg: Staudigel et al, 1986) and hydrothermal processes (Tlig et al, 1987). A considerable debate has been going on the influence of early diagenesis (Grandejean and Albarede, 1989) vis-a-vis late diagenesis (Toyoda and Tokonami, 1990) on the REE concentrations in fish teeth.

The present data, enable me to suggest that neither early diagenetic nor late diagenetic processes seem to have influenced the Ce concentrations of fish

debris after the initiation of the nodule growth. The fish teeth are said to

acquire most of the REE after their settling to the ocean floor, since the apatite from living animals has very low levels of REE concentrations (Arrhenius et al,

1957; Elderfield and Pagett, 1986 and Grandjean and Albarede,1989). Asssuming that REE enrichment in this phosphatic sample is post-depositional, the negative Ce anomaly is a result of fractionation by precipitation with Fe-Mn

oxides in an oxidizing condition. What is important in the present context is,

that inspite of the microenvironment created around .biogenic apatite with high Ce concentrations (in other words availability), the Ce apparently is unable to penetrate the apatitic structure. This seems to be in contrast with other REE which are said to have a capacity to diffuse into the fish teeth cavities (Toyoda

and Tokonami, 1990):

Another inference of this work is that the initial Fe/Mn oxide layers formed around the nucleus seem to control the Ce scavenging capacity and not the type of nucleus. This argument is supported by the fact that the two

nodules (MAR 8 and MAR 16) recovered from the same location (Fig 4.1) with 82 entirely different nuclei depict almost the same magnitude of Ce anomaly (Fig. 4.12; Table 4.12).

Sample 203B with three distinct layers show a gradual reduction of REE

concentrations towards the centre of the nodule (Fig. 4.12) without any relation to the major element variations (Table 4.12). This is analogous to the

behaviour of Th isotopes within the nodules which decrease exponentially with depth (eg: Ku et al, 1979). REE with a geochemical similarity to Th (Grandjean

and Albarede, 1989) could have been subjected to diffusion-like transport within the nodule.

4.433. Cerium in aluminosilicate phase (2M HCI leach)

In addition to the physical separation of nucleating material from oxides,

attempt was made to separate REE chemically into Fe-Mn oxide phase and aluminosilicate fraction using a 2M HCI leach. The main reason for undertaking

this exercise is to look for any effects of ridge rock weathering from east, west

and south directions and riverine input from the northern part of the Central Indian Basin on the REE concentrations of nodules. The leach residue percentages range from a minimum of 14% to as high as 46%. As expected

the REE concentrations of 2M HCI leaches are higher than the bulk values

(Table 4.13). The shale normalized patterns of bulk and leachates are more or less identical (Fig. 4.17), except that they are shifted up because of their higher

ratios. To estimate- the contribution of Ce from aluminosilicates, the ratios of

oxides over bulk concentrations are also plotted (Fig. 4.18). The ratio patterns are nearly flat on an arithmetic graph indicating that the bulk samples retain

virtually the same patterns of oxides. Analyses of four residues left out in the

leaching experiment have shown no cerium anomalies but have shown Sample / Sha le the manganese nodulesandtheirbulk counterparts. Fig. 20_ 10 12 15 10 18 3 2 4.17. 3_ 6_ 3_ La I The shalenormalizedpatterns of2MHCIleachates Ce I Nd

SmEu 0 Leach Bulk Gd Dy 'f i b L i u

• Bulk 0 Leach SK23-259A

SK23-252A le ha S /

le 10 mp Sa

2.5

7_ SS2-107D

2 1 I T 1 I 1 1 1 I La Ce Nd Sm Eu Gd Dy Yb Lu

Fig. 4.17.The shale normalized patterns of 2M HCI leachates of the manganese nodules and their bulk counterparts. Leachates / Bulk

2 • • • •20 • • • • • • . 090 • t : .1 . • . 1.---.A2 • • • i .122 1

I 4 I i 4 La Ce Nd Sm Eu Gd Dy Yb Lu

Fig. 4.18. The ratio patterns of 2M HCI leach over bulk concentrations. 83 europium anomalies which are very characteristic of feldspars (Fig. 4.19).

These analyses have also revealed that more than 95% of Ce is concentrated in leach solutions (Table 4.13). Thus, it can be concluded that the aluminosilicate phase has a minor control (<5%) of the total Ce anomaly of manganese nodules studied here.

CERIUM IN BULK NODULES

4.434. Mineralogical control

The nodules in this work can be categorized into three varieties on the mineralogical basis. Either nodules with solely b-Mn0 2 bearing or with b-Mn02 dominating over todorokite can be grouped under first type; second type containing predominantly todorokite and the third group having equal intensities of b-Mn02 and todorokite. The b-Mn02 bearing nodules originate either from the Western Indian Ocean or exposed on pelagic clays of the Central Indian Basin and crusts and nodules from topographic highs (Table 4.11). This group of nodules contain higher Ce anomaly values (avg. 0.4) and lowest Mn/Fe ratios (avg. 1.35) implying higher iron contents compared with todorokite rich nodules which have the lowest Ce anomalies (avg. 0.17) with high Mn/Fe ratios (avg. 3.93). All the three types of nodules and crusts studied here can be categorized either as hydrogenous or oxic diagenetic varieties of Dymond et al

(1984) based upon their Mn/Fe ratios, mineralogical characteristics and surface textural properties.

Although a good relationship is observed between b-Mn02 contents and the magnitude of Ce anomalies similar to other studies (eg: Piper, 1974; Addy,

1979), it is difficult to say whether Ce and other REE reside in the basal layers S AMPLE / SH AL E 0.04. 0'4 after leachingthenoduleswith2MHCI. Thestationnumbersare mentioned foreachpattern. Fig. La Ce 4.19. ShalenormalizedREEpatternsofthe residuesleftout Nd SmEuGd REE Dy Yb Lu 84 of this mineral. An important and unquantified mineral phase (with conventional XRD) in manganese nodules is the amorphous FeOOH.xH2O. The

FeOOH.xH2O and to-Mn02 are epitaxially intergrown to stabilize the latter's structure (Burns and Burns 1977). Since Fe is not a constituent of to-Mn0 2, the Fe enrichment in to-Mn0 2 nodules is thought to be controlled by FeOOH.xH 2O. Similarly, the Ce may be associated with FeOOH.xH 2O, because Ce exhibited a better relationship with Fe rather than Mn (Figs. 4.13 and 4.14) and the apparent Ce-b-Mn0 2 relationship may be by virtue of the above mentioned intergrowth.

Furthermore, an analogy can be drawn with the cobalt, another element exisits in two oxidation states, to locate the Ce. Co is enriched in nodules and crusts rich in 6-Mn02 due to the ability of this element in its higher oxidation state to substitute for Mn (IV) (Goldberg, 1961) or as Co(III)00H for Fe(III)00H

(Cronan, 1980 and references therein). In which case, a prerequisite for the enrichment in nodules would be its oxidation from normal Co (II) in seawater to

Co (III) state, which would be favoured under the highly oxidizing conditions which also favour the formation of to-Mn02. Similarly, Ce (III) has a tendency to segregate from trivalent REE due to its oxidation to Ce (IV) state. The analogy with Co ends here, since with its higher ionic radius (0.87 A), Ce will not be able to substitute either Mn (IV) (i.r=0.62 A) or Fe (III) (i.r=0.65 A). The considerable difference in ionic radii between these species would appear to limit the possibility of.. mineral lattice substitution, and adsorption of Ce 4+ on to

FeOOH.xH2O/ to-Mn02 is a more plausible explanation.

In contrast to the above mentioned epitaxial intergrowth, in a purely diagenetic environment, Fe reacts with biogenic silica to form Fe-rich smectites 85

(Lyle et al, 1977; Rao and Nath, 1988) leaving free Mn to form todorokite. The resultant todorokite has less iron oxide surfaces for Ce adsorption. Hence, a patch of todorokite rich nodules is found underlain by siliceous oozes/clays containing Fe-rich smectites (Rao and Nath, 1988) in the Central Indian Basin with low Ce anomalies (Fig. 4.20).

4.435. Diagenetic effects on Ce anomalies

A negative non-linear relation is observed between Mn/Fe and Ce anomalies (Fig. 4.13). Mn/Fe is usually used as an index of diagenetic influence on manganese nodules. In consistence with many earlier observations from Pacific (eg: Glasby et al, 1987; Calvert et al, 1987), the nodules from the Indian Ocean are showing lowest Ce anomalies in the nodules with highest diagenetic enrichment. This has either been attributed to fundamental dissimilarity between the geochemistries of Ce and Mn in manganese nodules (Glasby et al, 1987) or thought to be not involved in diagenetic reactions (Elderfield et al, 1981). In contrast to this commonly observed negative correlations between Mn/Fe or Mn and Ce in deep sea nodules, similar vertical concentration gradients (eg: German and Elderfield, 1990) and a close and constant relationship between oxidative removal pathways for Ce and Mn (Moffett, 1990) have been reported from diverse marine environments. In addition, a positive relationship between these two elements in shallow water manganese nodules (Ingri and Ponter, 1987) and a manganese oxide control on REE in shallow water encrustations (Aplin, 1984) have also been found. This shows that there is no fundamental dissimilarity in the geochemistries of Mn and Ce, but the processes controlling the diagenetic reactions are curtailing the Ce enrichment in Mn/Fe rich nodules. The Mn/Fe

9+ o <0.2 • 0.2-0.4 4 • 0.4-0.6 0 >0.6

8 +

V /131±1- 56 73: •-• _ -■ - 4 RIDGE 1 RIDGE

I • 16*

0 0

20+ 50 +90

Longitude °E

Fig. 4.20. Geographic distribution of Ce anomalies of the manganese nodules studied here. 86 rich nodules are mostly todorokite bearing and occur almost buried in various oozes and the environment in less oxidizing as a result of diagenesis

(Takematsu et al, 1989 and references therein). The creation of this less oxidizing conditions may also curb the oxidative removal of Ce from seawater. However, it does not imply that, this is the sole process controlling Ce depletion in diagenetic nodules.

4.436. Cerium relationship with Fe and P and their implications

A good positive relationship is observed between Ce anomalies and Fe

(r=0.62) on one hand and P (r=0.73) on the other (Figs. 4.14 and 4.15). This relationship was found by others too (eg: Elderfield et al, 1981; Glasby et al,

1987). While Elderfield et al (1981) explained this either due to the existence of two different phases (iron oxyhydroxide and fish debris/recrystallised biogenic apatite) or by chemisorption of phosphate on hydrous iron oxides, Glasby et al (1987) felt that the .division is unnecessary and attributed this relationship to a single iron phosphate phase. Piper (1988) favoured the partitioning of bulk of Ce into the Fe phase. To check whether the phosphatic fish debris has any role on the magnitude of Ce anomaly, the Ce anomalies are also plotted versus Ca (Fig. 4.16) which showed a poor relationship (r=0.36). Ca in nodules could be located in todorokite, lithogenous minerals or fish debris. When Ca is plotted against P and Fe (Figs. 4.21 and 4.22), more or less similar relations are revealed (r=0.50 and r=48 respectively), indicating that Ca may be partitioned into two phases (mineral matter and fish debris/recrytallised biogenic apatite). A second piece of evidence comes from the nodule with a shark tooth as nucleus having no effect on the bulk composition (Fig. 4.12) ruling out the possibility of a probable control of fish debris on the magnitude of Ce anomaly. 3.0

2.0_

0.4

Fig. 4.21. Relationship between Ca and P. The nuclei data are joined by a line with their respective oxide data. I I I I 5 10 15 20 Fe (%)

Fig. 4.22. Relationship between Ca and Fe. The nuclei data are joined by a line with their respective oxide data. 87

The second possibility of Ce association with a single iron phosphate phase (Glasby et al, 1987) is considered here. A plot of Fe against P (Fig. 4.23) yielded a very good correlation (r=0.86). It is tempting to agree with the above hypothesis, but no iron phosphate group minerals (vivianite etc.,) are reported to be present in manganese nodules (Burns and Burns, 1977). Neither, there minerals are detected in X-ray diffractograms. In addition, P is not an essential constituent of two major mineral phases todorokite and 6-Mn0 2. That leaves the possibility of chemisorption of P on the Fe00H-H 20 surfaces

(Rosier, 1987). The resultant chemisorbed phosphate may be occurring along with the Ce on the iron oxyhydroxide surfaces, since Fe oxyhydroxide phase in manganese nodules is responsible for an effective scavenging of Ce from seawater.

4.437. Cerium anomaly variation in the light of nodule forming processes

On a broader scale, two theories namely the adsorption and the lattice substitution are the major postulates on the nature of the occurrence of minor metals in nodules (Fuerstenau and Han, 1977). In addition to these sorptive and crystal chemical properties, other factors such as the availability and chemical form of the metals in seawater or interstitial water, the redox conditions of the depositional environments and the growth rates of nodules and crusts are also thought to be important (Takematsu et al, 1989). As far as the

Ce anomalies and Ce concentrations are concerned, the process of adsorption on FeOOH surfaces, as discussed earlier, seems to be dominating over the lattice substitution phenomenon. 0.1

I I 5 10 15 20 Fe (%) Fig. 4.23. Relationship between Fe and P. The nuclei data are joined by a line with their respective oxide data. 88

The high content of Ce and higher values of Ce anomalies in nodules and crusts of hydrogenous origin is attributed to the highly oxidizing environment. The low contents in nodules of diagenetic origin is owing to the lower redox potentials in the depositional environment. In addition, as the growth rate of manganese nodule is controlling other minor metal contents, it may control Ce variations also. Higher growth rates and lower fluxes of Fe and Co are found to be associated with diagenetic portions of the nodules (Banakar, 1990). Similarly, the diagenetic nodules in this study are also showing a lesser development of positive Ce anomalies implying a dependence on growth rates too.

Summarising the above results, the Ce anomaly variations in nodules also seems to be controlled by the same factors which govern the distribution of other trace metals. The smooth surfaced, slowly growing, 15-Mn0 2 bearing hydrogenetic nodules lying on pelagic clays in oxidizing conditions seem to possess more sites on the FeOOH surfaces facilitating the oxidative adsorption of Ce leading to greater positive anomaly values. In contrast, todorokite bearing, rough textured diagenetic nodules with a faster growth rate lying buried totally or partially in siliceous oozes with lower redox potentials have lower Ce anomalies since they are depleted in iron contents, eventually leading to less number of sites for adsorption. The mixed types of nodules show intermediate values of Ce anomalies.

In addition to ruling out the possibility of lattice substitution, it is also shown that the Ce and P relation is due to their sorption on the iron oxyhydroxides. The third factor which has been reassessed is the 15-Mn02 - Ce 89 association. This relationship seems to be by virtue of epitaxial intergrowth of

FeOOH and 45-Mn02, the former adsorb Ce.

4.44. Gadolinium - Terbium Anomalies

When compared to the neighbouring elements, the shale normalized Gd value shows a positive departure with a corresponding negative Tb anomaly in all the nodules and crusts studied here (Fig. 4.24). Not all the nodules and crusts so far studied have been measured for both Gd and Tb (Baturin, 1988).

Of the 171 nodules studied from the Pacific Ocean until 1986 (Haynes et al,

1986 as quoted by Baturin, 1988), Gd has been measured only in 57 samples. The published papers later (mainly Piper, 1988; Glasby et al, 1987; Calvert et al, 1987) report on Gd based on analyses by Neutron Activation. Most of the

NAA based papers report eight to nine REE. This incomplete data do not bring out certain variations and may generally show a smooth pattern. Eventhough the various normalizations are utilised to smoothen the zig-zag pattern, if all the

REE are considered, they may tend to show uneven patterns (eg. Murray et al, 1990). The average Gd/Tb works out to be 7.7; with a range between 7.1 and

8.7. The oceanic average and Indian Ocean average Gd/Tb ratios are 6.6 and 8.3, respectively (Baturin, 1988). Both are higher than the shale ratio of 5.2

(Sholkovitz, 1988). Elderfield et al (1981b) show a slight positive peak of Gd, but does not report Tb data, so a gradual slope down to Dy is depicted.

Similar distinct Gd positive anomalies and depressed Tb values have been reported for seawater (Fig. 4.24) by De Baar et al (1985b). They have also been reported peviously in Fe-Mn oxide crusts from the Pacific Ocean by Hein et al (1988) and De Carlo (1990). Gd anomalies in seawater have been attributed to a higher degree of complexation of Gd, resulting in reduced

REE PATTERNS SHOWING Gd-Tb ANOMALIES

seawater data from de Baar et al (1985)

20- LE

A 10- H /S MPLE SA

1 1 1 I 1 1 I I I I 1 1 1 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Pr SmGd Ho Yb 1. 0 SEAWATER 0.8 as 0.4 La Pr SmGd Ho Yb r 1.0 Q15 1 1 1 1 1 1 015 /-t 0.7 ___--e..,L 1 .4.1.1 j.N•-•""` , 0.15ck c 1.0 ,ems 0.8 • 7 /"- al 02 404/0 • L a2 •

/04- 0.2 6 of 0.1 0.1 6112/0" •A-9- 0.2

if\ / 01— 0:15 E * 10

SHAL 0.3 ER/

0.1 AWAT SE 0.06

Q02

CeNd EuTb Tm Lu CeNd Eu Tb Tm Lu

Fig. 4.24. Gadolinium-terbium anomalies observed in the present study compared to the anomalies observed by De Baar et al (1985b) in the seawater. 90 scavenging of dissolved Gd relative to its neighbours Eu and Tb (De Baar et al, 1985b). It may therefore be expected that marine authigenic deposits would, on average, exhibit a complementary negative Gd anomaly. But the REE patterns observed in the present study are similar to the seawater pattern. This is of interest, since the REE patterns of nodules are generally a mirror image of the seawater pattern. However, the positive Gd anomalies in seawater and its attribution to higher degree of complexation has been criticised by Cantrell and

Byrne (1987) since De Baar et al (1985b) have utilised the speciation model of

Turner et al (1981). Cantrell and Byrne (1987) felt that the Turner et al

(1981)'s large stability constants of Gd relative to Eu and Tb are grossly overestimated and the Gd anomalies observed cannot be related to degree of complexation. Later Taylor and McLennan (1988) have also questioned the Gd anomaly proposed by De Baar et al (1985b), Masuda and Ikeuchi (1979) and

Roaldset and Rosenquist (1971), mainly because the Gd and Tb data were based on Neutron Activation analysis. Taylor and McLennan (1988) further add that the samples analysed by refined isotope dilution techniques (Elderfield and

Greaves, 1982) do not show these anomalies. Brookins (1989) suggest that the critical samples analysed by De Baar et al (1985b) be cross-checked by isotope dilution techniques and if these positive Gd anomalies are true, must be due to other causes. If the aqueous complexing capacity of Gd is as high as reported by De Baar et al (1985 b), .the nodules would have shown a depleted behaviour, similar to HREE. Nevertheless, additional data from various locations is necessary to understand the behaviour of Gd in the oceanic environment.

