Volcanogenic and Hydrothermal Evidence from the Central Indian Ocean Basin Since 60 Ma

Volcanogenic and hydrothermal evidence from the Central Indian Ocean Basin since 60 Ma

A Thesis submitted to the Goa University for the Award of the Degree of DOCTOR OF PHILOSOPHY In the School of Earth, Ocean and Atmospheric Sciences

By Ankeeta Ashok Amonkar

Research Guide Dr. Sridhar D. Iyer Chief Scientist (Retired) (CSIR-National Institute of Oceanography, Goa 403004)

Goa University, Taleigao, Goa

2020 CONTENTS

Page No. Declaration Certificate Acknowledgements i-ii List of Publications iii-iv Acronym v-vi Preface vii-xii List of Figures xi-xiii List of Tables xiv-xv

Chapter 1 An overview of the Central Indian Ocean Basin: Structure, 1-14 Tectonic, Sediment and Volcanism 1.1 Study area - Central Indian Ocean Basin 2 1.2 Tectonic evolution of the CIOB 4 1.3 Sediment classification and distribution in the CIOB 7 1.4 Mineral deposits in the CIOB 12 1.5 Previous studies related to volcanism and hydrothermal 12 activity in the CIOB 1.6 Objectives 13 1.7 Summary 14 Chapter 2 Materials, Methodology, and Techniques 15-27 2.1 Sampling details 16 2.2 Sediment and rock processing 17 2.3 Microscopy 18 2.3.1 Binocular, Polarized 18 2.3.2 Scanning Electron Microscope (SEM) 19 2.4 Analytical Procedures 19 2.4.1 Energy Dispersive Spectrometry (EDS) 19 2.4.2 Electron Probe Micro Analyser (EPMA) 20 2.4.3 X-Ray Fluorescence (XRF) Spectrometer 20 2.4.4 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) 21 2.4.5 Magnetic susceptibility 21 2.5 Summary 22 Chapter3 Basalts and Vhm From Seamount Dominated Areas 23-56 3.1 Introduction 29 3.2 Literature review on seamounts 30 3.3 Study Area 32 3.4 Results 33 3.4.1 Morphology of the CIOB seamounts 33 3.4.2 Petrography of seamount basalts 33 3.4.3 Chemical composition of seamount basalts 36 3.5 Interpretations and Discussion 36 3.5.1. Seamount morphology 36 3.5.2 Petrography 36 3.5.3. Major oxides 38 3.5.4 Minor, trace and rare earth elements 40 3.5.5 Seamount emplacement and associated vhm 48 3.6. Conclusions 49 Chapter 4 Iron-Rich Magnetic Spherules 57-76 4.1Introduction 58 4.2 Earlier studies on Fe-rich spherules in the Indian Ocean 59 4.3 Study area 59 4.3.1 Sampling strategy 59 4.4 Results and Interpretations 61 4.4.1 Sediment characteristics and coarse fraction 61 description 4.4.2 Iron-rich magnetic Spherules 62 4.4.2.1 Spatial distribution 63 4.4.2.2 Physical characteristics- Shape, Size and 63 Texture 4.4.2.3 Chemical composition of spherules 67 4.4.3 Fe-rich Particles: physical and chemical parameters 69 4.4.4 Comparison with other deep sea Fe-rich spherules 69 4.5. Discussion 70 4.5.1 Formational process of Fe-rich spherules 72 4.5.2 Volcanic-hydrothermal activities in the CIOB 75 4.6 Conclusions 76 Chapter 5 Metal-Rich and Native Grains 87-105 5.1 Introduction 88 5.2 Study Area 90 5.3 Results and Interpretations 90 5.3.1 Titano-magnetite grains 90 5.3.2 Metallic grains: Zn-Cu-S, Zn-Cu 91 5.3.3 Metallic grains: Ba-S, Ba-Pb-S 94 5.3.4 Aluminium-rich spherules 94 5.3.5 Native silver grains 95 5.4 Discussion 95 5.4.1 Model for the formation of metal grains in the CIOB 100 5. 5 Conclusion 103 Chapter 6 Basalt Emplacement: Formation of baked sediments and 106-130 vhm 6.1 Introduction 108 6.2 Sediment core 108 6.3 Results and Discussion 109 6.3.1 Physical description and coarse fraction of the 109 sediment core AAS-22/7 6.3.2 Magnetic susceptibility measurements 110 6.3.3 Geochemistry of core AAS-22/7 sediments 114 6.3.4 Geochemistry of the entrapped rock 118 6.3.5 Effect of magmatic activity on the sediments 120 6.4 Conclusion 122 Chapter 7 Effect of low-temperature alteration: Palagonite formation 131-163 and Mass disappearance of radiolarian 7.1 Introduction 132 7.2 Background work 134 7.3 Study Area 135 7.4 Results and Interpretations 135 7.4.1 Core fractions description 135 7.4.2 Palagonite Grains: morphology and composition 138 7.4.3 Phillipsite Grains: morphology and composition 141 7.4.4 Composition of the core sediments 143 7.4.5 Magnetic susceptibility measurements 145 7.5 Discussion 149 7.5.1 Mass disappearance of radiolarians 149 7.5.2 Alteration of sediments / Palagonitisation 150 7.6 Conclusions 153 Chapter 8 Volcanic Glass Shards and Tephrochronology 164-190 8.1 Introduction 165 8.2 Earlier studies of the CIOB glass shards 167 8.3 Study area 167

8.4 Results and Interpretations 168 8.4.1 Core descriptions: 168 8.4.2 Morphology of shards (Shape, Size, Texture) 171 8.4.3 Chemical composition of glass shards 171 8.4.4 Sedimentation rate and age of the cores 172 8.4.5 Tephrochronology of the CIOB Shards 174 8.5 Discussion 177 8.5.1 Source and Formation of shards 179 8.6 Conclusion 180

Chapter 9 Summary and Conclusions 195-197 9.1 Summary and Conclusions 196 9.2 Scope for future study 196 References 198-212

CERTIFICATE

This is to certify that the thesis titled “Volcanogenic and hydrothermal evidence from the Central Indian Ocean Basin since 60 Ma” submitted to Goa University, by Ms. Ankeeta Ashok Amonkar for the award of the degree of Doctor of Philosophy in Marine Sciences is a record of original and independent work carried out by her during the period of September 2015- January 2020 under my supervision and the same has not been previously submitted for the award of any diploma, degree, associateship or fellowship or any other similar title.

CSIR-NIO, Goa January 2020

Dr. Sridhar D. Iyer Ph.D. Supervisor Formerly with CSIR-National Institute of Oceanography Dona Paula 403004 Goa, India

DECLARATION

As required under the University Ordinance OB-9A, I hereby declare that the matter embodied in this thesis titled “Volcanogenic and hydrothermal evidence from the Central Indian Ocean Basin since 60 Ma” submitted to Goa University, for the award of the degree of Doctor of Philosophy in Marine Sciences is a record of original and independent work carried out by me during the period of September 2015 – January 2020 under the supervision of Dr. Sridhar D. Iyer, CSIR-National Institute of Oceanography, Dona Paula and that it has not been previously formed the basis for award of any diploma, degree, associateship or fellowship or any other similar title.

CSIR-NIO, Goa January 2020

Ms. Ankeeta Ashok Amonkar CSIR-National Institute of Oceanography Dona Paula 403004 Goa, India

Goa University Taleigao, Goa, India

ACKNOWLEDGEMENTS

The successful completion of this thesis work would not have been possible without the support, encouragement, cooperation and assistance from many individuals who contributed immensely and stood constantly with me in these 4 years of long journey.

With great pleasure I take this opportunity to express my deep sense of gratitude to my research supervisor Dr. Sridhar D. Iyer, Retired Chief Scientist, CSIR-National Institute of Oceanography. His immense knowledge, valuable guidance and cool temperament helped me in evolving as a better person and in completing my research work successfully. I am really fortunate to have him as my Ph.D. supervisor.

I fall short of words to thank Dr. G. N. Nayak, Emeritus Scientist, School of Earth, Ocean and Atmospheric Sciences and Dr. R. Mukhopadhyay for their suggestions and advice during the doctoral committee meetings. Their inspirational words, scientific ideas and patience have equally helped me in completion of my research.

I am thankful to the Director, CSIR-NIO, Dr. V. Loveson (Head GOD and Project Leader), CSIR-NIO, The Vice-Chancellor Goa University and Dr. H. Menon, Dean of Faculty of School of Earth, Ocean and Atmospheric Sciences, Goa University for providing facilities and in administrative matters.

Wholeheartedly I am thankful to Dr. F. Badesab for permitting me to carry out magnetic susceptibility analysis, Dr. M. Kocherla for XRF analyses, Dr. A Mudholkar for thin section preparation, Dr. G. Parthiban for ICP-MS, Mr. A Sardar for SEM and SEM- EDS, Mr. V M Khedekar for EPMA,and Dr. Brenda and Dr. Pratima for allowing access to geochemistry and sedimentology laboratories and Dr S. M. Gupta, (Retired scientist) for his help and discussion.

I specially remember and thank Dr. N. G. Rudraswami, for rendering me his computer system and financial support to carry out analysis outside CSIR-NIO. His positive and kind nature has always encouraged me to keep going in low times.

I am thankful to Dr. E V S S K Babu from CSIR-National Geophysical Research Institute and his student S Manju for helping with the EPMA analysis of glass shards and spherules.

It is indeed a great pleasure and privilege for me to express my indebted sense of gratitude to Prof. S. Balakrishnan, Professor, Dept. of Earth Sciences, for allowing me to work in his clean laboratory at Pondicherry University.

I am thankful to Dr. J. N. Pattan for being there, supporting with his valued comments and suggestions during my research work and Dr. Anthony Veigas for conducting SRF evaluation.

I acknowledge Dr. B. N. Nath (PL- GEOSINK), and Dr. M. Prasad, Late Dr. R. Banerjee, Dr. B. Chakraborty, Dr. A. Saha (all PL- PMN Survey and Exploration).

I acknowledge CSIR-Direct SRF Fellowship (Grant No. 31/026/ (0306)/2018-EMR- I) for funding and to carry out the research work. Page | i

I am thankful to Mr. V. Gaikwad, Mrs. C. Desai and Mr. T. Salkar for their help in preparation of various figures and maps.

I would also like to thank Dr. Subhashree Mishra, Dr. Krushna Vudumala, Dr. Tyson Sebastian, Dr. Niyati Kalangutkar, Mr. Raghav Gadgil and Mr. Mayank Panday for sparing their valuable time in assisting and providing right suggestions.

I think words are incomplete to express my love and gratitude to Dr. Rajni Magotra for moral support, advice and motivation along with good food, that have kept me going in the final stage of my Ph.D. Thank you for constantly being with me.

I am also grateful to Mr. Aarbaz Khan, Mr. Allen, Mr. Kanishak and Miss. Shailijha for their support and help in various analyses.