The mechanism leading to this similarity in the shale normalized Gd patterns of seawater and manganese nodules may be analogous to the process 91 of retention of negative cerium anomalies from seawater by manganese nodules from various locations (Piper, 1974 a; Courtois and Clauer, 1980; Elderfield and Greaves, 1981; Glasby et al, 1987 and Calvert et al, 1987). Based on Sr

isotope studies, Courtois and Clauer (1980) suggest a seawater origin of

nodules and inheritance of properties of seawater (implying the negative Ce

anomalies too).

4.45. HREE enrichment in Sample 320 A

Sample 320 A shows a REE pattern (Fig. 4.9) markedly different from other nodules, with a significant HREE enrichment and no Ce anomaly. While examining the morphology and interior of this nodule, the type of nucleus was found to be different from many other nodules. This sample is composed of brownish, fibrous material with disseminated shining crystals. It is suspected that this brownish, fibrous material is palagonite and the crystals are phillipsite (?). The phillipsite crystals recovered from the centre of nucleus were more

whitish or pure and the ones picked up from the periphery (at the contact of nucleus and oxide) were coated by Fe-Mn oxides. Later this nucleus forming material was subjected to X-ray diffraction for mineral identification. As this was suspected to be palagonitic material, all the minerals expected like

phillipsite, beidellite, nontronite, plagioclase, diopside, chlorite, sericite, smectite,

mixed layer chlorite, limonite, chabasite and palygorskite during low temperation

alteration of basalts and other volcanic material and formation of palagonites

(Mahe and Frenzel, 1989; Honnorez, 1981 and references therein).

The XRD studies revealed quartz, illite, smectite, plagioclase and

kaolinite. Smectite was confirmed by the usual method of glycolation (Warshaw

and Roy, 1961). The exact mineral of smectite group could not be identified, 92 so it is not known whether it was illite, montmorillonite, saponite or beidellite. Phillipsite and caladonite could not be identified definitely due to the interference of kaolinite and illite respectively. Only one peak of K-feldspar was identified. So, this brownish fibrous nucleating material is identified as palagonite, since palagonite is considered to be a mixture of altered, hydrated, oxidized glass with authigenic minerals such as clay, zeolites and chlorites (Honnorez, 1981).

Eggleton and Keller (1982) suggest that the palagonite is composed of a

dioctahedral smectite with significant Mg in the octahedral sheet. Direct evidence regarding the mineralogical composition of palagonite is generally lacking (Zhou and Fyfe, 1989). This material may further be identified as

fibrous palagonite, 'since fibrous palagonite represents the age and maturity. Normally the surfaces of aged palagonites consist of randomly oriented smectite clay mineral flakes (Mahe and Frenzel, 1989) with comb like and frame textures. The surface tensions of these smectite clay mineral flakes form the

electrochemical basis for the growth of manganese crusts (Mahe and Frenzel, 1989). During the low temperature alteration of basalts and glasses (and

formation of palagonites), REE are immobile and mobile (Grauch, 1989 and the references therein). There is little agreement regarding the overall magnitude or the potential for fractionation among the REE during this alteration (McLennan, 1989 and references therein). The palagonites are said to enrich Ce during the alteration process (eg. Bonnot-Courtois, 1980). On the other hand, they also suggest that the halmyrolitic minerals smectite and phillipsite formed from

basalts display REE patterns similar to those of seawater (Despraires and Bonnot-Courtois, 1980), i.e., the depletion of Ce and HREE enrichment. If the

halmyrolytic minerals have a seawater pattern, the Ce should be depleted and

HREE be enriched. The REE pattern of 320A (Fig. 4.9) with no Ce anomaly 93 formed from halmyrolitic reactions involving volcanic lithogenous material and seawater (Piper, 1974a; Courtois and Hoffert, 1977). Depletion of Ce and HREE

enrichment are revealed by sample 320 A when plotted in the characterization

scheme (Fig. 4.11) of Kunzendorf et al (1988). These variations suggest to us the importance of REE in nucleus material and its probable effects on the bulk

REE contents. The importance of nucleus may be more significant in the nodules with low oxide/nucleus ratios. This further suggests that one should

exercise caution in comparing the results of oxide separated and bulk analyses.

4.5. SUMMARY

The REE distributions in the nodules and crusts of the Western Indian Ocean exhibit notable enrichments of 10-30% relative to the nodules from the more diagenetic environment of Central Indian Basin. For Ce, more pronounced enrichments of up to 220% are observed. The preferential enrichment of Ce and other REE in the nodules from the Western Indian Ocean is consistent with the oxygenating conditions created by the AABW flow. Superimposed on these oceanographic conditions are the redox conditions

within the sedimentary environment. Restriction of 1) higher REE contents and higher Ce anomaly values to .5-Mn0 2 nodules and crusts and vice-versa to

todorokite nodu!es and 2) inverse relation between REE and Mn/Fe suggests

that nodule REE chemistry is also linked to sediment diagenesis.

Inter-element correlations between Ca, Fe, P and trivalent REE supports

the idea of Elderfield et al (1981 b) that REE probably reside in two phases a) iron oxyhydroxide phase with phosphate chemisorbed on to it and b) a minor

apatitic phase. But, Ce seems to be limited to a single iron oxyhydroxide phase,

Mn02 catalyzing the oxidation of Ce to an insoluble tetravalent form. Table 4.1: Description of locations and samples.

Sample TYPE LAS LON G°E Depth Description No. (m)

GAR-EN Encrustation 12.500 76.333 5050- Thin brownish coloured oxide layer (<1cm) firmly encrusted over 5100 greyish brown copsolidated and underlain by mottled brown hardened mud. 0-Mn02 dominant.

649H Encrustation 12.644 76.282 5000 Black coloured very thin oxide layer (<1 cm). Rough (fine granular) surface texture. Loosely encrusted over brownish palagonitrc material. 0-MnO, dominant with todorokite traces.

PSET Encrustation 14.500 75.000 4881- Oxide and substratum are visually distinct, but firmly encrusted. 5173 0)dde black in colour (1.5 cm thick). Rough (coarse granular) surface te4ure. Substratum-light brown coloured hard mud/palagonite with dots. 0-Mn02 dominant.

MAR-1 Large Nodule 14.988 54.983 4300 Irregular elongated slab (10-11 cm). Gritty/knobby surface. Smooth from Shallow & rough surface texture. Nucleus-Irregular, elongated, dispersed Basin yellowish brown altered hardened mud, covering almost entire length of the nodule. 8-Mn02 dominant, feldspar and present.

MAR-3 Small Nodule 16.016 55.980 4300 Rectangular, irregular, small (3-4 cm); Smooth texture (granular) from Shallow Nucleus-Elongated, large, dispersed, yellowish brown coloured Basin ardened mud. Definite control of shape by nucleus. O-Mn02 with todorokite and quartz. MAR-22 Small Nodule 16.000 55.983 4130 Spheroidal to cubical (smoothened edges); Smooth surface texture. from an abyssal Nucleus-Triangular shkeped muddy material. No control of nucleus on hill of Shallow overall shape. Only 0-Mn02. Basin.

198-0 Nodule from 11.000 73.250 5031 Discoidal, medium sized (5.3 cm) with subcircular outline; Surface a flank of yery rough (coarse granular). Nucleus-clay (with quartz and illite). a Seamount 0-Mn02 dominant with minor amounts of todorokite.

142-0 Nodule from top 11.000 73.000 3580 Rhombohedral, small sized (3.8 cm) with smooth (fine granular) of a Seamount surface. Nucleus-clay (feldspar, phillipsite) 0-Mn0 2 dominant with minor amounts of todorokite.

219-0 Base of a 12.750 76.250 5350 Granular to coarse granular surface texture. Nucleus-Weathered rock Seamount fiagment (phillipsite, feldspar and pyroxene) todorokite dominant, 0-Mn 02 present.

320A Nodule from 12.248 75.501 5269 Rough textured medium sized nodule Nucleus-Fibroys palagonite with Siliceous clay/ phillipsite crystals (?) and smectite. Todorokite and 0-Mn0 2 almost ooze in quantities.

CO

Table 4.1 (Contd)

Sample TYPE LAT°S LONGS Depth Description No. (m)

284A Diagenetic 10.997 74.249 5204 Todorokite dominating over 6-Mn0 2. Nodule

75C Hydrogenetic 17.998 78.000 4904 Triangular, irregular, small sized (3.2 cm) nodule. Surface, texture- Nodule smooth. Nucleus-Altered rock material. Minor amounts of 0-Mn0 2.

134/10 Nodule from Somali-Seychelles Basin Smooth textured nodule. 6-Mn02 only. Shallow Basin

0-1T Top of an 13.087 75.019 5270 Medium sized (5-6cm) nodule with uneven top surface. Flat orierited bottom. Half Hamburger shaped. Serpulid growths on surface. Nodule

0-1 B Bottom of the above nodule Surface texture; Smooth (fine granular). Nucleus-Brownish yellow altered rock.

0-2T Top of another oriented nodule from Discoidal, medium sized (4-5 cm) nodule with a hump on top. the above location Three fourth of the nodule is stained with sediment Serpulid growths on top. Surface texture-smooth (fine granular). 0-2B Bottom of second oriented nodule Top: ti-Mn02 dominating over todorokite. Bottom: Todorokite and ti -Mn02.

SS2/107D Diagenetic 10.988 73.988 5200 Small, ellipsoidal nodule (2-4cm), rotigh textured. Nucleus - altered rock. Todorokite slightly more than 0-Mn02, quartz traces.

FA1-16F Pavement 11.002 74.996 5191 Medium sized (4-6cm), tabular, rough pavement. Numerous white serpulid(?) filamentous gro s on the surface. Hard mud/fibrous palagonite as its nucleus. -Mn02, quartz and phillipsite.

SS-2/9013 Hydrogenetic 14.997 75.996 4965 Medium sized (4-6 cm), irregular concretion. Smooth top and slightly rough bottom with sediment stains. Overall shale distinctly controlled by nucleus shape. Altered whitish yellow material with stains of Fe-Mn oxide intercalations. Similar to many nodules from Westem Indian Ocean. 0-Mn02 dominant, quartz, feldspar, phillipsite.

SK23/259A CIB 11.004 75.996 5212 Medium sjzed (4-6 cm), elongate, rough nodule. Altered rock as nucleus. 0-Mn02 equivalent to todorokite, quartz traces.

SS2/92A Hydrogenetic 17.032 76.012 4840 Small sized (2-4cm) triangular, smooth nodule. Altered rock fragment as the nucleus. 0-Mn02 minor, phillipsite and quartz.

SS5/367B Diagenetic 10.997 77.255 Small (2-4cm), ellipsoidal, rough nodule. Altered rock with very few phillipsite crystals inside the nucleus. Todorokite more than 0-Mn02, quartz traces.

CD Cn Table 4.1 (Contd)

Sample TYPE LAT°S LONG°E Depth Description No. (m)

SK23/252A CIB 11.001 79.998 5287 Small (2-4cm) elongated, smooth nodule. Probably older nodule as nucleus. 0-MnO, is equivalent to todorokite.

SS2/122C Nodule from 11.009 72.981 3580 Small (2-4cm) irregular, smooth shallow depths (coarse grained) Shallow depths nodule. Nucleus-Altered rock. Amorphous Iron-oxides, phillipsite, quartz.

SS1/35 CIB 11.014 81.999 5100 Small (2-4cm), angular, rough with sediment stains on one side and smoothooth another. Old nodule as nucleus. Todorokite equivalent to quartz traces.

SS1/20C ' CIB 11.042 80.995 Small (2-4 cm), discoidal, smooth nodule. Altered rock material as nucleus. Todorokite, 61-Mn02 minor, quartz.

Description of samples 198-0, 142-0 and 219-0 from Mukhopadhyay (1988) 97

Table 4.2: Details of nodule nuclei.

S.No. Nucleus type Weight in gms. oxide nucleus

MAR-8 Highly altered rock material 9.0211 1.7016

SS5/360B Less altered rock with 15.9036 5.5910 oxide penetration

FA2-73D Altered rock 6.073 4.354 FA5/203B Older Nodule 4.5915 1.0220 (Layer 1) (Layer 3) brown, leached and altered material between two nodules 3.7329 (Layer 2)

MAR-16 Shark tooth 2.3993 0.1229 Table 4.3: REE contents in U.S.G.S manganese nodule reference samples Nod-A-1 and Nod-P-1 (in ppm). Comparison of present results obtained using ICP-MS and ICP-AES with previously reported data...... Nodule Standard La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu and method

Nod-P-1:

This study: ICP-MS* 105 318 27.5 114 27.2 7.44 33.8 4.53 25.99 4.73 13.3 1.72 13.26 1.75 ICP-AES+ 104 314 128 32.3 7.99 29.3 25.8 13.27 1.83 Literature: SSMS 1 82 280 27.0 110 28.0 6.80 24.0 . 4.20 25.00 5.10 13.0 , 13.00 -- NAA 2 120 289 -- 113 30.4 6.57 29.4 -- - ' 13.80 1.85 NAA 3 107 278 82.6 -- 3.30 ------8.60 -- CP-OES 4 103 288 135 33.8 7.75 27.0 29.20 5.13 15.2 12.30 1.89 CP-OES 4' 107 296 -- 143 35.3 9.04 32.5 -- 27.00 4.93 13.4 -- 11.50 1.66 CP-AES 100 310 <30 124 20.0 7.00 21.0 <10 28.00 6.50 13.6 <5 11.80 1.70 CP-MS 6 99 305 30.4 130 30.8 10.30 37.6 4.40 25.70 4.80 12.8 2.10 12.70 2.10 CP-MS 6' 96 281 27.9 122 28.6 7.40 31.6 4.60 23.60 4.70 12.6 1.80 11.60 1.80 Nod-A-1

This study: ICP-MS* 115 656 21.7 94 20.4 5.81 34.3 4.20 25.80 5.09 15.6 2.19 15.40 2.21 ICP-AES+ 105 704 93.2 21.0 5.25 22.3 21.60 13.49 1.99 Literature: SSMS (1 130 >300 23.0 94 21.0 4.80 22.0 3.80 22.00 5.30 15.0 13.50 -- NAA 133 668 -- 85.3 20.9 4.48 26.5 -- -- 16.30 2.16 NAA 152 930 101 -- 4.10 ------• 16.00 -- CP-OES 112 676 -- 105 24.7 6.10 23.6 -- 22.40 4.70 14.4 -- 11.80 1.92 CP-AES 104 760 <30 81 18.0 4.30 29.0 <10 20.00 5.80 14.6 <5.0 13.20 2.80 CP-MS 6 115 771 25.3 101 25.2 5.50 28.7 4.30 24.10 5.20 15.2 2.40 13.80 2.20 CP-MS 109 755 24 98.3 22.4 5.70 27.7 4.00 23.00 5.20 15.0 2.20 13.50 2.10 CP-AES 115 682 23.2 102 -- 4.80 24.9 2.50 22.10 4.20 12.1 1.60 12.20 2.00 CP-AES 8 93 620 -- 93 22.0 5.00 26.0 -- 26.00 -- 11.7 -- 12.00 2.56

* Mean value of two independent analyses + Mean value of three independent analyses Rankin and Glasby (1979) Flanagan and Gottfried (1980) Baedecker (1980) Ingri and Ponter (1987) Fries et at (1984) Date et al (1987) 7) De Carlo (1990) (8) Govindaraju (1986)

CO , Table 4.4: Major and minor element data of manganese nodules and crusts (analysis performed on dried samples). concentrations in wt. %. ------S. No. Fe Mn 11 Ca K P Si AI Mg Cu Ni Co Zn Pb