I would like to express my gratitude to my colleagues Shruti, Dafilgo, Agnelo, Prem, Nupur, Arun, Satyajyoti, Biswajyoti, Sudhir, Sarboday, Vikram, Susheel, Tanvi, Jayesh, Prasad, Chayanika, Amol, Simontini, Bhagyasree, Snehal, Priya, Archana, Kartheek, Lincy, Pavan,, Saranya, Manuel, Surabhi, Lata, Sucharita, Muller, Nisha, Omkar, Sambhaji, Damodar and Trinadh.

A big thanks to Siddhesh, Anjan, Shital, Ajeesh, Bhupendra, Nitin, and Siddharth for tolerating my mood swings, guiding me time to time, planning outings more often and making my Ph.D. life easier.

I also extend my thanks to the administrative staff of CSIR-NIO and Goa University for their co-operation, support and assistance during this journey.

A special thanks to Sadhguru and Isha family for introducing me to yoga and helping me develop integrity and focus in life.

I bow to my family and to God for providing me one of the finest pieces of life and allowing me to grow exponentially.

Last but not the least I would like to thank those who have directly or indirectly contributed to my research work.

Ankeeta Amonkar

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List of Publications (Published/ Communicated/ Ready) 1. Amonkar, A., Iyer, S. D., EVSSK Babu., S. Manju (2020). Extending the limit of widespread dispersed Toba volcanic glass shards and identification of new in-situ volcanic events in the Central Indian Ocean Basin. Journal of Earth System Science, 129 (175), 0-24. 2. Amonkar, A, Iyer, S. D., EVSSK Babu., Sardar, A., Shailajha, N., S. Manju (2020). Fluid-driven hydrovolcanic activity along fracture zones and near seamounts: Evidence from deep sea Fe-rich spherules. Acta Geologica Sinica- Accepted in August 2020. 3. Amonkar, A. (2019). Metal-enriched particles in the Central Indian Ocean Basin: Characteristics, composition, origin and possible future resource. Presented and Submitted full-length paper to Indian Science Congress Association, 2019. (Young Scientist Awardee). 4. Iyer, S. D., Amonkar, A. and Das, P. (2018). Genesis of Central Indian Ocean Basin Seamounts: Morphological, Petrological, and Geochemical Evidence. International Journal of Earth Sciences. Doi: https://doi.org/10.1007/s00531-018- 1612-z 5. Pattan, J N., Parthiban, G., Amonkar, A., Shaikh, S. and Sankar, S J. (2017). Geochemical trace and ultra-trace elements and their association in ferromanganese nodules from Central Indian Ocean Basin. Marine Georesources & Geotechnology. Doi: 10.1080/1064119X.2017.1297878. 6. Amonkar, A. and Iyer, S. D. (2019). The enigma of disappearance of radiolarians in the Central Indian Ocean Basin sediments – causal factors or casual reasons? (Ready for re-submission).

Conferences and Seminar Presentations/ Abstracts

 Oral Presentation

1. Amonkar, A. (2018). Metal-Enriched Particles in the Central Indian Ocean Basin: Characteristics, Composition, Origin and Possible Future Resource. Presented a full-length paper at Indian Science Congress Association, 3-7 January 2019, Lovely Professional University, Phagwara, Jalandhar, Punjab, India. 2. Amonkar, A. and Iyer, S.D. (2018). Mass disappearance of radiolarians in the Central Indian Ocean Basin: response to paleo-events. Presented at National

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Seminar on Effect on Paleo- and Anthropogenic Events on Earth System, 19-21 Sept 2018, Periyar University, Tamil Nadu, India. 3. Amonkar, A. and Iyer, S.D. (2017). An account of silicic glass shards in the Central Indian Ocean Basin: terrestrial and in-situ origin. Presented at OSICON- 2017, 28-30th Aug. 2017, Thiruvananthapuram, Kerala, India.

 Poster Presentation

1. Amonkar, A. and Iyer, S.D. (2018). Influence of bottom water masses on volcanic and ferromanganese deposits, Central Indian Ocean Basin– weathering and alteration. Presented and was awarded 1st place for poster at Dynamics of Surface & Subsurface Geological Processes, Feb 8-9, 2018, Pondicherry University, India. 2. Amonkar, A., De Seixas, A., Iyer, S D. (2017). Preliminary results of two sediment cores from the Central Indian Ocean Basin: Source of the coarse fractions. Presented and was awarded 1st place for poster at National Conference on Young Researchers, 16-17 March 2017, Goa University, Goa, India. 3. Amonkar, A. and Iyer, S.D. (2016) Occurrence and origin of pyroclasts in the Central Indian Ocean Basin: are intraplate environments congenial for explosive volcanic eruptions? Poster at “The National Geo-Research Scholars Meet, June 1- 4, 2016 at Wadia Institute of Himalayan Geology, Dehradun, India.

 Only Abstract 1. Amonkar. A., Iyer, S.D. and Sardar, A. A., (2018). Fracture-controlled Phreatomagmatic activity in the Central Indian Ocean Basin: evidence from magnetic spherules. American Geophysical Union, Washington D.C. 2. Amonkar. A. and Iyer, S.D., (2018). Are ridge - extended fracture zones possible hydrothermal sites? A case study of the Central Indian Ocean Basin. SCOR-Inter Ridge Indian Ocean meeting, 14-18 November, CSIR-NIO, Goa, India. 3. Amonkar, A. and Iyer, S.D. (2017) Formation and significance of zeolites from the central Indian Ocean basin sediments. Participated at National Seminar Zeolites & Allied Mineral Deposits of India, 6-7th October 2017, Belgavi, India.

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List of Acronyms

ARM - AnhystericRemanent Magnetization AABW - Antarctic Bottom Water CCD - Carbonate Compensation Depth CR - Carlsberg Ridge CIOB - CentralIndianOcean Basin CIR - Central Indian Ridge DSDP - Deep Sea Drilling Program DBZ - Deformed Boundary Zone E-MORB - Enriched Mid-Oceanic Ridge Basalt EPMA - Electron Probe Micro Analyser EDS - Energy Dispersive Spectrometry EPR - East Pacific Rise FZ - Fracture Zones FCI - Fuel-coolant Interaction HDT - HaranggoalDacitic Tuff HREE - Heavy Rare Earth Element IONF - Indian Ocean Nodule Field IOTJ - Indian Ocean Triple Junction TJT-In - Indian Ocean Triple Junction Trace ICP-MS - Inductively Coupled Plasma-Mass Spectrometry IIOE - International Indian Ocean Expedition IODP - International Ocean Drilling Program IRM - Isothermal Remanent Magnetization LREE - Light Rare Earth Element MS - Magnetic Susceptibility MAR - Mid-Atlantic Ridge MTT - Middle Toba Tuff MOR - Mid-Ocean Ridge MFCI - Molten Fuel-coolant Interaction N-MORB - Normal Mid-Ocean Ridge Basalt ODP - Ocean Drilling Program

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OIB - Ocean Island Basalt OTT - Oldest Toba Tuff REE - Rare Earth Elements RTJ - Rodriguez Triple Junction SIRM - Saturation Isothermal Remanent Magnetization SEM - Scanning Electron Microscope SEIR - Southeast Indian Ridge SWIR - Southwest Indian Ridge SRH - Shard-Rich Horizons vhm - Volcanogenic-Hydrothermal Materials WDS - Wavelength Dispersive Spectroscopy XRF - X-Ray Fluorescence YTT - Youngest Toba Tuff

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PREFACE

Introduction The Indian Ocean is the world’s third largest water body, characterized by three active mid-oceanic ridges, subduction zones, and a deformed boundary zone. Indian Ocean has several basins amongst which the Central Indian Ocean Basin (CIOB) is the largest. This basin extends between the latitudes 6ºS and 20ºS and longitudes 72ºE and 80ºE and has an average water depth of 5,000 m. The basin is bounded by the Central Indian Ridge (CIR) on the west, Ninetyeast Ridge on east and Southeast Indian Ridge (SEIR) in the south. The basin hosts different morpho-tectonic features such as seamounts, abyssal hills, faults, and fracture zones (FZ). Seamounts of variable height (100 to 1,000 m, mostly <800 m) occur in chains and also in isolation and as mounds (<50 m) (Das et al., 2007) and were formed during the plate separation, reorganization and movement of India between 60 and 50 Ma. Extensive sampling in the CIOB helped to recover a variety of rocks such as basalts, ferrobasalts, spilites, pumice clasts and also sediments such as terrigenous, calcareous, siliceous and red clay. Basalts in the basin occur as pillows, large outcrops and as fragments on the slope and summit of the seamounts and abyssal hills. Compositionally, the basalts are Normal- Mid-Ocean Ridge Basalts (N-MORB) similar to those from the Mid-Atlantic Ridge and East Pacific Rise (Iyer, 1996; Mukhopadhyay et al., 2008; Das et al., 2012; Iyer et al., 2018). Besides N-MORB, ferrobasalts that were formed during a fast movement of the Indo- Australian Plate contain high Fe (>12 wt %) and Ti (>2%) and were recovered near topographic highs and high amplitude magnetic zones. These basalts have plagioclase (predominant), olivine (rare) and frequently small euhedral magnetite and hematite grains. The ferrobasalts were formed from Fe-rich fractionated melt that was emplaced at shallow crustal depth (Iyer et al., 1999a). Spilites (albitised basalts) occur near the Indrani FZ (79oE) and were formed due to low-temperature alteration of basaltic lavas (Karisiddaiah and Iyer, 1992). Pumices of variable colours, shapes, sizes, and vesicularities encompass a large field (600,000 sq km) in the CIOB and are trachyandesitic to rhyodacitic in composition. The origin of the pumice has been ascribed to in-situ intraplate volcanism (Iyer and Sudhakar, 1993; Iyer, 1996; Kalangutkar et al., 2011) as well as drifting from the 1883 eruption of Krakatao, Indonesia (Iyer and Karisiddaiah, 1988; Mukherjee and Iyer, 1999; Pattan et al., 2008; Kalangutkar et al., 2011).