GAR-EN# 14.53 8.70 0.61 1.34 1.74 0.19 14.43 5.12 1.21 0.19 0.22 0.14 0.064 0.044 649-H# 17.17 19.39 0.70 2.00 0.63 0.22 6.60 1.37 1.30 0.17 0.33 0.19 0.066 0.085 P-SET# 10.14 19.01 0.55 1.49 1.21 0.19 8.57 3.58 2.01 0.24 1.16 0.30 0.094 0.069 MAR-1 17.25 14.85 0.83 2.28 0.93 0.25 9.84 3.11 1.40 0.14 0.26 0.30 0.067 0.072 MAR-3 18.23 13.48 0.95 2.08 1.09 0.28 10.00 3.38 1.28 0.07 0.14 0.30 0.056 0.076 MAR-22 20.32 16.80 1.08 2.17 0.50 0.30 6.17 1.98 1.19 0.10 0.19 0.36 0.064 0.084 198-0 8.48 27.99 0.36 1.92 0.66 0.12 6.58 1.78 1.66 0.95 1.41 0.10 0.160 0.053 142-0 13.82 21.47 0.47 1.88 0.92 0.20 8.21 2.18 1.54 0.44 0.84 0.1p 0.093 0.079 219-0 8.99 28.46 0.36 1.85 0.69 0.14 6.36 1.96 1.57 0.98 1.34 0.15 0.140 0.053 320-A 8.11 24.58 0.33 1.45 0.81 0.09 • 8.95 2.99 2.03 1.12 1.31 0.14 0.139 0.042 284-A 7.51 25.24 0.34 1.59 0.85 0.09 9.00 2.73 1.98 1.21 1.51 0.09 0.146 0.044 75-C 10.94 6.47 0.61 0.65 2.74 0.07 18.84 6.32 1.65 0.13 0.32 0.10 0.065 0.041 134/10 19.04 17.09 0.70 2.08 0.64 0.25 7.79 1.75 1.19 0.12 0.28 0.21 0.068 0.076 0-1 TOP* 14.72 23.27 0.47 1.83 0.57 0.21 5.85 1.79 1.53 0.53 0.78 0.18 0.090 0.080 0-1 BOT* 14.51 22.45 0.46 1.69 0.59 0.18 6.49 2.13 1.57 0.61 0.76 0.18 0.089 0.079 0-2T0P* 20.19 26.97 0.83 2.03 0.36 0.23 5.06 1.42 1.51 0.47 0.90 0.33 0.170 0.144 0-2B0T* 15.98 30.76 0.95 2.09 0.32 0.18 5.15 1.80 1.80 0.98 1.88 0.24 0.199 0.137

# Ferromanganese crusts * Tops and Bottoms of two oriented nodules

------Table 4.5: Rare earth element concentrations in ferromanganese nodules and crusts (in p.p.m.). ----- S.No. La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

GAR-EN 126 546 26.70 110 24.14 6.23 31.37 4.40 22.35 4.51 11.32 1.92 11.02 1.72 649-H 243 860 49.93 201 48.82 12.49 58.24 7.33 40.53 6.90 19.76 2.74 16.76 2.38 PSET 107 872 22.57 93 20.94 5.29 32.17 3.68 21.21 3.72 10.21 1.48 10.17 1.41 MAR-1 160 989 34.21 137 28.64 7.52 38.58 5.20 28.57 5.44 15.30 1.91 13.34 1.93 MAR-3 197 1384 41.22 165 35.78 8.95 49.97 6.18 34.36 6.13 17.43 2.40 15.91 2.25 MAR-22 228 1517 44. 75 176 37.64 9.45 55.54 6.53 36.37 6.74 18.16 2.57 17.17 2.32 198-0 125 347 31.38 128 30.05 8.02 36.04 4.87 26.42 4.51 12.26 1.72 11.90 1.64 142-0 180 616 41.65 164 36.50 9.64 44.72 6.35 32.90 5.48 15.59 2.06 13.91 1.97 219-0 121 446 32.38 133 33.88 8.33 36.56 5.25 27.35 4.63 13.30 1.87 11.76 1.73 320-A 90 241 27.39 130 55.53 17.02 63.15 8.13 41.35 9.84 30.58 6.57 41.33 8.52 284-A 87 245 21.54 88 21.24 5.50 25.30 3.17 17.38 3.21 8.43 1.12 7.68 0.92 75-C 57 341 12.24 49 10.51 3.19 16.79 2.09 10.43 2.01 5.52 0.68 4.73 0.62 134/10 168 910 36.75 148 32.25 8.13 44.98 6.00 32.01 5.85 15.95 2.16 14.86 2.05 0-1TOP 181 715 43.74 177 41.22 10.96 53.35 7.21 37.84 6.50 17.17 2.49 16.21 2.07 0-1 BOT 175 726 41.35 170 38.64 10.49 52.40 6.81 35.90 6.26 17.55 2.39 16.03 2.24 0-2TOP 216 835 49.61 206 48.43 12.31 63.56 8.10 44.75 7.96 20.53 2.90 19.23 2.69 ------0-2BOT 147 549 ------34.27 -_-_-__139 ------30.81 8.33 40.29 5.21 29.71 5.27 13.96 1.65 13.45 1.71 Sample details are as described in Table 4.1 Table 4.6: Compositions of ferromanganese nodules reported by Elderfield et al, (1981b).

Sample Mn Fe P Co Cu Ni Mn/Fe La Ce Nd Sm Eu Gd Dy Er Yb ------.^------

------.------.------.------

1)

Top 20.6 9.20 0.176 0.300 0.537 0.882 2.2 117 624 170 42.5 9.25 41.4 33.4 16.3 Bottom 22.3 6.82 0.125 0.255 0.868 0.992 3.3 89.3 441 125 31.1 7.10 27.9 23.7 11.0 10.7

2)

Top 23.2 6.92 0.141 0.296 0.833 1.16 3.4 97.2 403 136 32.3 7.89 26.2 13.4 11.6 Bottom 22.7 6.68 0.137 0.269 0.823 1.15 3.4 94.1 394 133 32.9 7.78 29.7 25.5 12.7 11.4

3)

Top 23.2 5.92 0.159 0.235 0.768 1.03 3.9 83.8 258 110 28.1 6.80 26.3 24.0 12.6 11.6 Bottom 24.6 5.16 0.112 0.153 1.05 1.22 4.8 63.3 215 109 27.4 6.76 25.6 22.4 11.5 10.3 Table 4.7: Correlation coefficient matrix for complete data versus major elements in manganese nodules and crusts.

Fe Mn Ti Ca K P Si Al Mg Cu Ni Co Zn Pb

Mn -.26 Ti .88 -.32 Ca .59 .41 .46 K -.29 -.77 -.12 -.83 P .85 -.17 .75 .73 -.41 Si -.25 -.80 -.09 -.79 .97 -.40 Al -.26 -.77 -.05 -.80 .96 -.37 .96 Mg . -.70 .44 -.55 -.40 .03 -.72 -.02 .09 Cu -.67 .80 -.64 -.06 -.36 -.60 -.34 -.33 .62 Ni -.62 .84 -.52 -.04 -.36 -.49 -.40 -.35 .69 .90 Co .75 -.09 .85 .59 -.34 .79 -.37 -.23 -.29 -.53 -.33 Zn -.36 .86 -.26 .11 -.45 -.40 -.46 -.41 .57 .83 .91 -.18 Pb .67 .40 .62 .58 -.58 .52 -.60 -.55 -.14 -.09 .12 .64 .36 La .82 .07 .62 .75 -.59 .78 -.59 -.60 -.58 -.44 -.43 .62 -.24 .63 Ce .81 -.33 .81 .58 -.24 .89 -.26 -.19 -.59 -.71 -.63 .87 -.54 .39 Pr .71 .28 .45 .77 -.72 .67 -.71 -.73 -.45 -.22 -.25 .51 -.06 .65 Nd .67 .33 .41 .75 -.74 .61 -.73 -.74 -.38 -.16 -.20 .48 -.01 .64 Sm .30 .44 .09 .49 -.68 .24 -.63 -.63 -.02 .16 .03 .19 .17 .38 Eu .18 .43 -.01 .37 -.60 .11 -.54 -.54 .09 .24 .09 .08 .20 .28 Gd .50 .32 .30 .56 -.68 .43 -.64 -.61 -.13 -.04 -.12 .41 .04 .50 Tb .46 .36 .20 .57 -.69 .39 -.65 -.64 -.15 .02 -.09 .31 .07 .47 Dy .52 .38 .29 .63 -.73 .46 -.69 -.68 -.17 -.02 -.09 .40 .09 .55 Ho .37 .31 .21 .47 -.61 .30 -.53 -.50 -.04 .07 -.06 .29 .10 .37 Er .27 .29 .12 .40 -.55 .23 -.47 -.45 .02 .12 -.03 .21 .08 .26. Tm -.00 .22 -.09 .12 -.34 -.03 -.26 -.22 .20 .23 .06 -.00 .11 -.00 Yb .03 .27 -.05 .17 -.39 .00 -.31 -.27 .21 .26 .10 .04 .16 .07 Lu -.12 .21 -.16 .02 -.25 -.14 -.16 -.13 .29 .30 .13 -.08 .16 -.08

n = 17, values above 0.61 are significant at 99% level.

Table 4.8: Correlation coefficient matrix for rare earth elements in manganese nodules and crusts.

...... ------.M.------..... ------_-_-_-_------La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La 1 .74 .96 .93 .60 .45 .72 .70 .76 .54 .44 .14 .15 -.02 Ce 1 .58 .53 .19 .06 .41 .32 .40 .27 .20 -.03 -.01 -.13 Pr 1 .99 .74 .61 .82 .83 .87 .66 .57 .27 .29 .11 Nd 1 .82 .70 .88 .89 .92 .74 .65 .38 .39 .22 Sm 1 .98 .96 .97 .96 .95 .94 .82 .82 .72 Eu 1 .92 .93 .90 .96 .97 .90 .90 .83 Gd 1 .98 .99 .96 .92 .75 .76 .63 Tb 1 .99 .95 .91 .74 .76 .63 DY 1 .94 .89 .70 .71 .57 Ho t 1 .99 .89 .90 .81 Er 1 .94 .95 .88 Tm 1 .99 .98 Yb 1 .98

n = 17, values above 0.61 are significant at 99% level

Table 4.9: Correlation coefficient matrix for complete data versus major elements and important elemental ratios in manganese nodules and crusts. ------__ Fe Mn Ti Ca K P Si A Mg Mn/Fe Ce/La Ce/Nd La/Sm Yb/Sm

Mn/Fe -.78 .76 -.72 -.03 -.34 -.61 -.35 -.34 .66 Ce/La .37 -.61 .56 -.01 .35 .49 .27 .39 -.25 -.61 Ce/Nd .45 -.64 .63 .04 .33 .55 .27 .37 -.34 -.67 .99 La/Sm .63 -.59 .67 .19 .24 .63 .21 .19 • -.65 -.76 .67 .76 Yb/Sm -.16 -.14 -.05 -.24 .14 -.13 .22 .29 .29 .04 .12 .05 -.45 3+REE .68 .27 .44 .72 -.72 .62 -.69 -.69 -.37 -.24 -.13 -.08 .02 -.08 La .82 .07 .62 .75 -.59 .78 -.59 -.60 -.58 -.45 .08 .17 .40 -.34 Ce .81 -.33 .81 .58 -.24 .89 -.26 -.19 -.59 -.65 .69 .75 .66 -.08 Pr .71 .28 .45 .77 -.72 .67 -.71 -.73 -.45 -.26 -.12 -.05 .16 -.31 Nd .67 .33 .41 .75 -.74 .61 -.73 -.74 -.38 -.21 -.18 -.12 .05 -.22 Sm .30 .44 .09 .49 -.68 .24 -.63 -.63 -.02 .10 -.40 -.40 -.47 .24 Eu .18 .43 -.01 .37 -.60 .11 -.54 -.54 .09 .16 -.44 -.45 -.58 .39 Gd .50 .32 .30 .56 -.68 .43 -.64 -.61 -.13 -.10 -.19 -.17 -.26 .23 Tb .46 .36 .20 .57 -.69 .39 -.65 -.64 -.15 -.06 -.29 -.28 -.31 .19 Dy .52 .38 .29 .63 -.73 .46 -.69 -.68 -.17 -.08 -.24 -.22 -.24 .14 Ho .37 .31 .21 .47 -.61 .30 -.53 -.50 -.04 -.02 -.26 -.26 -.43 .45 Er .27 .29 .12 .40 -.55 .23 -.47 -.45 .02 .05 -.26 -.28 -.50 .53 Tm -.00 .22 -.09 .12 -.34 -.03 -.26 -.22 .20 .18 -.30 -.34 -.67 .73 Yb .03 .27 -.05 .17 -.39 .00 -.31 -.27 .21 .19 -.29 -.33 -.66 .73 Lu -.12 .21 -.16 .02 -.25 -.14 -.16 -.13 .29 .24 -.29 -.35 -.72 .81

------

n = 17, values above 0.61 are significant at 99% level. 103

Table 4.10: Important elemental ratios in nodules and crusts used for genetic interpretations.

S.No. Mn/Fe Ce/La Ce/Nd La/Sm Yb/Sm

GAR-EN 0.6 4.32 4.98 5.24 0.45 649-H 1.1 3.54 4.28 4.98 0.34 P-SET 1.8 8.17 9.41 5.10 0.49 MAR-1 0.9 6.17 7.20 5.59 0.47 MAR-3 0.7 7.01 8.41 5.51 0.45 MAR-22 0.8 6.67 8.63 6.05 0.46 198-0 3.3 2.77 2.72 4.16 0.40 142-0 1.6 3.43 3.76 4.91 0.38 219-0 3.2 3.70 3.35 3.56 0.35 320-A 3.0 ' 2.69 1.85 1.62 0.74 284-A 3.4 2.81 2.76 4.10 0.36 75-C 0.6 6.01 6,95 5.40 0.45 134/10 0.9 5.41 6.14 5.21 0.46 0-1TOP 1.6 3.95 4.05 4.39 0.39 0-1BOT 1.5 4.14 4.27 4.53 0.42 0-2TOP 1.3 3.87 4.06 4.76 0.40 0-2BOT 1.9 3.75 3.96 4.42 0.44

Sample details are as given in Table 3.

104

Table 4.11: Details of the crusts and nodules.

S. No. Water depth Mineralogy and Surface Texture Fe P Mn/Fe Ce & Location anom

GAR-EN* 5050-5100 6-MnO, - D 14.53 0.19 0.60 0.34 CIB 649H* 5000 CIB 6-MnO2 - D, TD - Tr; R 17.17 0.22 1.13 0.26 PSET* 4881-5173 6-MnO2 - D; R 10.14 0.19 1.87 0.62 CIB MAR-1 4300 MB 6-MnO2 - D; Q,F; S 17.25 0.25 0.86 0.49 MAR-3 4300 MB 6-MnO2 - D; TD,Q; S 18.23 0.28 0.74 0.55 MAR-22 4130 MB 6-MnO2 only; S 20.23 0.30 0.83 0.54 198-0 5031 CIB 6-Mn02 > TD; R 8.48 0.12 3.30 0.12 142-0 3580 CIB 6-MnO, > TD; S 13.82 0.20 1.55 0.23 219-0 5350 CIB TO > b-MnO2; R 8.99 0.14 3.17 0.23 320A 5269 CIB TD = to-Mn02; R 8.11 0.09 3.03 0.05 284A 5204 CIB TD > 6-Mn02; R 7.51 0.09 3.36 0.13 75C 4904 CIB 6-MnO2 minor; S 10.94 0.07 0.59 0.48 134/10 S O-SB 6-MnO2 only; .S 19.04 0.25 0.89 0.43 0-1 TOP 5270 CIB S 14.72 0.21 1.58 0.28 0-1 BOTTOM -do- S 14.51 0.18 1.69 0.30 O-2TOP -do- 6-MnO, > TD; S 20.19 0.23 2.03 0.28 0-2BOTTOM -do- TD = b-Mn02; S 15.98 0.18 2.09 0.26 SS2-107D 5200 CIB TD slightly > 6-MnO ; Q; R 6.63 0.12 3.95 0.17 FA1-16F 5191 GIB TD > 6-MnO ; Q,P; pavement 5.48 0.10 4.29 0.19 SS2-90B 4965 CIB 6-Mn02, Q,P, ; S 9.46 0.18 2.03 0.38 S K23-259A 5212 GIB 6-MnO2 = TD; Q; R 7.25 0.11 3.63 0.19 SS2-92A 4840 CIB 6-MnO, minor; P,Q; S 11.76 0.20 1.15 0.49 SS5-367B CIB TO > b-Mn02; Q; R 5.84 0.10 4.89 0.15 SK23-252A 5287 CIB 6-MnO2 = TD; S 10.09 0.16 2.50 0.26 SS2-122C 3580 CIB P,Q; S 11.02 0.14 0.88 0.32 SS1-35 5100 GIB TD = 6-Mn02;Q; R 9.07 0.15 2.88 0.31 SS1-20 CIB TD, to-MnO, minor; Q; S 14.16 0.21 1.65 0.31 Averages for 6-MnO2 dominating nodules 1.35 0.40 Averages for todorokite dominating nodules 3.93 0.17 Averages for nodules having equal intensities of 6-MnO2 and todorokite 2.38 0.24 TOTAL AVERAGE 2.12 0.33

* Encrustations ; depth in metres CIB - Central Indian Basin; MB - Mascarene Basin; SO-SB-Somali-Seychelles Basin D - Dominant ; Tr - Traces ; TD - Todorokite ; Q - Quartz ; P - Phillipsite ; F - Feldspar ; R - Rough ; S - Smooth ; Mineralogy, surface texture details of samples 198-0, 142-0 and 219-0 are from Mukhopadhyay (1988) 105

Table 4.12: Major elements and REE -concentrations in the seperated oxide and nucleating material.

S.No. Fe Mn Si Al Ca P La Ce Nd Ce anom SS5-360B oxide 8.60 27.81 6.57 2.15 1.66 0.14 114 413 137 0.21 nucleus 8.49 18.57 12.24 4.33 1.44 0.13 102 384 122 0.23 bulk* 8.57 25.38 8.06 2.72 1.60 0.14 111 405 133 0.22 MAR-8 oxide 18.95 17.74 7.25 2.30 2.14 0.33 212 1816 204 0.62 nucleus 6.11 2.94 22.55 8.58 0.99 0.12 69 314 77 0.33 bulk* 16.91 15.39 9.68 3.30 1.96 0.30 189 1578 184 0.61

FA2/73D oxide 6.70 29.22 6.32 2.08 1.62 0.12 96 319 116 0.17 nucleus 6.23 25.09 9.69 2.15 0.07 0.09 90 223 111 0.04 bulk* 6.50 27.49 7.73 2.11 0.97 0.11 94 279 114 0.12 FA5/203B Layer 1 9.09 27.11 6.86 2.09 1.76 0.16 113 447 141 0.24 Layer 2 10.80 23.05 7.59 2.92 0.16 0.21 86 319 108 0.22

Layer 3 -- 69 271 84 0.25

bulk* -- 97 377 122 0.23 MAR-16 oxide 17.46 15.93 7.90 2.91 2.42 0.33 191 1613 185 0.61

nucleus -- -- 203 228 250 -0.30

bulk* -- 192 1545 188 0.59

Major element data in %. REE data in ppm. * bulk concentrations are calculated with the weight proportions of oxide and nucleating material given in Table. 4.2. 106

Table 4.13: REE concentrations in bulk nodules and their corresponding acid leachates (in ppm).