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Hydrothermal activity at MOR is a common phenomenon and results in the formation of hydrothermal deposits (Beaulieu et al., 2013). Besides the MOR tectonic settings, hydrothermal activity is also reported in the intraplate (<3,000 m water depth) seamounts such as Macdonald, Loihi, and Vailulu’u (Samoan Hotspot related) that occur in the Pacific Ocean (White et al., 2003; Clague et al., 2009). In contrast to these settings, signatures of hydrothermal mineralization have been reported to be rare/sporadic in the intraplate seamounts of the CIOB (Iyer et al., 1997a, b, 1999; Kalangutkar et al., 2015). In addition to the above major volcanics, the presence of volcanogenic-hydrothermal materials (vhm) at the base of seamounts and near FZ in the CIOB suggests that the basin to be tectonically and hydrothermally active. The vhm are comprised of fragile bread-crust-like magnetic particles, glass shards (silicic and basaltic), palagonite grains, magnetite spherules, Fe-Ti spherules (Iyer, 2005 and references therein) and occurrence of spherules and particles of native aluminum (Iyer et al., 2007). It has been suggested that the vhm represent sporadic hydrothermal events in the CIOB since 625 ka (Iyer et al., 1997a,b; 1999b) to as recent as 100 yr (Nath et al., 2008). The detailed morpho-tectonic features and geologic materials of the CIOB have been reported by many researchers (Mukhopadhyay et al., 2018 and references therein) yet, there is a lacuna in our knowledge concerning the widespread presence of vhm that could constitute an important link to the several volcanic events that occurred in the CIOB since 60 Ma. Hence, in the present work, basalts from seamounts and vhm occurring in the sediments have been investigated to derive a link between these and the morpho-tectonics features. To achieve the objective, sediment coarse fractions along with other proxies (whole-rock and sediment geochemistry and magnetic susceptibility) have been used to trace the various volcanic events in the CIOB. In the above background, I propose to test some hypotheses concerning the vhm that occur in an intraplate environment of the CIOB where FZ and seamounts abound. 1. Is intraplate volcanism unique to the CIOB? If so, is it sporadic or episodic or continuum? 2. What is the mode of formation and mechanism of emplacement of the vhm? 3. Is there any relation between the occurrence of the vhm and morpho-tectonic features? 4. Were the co-existing hydrothermal materials formed contemporaneously with the volcanic products or are the hydrothermal events a later phenomenon (i.e., in time and space)? 5. Are the silicic glass shards occurring in the CIOB solely from the Indonesian Volcanic Arc (especially Toba eruptions) or is there a possibility of their in-situ formation in the basin?

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Objectives  Characterize the seamount basalt (petrography, geochemistry, origin)  Distinguish the vhm based on their colour, shape, size, texture and composition  Classify the vhm as pyroclasts, shards, spherules, metal particles  Comprehend the compositional variability of the vhm  Understand the formational mechanism of the vhm at abyssal depth  Decipher the role of morpho-tectonic features in the formation of the vhm  Model the volcanic and hydrothermal history of the CIOB.

Chapter 1 This chapter provides a general introduction about the CIOB in terms of its tectonic evolution, geology, sediment distribution, morpho-tectonic features, and ferromanganese nodule deposits. This chapter also collates the information of previous studies related to volcanism and hydrothermal activity, and the gap in our understanding between morpho- tectonic features and vhm. The chapter concludes with the importance of the present study, and the proposed scientific objectives, and hypothesis.

Chapter 2 This chapter describes the materials, methodology, and techniques that have been followed to fulfill the projected research objectives. The methods employed are onboard sampling for rocks and sediments and their recovery from different morpho-tectonic areas in the basin. This was followed by processing of the sediment samples for coarse fraction (CF, +63 µm), identification of the CF using binocular, petrological, and scanning electron microscopes (SEM). Chemical composition of several identified components was obtained using an SEM-EDS and Electron probe micro analyzer (EPMA). The major elements of whole rock and selected sediments were determined using an X-Ray Fluorescence (XRF), while the minor, trace and rare earth element were measured using an Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Magnetic susceptibility and related parameters such as Anhysteric Remanent Magnetization (ARM) and Isothermal Remanent Magnetization (IRM) were carried out for four sediment cores.

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Chapter 3 This chapter presents the description of a four seamounts based on their parameters such as height, basal area, summit area, volume, flatness, and slope. These parameters helped to morphologically classify the CIOB seamounts. Petrography and whole-rock geochemistry of seamount basalts were carried out to understand their variability. The basalts are mainly pillow lavas with a glassy veneer on the outer rim while the interior is holocrystalline. Compositionally, the basalts are low FeO* (8.0–10.5 wt %) and TiO2 (1.3–

2.0 wt %), and has variable K2O (0.1–1.0 wt %) contents. Further the whole-rock geochemistry of seamount rocks were compared with the surrounding seafloor basalts that have differing TiO2 (< 2.0 wt% and > 2.0 wt%) and ferrobasalts and also with basalts from the CIR, SEIR, and Deep-Sea Drilling Project (DSDP) sites 214, 215, 216 and 254 (all near the Ninetyeast Ridge). The study revealed that the seamount basalts are similar to those from the basinal seafloor and the CIR and SEIR, suggesting a sharing of a common magmatic source between 56 and 51 Ma. Details about the formation, source, and fractionation trends in the genesis of the CIOB seamounts basalts, and their association with the vhm are also discussed.

Chapter 4 This chapter details the extensive occurrence of iron-rich magnetic spherules in the CIOB. Sediments from two different geological sites – Seamounts (Site-1) and FZ (Site-2) – were investigated to understand the spatial distribution of the spherules. The spherules were characterized based on their shape, size, and texture. The composition of the spherules revealed high contents of iron, moderate amounts of titanium and minor contents of zinc, chromium, nickel, lead, and sulphur. Considering these evidence and presence of spherules near Sites-1 and 2, the mechanism of spherule formation is discussed. The results showed that these two areas were congenial for the formation of the spherules.

Chapter 5 In this chapter the new finding of metal-rich and native particles that occur in the CIOB are detailed. These particles were recovered from the surface and core sediments retrieved in the vicinity of a seamount and 79°E FZ. These particles are variably enriched in Zn-Cu, Ba-S, and Ba-S-Pb and in addition, there are particles of native Al and Ag. These metal-rich particles from the CIOB are ascribed to have formed from intraplate Page | x hydrothermal processes. Models for hydrothermal activity at FZ and seamount site are presented.

Chapter 6 This chapter describes the effect of magmatic activity controlled by the 79°E FZ. A well-preserved layer of altered basaltic rock was observed in a 5.6 m long gravity core retrieved from the siliceous sediment domain. The entrapped basalt occurs between 280 and 355, and 470 and 490 cm-bsf (below the seafloor). The core has been studied using proxies such as CF, magnetic susceptibility, and major, minor and trace element distributions. These parameters were important to understand the effect of magmatic activity on the host sediments and the subsequent formation of the vhm. The sedimentary components and elemental distribution indicate the presence of abundant magnetite spherules just above the basaltic layer. The geochemistry of the sediments and entrapped basalt reveal the effect of magmatic activity along the FZ on the sediment.

Chapter 7 In this chapter, emphasis is placed on the occurrence of abundant palagonite grains and their association with biota (radiolarians and diatoms). Palagonite grains occurring in two sediment cores located in the pelagic domain were studied in terms of their physical characteristics and chemical composition. The formation of palagonite is suggested to be a result of a low-temperature alteration of basaltic glass contained within the sediments. It was enigmatic to find that the presence of palagonite had an inverse relation with radiolarians in that, wherever radiolarians were present, palagonite grains were absent and vice-versa. The reasons for such an inverse relation is perhaps due to (i) the biogenic material going into dissolution during the process of palagonitisation, and/or (ii) the sediments suffered extensive alteration during the seepage of low-temperature fluid from the adjacent 79⁰E FZ.

Chapter 8 This chapter pertains to the extensive occurrence of volcanic glass shards in the CIOB sediments. A study of four sediment cores showed a variable abundance of glass shards at different core depth and these are referred to as shard-rich horizons (SRH). Detail observations were made of glass shards considering their morphology, size, abundance, and distribution in the sediment cores and the chemistry of glass shards was also taken into account. As it was important to link the SRH to known terrestrial volcanic events, hence Page | xi dating of the cores and sedimentation rates were determined by using the Australasian microtektites, occurring in the sediments that have a definite age of 0.77 Ma. Based on several evidence the tephrochronology of the CIOB glass shards has been derived in the present study. The 12 newly identified SRH have been correlated with the reported 16 SRH within the ODP Site 758 from the Northern Indian Ocean. Although the major source for these SRH is ascribed to be from the Toba eruptions. But two new insitu phreatomagmatic events have been identified based on morphology and chemistry of the glass shards.

Chapter 9 The Conclusions drawn from the present research work are presented in this chapter. Furthermore, scope and need for future volcanological studies in the CIOB are suggested.

A complete list of references cited in this work is presented in alphabetical order at the end of the thesis.

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List of Figures

Page No. Fig. 1.1 Major morpho-tectonic features and basins of the Indian Ocean. 2 Fig. 1.2 Detail bathymetry of the Indian Ocean. 4 Fig. 1.3 General multibeam map of the CIOB. 5 Fig. 1.4 Bathymetry map of the 79° fracture zone. 5 Fig. 1.5 Magnetic isochron data of the Central Indian Ocean 6 Fig. 1.6 Map of the Indian Ocean. 7 Fig. 1.7 Sediment distribution map of the CIOB. 9 Fig. 1.8 Manganese nodules occurring on the sediment of the CIOB 11 Fig. 2.1 Location map of the study area. 15 Fig. 2.2 Different components occurring in the CF 17 Fig. 3.1 Generalized map of the CIOB and the adjacent regions showing (a) 28 isobaths and tectonic elements and (b) age of the ocean floor. Fig. 3.2 Bathymetry of the three seamounts and location of the dredged 29 samples. Fig. 3.3 Bathymetry of the fourth studied seamount and location of the 30 dredged samples. Fig.3.4 Hand specimen photographs of the basalts recovered from the 31 seamounts. Fig. 3.5 Photomicrographs of the seamount basalts. 32 Fig. 3.6 TAS diagram shows the rock samples from the CIOB seamounts 35 and comparison with those from DSDP and SEIR sites. Fig. 3.7 Interrelationship between MgO and major oxides in seamount 37 basalt. Fig. 3.8 The relation between V vs. FeO (Fig. 3.8A), V vs. TiO2 (Fig. 3.8B) 38 Ba/Nb vs. MgO (Fig. 3.8C) and CaO/Al2O3 vs. MgO (Fig. 3.8D)in seamount basalt. Fig. 3.9 Plots of Zr v/s FeO and TiO2 in seamount basalt. 38 Fig. 3.10 Plots of Y v/s Zr.in seamount basalt. 39 Fig. 3.11 Plot of (La/Sm)N against TiO2 shows the clusters C1 and C2 (Fig. 39 11A). Ba/Nb vs. V (Fig. 3.11B), Ce/Y vs. Zr/Nb (Fig. 311C) in seamount basalt. Fig. 3.12 Spidergrams of incompatible trace elements of the CIOB 41 seamounts. A: sample /N-MORB. B: sample /primitive mantle in seamount basalt. Fig. 3.13 Spidergrams of incompatible trace elements of basalts from the 41 CIOB seamounts and other areas in seamount basalt. Fig. 3.14 Plots of rare earth elements of the CIOB seamounts. A:sample/N- 42 MORB. B: sample /primitive mantle in seamount basalt. Fig. 4.1 Left panel: General map (From Google earth). Centre panel: 56 Location of the study area.

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Fig. 4.2 Spatial distribution of the Iron-rich magnetic spherules from 55 57 locations in the CIOB. Fig. 4.3 Photograph of baked sediment from depth 280-355 cm, AAS-22/7. 57 Fig. 4.4 Down core variation in coarse fraction percentage in the two 58 studied cores from Site-2 of the CIOB. Fig. 4.5 The pie chart represents the classification of Fe-rich magnetic 60 spherules based on shape (A) and size (B). Fig. 4.6 Scanning electron micrographs of type 1: Smooth magnetic 61 spherules with no crystallinity. Fig. 4.7 Scanning electron micrographs of type 2: textured magnetic 62 spherules Fig. 4.8 Scanning electron micrographs of polished magnetic spherules (A- 63 H). Fig. 4.9 Scanning electron micrographs of Fe-rich particles that were 65 associated with the Fe-rich spherules. Fig. 4.10 A schematic diagram of magmatic activity associated with fracture 70 zone and seamounts and formation of magnetic spherules. Fig. 5.1 Location of reported hydrothermal sites in the Indian Ocean. 84 Fig. 5.2 Electron micrographs of metal-rich grains from the CIOB 87 sediments. A-Titano-magnetite grains; B and C-Well-crystallized titano-magnetite grains; D- buff orange Zn-Cu grain; E- lighter orange Zn-Cu-S; F-pale orange rich in Zn; G- Native Cu grains. Fig. 5.2 Electron micrographs of metal-rich grains from the CIOB sediments. 88 H-J = Barium-Sulphur grains; K-M = Native Pb grains with titaneferous iron oxide matrix; N= Aluminium spherule; O= Native silver grains embedded on Cu-S grain. Fig. 5.3 Model for hydrothermal deposits associated with seamounts in the 96 basin. Figure not to scale. Fig. 5.4 Model for hydrothermal deposits associated with the fracture zone 97 in the basin. Figure not to scale. Fig. 6.1 Hand specimen of basaltic pieces in the sediment core AAS-22/7. A 103 = Basaltic pieces at depth 345-355 cm; B = A specimen of basalt in which the cracks are filled with sediments; C = Highly altered rock pieces at depth 470-490 cm; D = Down-core variation of coarse fractions. Fig. 6.2 Magnetic susceptibility plot for cores A=AAS22/7 and B=AAS- 107 22/8. Fig. 6.3 Down-core variations in major oxides in the core AAS-22/7. 109 Fig. 6.4 Plot of Co, Ni, Cu, and Zn against depth in the core AAS-22/7. 111 Fig. 6.5A PAAS normalised plot of rare earth elements for the core AAS- 111 22/7

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Fig. 6.5 B N-MORB normalised Plot of rare earth elements for the core AAS- 112 22/7 Fig. 6.6 Classification of entrapped basalt based on TAS. 112

Fig. 6.7A The plot of Fe2O3–Al2O3–MgO for the characterization of clay. 114 Fig.6.7B Plot of Al2O3 versus Fe2O3 indicating mineralogy of the studied 114 core sediment. Fig. 6.7C Ternary diagram of K2O - Al2O3 – CaO + Na2O. 115 Fig. 7.1 Down core variation in the CF% for the core AAS-22/5 and AAS- 129 22/3. Fig. 7.2 Downcore variation in the abundance of components in the core 129 AAS-22/5 from a depth of 250 cm and below. Fig. 7.3 SEM Photographs of Palagonite grains. A-C=Palagonite grains; 131 D=phillipsite crystals growing out from the palagonite grain. Fig. 7.4 EPMA analysis of polished Palagonite grains. 132 Fig.7.5 Classification of palagonite based on TAS. 132 Fig. 7.6 Plot of (A) TiO2 vs FeO, (B) CaO+MgO vs K2O and (C) 133 CaO+MgO+MnO vs K2O for palagonite. Fig. 7.7 Plot of MgO, CaO, and K2O against LOI. 134 Fig. 7.8 Scanning electron micrographs of A=Phillipsite crystals showing 135 different forms of twinning, B= Overgrowth and outgrowth of phillipsite crystals on/from a grain of palagonite. Fig.7.8C EPMA analysis of polished phillipsite grains. 136 Fig. 7.9 Down-core variation in the major oxide data for the core AAS-22/5 137 and AAS-22/3. Fig. 7.10 The plot of Fe2O3–Al2O3–MgO for the characterization of clay. 138 Fig. 7.11 Magnetic susceptibility plot of core AAS-22/5 (A) and AAS-22/3(B). 141 Fig. 8.1 Map showing the major physiographic features of the CIOB and 160 sampling site for glass shards. Fig. 8.2 Down-core variation of CF% in the four studied cores of the CIOB. 161 Fig. 8.3 Electron micrographs of the volcanic shards. A:(Type I) cuspate 163 and with Y- shape intersections. B: (Type II) platy and blocky. Fig. 8.4 Schematic section of the core showing of Shard Rich Horizons 164 (SRH) at different depths within four cores Fig. 8.5 Chemical composition of SRH plotted on the TAS diagram. 166 Fig. 9.1 A schematic sketch shows different processes that are occurring in 186 the CIOB

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List of Tables

Page No Table 2.1 Compilation of locations and details of the studied sediments cores. 15 Table 2.2 Details and quantum of the work carried out. 20 Table 2.3 Details of CF% in all the four studied cores. 20 Table 3.1 Locations of basalts collected from four seamounts in the CIOB. 47 Table 3.2 Characteristics of the four studied seamounts wherefrom the samples 47 were dredged from the CIOB. Table 3.3 Petrographic details of the CIOB seamounts’ basalts. 48 Table 3.4 Whole-rock composition of basalts from four seamounts, CIOB. 49 Table 3.5 CIOB seamount basalts compared with those from the seafloor, CIR 51 and SEIR. Table 4.1A Details of 40 sediment sampling locations from Site-1. 73 Table 4.1B Details of iron-rich magnetic spherules occurring at 55 sediment 74 sampling locations from Site -1. Table 4.2 Morphological classification of Fe-rich magnetic spherules (A=Site-1 76 and B= Site-2). Table 4.3 Major element composition (wt%) of Fe-rich magnetic spherules from 77 (A=Site-1 and B= Site-2) and C=Minor elements from Site-1& 2. Table 4.4 Elemental composition (wt%) of A=major elements and B= minor 81 elements occurring in polished Fe-rich magnetic spherules. Table 4.5 Elemental composition of Fe-rich particles associated with spherules. 82 Table 4.6 Chemical data of iron-rich magnetic spherules from world ocean. 82 Table 5.1 Chemical composition of Fe-Ti grains. 99 Table 5.2 Chemical composition of A=buff orange grains rich in Zn, B=light 99 orange grains rich in S, C=Pale orange grains rich in Zn and D= Cu grains. Table 5.3 Chemical composition of A=metallic grains rich in barium, B= grains 100 rich in Ba-S-Pb, and C= lead grain. Table 5.4 Chemical composition of native aluminium spherules. 101 Table 5.5 Chemical composition of native silver grains. 101 Table 6.1A Magnetic susceptibility data for the core AAS-22/7. 117 Table 6.1B Magnetic susceptibility data for the core AAS-22/8. 119 Table 6.2A Down-depth major oxides (wt.%) of core AAS-22/7. 122 Table 6.2B Down-depth of trace and REE (ppm) of core AAS-22/7. 133 Table 7.1 A compilation of studies on disappearance of radiolarians in several 148 sediment cores from the CIOB. Table 7.2 Down-depth variations in the abundance (%) of the components in the 149 sediment core AAS-22/5.

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Table 7.3 Chemical composition of palagonite grains from the CIOB and from 150 the world ocean. Table 7.4 Chemical composition of phillipsite crystals from the CIOB. 151 Table 7.5 Down-depth major oxides (wt%) of sediment core A=AAS-22/5and B= 151 AAS-22/3. Table 7.6 Magnetic susceptibility data of core A=AAS-22/5and B= AAS-22/3. 153 Table 8.1 A compilation of reported glass shards along with their depth of 173 occurrence in the CIOB. Table 8.2 The coarse fraction components are listed in decreasing order of 175 abundance in SRH. Table 8.3 EDS composition of the glass shards recovered from different SRH in 176 the four studied cores. Table 8.4 Compilation of compositions of the CIOB glass shards. 181 Table 8.5 The occurrence of SRH at variable depths within the four cores along 182 with the calculated ages. These ages are compared with the paleomagnetic (Diehl et al., 1987) and δ18O dates from Dehn et al. (1991). Table 9.1 Gist of different volcanogenic and hydrothermal events from the CIOB. 187

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CHAPTER 1

AN OVERVIEW OF THE CENTRAL INDIAN OCEAN BASIN: STRUCTURE, TECTONIC, SEDIMENT AND VOLCANISM

Chapter 1

1.1 Introduction

The Indian Ocean is the third largest ocean covering 19.8% area of the world ocean. Geographically, the Indian Ocean extends from latitudes 22º N and 65º S and longitudes 20º E and 125º E and has an area of 74 million sq. km. The water depth ranges from 1,800 to 5,400 m. The Indian Ocean was formed as a result of break-up of Gondwanaland during 95 ± 5 Ma. In the framework of plate tectonics the active mid-ocean ridge system (MOR) in the Indian Ocean consists of the Carlsberg Ridge (CR), Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR), and Southeast Indian Ridge (SEIR) and form boundaries between the India, Africa, Antarctica and Australia plates (Fig. 1.1). The CIR, SWIR and SEIR meet at latitude 25°S and longitude 70°E giving rise to the Rodriguez Triple Junction (RTJ) or Indian Ocean Triple Junction (IOTJ) (McKenzie and Sclater, 1971). This RTJ has a ridge-ridge-ridge type configuration (Tapascott et al., 1980). The trace of the plate movement post-breakup of the Gondwanaland is preserved in the Indian Ocean basin as Indian Ocean Triple Junction Trace (TJT-In) (Lisitzyn and Gurvich, 1987; Mitchell and Parsons, 1993; Dyament, 1993). The northern branch of the MOR comprises of two segments: the CIR that runs north to south and the CR which joins the CIR and abuts at the Owen Fracture Zone in the northwest. The SWIR continues westward and is known as the Southern Mid-Atlantic Ridge (MAR) in the Atlantic Ocean. The SEIR extends southeast to join the Pacific- Antarctic MOR system south of Australia (Fig. 1.1). Much of the knowledge about the Indian Ocean has emanated from the geophysical studies carried out in early 1960s during the course of the International Indian Ocean Expedition (IIOE) between 1959 and 1965. Later the international scientific campaigns by Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP) and International Ocean Drilling Program (IODP) drilled tens of sites in the Indian Ocean. These programmes provided large amount of bathymetric data and samples (rocks, sediments) that helped to validate the inferences drawn from geophysical and geological investigations of the IIOE.

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Chapter 1

Fig 1.1 Major morpho-tectonic features and basins of the Indian Ocean. (Source: Yatheesh et al., 2019). Red line = spreading centers; Blue lines = subduction zones; Pink lines = transform boundaries; Pale yellow = diffuse plate boundaries; Violet dots = Indian Ocean Triple Junction; Black lines = prominent fracture zones.