S.No. La Ce Nd

FA1-16F-Bulk 75 262 93 leach 119 383 142 residue 14.8 29.9 7.98 % on leach 95 96.7 97.5

SK23-252A-Bulk 126 501 148 leach 180 703 212 SK23-259A-Bulk 84 297 106 leach 124 438 156 SS-1-20C-Bulk 150 681 175 leach 294 1359 323 SS-1-35-Bulk 127 557 143 leach 184 671 217

SS-2-90B-Bulk 102 538 117 leach 180 936 203 residue 7.8 20.8 3.8* % on leach 98.4 99.2 99.3 SS2-92A-Bulk 129 827 126 leach 196 1280 194 residue 6.97 10.8 1.9* % on leach — 99.1 99.8 99.74 SS2-107D-Bulk 94 306 109 leach 126 403 151

SS2-122C-Bulk 138 618 153 leach 174 719 201 residue 6.15 13.3 3.42 % on leach 96.9 98.4 98.5

* Values based on semi-quantitative analysis CHAPTER 5

RARE EARTH ELEMENTS IN HYDROTHERMAL FERROMANGANESE CRUSTS 107

RARE EARTH ELEMENTS IN HYDROTHERMAL FERROMANGANESE CRUSTS

5.1. INTRODUCTION

Ferro-manganese oxide crusts are precipitates that coat the available strata. For instance, they coat the strata near hydrothermal submarine volcanic edifices such as seamounts, guyots and ridges (eg: Futa et al, 1988; Ingram et al, 1990). Based on the mineralogical and geochemical studies and the tectonic setting of the sample locations, three types of crusts are commonly described. They are 1) hydrogenetic crusts, 2) hydrothermal strata-bound deposits and 3) a mixture of hydrothermal and hydrogenetic Fe-Mn oxides (Hein et al, 1988).

Manganese nodules grow by both diagenetic and hydrogenetic processes and accretion occurs both on the upper and lower surfaces. In contrast, Fe-Mn crusts grow by hydrogenetic or hydrothermal processes only on their upper surfaces at the rock- seawater interface (eg: Varentsov et al, 1991). Hydrogenetic crusts form mostly on seamounts and other ridges, plateaus, abyssal hills and plains, away from active ocean spreading centers and other sources of hydrothermal activity. These crusts form by slow precipitation of inorganic colloidal particles of hydrated metal oxide from near-bottom seawater

(Reyss et al, 1985; Halbach, 1986). The rate of growth or accumulation is as slow as 1 - 10 mm/Ma for these crusts. Rare earth elements (REE), alongwith other elements, such as Co and Pt, are highly concentrated in these crusts compared to hydrothermal crusts and manganese nodules (Aplin, 1984; Hein et al, 1988). They are characterized by high positive cerium anomalies and light 108

REE enrichment (eg: Nath et al, 1992b). REE concentrations vary with the water depth, latitude as well as within the layers of the Fe-Mn crusts (Aplin, 1984; Hein et al, 1988; De Carlo and McMurtry, 1992). The principal minerals present in these crusts are vernadite ( 45-MnO) and x-ray amorphous iron oxyhydroxides with minor amounts of goethite, quartz, zeolites and feldspars

(eg: Cronan, 1980).

Hydrothermal ferromanganese crusts and stratabound deposits accumulate at active spreading centers and in volcanic arcs. They form either within volcaniclastic sedimentary sections or within hydrothermal mud in island arcs, and are commonly associated with pillow basalts and hydrothermal mud at spreading centers (Ingram et al, 1990 and references therein). Hydrothermal ferromanganese crusts are also precipitated from seawater which has circulated through the igneous lithosphere and reacted with it at elevated temperatures (Fleet, 1983). They grow at much faster rate (upto 10,000's mm / Ma) than the hydrogenetic crusts due to the high flux of Mn in hydrothermal fluids (eg: Hein et al, 1990). Hydrothermal ferromanganese crusts are characterised by lower REE abundances and negative Ce anomalies (Elderfield and Greaves, 1982; Fleet, 1983). Well-crystallized todorokite, birnessite and vernadite (45-Mn0 2) are the predominant manganese minerals. They possess lower concentrations of transitional metals like Cu, Ni, Co and have higher contents of Mo, Ba, Zn, As,

Sb and Hg when compared to the hydrogenous crusts and manganese nodules (Toth, 1980).

The third type consists of crusts which are transitional between hydrothermal and hydrogenetic crusts. They may have mixed mineralogical and geochemical characters. These crusts form at tens of meters to tens of 109 kilometers from active hydrothermal sites. The chemistry may vary with increasing distance from the hydrothermal source (Ingram et al, 1990).

The study of marine hydrothermal deposits assumes importance in the context of REE mass balance calculations. Piper (1974) made mass balance computations for the REE by comparing input rates with mass accumulation rates in various Pacific pelagic sedimentary phases. His calculations have shown an imbalance between the REE input and removal rates in the oceans.

He predicted that the REE removal rates were 11-18 times greater than the river input rates or 11-14 times greater than river-plus- basalt inputs. So, Piper

(1974) has given three possible explanations for this imbalance. Firstly, since only one measurement of dissolved REE concentrations in river available at that time (for Gironde river in France by Hogdahl et al, 1968), the estimates of

REE river input may have been too low. Secondly, the REE may be enriched in surficial sediments due to remobilization during diasenesis. Thirdly, significant amounts of REE might be entering the oceanic system from submarine volcanic activity (Owen and Olivarez, 1988). The direct evidence for the third explanation came from the initial discovery of active hydrothermal venting in 1977 (Corliss et al, 1979). Subsequently, seafloor hydrothermal activity has been recognized as a significant mechanism in controlling the marine geochemical processes (Rona et al, 1983). High temperature venting has now been documented in association with a variety of tectonic settings such as slow to fast spreading ridges, seamounts, back arc basins in both sediment-rich and sediment-starved areas (McDuff, 1987). As far as REE is concerned, Michard and Albarede (1986) and Michard (1989) have reported highly enriched REE concentrations in hydrothermal vent fluids. The REE concentrations are enriched between 14 and 4200 times relative to their 110 concentrations in seawater at similar water depths. German et al (1990) reported REE removal from seawater by freshly formed hydrothermal precipitates at the TAG hydrothermal vent, Mid-Atlantic ridge. These direct observations of seawater are supported by data from EPR metalliferous sediments which indicate that the ridge- proximal seawater is experiencing a net depletion of REEs due to enhanced scavenging by iron oxyhydroxides introduced at the ridge (Barrett and Jarvis, 1988; Owen and Olivarez, 1988;

Olivarez and Owen, 1989). Since the Mn-Fe oxide encrustations are the sinks for the REE removed from the seawater at the vents, the study of hydrothermal iron-manganese encrustations assumes importance in the sense, it may give an idea about the net depletion of REE from seawater through time.

For the formatiOn of the third type of ferromanganese crust described in the previous paragraph, which is a mixture of hydrothermal and hydrogenous material, both the processes may contribute. It is expected that a continuum between these two "end-members" may exist (Cronan, 1980). Fleet (1983) attempted to study this continuum using REE evidence. Since the publication of Fleet's paper, a number of new discoveries of hydrothermal sites are made.

I have analyzed some more hydrogenous and hydrothermal ferromanganese crusts from new areas which Fleet (1983) has not considered in his review.

Todate, hydrothermal crusts have been reported and REE have been analyzed from all the major oceanic ridges except that from the Indian Ocean.

Only recently, the discovery of the signatures of hydrothermal activity in the Indian Ocean has been reported (Herzig, PlOger and cruise participants, 1988;

PlOger et al, 1990; Jean-Baptiste et al, 1992). Although tectonic and geochemical features (in the water column), which are characteristic of 111 hydrothermal activity, are reported in the literature, actual hydrothermal mineralization is not yet found in the Indian Ocean (excluding the Red Sea). I report here for the first time, the REE evidence for hydrothermal nature of the ferromanganese crusts recovered from the Indian Ocean triple junction

(Rodriguez triple junction). A comparison is also made with the purely hydrogenous ferromanganese crusts recovered from Central Indian Basin and

as well as those from Central Pacific.

A considerable debate is currently in vogue about the mineral / elemental phases hosting the REE in marine ferromanganese deposits. Elderfield et al

(1981a), for example, have found that most of the REE is in association with iron oxide and phosphate phases. Glasby et al (1987) have proposed that REE in manganese nodules are hosted by Fe-oxide phases. A good correlation

between REE and either Mn or Fe is taken to reflect the dominant role of sorption by Mn or Fe oxyhydroxides, respectively (Craig, 1974; Piper, 1974; Elderfield et al, 1981a, b; Li, 1981; Whitfield and Turner, 1982, 1984; Aplin, 1984; Ruhlin and Owen, 1986; Glasby et al, 1987; Calvert et al, 1987;• Koeppenkastrop and De Carlo, 1988, 1990, 1992; Nath et al, 1992b). Koeppenkastrop and De Carlo (1992) have found that while trivalent REE are

associated with Fe-oxides, Ce was found to be associated with Mn.

Hydrothermal ferromanganese crusts consist of almost pure Mn-oxides (with -40 % MnO and 1-2% Fe2O3 in some cases) or pure Fe-oxides (MnO < 1%). Thus, a study of these hydrothermal ferromanganese crusts gives an

opportunity to study the REE association with each of this pure oxide phase.

The rare earth element behaviour in hydrogenous crusts is fairly well

known with their positive Ce anomalies. It would be interesting to study and 112 compare those with hydrothermal iron and manganese crusts which may explain:

(1) end-member REE compositions of pure manganese oxides, pure iron oxides of hydrothermal origin and purely hydrogenetic ferromanganese

crusts,

(2) hypothetical gradations that may exist between these two end members,

(3) REE scavenging capacity of precipitated oxides (i.e., crusts) at different

redox conditions (since Ce is particularly redox sensitive) and

(4) Location of REE in various phases (broadly speaking a) adsorbed and b) substituted in mineral lattice - if located in (b), which mineral phase).

Significant differences in the magnitude of Ce anomalies with the varying oceanographic conditions are found earlier (eg: Nath et al, 1992b), such as high Ce anomalies in AABW influenced deposits. As a continuation, it would be interesting to examine the Ce anomalies in ferromanganese crusts of hydrothermal origin which may have formed definitely at higher temperatures than the hydrogenetic nodules.

To our knowledge, not many publications exist which compare the REE geochemistry of hydrogenetic and hydrothermal ferromanganese deposits of the ocean floor (Fleet, 1983; Ingram et al, 1990).

5.2. DESCRIPTION OF THE STUDY AREAS AND SAMPLES

Thirty ferromanganese crusts were selected from a variety of geologic, tectonic and geochemical (such as hydrothermal, hydrogenetic and mixed

(hydrogenetic-hydrothermal) environments. Sampled locations include mid plate seamounts and ridges (Southeastern seamount, East Pacific Rise, Galapagos 113

Ridge) and a back-arc spreading ridge (Lau Basin, Valu-Fa ridge), that are sites of hydrothermal precipitation of Fe-Mn oxides, deep-sea abyssal plains (and seamounts therein), that are sites of hydrogenous precipitation (Central Pacific and Central Indian Basin) and Indian Ocean triple junction (Rodriguez triple junction) to represent the site of mixed hydrothermal - hydrogenetic deposition. The type of tectonic setting of each of these samples is described below.

5.21. Hydrothermal Crusts

5.211. Southeastern seamount, East Pacific Rise (EPR)

Since 1981, numerous detailed studies have been conducted on the hydrothermal area from 12 °40'N to 12°52'N on the East Pacific Rise on a segment where the spreading rate is 12 cm / yr (Fouquet et al, 1988).

The axial zone of EPR consists of an axial graben between 200 and

600m wide and 20 - 50m deep (Fig. 5.1). The average water depth at the graben floor is 2630m (Hekinian et al, 1983). The western side of the ridge consists of a series of small horsts and grabens parallel to the ridge, while on the eastern flank, earlier tectonic features are obliterated by two seamounts

(Hekinian et al, 1983). Both the mounts are about 6km away from the axis. First one is Vauban seamount and the second one is Southeastern seamount which is connected to the ridge by a marginal high. The marginal one is an young seamount whose growth has continued since it is proximal to the axis (Fouquet et al, 1988). Gossan-type sample of Fe-rich oxides, consisting of goethite and limonite, and Mn oxides, containing nontronic material coated with a thick black Mn oxide layer of todorokite in the core (hydrothermal in nature) 10 00 103 56

12 0 54

MM.

FIG.2 12 ° Site A 50 Pogonord ( 100°c) Actinoir Pogosud 11 10y)7fiaiN Pang° 84 38 01 (II)

.01 882 201 1 000 :3 (((I 11;2) Pogoroux 82 82 21 07 (IVaY 04 (IVa) 82 01 01 (lb) , Brice Canyon ( 350°c) 02 (Ib) Site 04 (284C) 82 21 02 (Ib) —

12 0 03 (111b)I 05 (la) 46

82 27 04 (II) 82 09 01 (Vb) DR 03 (Va) 82 27 03 (II) 82 15 01 (Vb) 82 14 4 (Va) (2100y) 82 14 6 (XI)

24 04 (Villa 82 09 02 (II) 06 (Va) (vg" ...74.---\(20 0°0° ) (1900y) 12 44 S E M ARGINAL EAMOU T HIGH

0 2 Km

Fig. 5.1. Bathymetric map at 12°15'N on the East Pacific Rise showing the location of the studied samples. This map was prepared during the Clipperton cruise (1981) using a multibeam echo sounder (SEA BEAM) (from Fouquet et al., 1988). 114 and b-MnO, in the outer zone in contact with seawater (hydrogenous precipitate), are recovered from the Southeastern seamount (Fouquet et al, 1988).

Mineralogical and geochemical data are presented in Tables 5.1 and 5.2.

5.212. Valu Fa Ridge, Lau Basin

Volcanic arc mineralization is the least studied of the major types of metal enrichments found in the ocean basins (Hein et al, 1990). The Tonga

Ridge-Lau Basin has been the most extensively studied (Hein et al, 1990)

among the arc systems such as Mariana Islands arc, Bonin arc, Sanghihe arc, Fiji Basin, Solomon- islands - New Caledonia region, the Tyrrhenian Sea and the Lesser Antilles. Rona (1984) has reviewed the results of studies on

hydrothermal deposits. Among the 63 locations reported till then at various seafloor spreading centers, only four of these deposits were located in back-arc spreading centers.

The Tonga Ridge consists of the Tonga Ridge, which is a fore- arc terrace, the Tofua Ridge which is the active arc axis, the back-arc Lau Basin including the Valu Fa Ridge spreading axis, and the Lau Ridge, a remnant arc (Fig. 5.2). The samples studied in this work were collected during the 35th and

48th cruises of R.V. Sonne held during the years 1984-85 and 1987. Sample

80KD representing a pure Mn-oxide was collected during Sonne-35th cruise

from a seamount S3, which is 6km east of the Central Valu Fa Ridge (CVFR). The black material of the crusts was silver gray in colour when freshly fractured

(von Stackelberg et al, 1990). The black material is composed of Mn-oxide

which is either massive or compact. The growth cusps reveal very fine

laminations (von Stackelberg et al, 1990). Birnessite is the dominant mineral

with frequent association of todorokite and 8-Mn0 2. It consists of 64% of MnO Fig. 5.2. Bathymetric map of the Lau Basin and the vicinity (from von Stackelberg and von Rad, 1990). Solid triangles = DSDP sites and ODP sites. Black star - massive sulfide deposit. NLSR = Northern Lau spreading ridge. VFR = Valu Fa ridge. 115 with traces of minor metals such as Co, Cu and Ni. Such mineralogical and geochemical properties reveal a hydrothermal nature. Samples 93KD, 110KD and 116KD, recovered during the 48th cruise of R.V. Sonne, are ochre coloured and represent pure Fe- rich oxide precipitates. All are X-ray amorphous in nature with only 110KD having goethite contents (Herzig et al,

1990). Fe2O3 range between 35 to 58% with almost negligible quantities of MnO (ranging between 0.12 and 1.71% - Herzig et al, 1990). Mineralogical and geochemical data are presented in Tables 5.1 and 5.2.

5.213. Galapagos Spreading Center

Since 1972, the Galapagos Rift near 86°W has been a major target for the exploration of hydrothermal activity. Two cruises in connection with the project GARIMAS ("Galapagos Rift Massive Sulfides") were carried out in 1984 and 1985 onboard the German research vessel R.V. Sonne (Herzig et al, 1988). During these operations, the Galapagos rift between the Inca Fracture zone and the De Steiguer Deep was mapped at various scale and several new hydrothermal sites were found. The Fe-Mn crusts used in the present study were collected during these cruises from the 'Cauliflower Garden' (falling in the area 0°46.05' - 0°46.15'N; 85°53.39' - 85°53.55'W - Fig. 5.3) hydrothermal field. It occupies an area of about 250 m x 250 m of the rift valley floor, extending from the axis to the base of the northern talus slope. The bed rock is mainly formed by scrambled sheet lava, which is more or less covered by sediments and hydrothermal products (Herzig et al, 1988).