Apart from the active MOR, the Indian Ocean also consists of many aseismic ridges (Fig. 1.1) such as the Chagos-Laccadive Ridge formed by the Reunion hotspot and the Ninetyeast Ridge by the Kerguelen hotspot. The other aseismic ridges are the Broken Ridge, Madagascar Ridge, Mozambique Ridge, Kerguelen-Gaussberg Ridge and Laxmi Ridge. The Indian Ocean hosts a prominent deformed boundary zone (DBZ) occurring above the Indian Ocean Nodule Field (IONF) (Mukhopadhyay et al., 2008). The Andaman back-arc basin and Indonesian trench are subduction zones that are regimes of active tectonics in the Indian Ocean. The ocean also has microcontinents such as Socotra with Error seamount near ridges such as Madagascar, Agulhas, and Crozet. There are also

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Chapter 1 oceanic plateaus such as Kerguelen-Head and the Wallaby Plateau (Fisher et al., 1971). Madagascar, Seychelles, Sri Lanka are among the well known continental islands occurring in the Indian Ocean. Seamounts in the Indian Ocean are Error Seamount (near Africa); Sagar Kanya Seamount, Panikkar Seamount, Wadia Seamount (West coast of India); Alcock Seamount and Sewell Seamount (Andaman) (Iyer et al., 2012). Apart from the above geologic features, the Indian Ocean also has several basins such as Bengal Basin, Arabian Basin, Wharton Basin, Somali Basin, Madagascar, Laccadive and Central Indian Ocean Basin (CIOB) (Fig. 1.1). The CIOB has drawn attention of many researchers because of its complex tectonic and morphologic fabrics, vastness in latitudinal extent, presence of a variety of sediments and rocks, amongst others. Most importantly the basin hosts large and rich deposits of ferromanganese or polymetallic nodules that are second after the North Pacific nodule belt (Mukhopadhyay et al., 2018 and references therein).

1.2 Study area - Central Indian Ocean Basin

Among the sedimentary basins mentioned above, the CIOB is the largest in the Indian Ocean. This basin extends between the latitudes 6ºS to 20ºS and longitudes 72ºE to 80ºE and has an average water depth of 5,000 m. The basin is bounded by the CIR on the west, Ninetyeast Ridge on east and SEIR in the south. The basin has different morpho- tectonic features such as seamounts, abyssal hills, faults, crenulations, lineations, and fracture zones (FZ). The northern part of the basin (below the equator to 4ºS) has relatively a smooth seafloor with buried hills and the Afanasy-Nikitin Seamount complex. The sub- seafloor sediments are highly deformed, folded and faulted (Krishna, 2003). Based on bathymetry, Kodagali (1989) divided the basin into three major domains (Fig.1.2). 1. The Eastern part with a medium relief extends from 79º-82ºE and has a water depth of 5,000-5,500 m. 2. The Middle area is an abyssal plain from 74º-79ºE with water depth of 4,900-5,100 m. 3. The Western portion with the most rugged topography and variable relief extends from 71º-74º E and water depth is 2,900-5,000 m.

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Chapter 1

Fig. 1.2 Detail bathymetry of the Indian Ocean (after Kalangutkar, 2012). The map was produced from NIO dataset-www.nodc.noaa.go. Shallow depth to deeper water depth is represented by green to brown coloured scale on the right.

Seamounts of the CIOB have been used to understand the evolutionary history of the basin. Mukhopadhyay and Khadge (1990) reported the occurrence of seamounts based on the single-beam bathymetry data and suggested their origin could be from a hotspot. Later Kodagali (1991) opined that these seamounts may have resulted from mid-plate volcanism. Seamounts occur in chains and also in isolation and as mounds (<50 m) and are clustered in the eastern portion between 10°S and 14°S. Most of the 200 seamounts in the CIOB are oriented in an N-S direction and are located along eight propogative FZ (Fig. 1.3). The seamounts either have a single-peak, multi-peaks or composite peaks and these indicates multiple volcanic episodes in the CIOB (Das et al., 2005, 2007). This is attested by relatively fresh basalts on the slope and E-W enlargement of the base of some seamounts. This fact indicates that some of the seamounts may have been active even after their emplacement that took place several million years earlier (Iyer, 1996; Iyer et al., 2018). In this thesis, the seamounts are detailed in Chapter 3 with emphasis on their evolution based on morphology, petrology and geochemical evidence and presence of volcanogenic-hydrothermal materials (vhm).

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Chapter 1

Fig. 1.3 General multibeam map of the CIOB. Note the approximate N-S alignment of most of the seamounts (after Das et al., 2005). The seamounts enclosed in the rectangular box are detailed in Chapter 3.

The other important geologic and quite prominent features in the CIOB are the FZ. These FZ and ridge-normal lineaments are oriented in an N- S or NNE-SSW direction and are mostly perpendicular to the SEIR. The prominent FZ are the 73oE (Vishnu FZ), Triple Junction Trace on Indian Plate, 79oE (Indrani FZ) and 83oE (Indira FZ).

Fig. 1.4 Bathymetry map of the 79°E FZ. The inset shows cluster of seamounts to the south. (Source: Das et al., 2005).

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Chapter 1

The 79⁰E FZ also known as Indrani FZ (or L’Astrolabe) originated from the SEIR and traces of this FZ lie on either side of the SEIR. Geophysical data indicate a ridge and trough topography of the FZ (Fig. 1.4). During the global plate reorganization in the Indian Ocean the magnetic anomalies A26 to A21 underwent a change in direction (Dyment, 1993). The FZ has a throw of 100 m to the east and within the trough there are 14 seamounts which may have been produced during the reactivation of the FZ (Kamesh Raju et al., 1993; Das et al., 2005). Based on the geological setting and significance, sediment samples were collected adjacent to this FZ to understand its role in volcanic and hydrothermal activity.

1.3 Tectonic evolution of the CIOB

The formation of the CIOB commenced during the third phase of Gondwanaland separation that occurred in the Cretaceous-Paleocene time i.e. around 60 Ma. The spreading rate during this time was around 95 mm/year (half rate; henceforth half rate would be used) with India moving towards Eurasia. The spreading rate after the collision of the Indian Plate drastically reduced to 26 mm/yr during 50.8 Ma. Yatheesh et al. (2019) provided an updated magnetic anomaly map for the CIOB.

Fig. 1.5 Magnetic isochrons data of the Central Indian Ocean. Chron numbers are shown in different colours. Solid black line =MOR, dash line = fracture zones. (Source: Yatheesh et al., 2019).

Based on the magnetic data it is obvious that the northern part of the

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Chapter 1

CIOB was formed before the collision of the Indian Plate with the Eurasian Plate (Fig. 1.5). The displacement in the magnetic anomaly pattern near the prominent FZ indicates that the FZ were active and resulted in block faulting

Since my samples were collected from the IONF, it is important here to understand the formation of the IONF within the CIOB. Based on magnetic anomaly data of Royer et al. (1989), Mukhopadhyay et al. (2008) divided the IONF into four sectors (Fig. 1.6).

Fig. 1.6 Map of the southern part of the Indian Ocean. The rectangular box indicates the Indian Ocean Nodule Field (IONF), which is divided into four sectors A-D. Above the IONF is the Deformed Boundary Zone (DBZ). Dash lines are the FZ and solid lines are the Indian Ocean ridge systems.

(Source: Mukhopadhyay et al., 2008).

The northern part (Sector A) extends from 9⁰S to 10.26⁰S and was generated when the fast-spreading rate was 90 mm/yr. This sector occurs above chron 26 which corresponds to an age of 57.9 Ma. Below this sector is the north-central part (Sector B) extending from 10.26⁰S to 10.96⁰S that formed during an intermediate spreading rate of 55 mm/yr. This sector with chrons 26 and 25 has an age of 57.9-55.9 Ma. The south-central (Sector C) from 10.96⁰S to 13.75⁰S was formed during a fast spreading rate of about 95 mm/year between chrons 25 and 23 and has an age of 55.9-50.8 Ma. The southernmost part of the IONF (Sector D) extending from 13º 76’S to 16º 30’S was reported to have formed due to a very slow spreading rate of 26 mm/yr. This sector has a chron younger than 23 which corresponds to an age less than 50.8 Ma (Mukhopadhyay et al., 2008).

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Chapter 1

The seamounts sampled for the present study fall in Sector C which was formed during a phase of fast-spreading rate.

Subsequent to the formation of the CIOB, the Indian Ocean witnessed a second phase of plate restructuring between 45 and 38 Ma which must have led to the activation of the FZ (Mukhopadhyay et al., 2018). This second phase resulted in ceasing of spreading in the Wharton Basin (Fig. 1.1). Later there occurred the opening of the Gulf of Aden at 10 Ma and deformation of Indian and Australian plates at 8-7 Ma.

1.4 Sediment classification and distribution in the CIOB

The first detailed study of marine sediments was done in the 1870s during the “HMS Challenger” expedition led by Sir Murray and Renard who dredged the bottom of the Pacific Ocean systematically and described the sediments. Murray and Renard (1891) classification of the marine sediments was modified by Nyström et al. (2016) as follows:

1. Red clays – contain less than 30% organic material 2. Calcareous oozes – contain more than 30% of calcium carbonate obtained from skeletons of minute planktonic animals and plants. A. Globigerina ooze – formed from the tests of pelagic foraminifera B. Pteropod ooze – formed from the shells of pelagic molluscs C. Coccolith ooze – formed from the accumulation of tests of minute coccolithophoridae. 3. Siliceous oozes – contain more than 30% of silica obtained from siliceous planktonic creatures. A. Diatom ooze – contains remains of planktonic plants in the form of frustules. B. Radiolarian ooze – contains the skeletons of radiolaria.

Authigenic components also contribute to the oceanic sediments but in a small proportion. Such components are either directly precipitated from the seawater (i.e., in the water column or at the seawater-sediment interface) or formed during burial or diagenesis of sediments. The authigenic materials include evaporates (e.g. like gypsum, anhydrite, and halite), zeolite minerals (phillipsite, clinoptilolite, etc.) and hydrogenous Fe-Mn oxy- hydroxides that usually are present as a coating on rocky substrate to form ferromanganese nodules and crusts. The other components that are present in less abundance in deep-sea

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Chapter 1 sediments are extra-terrestrial materials like cosmic spherules and cosmic dust, tektites and microtektites; and range in size from 0.1 to 1 mm and are typically spherical in shape. It is estimated that annually about 4-6 x104 tons of these particles accumulate in the oceanic sediments (Seibold and Berger, 1996).

The nature and distribution of seafloor sediments in the Indian Ocean are mainly influenced by five interrelated factors. These are climatic and current patterns, nutrient and organic production in surface waters, relative solubility of calcite and silica, submarine topography and detrital input. The Carbonate Compensation Depth (CCD) in the Indian Ocean varies between water depth 3,700 and 4,300 m (Mukhopadhyay et al., 2008).

The wide extent of the CIOB has resulted in a change in the sediment composition and lithofacies. Sediments occurring in the basins have been classified into four major types (Udintsev, 1975; Kolla and Kidd, 1982).

Fig. 1.7 Sediment distribution map of the CIOB (Source: Mukhopadhyay et al., 2008). a) Terrigenous sediments: These sediments are derived from neighbouring landmasses and transported by high-velocity rivers that drain into the Indian Ocean. Terrigenous sediments are mainly sourced from the Indo-Gangetic plain and the Himalayas and discharged into the CIOB basin by the Ganges-Brahmaputra river system at a rate of 1.670 x 106 tons/yr (Nath, 2001). Terrigenous sediments are found only in the northern part of the basin (Fig. 1.7) (Nath et al., 1989).

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Chapter 1

b) Siliceous sediments: Siliceous sediments are composed of diatoms, radiolarian, silico-flagellates and sponge spicules. In the Indian Ocean, diatoms are primary producers of silica followed by radiolarians in the CIOB. The region between 5ºS and 15ºS is marked by a clear zone of siliceous ooze with more than 70% of radiolarians (Fig. 1.7). These sediments are associated with high biogenic productivity region. Manganese nodules and crusts that are closely associated with siliceous sediments, form economically potential deposts (Mukhopadhyay et al., 2002).

c) Calcareous sediments: These sediments are composed of calcareous oozes such as foraminifera and pteropods. Calcareous sediments are found to occur above the CCD and accounts for more than 54% of the surface sediments in the Indian Ocean floor. A small patch of calcareous sediment is observed at around 12-14°S and 82.5-83.5°E in the CIOB. Nath et al. (2012) reported occurrence of calcareous sediments on the top of a seamount. That indicates that the CCD is shallower at the seamount top (water depth 3992-4252 m).

d) Red clays: Red clays or pelagic sediments are found below 15ºS and upto 25º in the CIOB. These sediments occur below the CCD and have very low sedimentation rates (Mukhopadhyay et al., 2018).

For my study a total of 95 surface samples were collected from the siliceous sediments while four gravity cores were recovered along a transect line adjacent to the 79°E FZ. The cores fall in siliceous, transition (siliceous-pelagic) and pelagic clays. The core details are provided in Chapter 2 (Table 2.1).

The rate of sedimentation in the CIOB is reported to have varied as determined using different methods. Goldberg and Koide (1963) recorded a sedimentation rate of 2.75 mm/1000 yr using Ionium/Thorium methods. Banakar et al. (1991) and Borole (1993) recorded a rate of 1-5 mm/1000 yr using 230Th fluctuation method. Gupta (1991) recorded a sedimentation rate of 2.5 mm/1000 yr based on the radiolarian biostratigraphy technique. Therefore, it is seen that the rate of sedimentation varies between 1 and 2.75 mm/1000 yr. The sedimentation rate for the CIOB and its implication are discussed in Chapter 8.

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Chapter 1

The volcanic sediments in the CIOB are mainly composed of pyroclastic materials brought by wind from nearby terrestrial volcanic eruptions or where also derived from in- situ basinal volcanism. In the CIOB the volcanic components are mainly Fe-rich magnetic spherules, metal-rich particles, volcanic glass (mafic and silicic), and palagonites.

1.5 Mineral deposits in the CIOB: The CIOB consist of the world’s second-largest and second richest manganese nodule deposit (Fig. 1.8). The nodules occur abundantly between 9°S to 16°30’S and 72°E to 80°E. This area, referred to as the IONF, has a coverage of 739,260 km2 and consists of commercially exploitable manganese nodules. The nodules have been detailed in terms of morphology, distribution, mineralogy, chemical composition and origin. The above parameters of the nodules

are influenced by the depth of the basin, sedimentation rate, and oxic bottom water condition amongst other factors (Mukhopadhyay et al., 2002).

Fig. 1.8. Manganese nodules occurring on the sediment of the CIOB collected during my participation on SSD-48 cruise, April 2018.

The CIOB nodules are rich in Mn (24.58%), Fe (7.89%), Co (0.14%), Ni (1.16%), and Cu (1.12%). The combined contents of Ni+Cu+Co ≥ 2.0% suggests that these nodules are of high metal grade (Mukhopadhyay et al., 2018).

1.6 Previous studies related to volcanism and hydrothermal activity in the CIOB The CIOB has witnessed several episodes of volcanic events as evident from the presence of basalts, ferrobasalts, spilites, and pumice clasts. The basalts occur as large outcrops and fragments on the slope and summit of the seamounts and abyssal hills and on the seafloor. Compositionally, the basalts are N-MORB similar to those from the MAR and

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Chapter 1

EPR (Iyer, 1996; Mukhopadhyay et al., 2008; Das et al., 2012). Besides N-MORB, ferrobasalts formed by the fast movement of the SEIR contains high Fe (>12 wt %) and Ti (>2%) and were recovered near topographic highs and high amplitude magnetic zones. The ferrobasalts were formed from fractionated melt that was emplaced at shallow crustal depth (Iyer et al., 1999a). Spilites (albitised basalts), although rare in the oceans, occur near the Indrani FZ (79oE) and were formed due to low-temperature alteration of basaltic lavas (Karisiddaiah and Iyer, 1992).

Pumices of variable colours, shapes, sizes, and vesicularities encompass a large field (600,000 sq km) in the CIOB and are trachyandesitic to rhyodacitic in composition. The occurrence of pumices in the CIOB sheds light on the possibility of intraplate silicic volcanism (Iyer and Sudhakar, 1993; Iyer, 1996; Kalangutkar et al., 2011; Kalangutkar, 2012). But the possibility of drift pumice from the phenomenal 1883 eruption of Krakatao volcano, Indonesia was not ruled out (Iyer and Karisiddaiah, 1988; Mukherjee and Iyer, 1999; Pattan et al., 2008; Kalangutkar et al., 2011).

Iyer (2005) defined the vhm in the CIOB as a package comprising of fragile bread- crust-like magnetic particles, magnetite spherules, spherules of Fe-Ti and Al composition, glass shards (silicic and basaltic), and palagonite grains. The vhm at the base of seamounts and near FZ also highlight the fact that there could be some ongoing volcanic and hydrothermal activities in the basin. The presence of vhm suggests sporadic hydrothermal events in the CIOB since 625 ka (Iyer et al., 1997a,b; 1999b) to as recent as 100 yr (Nath et al., 2008).

1.7 Objectives

In the last more than three decades, the scientific community has detailed extensively much of the above aspects of the CIOB, yet there is a lacuna in a holistic understanding of the volcanic and hydrothermal history of the CIOB. Hence, I intend to trace the volcanic and hydrothermal events that occurred in the CIOB, post-its formation after 60 Ma.

Hence, in the present work a test of hypotheses in connection with the objective of the study is proposed to understand the volcanogenic and hydrothermal activity in the intraplate environment of the CIOB.

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Chapter 1

 Is intraplate volcanism unique to the CIOB? If so, is it sporadic or episodic or continuum?  What is the mode of formation and mechanism of emplacement of the vhm?  Is there any relation between the occurrence of the vhm and morpho-tectonic features?  Were the co-existing hydrothermal materials formed contemporaneously with the volcanic products or are the hydrothermal events a later phenomenon (i.e., in time and space)?  Are the silicic glass shards occurring in the CIOB solely from the Indonesian Volcanic Arc (especially Toba eruptions) or is there a possibility of their in-situ formation in the basin?  In the above background, the following are the objectives of my thesis:

 Characterization of the seamount basalts based on morphology, petrography, geochemistry to decipher their origin.  Identification and classification of vhm based on their colour, shape, size, texture, and composition.  The vhms were detailed based on the composition to understand their variability in different geological settings in the basin.  Process of formation of vhm at abyssal depth.  The role of morpho-tectonic features in the formation of the vhm.  Finally, a model for the volcanic and hydrothermal history of the CIOB is proposed based on the current findings.

1.8 Summary A brief review of the Indian Ocean and the study area - Central Indian Ocean Basin has been detailed in this chapter highlighting the bathymetry, tectonic features such as the seamounts and FZ. Information about sediment types, sedimentation rate, mineral deposit and previous works are provided. The objectives of my investigations are also defined.

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CHAPTER 2

MATERIALS, METHODOLOGY, AND TECHNIQUES

Chapter 2

To accomplish the objectives of this study, several methods of investigations were undertaken for the samples recovered from the CIOB. The methods employed were sampling and processing of sediments and rocks, separation of coarse fractions (CF), microscopy (Binocular, Polarized, and Scanning Electron Microscope SEM). Analytical procedures include determination of chemical composition using Energy Dispersive Spectrometry (EDS) and Electron Probe Micro Analyser (EPMA), whole-rock and sediment geochemistry for major elements by X-Ray Fluorescence (XRF), minor and trace elements including Rare Earth Elements (REE) were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). In addition, Magnetic susceptibility (MS) studies of four sediment cores were carried out.

2.1 Sampling Details

Based on the literature review and multibeam maps, location and morphology, of four seamounts were identified for close-spaced sampling for rocks. Sampling of these seamounts was done using a chain bag dredge. Several rock pieces (boulders to fragments) were recovered near the summit, flanks, and base of the seamounts. Twelve samples were selected from four seamounts for petrography and petrochemical studies. Detail geological setting and locations of sampling sites are discussed in Chapter 3. Sediments 1. Forty surface sediment samples were used which were collected using an Okean grab from the abyssal plain in the vicinity of seamounts during previous cruises. Further, samples were also collected from the base of a seamount during the 48th expedition of RV Sindhu Sadhana in April 2018. The data are tabulated in Chapter 4. 2. Fifty five sediment samples were collected onboard the vessel RV A.A. Sidorenko (Cruise No 38) in the year 2001 during the bulk sampling for FeMn nodules and deep- towed operations. The sediments were recovered in close-grid pattern (10.25⁰S to 12.75⁰S and 74⁰E to 76⁰E) using an Okean grab. The data are tabulated in Chapter 4. Both of the above locations are referred to as Site-1 in Chapter 4. 3. The four gravity sediment cores (Fig. 2.1) used in this study were collected during the 22nd cruise of RV A.A. Sidorenko in the year 1999. These cores, located adjacent to the 79ºE FZ, are referred to as Site-2 in Chapter 4.

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Table 2.1 Compilation of locations and details of sediments cores. Core name Latitude S Longitude E Water Depth (m)

AAS 22/8 7o 05.289’ 78o 07.712’ 5,180

AAS 22/7 10o 00.988’ 78o 08.971’ 5,240

AAS 22/5 15o 06.619’ 78o 11.742’ 5,030

AAS 22/3 16o 59.590’ 77o 59.665’ 4,760

Fig.2.1 Location of the study area. The yellow solid circles represent 55 sediment locations and the blue diamond represents 40 sediment sites. The red stars are the location of the gravity cores AAS-22/7 (north) and AAS-22/3 (south). Green solid circle indicates the location of studied seamounts. Dotted lines indicate the fracture zones.

2.2 Sediment and rock processing

All the four sediment cores (sub-sampled onboard at 5 cm interval) were oven-dried at 60-70 oC. About 25 g each of these dried samples was treated with 20 ml of 10% of Na- hexametaphosphate and kept overnight to disperse the clay and subsequently wet sieved (using 63 µm mesh). The CF recovered after sieving were again oven-dried and prepared for

Page | 15 Chapter 2 further examination. The percentage of CF was calculated based on the recovered fraction subtracted by the loss of dry sediment weight. The CF (surface and core sediments) were examined using a binocular microscope. To determine the volume percentage of volcanic glass shards (henceforth glass shards would be used), 0.10 mg of the CF was coned and quartered and from this the glass shards were counted under the microscope. The data are presented in Chapter 8. The magnetic fractions were separated from the total CF using a bar magnet and seen under an SEM. Apart from these, Fe-rich magnetic spherules were mounted on an epoxy stub and polished to an extent at which the internal surface was well exposed, and then was carbon-coated for SEM-EDS analysis. Few interesting sections from the cores were hand pulverized using an agate mortar for geochemical analysis. The details are given in Chapter 6. Twelve rock samples were sliced using a diamond embedded saw and the slices were abraded with sandpaper to remove traces of sawdust and visible alteration. The sample slices were cleaned with acetone and distilled water and were coarsely crushed in a hydraulic piston crusher prior to powdering in a tungsten-carbide ring mill. The details of the rock samples are discussed in Chapter 3.