Six ferromanganese crusts were used in this study and are composed of pure Mn-oxides (#K 10, #K 14) with about 82% MnO and rich in birnessite and todorokite. Two samples are iron rich (45 - 55% Fe 2O3) and are composed of ma

W 85°54' W 85°53' - ,,Noi Nippr■ lli4■1111•1 ■11110 giiiiiiiiiummiiiiiiiiii

10, 11,114,1111...11 ' 1"mom-En-a-.%%,1 % ...„01.1

.. MI A 4: MS . v. • Siii 1 *At ■ N 0°46' A 1016 . OMR MaPilli 481

Filr • :IU 1114111111361111."H4411 4iill K 111. A M.4.11.1°P' i r . - -..- 61‘1 1 111111111111all- 1101.111111111411l l % . • • \ \. , .. 1. e V11.71gra iii".1.1.1 4t; 1611- 104 1 ' an MEN wII .44 ' 4r,°I UST TILE. m II I cornsos o. 411•RVIS ) a C:. • ... ' SIALPfl 118:•:61t'g 111 AM: •: -: -MS ..„3. MiRSIMISMNI IM • , 14.188118111-: -MS i is •-: RUM ______..) IIIIMMiiiiin t • - • •••11 --..------,---- IMMO- • • T.P!ttv :. mamma ------, ? iso 2,0 no 90 no° RIME MU. .A!A "I`-• GUM Fig. 5.3. Location map of 'Cauliflower garden' hydrothermal field (from Herzig et al, 1988). Seabeam morphology, contour intervals 10 m. Cross-hatched area delineates the hydrothermal field of location C (Cauliflower garden). Full triangles indicate other hydrothermal products outside this field. At locations marked by full dots faunal elements related to hydrothermal vents were observed. The vertically hatched areas are covered I by ponded lavas. 116 amorphous iron oxides, birnessite, montmorillonite / illite (Becker, 1991 - personal communication).

5.214. Indian Ocean Triple Junction

Since the initial discovery of ore-forming hydrothermal processes on the modern sea-floor (eg: Corliss et al, 1979), exploration has been concentrated mainly on the divergent plate boundaries in the Pacific Ocean. This has resulted in the discovery of numerous hydrothermal sites along the East Pacific Rise and Galapagos spreading centres mentioned earlier (in EPR section). As it was thought that the slow and medium spreading ridges represent less favourable targets for exploration, Atlantic and Indian Ocean ridges were poorly investigated (Pluger et al, 1990). After the discovery of hydrothermal activity (in the form of massive sulfides, black smokers and vent biota) at the slow-

spreading TAG area on the Mid-Atlantic Ridge in 1985 (Rona et al, 1986), a series of cruises have been undertaken for studying the hydrothermal activity in the Indian Ocean triple junction (Herzig, Pluger and ship-board scientific parties,

1988; Pluger and Herzig, 1986; Rao and Pattan, 1989; Pluger et al, 1990;

Chaubey et al, 1990). These efforts have revealed indications of hydrothermal mineralization.

The Rodriguez Triple Junction (RTJ) or the Indian Ocean triple junction,

located at about 25°30'S and 70 °E (Fig. 5.4), represents the convergence point of the Indian, African and Antarctic plates. The three associated ridges are

the Central Indian Ridge (CIR), the Southeast Indian Ridge (SEIR) and the Southwest Indian Ridge (SWIR) (Herzig, Pluger and shipboard scientific parties,

1988). Fig. 5.4. Simplified map of the Indian Ocean ridge system indicating the study area north of the Rodriguez triple junction (RTJ). CR = Carlsberg ridge, CIR = Central Indian Ridge, SWIR = Southwest Indian Ridge, SEIR = Southeast Indian Ridge (from Plueger et al, 1990). 117

The ferromanganese crusts used in this study were collected during the

German expedition GEMINO-3 onboard R.V. Sonne cruise SO-52 (Pluger,

1990). The samples were collected from area LX (Fig. 5.4) from the rift valley. The samples are spongy, porous, 2cm thick Fe-Mn oxide encrustations over basaltic pillow lavas. The samples are rich in 6-Mn0 2.

5.22. HYDROGENOUS CRUSTS

5.221. Central Pacific

Among the number of ferromanganese crusts collected, the sample with the thickest manganese crust ever recovered' has been used in this study. The sample was collected from the Central Pacific Ocean during the cruise R.V.Valdivia - 13/2 in 1976 (von Stackelberg, 1976; Friedrich et al, 1976). It is recovered by dredging near the summit of a submarine peak at a depth of 4830m. The external and internal features, elemental distribution in the growth zones of this encrustation are studied by Friedrich and Schmitz-Wiechowski

(1980). The crust is later studied for paleoenvironmental reconstruction by Segl et al (1984).

The encrustation has three visibly separable zones. The top and middle zones consist of compact layers of hard, black ferromanganese material. The bottom zone contains silicate minerals between the ferromanganese layers

(Friedrich and Schmitz-Wiechowski, 1980). Manganese to iron ratios varied from about 0.5, in the bottom layers, to about 1.5, towards the top with a significant break in between.

118

5.222. Central Indian Basin

In contrast to the detailed studies carried out on the Pacific Fe-Mn crusts (Fein and Morgenstein, 1973; Glasby and Andrews, 1977; Toth, 1980; Craig et al, 1982; Halbach et al, 1983; Halbach and Puteanus, 1984; Aplin, 1984; Segl et al, 1984; Hein et al, 1985; De Carlo et al, 1987; and many others), Indian Ocean crusts are least studied (Cronan and Moorby, 1981; Rao and Pattan,

1989; lyer, 1991). This is because of two reasons. The crusts are not as economical as the Pacific crusts. The Pacific crusts are enriched in cobalt and

platinum. Secondly, not many seamounts and abyssal hills are reported until

the discovery of the former in the Indian Ocean region (Mukhopadhyay and Khadge, 1990). As far as the REE studies on the Indian Ocean crusts are

concerned, Nath et al (1992b) have studied three manganese crusts from the

Central Indian Basin. They found significant positive Ce anomalies in these crusts. This study considered nine crusts recovered from various locations in the Central Indian Basin.

5.3. ANALYTICAL METHODS

Rare earth element concentrations are measured at University of Liege, Belgium following the methods described in chapters 3 and 4 using Inductively

coupled plasma - atomic emission spectroscopy (ICP - AES). The accuracy and precision are described in detail in Chapter 4 and Table 4.3.

5.4. RESULTS AND DISCUSSION

5.41. REE contents

The concentrations of major, trace and rare-earth elements of the thirty 119 crusts studied here are presented in Table 5.2. Among the hydrothermal crusts studied, the samples from the East Pacific Rise and Galapagos Ridge show lower REE concentrations compared to the Lau Basin crusts. The TREE (total REE) concentrations range from 1.04 to 2.42 ppm in the hydrothermal portion of the EPR crust; from 1.29 to 8.99 ppm in case of GARIMAS samples and range between 5.8 and 30.6 ppm in case of Lau Basin samples. Among the REE, La is detected in all the samples. Many of the elements (between Ce and Dy) were below their detection limits in the purely hydrothermal crusts from

East Pacific Rise and Galapagos regions (Table 5.2). Excepting one sample from Galapagos (# K14 < 0.06ppm), Lu could be detected in all the samples.

The outer b-Mn02 bearing hydrogenous portion of the East Pacific Rise crust show almost 80-fold enrichment (TREE - 168 ppm) compared to the inner birnessite/todorokite rich hydrothermal portions (TREE - maximum 2.42 ppm ).

The hydrogenous crusts from Central Indian Basin and Central Pacific show very high REE contents. TREE in CP-1 (Central Pacific) crust are close to 2000 ppm of TREE. The TREE contents in nine Central Indian crusts range between 1188 and 2757 ppm with an average of 2220 ppm. Therefore, the hydrogenous crusts are about 800 - 1000 times enriched in REE than the hydrothermal crusts (Table 5.2).

The mixed crusts from Rodriguez triple junction (GEMINO) show slightly

lower trivalent REE concentrations compared to the hydrogenetic crusts of

Central Indian Basin. But because of very low Ce concentrations, the TREE

concentrations in RTJ crusts are about one third lower than the CIB crusts

(Average of 648 ppm of TREE in RTJ samples compared to 2200 ppm of CIB

crusts). 120

5.42. REE patterns

The shale-normalized REE patterns of all the samples are shown in Figs.

5.5 and 5.6. Striking differences are noticed between the hydrogenetic, hyudrothermal and mixed-hydrogenetic- hydrothermal portions of the crusts.

The patterns of hydrogenetic crusts resemble the deep-sea manganese nodules and crusts described in chapter 4 and other publications (Piper, 1974; Elderfield et al, 1981a; Glasby et al, 1987; Aplin, 1984; De Carlo and McMurtry, 1992). The patterns display a characteristic convex patterns (excluding Ce) with a clear enrichment of intermediate or middle REE relative to the light and heavy ones. Piper (1974) observed that the deep-water nodules (>3000 m) invariably are depleted in heavy REE relative to the intermediate ones and that shallow water deposits (<3000 m) showed nearly the opposite trend. Therefore, the shale-normalized patterns of the hydrogenous crusts studied here, fall closer to Piper's deep-water group with convex patterns (Fig. 5.6). The positive Ce anomalies and heavy REE enrichment as described in Chapter 4 are clearly opposite to their trends in the seawater. The process of oxidation is responsible for the positive Ce anomalies in crusts. Patterns showing HREE depletion result from the HREE forming more stable complexes in seawater than light REE and are consequently more difficult to fix in the crust mineral phases than the REE.

The shale-normalized distribution patterns of the hydrothermal crusts are broadly similar to seawater, being depleted in Ce and enriched in heavy REE

(Fig. 5.5) but differ in absolute abundances. Ce depletions are mostly greater in Mn - rich crusts, while no pronounced differences in heavy REE enrichment

/ light REE depletion are noticed between the Fe and Mn oxide crusts. Most of

SHALE - NORMALIZED REE PATTERNS OF HYDROTHERMAL FERROMANGANESE CRUSTS

1., ,11,1_. 47' .' 411'1.-'.g.,- 'Ili' . ,.. 1.111' 1.1.1.1•''• - Garimas-K2 :t, , 1,,, , . 4 , 1 elill '''‘.. ■11 ,:.1.•; • II 1 . , ,'7:.• 74''.. 1:.. ‘', --.1,' ,•': .,1; ,.111..41, ;'•• -: • ill:, 111'.. ::.',.. .,.,.1; „ , : , 1-.77—..:, • 11, •I .i"!!.., illf1, 'i•11, ,11,1 '.',; i ' • ; , .' .' ...., , ,..i. '? : ::.',1'. 11, "41:I',' 1,-: 1 L.: 11..1,,i —_1„..ir, ii i',, : .-.::.•,,i, .re.• .• 1i; 1 -.11. ;', .1:, :.,. ,,.- 1 •':iii.;111;.1, ,1,, ,• 4 4;, 111,, 4.' ''... 1.110.'11 ..•,i ''.• ' 1 ii• ••,1 • -• ' ' I - '' Garimas-K3 • ' !• ' • c' ; ,1 ; • .;! :I • i•111:1; : •• 1 '1 tV' ll..' 41.. .•^' '... . ii.ir 1.1: -'' 11' ; i . i ■ ''' 41.: 11!4' ■■ 11:: ':;■': 1:. , ;. i :. ': !,... L Iil!. .';', :1: 'Ili': .ii' Garimas-K8 Ir.: 1.1''' 'ii■ .' rl. it I ■■1 1 :- '' J; i4;. • . I; . •:ti - GarimasK1 0 EPR-A - EPR-B ,-.ii. • " "' ..;,•-"i..1 ,. ■ • 1. i. EPR-C r'•, . . ir-11,,.,i.. iiiit1:.i1...:j li:1;1. I. -!;1:;•-i, LI , .!..1i,T.ii,‘ . :',•.! -Fr- LB-80-KD CZIE .1.-. •• ••.• ■• 4I: a , . 1!; .!1:-i,,-...!ii:• ill ' ,4!:,...11,1,': !..4.: .1 ; ';', , •11 — ii•••- j :.":1:1! .. 1:1;11.+1: 11. '1,1,1::.'•'... i : ti :: ::i : . ,•,1;i•1 1.;;" : ,•'' - ii,.. I/ ••• •','' I!' - 1!•'• li1: C.' ,! Ii'.ll 11: ,,,i, ii, , .'1 1 : ! „' ,•, •;' • LB-93-KD ;•,.. • : ' • • •.' 1 ' ' :•i. ! • • ,-7 1!1, .,': 1.1 r. ''' -':-.!,- .4 ir' ri' ••• iti ‘A: 'i, ,', • " • ::.- Ir.' • 1- !,0 --,".• "';'E-.. 1- 111", •i4 ., I. ■ ,, •, ..,'. il ,:1• ,. .,,, •-• " ' ,.,, •,,. '," , !•-, ,. , .,.. 11 i• 1". ill • ,: I'. -.1.. ;,—•,,1 .. !;: :'.., •,. .1 :: ,,..,,:.' 1... 1 , ■ i•- :.• •41. •-• .i •11; ",, 0, li ‘'l' 1• '" •. 1!" 1•:" " 'I,..! '.1 I, 1 ..; .11 I ', 1, iv, I LB-11 O-KD I. 11. l' • I 1 , 1 J.- .. .. - ,... I. . ,. i• . • . . , '1, 1 . '; ,• 1 !! , i. .. r '1,- - , ;: . ,:. ,..; ;'; •/,. ii. '' i ; '•••• 'I' •'' • . ,.. , • 1 ,• ., .. ., .- .i• 1 • .. ', PI •I'i 1, ;i11" '- ' •' ' • 0.01 LB-116-GA La Ce Nd SmEuGd Dy Yb Lu - Fig. 5.5. Shale-normalized REE patterns of hydrothermal ferromanganese crusts from Galapagos spreading center (Garimas K2 - K10), East Pacific Rise (EPR - A to C), Lau Basin (LB - 80 to 116 GA). All of them display a HREE enriched pattern similar to seawater. Ce is either not detected in some cases, or when I detected show a negative anomaly. Some of the samples (# Garimas - K2, K3, Lau Basin - 93 KD) show positive Eu anomalies which is a typical feature of hydrothermal plume solutions. REE in Indian Ocean crusts

-- Gemlno-K6

Gemino-K9

-.- Gemlno-K10

-7-- ,Gemino-K17 + CIB-1

-ay CIB-2

E CIB-3

-.CIB-4 ' - CIB-5 CIB-6 CIB-7

4P- CIB-8 1 La Ce Nd SmEu Gd Dy Yb Lu 0 — CIB-9

Fig. 5.6. Shale-normalized REE patterns of hydrogenous Central Indian Basin crusts (CIB - 1 to CIB - 9) and mixed hydrothermal-hydrogenous crusts from Rodriguez triple junction (GEMINO - K6 to K17). The patterns are same with convex - bowed up shape IREE enrichment except Ce anomalies. Ce anomalies in hydrogenetic crusts are typically positive. The hydrothermal ( or mixed ) crusts from ridge area display a mirror-image pattern as far as Ce is concerned. Ce anomalies are all characteristically negative which indicate a hydrothermal origin. 121 the samples show typical positive Eu anomalies (compared to it's immediate neighbours) which are characteristic of mid-ocean ridge hydrothermal fluids

(Michard et al, 1983; Piepgras and Wasserburg, 1985; Michard and Albarede, 1986; Michard, 1989; Derry and Jacobsen, 1990). Few samples (#K2 - Galapagos) show much larger Eu anomalies than those found in basalts which are caused by the preferential solution of Eu 2+ during alteration of basalt by reducing hydrothermal fluids (Derry and Jacobsen, 1990).

The mixed hydrogenetic-hydrothermal crusts from Rodriguez triple junction display similar REE patterns to the hydrothermal crusts as far as the negative Ce anomalies are concerned (Fig. 5.6). But, the convex patterns (excluding Ce) with respect to intermediate REE enrichment are an evidence of a transition towards an authigenic REE pattern (found in hydrogenetic crusts): a pattern characteristic of slowly growing hydrogenetic crusts with no hydrothermal component.

5.43. HREE enrichment

Most of the hydrothermal samples from Galapagos spreading center, East Pacific Rise and Lau Basin share several important features: some have positive Eu anomalies, all are HREE enriched. Shale normalized La / Lu ratios (Lan / Lu,) are mostly lower than 1. La / Lu ratios vary between 0.09 and 0.19

(barring 3.82 in #K8) for GARIMAS sample, range between 0.15 to 0.61 for EPR crusts and from 0.12 to 0.29 for Lau Basin samples. The HREE enrichment and the La„ / Lu n < 1 are features that are characteristic of seawater (except for Eu) and closely resemble modern deep-sea metalliferous sediments found on the ridge flanks (eg: Ruhlin and Owen, 1986). Results from modern environments show that the precipitation of Fe oxy-hydroxides 122 from seawater fractionates the REEs (Derry and Jacobsen, 1990). For example, Klinkhammer et al (1983) found systematic differences in the

behaviour of the LREEs and HREEs in samples of Pacific seawater collected in regions affected by hydrothermal discharge. Their samples are more LREE

depleted with increasing depth. They attributed this to preferential scavenging

of LREEs by Fe and Mn oxy-hydroxides in the basinal areas. Therefore, the HREE enrichment observed here is consistent with the precipitation of Fe and scavenging of REEs from a water mass with a REE pattern similar to modern

seawater at hydrothermal vents.

5.44. Cerium anomalies

The prominent feature of the REE patterns of the hydrothermal crusts is the occurrence of a negative cerium anomaly. Cerium anomaly is quantified using the formula proposed by Elderfield and Greaves (1981) and has been

described in previous chapters.