2.3 Microscopy 2.3.1 Binocular, Polarized The CF recovered after sieving of the sediments on a 63 µm size mesh were observed under an Olympus SZX7 binocular microscope at magnifications ranging between ×8 and ×56. This study was important to understand the type of components and their down-core variation and surface distribution. Thin sections of the rocks were observed using Leica DMLP, Carl Zeiss and Nikon microscopes and photomicrographs were obtained with an SLR camera. The magnifications ranged between ×20 and ×100.

The CF with maximum occurrence of spherules and glass shards were counted using a grid plate. The details are given in Chapters 4 and 8, respectively. While, the CF counts performed for core AAS-22/5 for depth below 250 cm to 500 cm (core end) are detailed in Chapter 7.

Apart from this, the CF was composed of biotite flakes, phillipsites, manganese micronodules, microtektites, glass shards, shark tooth and radiolarians (Fig. 2.2).

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Fig. 2.2 Different components occurring in the CF. A=biotite flakes, B=phillipsites, C=manganese micro nodules, D and H= biota, E=microtektites, F=glass shards, and G=shark tooth.

2.3.2 Scanning Electron Microscope (SEM) Several components, such as glass shards from each SRH (Shard-rich Horizons), Fe-rich spherules, palagonite, metal-rich particles, and basaltic glass were handpicked, affixed on carbon adhesive tape that was pasted over plastic mounts. The mounts were carbon-coated so as to improve the conductivity during imaging. The mounted grains were examined under an SEM, Leica Model 440 with a working distance of 20 mm and accelerating voltage of 20 kV and a dead time of 20%.

2.4 Analytical Procedures 2.4.1 Energy Dispersive Spectrometry (EDS) The major oxide data for the CF components were obtained by using an Energy Dispersive Spectrometer (EDS, OXFORD ISIS) attached to an SEM JEOL (JSM-5800 LV). During the analysis, the working distance was 12 mm, counting time of 100 sec, counting rate varied between 1,500 and 3,000 cps and the dead time was below 2%. The mineral standards (albite and orthoclase) were used to check the accuracy of the analysis which was noted to be less than 2%.

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2.4.2 Electron Probe Micro Analyser (EPMA) An EPMA is a fully qualitative and quantitative method for non-destructive elemental analysis of micron-sized particles with sensitivity at the level of ppm. An EPMA works by bombarding a micro-volume of a sample with a focused electron beam and collecting the X- ray photons thereby emitted by the various elements. Because the wavelengths of these X- rays are characteristic of the emitting elements, the sample composition can be easily identified by recording the WDS spectra (Wavelength Dispersive Spectroscopy). WDS spectrometers are based on the Bragg's law nλ=2d sinƟ. The grains of interest were mounted on glass stubs and later transferred to a newly prepared plastic stubs. This plastic stubs was prepared using Trans-optic powder and were polished using a very fine-grained master met 2 (Non-Crystallizing Colloidal Silica Polishing Suspension) and aluminium polishing powder with the help of a polishing machine until the surface of the grains was visible under the binocular microscope. Also, care was taken to avoid polishing scratches. The polished stubs were cleaned in an ultrasonic bath. Later, the stubs were coated with carbon under vacuum. The analytical conditions for the CAMECA SXFIVE probe containing filament LaB6 (Lanthanum Hexaboride) were an operating voltage of 15 kV, a beam current of 12 nA and a beam diameter of 1 μm. The compositional data were obtained by standardizing the value with international mineral standards. After calibration and ZAF corrections the final output in weight percentage of each oxide was obtained. Depending on the grain size and nature 5-10 probe spots were analyzed and the values were averaged.

2.4.3 X-Ray Fluorescence (XRF) Spectrometer X-ray fluorescence (XRF) is the emission of characteristic X-rays (or fluorescence) from a material that has been excited by bombarding with high-energy X-rays or gamma rays. Glass beads were prepared by thoroughly mixing 0.55 gm of each powdered samples (rocks, sediments) with spectromelt A12® a Lithium Borate and flux in a platinum crucible. Then the platinum crucible was covered with a platinum dish in the Minifuse®, which is an induction oven. The mixture was melted and converted into a glass bead which was air cooled and then was loaded in the XRF (Philips XRF) to analyze major oxides following the methods described by Rhodes (1996). The standards used for rock analysis were Icelandic basalt (BIR-1) and River Green Shale (SGR-1B) while standards used for the sediments were marine sediments standards

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(JMS-1, GSR-5, and NCS DC-7430).The accuracy for major oxides is within ±5% and standard deviations were <±1%

2.4.4 Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) The concentrations of trace and REE were determined with an ICP-MS (X-II Series) following the standard procedure of Balaram and Rao (2003). Finely ground rock and sediment samples were digested by dissolving 50 mg powder in a mixture of HF, HNO3, and

HClO4. A minimum of two cycles of drying of the acid and sample mixture were carried out during the digestion. The residue was dissolved in diluted (1:1) HNO3 and the aliquot was made upto 100 ml using distilled water. Rhodium was used as an internal standard. The standard reference materials used for rock and sediment analysis were Icelandic basalt (BIR-1) and Marine Sediment (MAG-1). The precision for trace elements is less than 1%, accuracy was within ±5 % while standard deviations for trace and RE elements were <3%. The REE data were normalized using chondrite values after Sun and McDonough (1989).

2.4.5 Magnetic susceptibility A total of 436 dried samples from four sediment cores were packed in 1-inch cylindrical plastic bottles. To measure magnetic susceptibility of the sediments a Bartington MS2B dual-frequency susceptibility meter was used at two different frequencies i.e., 50.47 kHz (Low) and of 54.7 kHz (High). Later, Anhysteric Remanent Magnetization (ARM) was imparted using 100 mT. Anhysteric Forward field was superimposed with a fixed DC bias field of 50 mT (microtesla) and drop down in the field intensity was measured using a Molspin Mini spinner magnetometer. Isothermal Remanent Magnetization (IRM) of 20, 100, 700, and 1T (Tesla) in the forward direction and -30, and 300 mT in the backward direction were imparted to the sediment samples using an MMP10 pulse magnetizer and measured with a Molspin Mini spinner magnetometer. Mass normalized IRM obtained from the peak field of 1T is assumed to be the Saturation Isothermal Remanent Magnetization (SIRM).

2.5 Summary This chapter provides a brief description of the various methods used, beginning with the sampling, petrographic description of rocks and analytical methods employed for different components, whole-rock and sediment geochemistry. Also, magnetic susceptibility

Page | 19 Chapter 2 procedures carried for the CIOB sediments have been discussed. The quantum and type of work carried out are shown in Table 2.2. The CF% of all the four cores are shown in Table 2.3.

Table 2.2 Details and quantum of the work carried out. Core name Length of Sieving, Coarse SEM and XRF ICPMS EPMA Magnetic core (m) fractions SEM-EDS analysis AAS 22/8 5.65 113 70 7 - 10 113 AAS 22/7 5.6 112 120 28 16 5 112 AAS 22/5 5.0 100 82 7 - 10 100 AAS 22/3 5.6 112 150 12 - 13 112 SSD-48 Surface 45 80 - - sediments Total 482 502 54 16 38 482

Table 2.3 Details of CF% in all the four studied cores. NA= sample not available

Sediment Core AAS-22/8 Sediment Core AAS-22/7 Depth(cm) Sample Coarse Coarse Depth(cm) Sample Coarse Coarse weight Fraction Fraction weight Fraction Fraction (gm) (gm) (%) (gm) (gm) (%) 0-5 13.288 0.161 1.212 0-5 17.452 0.91 5.214 5-10 14.577 0.331 2.271 5-10 17.378 1.383 7.958 10-15 15.041 0.255 1.695 10-15 19.9 1.529 7.683 15-20 15.607 0.257 1.647 15-20 16.851 0.612 3.632 20-25 14.767 0.175 1.185 20-25 19.931 0.609 3.056 25-30 14.829 0.172 1.160 25-30 20.001 1.575 7.875 30-35 14.459 0.243 1.681 30-35 20.408 1.225 6.003 35-40 10.087 0.143 1.418 35-40 18.977 0.815 4.295 40-45 17.143 0.195 1.137 40-45 22.097 1.066 4.824 45-50 15.08 0.24 1.592 45-50 27.427 1.141 4.160 50-55 15.643 0.206 1.317 50-55 27.474 1.053 3.833 55-60 17.086 0.283 1.656 55-60 21.016 1.166 5.548 60-65 13.003 0.173 1.330 60-65 17.146 0.625 3.645 65-70 12.872 0.165 1.282 65-70 14.546 0.587 4.035 70-75 15.852 0.235 1.482 70-75 19.948 0.533 2.672 75-80 15.206 0.168 1.105 75-80 21.14 0.817 3.865 80-85 12.904 0.167 1.294 80-85 13.558 0.777 5.731 85-90 13.253 0.178 1.343 85-90 17.281 1.383 8.003 90-95 13.456 0.192 1.427 90-95 14.332 0.997 6.956 95-100 16.403 0.193 1.177 95-100 18.475 0.688 3.724 100-105 14.059 0.243 1.728 100-105 14.706 0.707 4.808 105-110 16.549 0.295 1.783 105-110 12.137 0.472 3.889 110-115 16.616 0.297 1.787 110-115 19.181 0.922 4.807 115-120 13.885 0.273 1.966 115-120 17.337 0.6 3.461 120-125 15.619 0.407 2.606 120-125 18.325 0.569 3.105 125-130 13.57 0.428 3.154 125-130 19.402 2.074 10.690