Ce anomalies calculated above are summarized in Table 5.2. Ce anomalies could not be calculated for few purely hydrothermal crusts since

either Ce or Nd was below the levels of detection. Although, the Ce anomaly cannot be calculated for these crusts (#K8, K10 and K14 of GARIMAS and A,

B, C of East Pacific Rise), cerium is depleted with respect to La. Therefore, all

the hydrothermal crusts show negative Ce anomalies compared to the hydrogenetic crusts which display a mirror-image Ce profiles. Negative Ce

anomalies have been reported in many hydrothermal crusts and only few diagenetic nodules. The Ce anomaly in hydrothermal crusts range between

-0.066 (#93KD of Lau Basin) and -1.08 (the largest negative Ce anomaly in

#EPR-D). The pure Mn-oxides in general, show the lowest Ce concentrations 123 and anomalies (#K10, K14 of GARIMAS; #80KD of Lau Basin). GEMINO samples from the Rodriguez Triple Junction show consistent negative Ce anomalies without significant variations (-0.277 to - 0.411). All of them are less than the average seawater anomaly of -0.74, except the EPR-D crust which show a close resemblance to the -1.2 value of hydrothermal water of Southeast

Pacific (Klinkhammer et al, 1983). The pronounced negative Ce anomaly of seawater has been attributed to oxidative scavenging of Ce(IV) onto particulate phases, especially ferromanganese nodules (eg: Piper, 1974a). Therefore, our data in addition to the data on metalliferous sediments from Pacific suggest that the hydrothermal precipitates (Ruhlin and Owen, 1986) may also be important in this regard.

5.45. Significance of Eu anomalies in hydrothermal crusts

Positive Eu anomalies compared to the immediate neighbouring elements Sm and Gd on a shale-normalization basis are observed in #GARIMAS - K2,

K3 (Fe-Mn oxides) and LB - 93KD (Fe oxide) (Fig. 5.5). The largest positive

Eu anomaly is observed in GARIMAS - K2 crust. In addition, even in other hydrothermal crusts too, Eu abundance is much higher than Sm and Gd which are below the defeCtion limits. The positive Eu anomalies observed in the hydrothermal crusts can result from the input of Eu-enriched hydrothermal fluids into the water column. Modern seawater does not typically display an Eu anomaly except near hydrothermal vents (eg., Elderfield, 1988). The presence of anomalous Eu concentrations in Galapagos sample indicates that the hydrothermal REE flux comprised a much larger fraction of the total REE in this area than other areas. The absence of Eu anomalies in other crusts and seawater imply a preferential removal of Eu from the hydrothermal plume at or 124

near the vent sites. This results in a much lower effective flux of Eu to the oceans (Derry and Jacobsen, 1990). Europium exists in divalent and trivalent

oxidation states depending upon the redox conditions. Scavenging of divalent

Eu at or near the hydrothermal vent sites is a likely mechanism which may

limit the large Eu flux to the oceans from hydrothermal systems. Divalent Eu species have been found to be stable form in the hydrothermal solutions until

relatively large dilutions with ambient seawater (Sverjensky, 1984). Hydrothermal minerals such as barite and anhydrite may precipitate when the

Eu is in divalent state (Derry and Jacobsen, 1990). As the REE concentrations

in such mineral phases are much greater than the hydrothermal fluids, a relatively modest amount of Eu-enriched precipitate could significantly reduce the net hydrothermal flux of dissolved Eu to the oceans (Derry and Jacobsen,

1990).

Positive Eu anomalies are also observed in geological material rich in

feldspars or basalt debris (eg: Nath et al, 1992b and references therein). Such a reason for the positive Eu anomalies in these hydrothermal crusts is ruled out since the Al concentrations are either below the detection limits or very low (Table 5.2).

5.46. REE and other geochemical constraints on the hydrothermal mineralization in the Indian Ocean area

In the Indian Ocean, only sporadic investigations have been carried out

on the mid-oceanic ridge area with reference to ore mineralization that resulted in the discovery of a few sites of hydrothermally formed mineral deposits

(Rozanova and Baturin, 1971; Backer, 1975; Rona et al, 1983). These investigations revealed 125

1) high intensity of hydrothermal activity at sites of anomalously high

thermal gradients such as Vityaz fracture zone on Carlsberg ridge

(5°21'S to 10°N) in the northwest Indian Ocean. This fracture zone exposes the polymetallic sulfide, disseminated and stockwork forms such

as in TAG hydrothermal field (Rona et al, 1983),

2) basal sediments from DSDP site 236 in the Western Indian Ocean

(1°40'S and 57°38'E) are found to be rich in Fe upto 28% (Cronan et al,

1974) and are analogous to the iron rich deposits of hydrothermal origin of Mid-Atlantic Ridge and East Pacific Rise, 3) metalliferous sediments have been found in DSDP sites 245 (31°22'S,

52°18'E), 248 (30°, 36°E) and 249 (24°, 60°E).

In 1983, the project GEMINO (Geothermal metallogenesis Indian Ocean) was initiated as the first large scale exploration programme for polymetallic

sulfide deposits in the Indian Ocean. A 42-day reconnaissance survey on board the vessel R.V. Sonne along the Carlsberg ridge and Central Indian

Ridge was undertaken to verify previously reported hydrothermal mineralization

at 6°N, 1.5° and 5.5°S (Backer, 1975; Rona et al, 1983, Rozanova and Baturin, 1971).

Based on the above reconnaissance survey and the earlier reports on

spreading rate and high heat flows (Udinstev, 1975), Indian Ocean triple junction area has been chosen for further exploration by German and as well

as Indian ships. The ridge sediments had marine micronodules which are poor in Cu, Ni, Co and Zn indicating hydrothermal activity. Factor analyses of the

elemental data of sediment has also revealed a probable hydrothermal factor

much similar to that for the East Pacific Rise sediments (Herzig, Pluger and 126 shipboard scientific parties, 1988). Further surveys in the area has revealed an inactive hydrothermal field and a hydrothermal plume site about 200 km northwest of the Rodriguez triple junction (Pluger et al, 1990). The fossil deposit (SONNE hydrothermal field) at 23°23.6'S consists of hydrothermally influenced basalts and sediments, layered ferromanganese precipitates and probably blocks of massive sulfides (all based on visual observations from OFOS photos - Pluger et al, 1990). The hydrothermal plume with maximum concentrations of 202 m1/I CH 4 and a temperature anomaly of +0.05°C was

delineated at 24°00:3'S (hydrothermal plume site). The total dissolved manganese (TDM) concentrations in the seawater reach a maximum of 23.1 nmol / kg and show a significant positive correlation with methane. All these

factors indicate a possible hydrothermal activity in the area. In addition, bottom

photos / TV studies revealed young tectonic features which may suggest the seismic activeness of the area (Pluger et al, 1990).

The exploration activity carried out by French in this area also discovered a hydrothermal helium-3 plume north of RTJ at 19°29'S. Water column 3He anomalies have shown a spike of 33.8% at 2460m depth, i.e., the highest 3He

anomaly recorded so far in the Indian Ocean (Jean-Baptiste et al, 1992). These methane / helium ratios are closer to the data reported for the East Pacific Rise waters.

It may be mentioned here that the previous data on heatflow and thermal gradients of the triple junction area have shown higher values than the other

ridge areas in this ocean (Udinstev, 1975). Rao and Pattan (1989) have

reported hydrothermal characteristics of the Fe-oxide coatings on the basalts. 127

Inspite of all this, hydrothermal mineralization in terms of samples has not been reported to be collected so far.

During the GEMINO-3 cruise, Fe-Mn crusts have been recovered from the Indian Ocean triple junction area (Becker, 1990). At station 106KG, which is located on the western wall of the rift valley, 2cm thick crusts were recovered. The crusts are porous, friable in nature. X-ray diffraction studies

revealed b-Mn02 at 2.4 and 1.4A d-spacings. No todorokite or birnessite could be recognized. Some calcite peaks were observed which were later identified

as foraminiferal shells. Microscopic examination of polished section revealed

dendritic growth structures.

The REE patterns of these crusts display very significant negative Ce

anomalies. The patterns are similar to the hydrogenetic crusts from Central Indian Basin except for the negative Ce anomalies (Fig. 5.6). Ce anomaly

values ranged between -0.277 and -0.411 (Table 5.2) in comparison to the positive values between 0.352 and 0.637 for the hydrogenetic crusts from Central Indian Basin. Negative Ce anomalies are characteristic of hydrothermal Mn-crusts. On the other hand, the convex shaped IREE enrichment and HREE depletion is consistent with hydrogenous nature. Thus these crusts must have been originated hydrothermally and later on interaction

with ambient seawater must have acquired hydrogenetic signatures.

When La concentrations are plotted against Ce, the values for GEMINO crusts fall close to the seawater ratio and on a line defining hydrothermal

Mn-rich, Fe-rich and Mn-Fe crusts (Fig. 5.7). On the other hand, the CIB crust

data have a different trend close to the nodule and hydrogenous crust data

(Fig. 5.7). When the IREE (sum REE) are plotted against the Cu+Ni+Co

10- ■

SEA WATER

■

5 - • ■ ci

■ 0 ■

0 l 2 I0 20 500

ISEA WATER ( La /Ce z 2.8)

E Ce ENRICHMENT O. • O. • / • • • _ • - - - - 1! --- _ IL i) At, . .----! _ - - 3 ■ Mn -RICH CRUSTS - --- - - __ 0 Fe - RICH CRUSTS loop ../L..1"• (ii:: - - - • Mn- Fe CRUSTS ,A, NODULES LS' C.I.B. Fe -Mn CRUSTS

Fig. 5.7. La versus Ce concentrations (adopted from Toth, 1980). Hydrothermal crust La / Ce ratios are similar to seawater, approaching an apparent lower limit of La / Ce -0.25. The GEMINO crusts (the field encircled with small dots) fall close to the line describing hydrothermal deposits. 128 concentrations, the GEMINO crust values fall closer to the metalliferous sediment data (Fig. 5.8) from FAMOUS, Galapagos, Cyprus, Leg 54, Bauer deep, Marquesas zone reported in Clauer et al (1984).

Additional support for a hydrothermal nature for the crusts comes from other data sets as well. Detailed geochemical analyses of both surficial sediments (Dymond, 1981), DSDP sediment cores (Ruhlin and Owen, 1986) have shown that, the Fe and Mn contents of dispersed hydrothermal precipitates remains highly stable (Chen and Owen, 1989). Therefore, the Mn/Fe ratio can serve as a characteristic signal to distinguish the presence of hydrothermally derived material from that of other sources in sediments of widely varying composition (Dymond, 1981). The major elements Fe and Mn composition of the hydrothermal precipitates becomes fixed as they form and remains essentially constant during their dispersal as colloidal-size particles and subsequent accretion by the nodules/crusts (Chen and Owen, 1989). Several investigations (Dymond and Eklund, 1978; Dymond, 1981; Graybeal and Heath,

1984; Ruhlin and Owen, 1986) have demonstrated that the presence of a hydrothermal component in a sediment mixture can be readily identified and distinguished from materials from other sources on the basis of highly characteristic Mn / Fe ratio of -0.3. Mn/Fe ratios close to unity are diagnostic

of hydrogenous crusts / nodules (Cronan, 1980). Mn / Fe ratios of K6, K7, K8, K9 and K10 samples (all GEMINO) are 0.4, 0.3, 0.5, 0.5 and 0.4 and close

to the typical hydrothermal ratio. Slightly higher ratios (above 0.3) in these

samples may indicate an additional hydrogenous component in them. This is

consistent with the mixed REE pattern discussed above. I II I i I 7 I Cu NI Co( ppm.) 10,000 D9(0-4 0 5,000 Metalliferous deposits • 0 0

Hydrothermal • O 500 deposits D 9(10 , 15) • • GEMINO] • • • S CRUSTS • op • 100 • • • D6 (0-3.) 50 IP • • • Sum REE (P.P.m.)

5 10 50 100 500 1,000 Fig. 5.8. Correlation between XREE and Cu+Ni+Co concentrations in hydrothermal deposits and metalliferous sedimentary deposits (adopted from Clauer et al, 1984). Data characterized by smaller open and filled circles, are from Bonnot-Cortois (1981), corresponding to hydrothermal and metalliferous material from FAMOUS, Galapagos, Cyprus, Leg 54, Bauer deep, Marquesas zone, N. Pacific (Clauer et al, 1984 and references therein). GEMINO crusts (closed triangles) fall in the field of metalliferous deposits. 129

Co/Zn ratio is another diagnostic feature of hydrothermal crusts. Goan values of GEMINO samples are close to the Mn-crusts of hydrothermal nature

(Fig. 5.9). Low transitional element (Cu, Co and Ni) contents in the samples also point out a hydrothermal nature.

Marine sediments have a Si / Al ratio of 3 and manganese crusts have higher Si / Al ratios of 5.1 (Toth, 1980). GEMINO samples also reveal Si/AI ratios close to other Fe-Mn crusts (Fig. 5.10).

In the triangular diagrams, one with (Co+Cu+Ni)*10, Fe, Mn on three axes (Bonatti et al, 1972) and the other with Si*2, Fe and Mn (Toth, 1980) on its axes, the GEMINO values fall in the hydrothermal field close to the EPR metalliferous sediments (Figs. 11a and b).

All these features support a claim for a first report on the characteristic hydrothermal mineralization in the Indian Ocean area.

5.47. REE - Major element associations - Discrepancy in the sorption experiment results

As it has been mentioned in Section 5.1, one of the objectives of the present investigation is to decipher the preference of Ce and other REE for various pure mineral phases. Pure mineral phases exist in hydrothermal manganese deposits. The samples from Galapagos spreading center: Lau

Basin and East Pacific Rise have either pure Mn-oxides or Fe-oxides (with some hydrothermal silica component).

Aplin (1984) has attributed the depletion of LREE relative to HREE observed in shallow water crusts from Pacific to preferential scavenging of

HREE by Mn-oxides (more abundant in shallow water deposits), whereas •

• C N 3 • 0 • • mn- RICH CRUSTS o Fe -RICH CRUSTS • Fe-Mn CRUSTS GEMINO CRUSTS

2000 4000 6000 8000 CO + Ni i-Cu (ppm)

Fig. 5.9. Co/Zn versus Co+Ni+Cu concentrations (adopted from Toth, 1980). A good correlation of these parameters is shown for hydrothermal and hydrogenous ferromanganese crusts._ ..The low values for hydrothermal crusts indicate low concentrations of seawater derived Co, Ni, and Cu, but relatively high hydrothermally derived Zn contents. GEMINO values fall more closer to hydrothermal field. #K14 and #K8 are from Galapagos spreading center. •

• ■ Mn - RICH CRUSTS ci Fe- RICH CRUSTS • Fe-Mn CRUSTS o NODULES ••:GEMINO CRUSTS 1 LAU BASIN Fe RICH CRUSTS

1.0 2.0 3.0 4.0 5.0 6.0 7.0 AI(wt%)

Fig. 5.10. Si versus Al concentrations (adopted from Toth, 1980). Si / Al ratios of nodules are more similar to marine sediments. Ferromanganese crusts and Fe-rich crusts show enrichments in Si. Average marine sediment value is from Turekian and Wedepohl, 1961. GEMINO values fall in a zone in between the two lines. ( Co + Ni + Cu ) X 1 0

M n Fe EPR METALLIFEROUS HYDROTHERMAL S E DS Fig. 5.11 a. Ternary diagram of Fe versus Mn versus (Cu+Ni+Cu)*10 (adopted from Toth, 1980). Illustrated are 1) low trace-metal content of hydrothermal crusts, 2) the similar Fe enriched trace-metal depleted compositions of ferromanganese crusts and EPR metalliferous sediments. "Hydrogenous" and "hydrothermal" fields are from Bonatti et al (1972). EPR metalliferous sediment data are from Corliss and Dymond (1975). GEMINO samples fall just at the base of EPR metalliferous data and in hydrothermal field. Si X2

41I■ AM

411.••16,Att • i 4/11111 ■1110A //411111111111111•1m daNIIIIIMINIENI Fe-Mn CRUSTS ailnillrom .11.a 11.113.1111111 aiffINNA•••iimiaMill. OM= NODULE /Milo Illraws SEMI N0111 III IIIIIIIIIII CRUSTS At III EPR METALLIFEROUS SEDS Mn R CH CRUSTS (HYDROTHERMAL) Fe M n Fig. 5.11b. Ternary diagram of Fe versus Mn versus Si*2. The trend of increasing Si with Fe content is shown. Bauer basin smectite data are from Eklund, 1974. GEMINO crusts fall in the EPR metalliferous sediment field. The open squares in the 4 hydrothermal Fe-rich crusts / oxides are from Lau Basin. The filled squares and Mn-rich crusts from Galapagos, EPR and Lau Basins. 130

Fe-oxides (more abundant in deep- water deposits) preferentially sorb LREE.

De Carlo and McMurtry (1992) have noted that such an association is contrary to what would be predicted by the electrical double layer theory. The Mn0 2 surface has a point-of-zero charge (pzc) at pH 2 (Parks, 1965, 1967) that results in a development of a significant negative residual charge at the pH of seawater, hence it should attract positively charged species such as free REE and monocarbonato-LREE complexes rather than negatively charged dicarbonato HREE complexes. The pzc for FeOOH lies in the range of pH 6.5 - 7.5 (Parks, 1965, 1967) which leads to relatively weak negative surface charge under seawater conditions. Therefore FeOOH would not prefer LREE over HREE complexes (De Carlo and McMurtry, 1992). Further sorption experiments carried out using synthetic mineral particles such as 6-Mn0 2, goethite and apatite by Koeppenkastrop and De Carlo (1992) have shown that positive Ce anomalies are associated with Mn0 2 surfaces. Fractionation of LREE from HREE was observed on both vernadite and goethite particles.

As far as Ce anomalies are concerned in this work, among the Lau Basin samples #80 KD which is a pure Mn-oxide (todorokite rich - 49% Mn; <1% Fe) has the highest negative Ce anomaly compared to the Fe-oxide samples (#93KD, 110KD and 116GA). This implies that comparatively more Ce

is associated with FeOOH (or goethite) and less with Mn0 2 surfaces. In addition, other pure Mn-oxides such as #EPR-B (crystalline Mn0 2), K10 and

K14 of Galapagos (64% Mn and <1% Fe) have Ce levels below their detection

(<0.9 ppm). Comparatively, Fe oxides have higher Ce concentrations. The

sample #110 KD of Lau Basin with highest Fe contents (40% Fe and <0.1%

Mn) has the lowest Ce anomaly values (Table 5.2). Therefore, these results on

REE contents of pure mineral phases are in disagreement with the sorption 131 experimental results on synthetic particles (Koeppenkastrop and De Carlo, 1992). I conclude that experimental data cannot yet quantitatively account for the actual features of the crust REE patterns.

As far as the fractionation of LREE from HREE are concerned, no particular sorption affinity is shown for either Mn-oxides or Fe-oxides.

5.5. SUMMARY

The REE contents of hydrogenous Fe-Mn crusts are distinctly higher than those of hydrothermal samples, at times enriched as much as 1000 times. Furthermore, significant differences are observed in the shale-normalized patterns. Hydrogenous crusts show the well-established convex patterns (with middle REE enrichment) and positive Ce anomalies, whereas the patterns for

hydrothermal precipitates resemble those of seawater with HREE enrichment and negative Ce anomalies. Some of the hydrothermal Fe-Mn crusts also display Eu anomalies which are characteristic of hydrothermal fluids. Analyses of pure Mn-oxide mineral phase reveal lower Ce contents than the pure Fe-oxide phase and contradicts the sorption experimental results on synthetic

mineral phases. Negative Ce anomalies, Mn/Fe ratios, geochemical

characterization plots of the crusts from Rodriguez triple junction suggest a

hydrothermal nature. This forms the first report of hydrothermal mineralization

in this area. 132

Table 5.1: Details of the samples studied.

S.No. Location Sample type Mineralogy

124GTVC-K2 Galapagos Rise Fe-Mn crust Bir, To, amorp (Fe = Mn) 40GTVC-K3 - do - Fe-Mn crust Bir, To, Mo/III (Fe > Mn) 119GTVC-K8 - do - Fe-Mn crust amorp, Bir (Fe > Mn) 135GTVC-K10 .00 - Mn-crust Bir, To 134GTVC-K14 - do - Mn-crust Bir CY82146 A East Pacific Inner part of Nontronite the crust B - do - Fresh inner crystalline Mn02, To. part Fe-Mn oxide C - do Mixture of A&B Nontronite + Mn02 D do - Outer hydro- 2 6-Mn0 genous part CLDR3-3 - do - Altered sulfide amorp Fe-oxide

80KD Lau Basin Mn oxide Bir, To, 6-Mn02 93KD - do ochre Fe-rich amorp 110KD do - ochre Fe-rich amorp, goethite 116KD - do - ochre Fe-rich amorp

112DR-K6 Rodriguez Triple Junction Fe-Mn crust 6-Mn02, 151DR-K8 . - do - - do - - do -, calcite 159DR-K9 - do - - do - 168DR-K10 - do - - do - 168DR-K17 - do - - do -

NRS 1-9 Central Indian Hydrogenous 6-Mn02, amorp Basin Fe-Mn crusts CP-1 Central Pacific Thick Fe-Mn 6-Mn02 crust

Bir - Birnessite, To - Todorokite, amorp - amorphous material (probably Fe-oxides), Mo - montmorillonite, Ill - illite. 133

Table 5.2: Chemistry of hydrogenous and hydrothermal crusts.

Galapagos spreading center ( GARIMAS ) - Hydrothermal deposits

124GTVC K2 40GTVC K3 119G1VC K8 135GTVC K10 134GTVC K14

Si% 20.52 13.30 9.94 0.31 0.28 Al% <2 <2 <2 <2 0.13 Fe% 17.98 31.54 39.04 0.15 0.82 Mn% 18.59 14.88 11.45 63.69 63.74 Cu (ppm) 25 361 2509 39 230 Co (ppm) <20 <20 23 <20 59 Ni (ppm) 42 52 29 186 - 106 Zn (ppm) 49 1211 2240 203 87

La (rpm) 1.61 1.81 1.73 1.24 1.29 Ce ppm) 1.82 2.04 1.14 <0.9 <0.9 Nd ppm) 0.94 1.27 <0.9 <0.9 <0.9 Sm (ppm) <0.5 <0.5 <0.5 <0.5 <0.5 Eu (ppm) 0.66 0.30 0.07 0.05 <0.05 Gd (ppm) 0.68 1.02 <0.3 <0.3 <0.3 Dy (ppm) 0.78- • 1.10 <0.3 0.76 <0.3 Yb (ppm) 0.53 1.16 0.22 <0.2 <0.2 Lu (ppm) 0.13 0.29 0.07 0.15 <0.06

TREE 7.15 8.99 3.23 2.2 1.29 LadLun 0.19 0.09 3.82 0.12 -- Ce anomaly -0.20 -0.20

East Pacific Rise - Hydrothermal deposits

CY82146 A B C D 3

Si% Al% Fe% Mn% Cu (ppm) C (ppm ) Ni (ppm) Zn (ppm)

La (rpm) 0.68 1.18 1.37 61.0 4.76 Ce (ppm) <0.9 <0.9 <0.9 9.9 5.6 Nd ppm) <0.9 <0.9 <0.9 48.4 3.31 Sm (ppm) <0.5 <0.5 <0.5 10 1.14 Eu (ppm) <0.05 0.05 0.06 3.01 0.45 Gd (ppm) <0.3 <0.3 <0.3 10.6 1.66 Dy (ppm) <0.3 0.42 0.56 13.6 1.86 Yb (PP m) 0.29 0.3 0.31 9.33 1.44 Lu (ppm) 0.07 0.09 0.12 1.5 0.29

TREE 1.04 2.04 2.42 167.3 20.51 LaiLun 0.15 0.2 0.17 0.61 0.24 Ce anomaly ------1.08 -0.199

134

Table 5.2 (Contd)

Lau Basin - Valu Fa Ridge - Hydrothermal deposits

80KD 93KD 110KD 116GA

Si% 1.38 7.43 5.5 12.13 AI% 0.42 0.06 1.1 2.48 Fe% 0.57 32.51 40.44 24.58 Mn% 49.63 1.01 0.09 1.32 Cu (ppm) 25 54 729 166 Co (ppm) -- -- -- -- Ni (ppm) 192 -- -- -- Zn (ppm) 35 38 169 60

La (ppm) 2.74 3.34 4.88 1.47 Ce (ppm) 1.48 5.57 6.69 1.61 Nd (ppm) 2.25 2.68 5.10 1.00 Sm (ppm) 0.79 0.72 2.28 <0.5 Eu (ppm) 0.28 0.26 0.73 0.12 Gd (ppm) 1.23 0.90 3.23 0.54 Dy(ppm) 2.00 0.92 3.64 0.49 Yb (ppm) 1.00 0.86 3.43 0.45 Lu (ppm) 0.27 0.17 0.60 0.12

FREE 12.04 15.42 30.58 5.80 " LadLun 0.15 0.29 0.12 0.18 Ce anomaly -0.55 -0.07 -0.19 -0.24

Indian Ocean - Rodriguez Triple Junction Mixed hydrothermal - hydrogenous deposits

GEMINO-K6 GEMINO-K8 GEMINO-K9 GEMINO-K10 GEMINO-K17

Si% 4.91 5.05 5.18 6.24 AI% 1.33 1.05 1.13 1.63 Fe% 29.54 23.67 22.76 20.92 Mn% 11.29 11.54 11.62 9.30 Cu (ppm) 500 300 300 300 Co (ppm) 700 900 900 700 Ni (ppm) 100 B.D 100 B.D Zn (ppm) 660 580 1080 760

La (ppm) 204 183 202 151 176 Ce (ppm) 187 195 173 117 147 Nd (ppm) 183 168 179 135 157 Sm (ppm) 39.8 36.1 37.4 28.8 34.6 Eu (ppm) 10.14 9.20 9.59 7.59 8.86 Gd (ppm) 42.8 37.5 37.9 30.5 37.3 Dy(ppm) 40.3 34.1 35.0 28.4 35.7 Yb (ppm) 16.86 16.37 14.04 19.45 Lu (ppm) 3.16 2.42 2.37 2.95

FREE 732 682 693 514 619 La„/Lu„ 0.96 1.12 1.27 1.12 0.89 Ce anomaly -0.34 -0.28 -0.37 -0.41 -0.38 135 • Table 5.2 (Contd)

Hydrogenous crusts from Central Indian Basin and Central Pacific

NRS-1 NRS-2 NRS-3 NRS-4 NRS-5 NRS-6 NRS-7 NRS-8 NRS-9 CP-1

La (ppm) 144 262 240 258 288 233 234 232 243 283 Ce (ppm) 724 1561 1507 1573 1345 1360 1977 2092 1709 1139 Nd (ppm) 176 243 226 265 300 263 245 231 250 302 Sm (ppm) 45.1 55.1 51.5 61.7 72.6 66.7 61.8 56.5 60.7 70.1 Eu (ppm) 11.1 13.6 12.8 15.5 18.0 16.4 15.2 14.3 15.2 17.4 Gd (ppm) 37.9 50.4 47.1 55.8 64.2 57.9 54.8 52.0 55.1 62.6 Dy (ppm) 33.4 46.6 43.2 50.6 58.4 52.6 50.5 49 51.1 58.8 Yb (ppm) 14.31 22.99 21.41 22.49 26.15 22.65 22.94 26.52 24.83 27.75 Lu (ppm) 2.07 3.35 3.09 3.21 3.74 3.43 3.48 3.66 3.53 3.80

)REE 1188 2258 2152 2305 2176 2076 2665 2757 2412 1964 Lan/Lun 1.04 1.16 1.15 1.2 1.15 1.01 1 0.94 1.02 1.11 Ce anomaly 0.35 0.49 0.49 0.46 0.35 ' 0.43 0.60 0.64 0.52 0.54 CHAPTER 6

COMPARISION OF THE REE UPTAKE BEHAVIOUR BY VARIOUS AQUEOUS SEDIMENTARY PHASES IN THE INDIAN OCEAN 136 COMPARISON OF THE REE UPTAKE BEHAVIOUR BY VARIOUS AQUEOUS SEDIMENTARY PHASES IN THE INDIAN OCEAN

6.1. ENRICHMENT FACTORS

The mean REE contents of all the types of sediments and ferromanganese deposits such as fluvial, brackish, continental shelf, deep-sea terrigenous, siliceous, calcareous, red clay sediments; hydrogenetic and diagenetic nodules; and ferromanganese crusts of hydrogenous as well as mixed

hydrogenous-hydrothermal nature, all recovered from the Indian . Ocean are used for calculating the enrichment factors (E). E, is determined by normalizing the REE concentrations of each type of sedimentary phase with that of the upper continental crust with the formula

= REEsp / REEuc, where REE.p represents the mean REE contents of an individual type of sedimentary phase and REE uc denotes the REE content of average upper continental crust. The average upper continental crust values are from Taylor and McLennan (1985).

The enrichment factors calculated as above are presented in Fig. 6.1. Like several other minor elements which have been studied extensively (eg: Cu,

Co, Ni, As, Pb, Zn), REE are also enriched in deep-sea ferromanganese deposits compared with either nearshore sediment or deep-sea sediments. The enrichment factors decrease in the order of Hydrogenous crusts > hydrogenetic nodules > mixed hydrogenetic-hydrothemal crusts (except cerium) > diagenetic nodules > pelagic clays > fluvial sediments > brackish water sediments > shelf sediments > terrigenous deep-sea sediments > siliceous clays > calcareous

ENRICHMENT FACTORS Concentration (sample) / Concentration (upper crust)

Fig. 6.1. Figure showing the comparison of REE enrichment factors (REE sample / REEupper crust) of the sedimentary phases considered for the study. 137 sediments. Among the deep-sea sediments, only pelagic red clays are enriched compared with nearshore sediments as far as the middle and heavy REE are concerned. If the deep-sea sediments contain minor elements predominantly of continental origin, will have similar concentrations in deep-sea and near-shore sediments (Elderfield, 1977). As the pelagic clays studied here are enriched in some REE compared to the nearshore sediments, they may contain a substantial hydrogenous component (Goldberg, 1954) derived either directly from seawater or from the weathering of the ridge/ seamount basalts (Nath et al, 1992a). Deep-sea terrigenous sediments and siliceous clays from Central Indian Basin have relatively lower REE contents than the near-shore sediments studied

here. This is in contrast to the Pacific deep-sea sediments wherein Elderfield et al (1981a) have observed higher concentrations than either shale or the near-shore sediments. Low REE concentrations in siliceous sediments and calcareous clays is probably due to the dilution from biogenic debris present in

these sediments. Elderfield et al (1981a) have found markedly lower REE

contents in both diatoms and foraminifera (FREE less than 25 ppm and 10 ppm

respectively). Average biogenic silica contents estimated by normative method. (Bischoff et al, 1979) is 28% for the siliceous clays / oozes in the Central Indian

Basin (Nath et al, 1989). Alkali extraction method has yielded biogenic silica

contents of 30 - 35% for the area between 11 ° and 13°S in the Central Indian Basin (Pattan et al, 1992). Therefore, about 30% of these sediments would contain only about 25 ppm FREE compared to -250 ppm of FREE contents in terrigenous sediments and this would certainly dilute the bulk REE contents of these sediments.

The behaviour of Ce in ferromanganese deposits is different from other

REE when compared with other sedimentary phases of the Indian Ocean region. 138

Ce is either markedly - enriched (in nodules and hydrogenous crusts) or depleted (in hydrothermal crusts and foraminiferal oozes) compared with its immediate trivalent REE neighbours. The Ce enrichment has been attributed to the oxidation of Ce3+ to tetravalent CeO2 (Goldberg, 1961) and the depletion in hydrothermal crusts is due to retention of seawater signatures. Ce in the sedimentary phases of predominantly detrital / terrigenous origin (both near- shore and deep-sea) with less biogenic and authigenic components in them follow the trend of its REE neighbours. This suggests that Ce in these phases is mainly in trivalent state.

6.2. CERIUM AS AN ADDITIONAL RESOURCE FROM MANGANESE NODULES

Rare-earths have been used extensively in the later part of nineteenth century in the development of incandescent gas mantles. In addition, they are used in a mixed form in metallurgy, chemicals and catalysts, glass manufacture and as polishing compounds (Neary and Highley, 1984). Growth in demand, as revealed by increase in production has been about 7 - 10% in the last four decades (Neary and Highley, 1984). Historically, most conventional commercial sources of REE have been monazite deposits in the form of placers formed by the natural agencies such as weathering, erosion, transportation and deposition

(Neary and Highley, 1984). Besides, bastnaesite, xenotime and apatite are the other REE bearing economic mineral deposits (Mariano, 1989).

Among the sedimentary phases considered for this study, only ferromanganese nodules and crusts, as discussed in the previous section, have shown considerable enrichment over the upper crustal abundances. Among these two types of sedimentary phases, crusts are enriched much more than the 139

nodules. But the details regarding the distribution and abundance of manganese crusts are not available. Nevertheless, the manganese nodules from the Central Indian Basin have been mapped very systematically and potential sites of commercial deposits have been identified (eg: Gujar et al, 1988, Sudhakar, 1993). India has been allocated an area of 150,000 sq.km by the PrepCom (UN) as the pioneer area. The nodule deposits of this Indian Pioneer area are enriched in cerium by atleast 5 to 17 times compared to the crustal abundances.

Here, an attempt has been made to quantify the resources of cerium in manganese nodules of the Central Indian Basin with particular reference to the

Indian pioneer area. Total reserves of nodules (dry) estimated for the pioneer area are of the order of 607 x 106 metric tonnes (Sudhakar, 1993). Considering an average Ce content of 524 ppm in the Central Indian Basin nodules (data from Nath et al, 1992b; Patten and Banakar, 1993 and chapter 4 of this thesis), cerium resources are computed. The cerium resources range from 0.15 to 0.51 x 106 metric tonnes with an average of 0.32 x 10 6 metric tonnes. Such a magnitude of Ce resources which form nearly 1/3 of the cobalt reserves (0.85 x 106 metric tonnes; Sudhakar, 1993) pose a question, whether these Ce resources could be tapped during the extractive metallurgical processes of manganese nodules for other important metals (mainly Cu, Ni and Co).

However, the figures attract ones attention to perceive a possibility of extracting Ce as a by product. CHAPTER 7

SUMMARY AND CONCLUSIONS 140 SUMMARY AND CONCLUSIONS

One hundred and fifty three analyses have been conducted on the various types of sedimentary phases such as fresh water and brackish water sediments from the Vembanad lake, Kerala, inner continental shelf sediments off Cochin, Kerala; deep-sea terrigenous, calcareous, siliceous and pelagic clays from the Central Indian Basin; hydrogenetic manganese nodules from the Central Indian, Somali-Seychelles and

Mascarene Basins, diaganetic nodules from Central Indian Basin, physically separable and chemically separable mineral phases of nodules from Central and Western Indian Oceans; pure hydrothermal

Mn and Fe oxides from Lau Basin, SW Pacific, a back-arc spreading center and Galapagos and East Pacific Rise, two fast spreading ridges; mixed hydrothermal- hydrogenous Fe-Mn crusts from the

Rodriguez triple junction, Indian Ocean, which is an intermediate spreading ridge and hydrogenetic ferromanganese crusts from the Central Indian Basin and Central Pacific for rare-earth elements or lanthanides. The basic objective of the work has been to study the geochemical processes leading to the variable uptake by different aqueous sedimentary phases of the Indian Ocean region and also to look into the processes responsible for their fractionation. This study contributes to the understanding of 1) continental and hydrothermal input patterns / fluxes of REE into the ocean, 2) enrichment of REE in these phases in comparison to the upper crust composition and 3)

REE sorption mechanisms.

141

2.1. Multielement analyses were conducted by X-ray flourescence spectroscopy (XRF) and Instrumental Neutron activation analysis

(INAA) for Fe, Al, Ca, Na, K, Cr, Co, Zn, As, Rb, Cs, Hf, Sc, Th, U and eight rare earth elements (REE) on the clay size (< 4 ) fractions of modern sediments from Vembanad lake, a tropical coastal estuary/lagoon from the southwest coast of India and its adjoining representing the second leg and third leg of the sedimentary cycle. The sediments represents three sub- environments namely 'forced

fluvial', brackish/lagoonal and nearshore marine. Physico-chemical

factors like dissolved oxygen, pH and salinity vary with the type of

environment.

2.2. The average concentrations of almost all the elements (except sodium and chromium) from the continental shelf region are lower than the averages of lagoonal sediments. Sodium is highest in the shelf

sediments, moderate in northern part of the lagoon dominated by

brackish water and lowest in the sediments south of Tannirmukhan bund/barrage which is constructed to curtail the tidal influx from north.

The metal enrichment in the lagoon compared to the shelf is in consistence with the previous findings based on textural,

sedimentological and mineralogical studies, the estuaries/lagoons apparently have filtering effect on the geochemical properties too.

2.3. Lagoonal sediments show a good positive relation with Fe for the shelf sediments, whereas only light REE covary with Fe for the shelf

sediments. This suggests a mobilization of REE during the

weathering-of- acid igneous rocks and formation of kaolinite and iron

142

oxyhydroxides on one hand and the susceptibility of HREE to form carbonate, chloride and organic complexes in seawater on the other seems to be controlling the REE distribution in lagoonal sediments affected by tidal processes and shelf sediments.

2.4. REE patterns and La/Lu ratios compared to average shale reveal a significant light REE (LREE) enrichment over the heavy REE for all these nearshore sediments. The LREE enrichment factor (La n/Lu„)

ranges between 1.55 to 2.94. The degree of REE fractionation has been found to be related to the size sorting of minerals as well as the

physico-chemical conditions of the environment.

2.5. Since the technique employed here for analysing REE is non- destructive (INAA - Instrumental Neutron Activation Analyses), the

HREE depletion observed in these sediments gives a true picture and

is not influenced by incomplete sample dissolution as suggested

(Condie, 1991) for other nearshore sediments.

2.6. The REE distributions, La/Th and Th/Sc ratios in these nearshore

sediments are significantly different from shale and from the average upper continental crust. This raises once more the question whether the terrigenous REE input into the ocean really represents the average upper continental crustal composition.

3.1. Rare earth element concentrations have been determined in the

surface sediments of the Central Indian Basin to represent a deep-

sea abyssal basinal environment. The Central Indian Basin is chosen because it is relatively well-constrained as regard to the sedimentary

143

and oceanographic sources are concerned. The samples represent various sediment types namely terrigenous, siliceous (nodule barren

and nodule bearing), calcareous and red clays all from this basin.

3.2. The shale-normalized REE patterns are found to be characteristic for each sediment type with flat shale-normalized patterns associated with terrigenous sediments, positive Ce anomalies with siliceous sediments, negative Ce and positive Eu anomalies with calcareous sediments,

and LREE depleted patterns with red clays.

3.3. REE fractionation indices such as Lan / Yb„, Lan / Gdr, and Gdr, / Yb„ are characteristically different for each sediment type. These observations (3.2 and 3.3) suggest a lithological control on the REE

patterns.

3.4. The sediments collected from a sub-environment dominated by oxic diagenetic processes possess highest positive Ce anomalies which are probably acquired during early diagenetic processes. During this

process, the sediments seem to be acquiring Ce efficiently compared

to the nodules thereby leading to the smaller positive anomalies in the associated nodules.

3.5. On comparison with the bottom water oxygen and temperature conditions, no relation is observed between the REE fractionations in

the bottom sediments to the bottom water redox conditions.

3.6. These results therefore, indicate that the REE signatures in marine sediments are not only related to depositional setting, but also to the

lithological variations and surficial diagenetic processes. Therefore,

144

these constraints indicate that REE fractionations for paleotectonic and paleoredox reconstructions should be used with caution.

4.1. A number of manganese nodules and ferromanganese crusts from the Central Indian, Mascarene and Somali-Seychelles Basins of the Indian Ocean were analysed for major, minor and the fourteen naturally

occurring rare earth elements. The REE were analysed by Inductively

Coupled Plasma - Mass Spectroscopy (ICP - MS) and Inductively

Coupled Plasma - Atomic Emission Spectroscopy (ICP - AES). The samples were selected systematically from the Western Indian Ocean

and the Central Indian Basin, to represent a seamount top, slope, an abyssal hill, siliceous sediment affected by terrigenous influx, a highly productive siliceous environment, red clay and carbonate sedimentary domains. A systematic approach has been employed to identify the

processes controlling REE and the Ce anomaly variations and the phases hosting the Ce in the manganese nodules. While the general

interpretation of Goldberg (1961) that Ce is preferentially taken up by manganese nodules is undisputed, our approach has been to separate a) the oxides and nuclei physically and b) Fe-Mn oxides from

aluminosilicate phase chemically (2M HCI leach) to specifically locate

the Ce and relate this to the nodule forming processes.

4.2. Although rare earth element zonation is observed in one oriented

nodule with enrichment in the top, evidence of top-bottom fractionation

is apparently being obliterated when the nodule turns over.

4.3. Correlations between Ca, P, Fe and REE in nodules suggest that the

REE reside in the iron oxyhydroxide and phosphatic phases. An

145

authigenic origin is attributed to these elements. On the other hand, interelement relations have shown that the Ce anomalies are controlled by the amorphous mineral phase FeOOH.xH2O which in turn depends upon the redox conditions of the depositional environment. A study of Fe, P, Ce anomaly and b-Mn02 relationship has revealed that Ce and P probably are chemisorbed onto iron oxyhydroxides which

are epitaxially intergrown with b-Mn0 2.

4.4. The nodules and crusts from the Western Indian Ocean and the shallower depths of the Central Indian Basin are b-Mn0 2 rich. They

are characterised by high concentrations of REE, high positive cerium

anomalies, higher Ce/La ratios and LREE enrichments. These are attributed to the oxidative scavenging of Ce by particles and its subsequent incorporation into manganese nodules.

4.5. A study on the cerium speciation into oxide, nuclei and various

phases have revealed that, although the Ce concentrations are

considerably lower in the nuclei, they seem to make a little impact on

the overall magnitude of Ce anomalies in nodules. The ratios between the concentrations in the acid leachates and the

corresponding bulk values show flat patterns indicating that

aluminosilicate phase has a negligible role in the formation of Ce anomalies.

4.6. The spin-off findings from the studies of the nucleating material are a) no Ce diffusion into fish teeth even with an availability of Ce rich microenvironment b) diffusion like transport of Ce within the nodule

146

and c) the scavenging capacity of the initial oxide layers rather than the nucleus is important in regulating the behaviour of Ce.

4.7. The data normalized to shales depict gadolinium-terbium anomalies.

The anomaly is made up of both elevated Gd and depressed Tb concentrations which were hitherto reported in seawater. These are

assumed to be due to the retention of seawater properties.

4.8. A diagenetic nodule with a palagonitic, smectite-rich nucleus exhibits

an unusual HREE enrichment.

4.9. The spatial distribution of the Ce anomaly values appear to depend

more or less on all the factors responsible for the uptake of other

minor metals during the nodule formation.

5.1. Major element, REE compositions and mineralogical characteristics

were determined for thirty crusts comprising hydrogenetic Fe-Mn crusts, hydrothermal Mn oxides, Fe oxides and mixed

hydrothermal-hydrogenous crusts. The samples were recovered from

diverge tectonic settings such as mid plate seamounts and fast-spreading ridges such as East Pacific Rise and Galapagos spreading center, medium spreading Rodriguez triple junction, Indian

Ocean, a back-arc spreading centre (Lau Basin, Valu-Fa ridge) that are sites of hydrothermal precipitation of Fe-Mn oxides. Samples

recovered from less dynamic abyssal environments such as Central

Indian Basin and Central Pacific which represent the sites of

hydrogenous Mn-Fe oxide precipitation.

147

5.2. The total REE contents of hydrothermal crusts are very low and range

between 1.04 and 30.6 ppm. The concentrations of some of the elements (Ce, Nd, Sm, Eu, Gd and Dy) were lower than their detection limits. On the other hand, REE contents were very high

with an average TREE close to 2200 ppm and about 800-1000 times

enriched than the hydrogenous crusts. The mixed crusts from Rodriguez triple junction, Indian Ocean show only slightly lower

trivalent REE contents, but huge Ce depletions compared to their

hydrogenous counterparts of Central Indian Basin.

5.3. Significant differences are noticed in the shale-normalized REE

patterns between the hydrothermal, hydrogenous and mixed hydrogenous-hydrothermal portions of the crusts. Hydrogenous crusts typically possess the convex pattern with a clear enrichment of IREE.

The patterns for the hydrothermal crusts are broadly similar to seawater, depleted in Ce and enriched in heavy REE. The Rodriguez

triple junction show a pattern resembling hydrogenous crusts as far as

the trivalent REE are concerned, but display a negative Ce anomaly which is a characteristic of hydrothermal crusts.

5.4. The purely hydrothermal Fe-Mn crusts from Galapagos spreading

centre, East Pacific Rise and Lau Basin are typically HREE enriched. Lan / Lun values are 0.09 to 0.29 for these crusts. The HREE

enrichment and Lan / Lu , ratios <1 are characteristic of seawater and the metalliferous sediments on the ridge flanks.

5.5. Hydrothermal Fe-Mn crusts are characterised by negative Ce

anomalies in contrast to hydrogenetic crusts which display a mirror

148

image positive Ce anomalies. The Ce anomalies for these crusts are all lower than the average seawater value of -0.74 and much less than the -1.2 value of hydrothermal water of Southeast Pacific. Our data suggest that the hydrothermal precipitates, like the seawater are one of the sources for Ce for the authigenic Fe-Mn oxides.

5.6. Positive Eu anomalies (compared to its neighbours Sm and Gd) are observed in some of the hydrothermal crusts. The positive Eu anomalies observed in the shale-normalized REE patterns of

hydrothermal crusts can result from the input of Eu-enriched

hydrothermal fluids into the water column.

5.7. REE contents, shale-normalized patterns, negative Ce anomalies, Mn/Fe ratios and geochemical characterization plots such as 1) La v/s Ce, 2) sum REE v/s Cu+Ni+Co, 3) Co/Zn v/s (Cu+Co+Ni), 4) Si v/s

Al, and ternary diagrams of 5) (Cu+Ni+Co)*10 v/s Fe v/s Mn, 6) Si*2 v/s Fe v/s Mn suggest that the Fe-Mn crusts from Rodriguez triple

junction are hydrothermal in nature. These constraints help me to

report a first direct evidence of hydrothermal mineralization for this

area. This supports the indirect evidences such as water column hydrothermal tracers of high TDM (total dissolved Mn), methane and

He anomalies, temperature anomalies, sediment heat flow, sediment chemistry data (hydrothermal factor) reported earlier.

5.8. Analyses of pure Mn-oxides and relatively pure Fe-oxides allowed me

to study the sorption affinity of REE for these phases. Ce contents are very low (below their detection) and Ce anomalies are less

developed on the Mn-oxides compared to the Fe-oxides. No particular

149

sorption affinity among the REE series (fractionation of LREE from HREE) is noticed. These results are in direct contradition with the results on sorption experiments carried out on synthetic Mn-oxide and

Fe-oxide particles by Koeppenkastrop and De Carlo (1992).

6.1. In order to compare the variable uptake behaviour of different sedimentary phases considered for this study, enrichment factors (E t) are determined for all the sediment types studied here. E t determined

by normalizing the REE concentrations in sediment with that of the upper continental crust are highest for hydrogenous ferromanganese

crusts and lowest for calcareous clays. The variation is regulated by different factors such as terrigenous and volcanogenic sediment supply, authigenic and diagenetic processes, adsorption and scavenging mechanism and dilution from REE-poor biogenic phases.

6.2. When the REE contents in these sedimentary phases are compared in

an economic point of view, they are much lower than the

concentrations in the conventional land resources such as placers or bastaesite. However, cerium in ferromanganese crusts is enriched by atleast 5 to 25 times compared to the crustal abundances. A

conservative estimate of Ce resources for the Indian Pioneer area in

the Central Indian Basin (average 0.32 X 10 6 metric tonnes) suggest

that it could be a potential by product during the metal extraction from

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1. H. N. Siddiquie, A.R. Gujar, N.H. Hashimi, A.B. Valsangkar and B. Nagender Nath (1986) Mineral resources of the Indian Ocean and related scientific research. IOC Workshop Report on Regional Co-operation in Marine Sciences in the Central Indian Ocean and adjacent seas and gulfs No. 37, Supplement, 123-152.

2. B. Nagender Nath, V.S. Rajaraman and A.V. Mudholkar (1988) Modified interstitial water squeezer for trace metal analysis. Indian Journal of Marine Sciences, 17: 71-72. 3. V. Purnachandra Rao and B. Nagender Nath (1988) Nature, distribution and origin of clay minerals in grain size fractions of sediments from manganese nodule field, Central Indian Ocean Basin. Indian Journal of Marine Sciences, 17: 202-207. 4. R. Mukhopadhyay and B. Nagender Nath (1988) Influence of seamount topography on the local facies variation in ferromanganese deposits in the Indian Ocean. Deep-sea Research, 35(8): 1431-1436.

5. A.R. Gujar, B. Nagender Nath and R. Banerjee (1988) Marine Minerals - The Indian Perspective. Marine Mining, 7(4): 317-350. 6. B. Nagender Nath and A.V. Mudholkar (1989) Early diagenetic processes affecting nutrients in the Central Indian Ocean cores. Marine Geology, 86: 57-65.

7. B. Nagender Nath and S.D. lyer (1989) Basalt microlapilli in deep sea sediments of Indian Ocean in the vicinity of Vityaz fracture zone. The Journal of the Geological Society of India, 34(3): 303-309.

8. B. Nagender Nath, V. Purnachandra Rao and K.P. Becker (1989) Geochemical evidence of terrigenous influence upto 8°S in the Central Indian Basin. Marine Geology, 87: 301-313. 9. B. Nagender Nath and M. Shyam Prasad (1991) Manganese nodules in the Exclusive Economic Zone of Mauritius. Marine Mining, 10(4): 303-335.

10. B. Nagender Nath, V. Balaram, M. Sudhakar and W.L. Pluger (1992) Rare earth element geochemistry of ferromanganese deposits from the Indian Ocean. Marine Chemistry, 38: 185-208. 11. B. Nagender Nath, I. Roelandts, M. Sudhakar and W.L. Pluger (1992) Rare earth element patterns of the Central Indian Basin sediments related to their lithology. Geophysical Research letters, 19(12): 1197-1200. 175

Symposium / Seminar Papers:

1. H.N. Siddiquie, A.R. Gujar, N.H. Hashimi, A.B. Valsangkar and B. Nagender Nath (1985) Mineral resources of the Indian Ocean and related scientific research. IOC Workshop Report No.37, Annex II, p.13-15. 2. V. Purnachandra Rao and B. Nagender Nath (1986) Distribution of clay minerals in the Central Indian Ocean Basin. VI Convention of Indian Association of Sedimentologists, (Abstracts), p. 77. 3. B. Nagender Nath and R. Mukhopadhyay (1987) Hydrothermal mineralization in the Indian Ocean. National Symposium on Marine resources, techniques, evaluation and management, (Abstracts), p. 35.

4. A.R. Gujar, B. Nagender Nath and R. Banerjee (1987) Marine minerals - The Indian Perspective. Diamond Jubilee National Symposium on Development of India's mineral and fuel resources. Geological and environmental aspects, (Abstracts), p. MMR-1.

5. B. Nagender Nath and V. Purnachandra Rao (1988) Mineralogical and geochemical evidences of terrigenous influence in deep sea sediments of the Central Indian Basin. First International Conference on Asian Marine Geology, Shanghai, China, (Abstracts).

6. B. Nagender Nath, H. Kunzendorf and Y.L. Dora (1990) Behaviour of major, minor and rare earth elements of the modern sediments in brackish water conditions and at increased salinity. 13th International Sedimentological Congress, Nottingham, U.K, (Abstracts), p. 377. 7. B. Nagender Nath, V. Balaram, M. Sudhakar and W.L. Pluger (1990) The distribution of rare earth elements in ferromanganese deposits from the Central and Western Indian Ocean. 68th Annual meeting of the German Millikalogical Society, Wurzburg, Germany, Beihefte zum European Jour. Mineralogy., 2(1): p. 186. 8. B. Nagender Nath and M. Sudhakar (1992) Processes controlling the characteristics of ferromanganese nodules of the Central Indian Basin, Indian Ocean. 29th International Geological Congress, Kyoto, Japan, (Abstracts).

9. B. Nagender Nath, M. Sudhakar, I. Roelandts and W.L. Pluger (1992) Validity of sediment REE data in probing paleoceanography: A case study from Central Indian Basin. Second International Conference on Asian Marine Geology, Tokyo, Japan, (Abstracts), p. P3-24. 176

Reports:

1. Seabed surveys along the proposed alignments for the Bombay Mainland Link Project - 1983. 2. Approach paper on the proposed Oceanographic studies in the Exclusive Economic Zone of Mauritius - 1985. 3. Shallow seismic surveys in Vembanad Lake, Kerala - 1986.

4. Report on the Oceanographic Surveys in the Exclusive Economic Zone of Mauritius. - 1988.

5. A note on second international conference on Asian Marine Geology (1992), The Journal of Geological Society of India, V. 40. Textbook / Monograph:

1. A.B. Valsangkar, B. Nagender Nath and R. Sharma (1988) Manpower. In: S.Z. Qasim and R.R. Nair (Editors) From First Nodule to First Mine Site, An account of the Polymetallic nodules project. Department of Ocean Development and National Institute of Oceanography, India, 27-28.

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