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130-135 15.357 0.26 1.693 130-135 20.696 0.775 3.745 135-140 19.29 0.238 1.234 135-140 13.475 0.566 4.200 140-145 14.581 0.199 1.365 140-145 18.516 1.628 8.792 145-150 15.853 0.274 1.728 145-150 14.863 1.273 8.565 150-155 17.247 0.295 1.710 150-155 22.087 0.452 2.046 155-160 16.041 0.255 1.590 155-160 12.886 0.881 6.837 160-165 15.74 0.261 1.658 160-165 16.191 0.903 5.577 165-170 14.973 0.323 2.157 165-170 14.221 0.613 4.311 170-175 27.659 0.722 2.610 170-175 19.691 0.644 3.271 175-180 26.1 0.55 2.107 175-180 13.202 0.426 3.227 180-185 29.167 0.778 2.667 180-185 15.314 0.722 4.715 185-190 27.322 0.873 3.195 185-190 16.645 0.891 5.353 190-195 24.357 0.762 3.128 190-195 23.038 0.876 3.802 195-200 26.474 0.836 3.158 195-200 18.44 0.477 2.587 200-205 22.998 1.003 4.361 200-205 15.704 0.756 4.814 205-210 26.82 1.578 5.884 205-210 20.53 1.306 6.361 210-215 27.498 1.414 5.142 210-215 19.553 0.98 5.012 215-220 -74.263 1.682 -2.265 215-220 23.635 1.776 7.514 220-225 25.285 1.77 7.000 220-225 24.043 1.628 6.771 225-230 31.584 1.395 4.417 225-230 25.665 1.472 5.735 230-235 27.308 0.789 2.889 230-235 19.039 1.643 8.630 235-240 27.458 0.817 2.975 235-240 15.766 0.983 6.235 240-245 23.679 0.709 2.994 240-245 24.25 1.455 6.000 245-250 26.208 0.653 2.492 245-250 24.704 1.555 6.295 250-255 20.975 0.706 3.366 250-255 22.211 1.095 4.930 255-260 25.679 0.612 2.383 255-260 24.375 1.74 7.138 260-265 27.515 0.549 1.995 260-265 26.329 1.951 7.410 265-270 28.147 0.436 1.549 265-270 20.125 1.899 9.436 270-275 28.603 0.335 1.171 270-275 22.867 1.059 4.631 275-280 26.799 0.38 1.418 275-280 22.758 1.504 6.609 280-285 25.38 0.525 2.069 280-285 18.91 2.444 12.924 285-290 25.946 0.587 2.262 285-290 22.585 1.557 6.894 290-295 28.368 0.627 2.210 290-295 14.77 0.346 2.343 295-300 27.038 0.661 2.445 295-300 7.6901 0.216 2.809 300-305 28.101 0.46 1.637 300-305 17.834 0.835 4.682 305-310 24.833 0.744 2.996 305-310 14.426 0.82 5.684 310-315 25.214 0.544 2.158 310-315 20.893 0.425 2.034 315-320 29.093 0.761 2.616 315-320 20.061 1.457 7.263 320-325 26.858 0.833 3.101 320-325 14.175 0.345 2.434 325-330 28.032 0.818 2.918 325-330 19.916 0.793 3.982 330-335 29.4699 0.843 2.861 330-335 22.808 0.592 2.596 335-340 24.659 0.151 0.612 335-340 15.341 0.15 0.978 340-345 27.95 1.254 4.487 340-345 20.48 1.325 6.470 345-350 29.946 1.339 4.471 345-350 23.27 1.798 7.727 350-355 30.181 0.988 3.274 350-355 37.263 3.273 8.784 355-360 27.98 1.136 4.060 355-360 21.341 0.774 3.627 360-365 33.695 1.198 3.555 360-365 18.193 1.111 6.107 365-370 26.049 1.325 5.087 365-370 19.21 0.491 2.556 370-375 25.908 0.68 2.625 370-375 17.303 0.347 2.005 375-380 30.006 0.838 2.793 375-380 18.331 0.361 1.969 380-385 31.803 0.94 2.956 380-385 16.586 0.975 5.878 385-390 28.924 0.783 2.707 385-390 15.302 0.4 2.614

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390-395 28.574 0.659 2.306 390-395 21.535 0.393 1.825 395-400 29.284 0.573 1.957 395-400 20.453 1.917 9.373 400-405 17.353 0.457 2.634 400-405 16.735 0.505 3.018 405-410 15.141 0.411 2.714 405-410 27.563 1.221 4.430 410-415 16.498 0.497 3.012 410-415 17.75 0.343 1.932 415-420 18.649 0.405 2.172 415-420 19.64 0.795 4.048 420-425 16.474 0.395 2.398 420-425 19.857 1.025 5.162 425-430 18.129 0.62 3.420 425-430 18.481 0.492 2.662 430-435 21.615 1.419 6.565 430-435 17.705 0.846 4.778 435-440 8.702 0.148 1.701 435-440 16.505 0.594 3.599 440-445 11.161 0.202 1.810 440-445 17.695 0.283 1.599 445-450 8.284 0.113 1.364 445-450 16.45 0.183 1.112 450-455 14.243 0.366 2.570 450-455 16.47 0.312 1.894 455-460 13.485 0.182 1.350 455-460 11.912 0.51 4.281 460-465 13.952 0.345 2.473 460-465 14.763 0.421 2.852 465-470 17.285 0.336 1.944 465-470 14.381 0.492 3.421 470-475 12.017 0.215 1.789 470-475 14.719 0.385 2.616 475-480 16.054 0.236 1.470 475-480 18.864 0.377 1.999 480-485 16.62 0.32 1.925 480-485 19.23 0.245 1.274 485-490 16.527 0.419 2.535 485-490 28.51 0.352 1.235 490-495 14.669 0.421 2.870 490-495 17.335 0.37 2.134 495-500 17.979 0.617 3.432 495-500 18.135 0.35 1.930 500-505 16.786 0.726 4.325 500-505 18.571 0.246 1.325 505-510 12.039 0.577 4.793 505-510 24.19 0.226 0.934 510-515 15.892 1.008 6.343 510-515 19.243 0.275 1.429 515-520 21.902 1.646 7.515 515-520 14.779 0.227 1.536 520-525 13.812 0.613 4.438 520-525 14.994 0.199 1.327 525-530 25.764 1.714 6.653 525-530 15.199 0.417 2.744 530-535 11.783 0.254 2.156 530-535 15.346 0.244 1.590 535-540 11.525 0.264 2.291 535-540 15.412 0.39 2.530 540-545 14.196 0.273 1.923 540-545 16.034 0.274 1.709 545-550 13.237 0.263 1.987 545-550 19.945 0.915 4.588 550-555 8.491 0.151 1.778 550-555 17.643 0.621 3.520 555-560 12.117 0.22 1.816 560-565 6.361 0.155 2.437

Sediment CoreAAS-22/5 Sediment Core AAS-22/3 Depth Depth(cm) Sample Coarse Coarse Sample Coarse Coarse (cm) weight Fraction Fraction weight Fraction Fraction (gm) (gm) (%) (gm) (gm) (%) 0-5 2.830 0.280 9.894 0-5 18.446 0.939 5.091 5-10 2.370 0.150 6.329 5-10 12.985 0.702 5.406 10-15 5.000 0.280 5.600 10-15 19.872 0.959 4.826 15-20 7.130 0.430 6.031 15-20 20.227 0.937 4.632 20-25 7.600 0.450 5.921 20-25 19.798 0.571 2.884 25-30 8.860 0.430 4.853 25-30 20.301 0.389 1.916 30-35 7.940 0.340 4.282 30-35 18.109 0.393 2.170 35-40 5.330 0.230 4.315 35-40 17.479 0.467 2.672 40-45 8.330 0.540 6.483 40-45 17.234 0.441 2.559

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45-50 5.370 0.220 4.097 45-50 18.912 0.440 2.327 50-55 10.920 0.360 3.297 50-55 18.300 0.386 2.109 55-60 11.120 0.380 3.417 55-60 18.565 0.333 1.794 60-65 11.020 0.410 3.721 60-65 NA NA NA 65-70 9.940 0.400 4.024 65-70 NA NA NA 70-75 7.680 0.210 2.734 70-75 NA NA NA 75-80 8.110 0.290 3.576 75-80 22.825 0.251 1.100 80-85 11.530 0.370 3.209 80-85 19.534 0.182 0.932 85-90 11.870 0.420 3.538 85-90 20.247 0.277 1.368 90-95 11.670 0.340 2.913 90-95 21.072 0.212 1.006 95-100 12.380 0.570 4.604 95-100 19.235 0.168 0.873 100-105 10.750 0.430 4.000 100-105 22.608 0.187 0.827 105-110 11.600 0.470 4.052 105-110 -12.628 0.144 1.140 110-115 11.140 0.430 3.860 110-115 16.653 0.157 0.943 115-120 11.380 0.420 3.691 115-120 17.484 0.148 0.846 120-125 11.980 0.550 4.591 120-125 21.693 0.175 0.807 125-130 11.500 0.480 4.174 125-130 20.343 0.152 0.747 130-135 11.000 0.360 3.273 130-135 29.712 0.085 0.286 135-140 11.100 0.340 3.063 135-140 22.897 0.147 0.642 140-145 11.270 0.360 3.194 140-145 18.452 0.131 0.710 145-150 11.460 0.350 3.054 145-150 19.471 0.166 0.853 150-155 11.810 0.330 2.794 150-155 24.551 0.188 0.766 155-160 12.100 0.370 3.058 155-160 19.434 0.128 0.659 160-165 11.770 0.320 2.719 160-165 25.110 0.138 0.550 165-170 11.410 0.300 2.629 165-170 20.841 0.129 0.619 170-175 11.490 0.340 2.959 170-175 20.502 0.117 0.571 175-180 10.410 0.310 2.978 175-180 24.662 0.147 0.596 180-185 10.060 0.240 2.386 180-185 26.447 0.159 0.601 185-190 10.160 0.210 2.067 185-190 25.542 0.143 0.560 190-195 10.170 0.210 2.065 190-195 22.106 0.212 0.959 195-200 10.000 0.190 1.900 195-200 24.681 0.113 0.458 200-205 10.130 0.180 1.777 200-205 17.155 0.102 0.595 205-210 10.310 0.170 1.649 205-210 19.143 0.120 0.627 210-215 10.110 0.170 1.682 210-215 18.311 0.111 0.606 215-220 9.980 0.150 1.503 215-220 22.600 0.130 0.575 220-225 10.030 0.180 1.795 220-225 28.246 0.175 0.620 225-230 9.980 0.140 1.403 225-230 17.957 0.090 0.501 230-235 9.930 0.130 1.309 230-235 15.220 0.088 0.578 235-240 9.600 0.100 1.042 235-240 19.579 0.107 0.547 240-245 10.780 0.090 0.835 240-245 15.931 0.086 0.540 245-250 9.950 0.060 0.603 245-250 15.662 0.084 0.536 250-255 10.270 0.090 0.876 250-255 16.998 0.078 0.459 255-260 11.770 0.080 0.680 255-260 18.398 0.107 0.582 260-265 10.390 0.060 0.577 260-265 15.514 0.100 0.645 265-270 9.990 0.060 0.601 265-270 16.712 0.121 0.724

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270-275 10.380 0.060 0.578 270-275 16.762 0.092 0.549 275-280 10.300 0.050 0.485 275-280 17.297 0.604 3.492 280-285 10.080 0.040 0.397 280-285 16.355 0.131 0.801 285-290 10.310 0.030 0.291 285-290 18.635 0.115 0.617 290-295 6.980 0.040 0.573 290-295 18.407 0.132 0.717 295-300 7.200 0.030 0.417 295-300 21.483 0.652 3.035 300-305 6.470 0.050 0.773 300-305 20.617 0.351 1.702 305-310 6.400 0.060 0.938 305-310 16.882 0.287 1.700 310-315 6.600 0.060 0.909 310-315 19.296 0.367 1.902 315-320 6.560 0.070 1.067 315-320 12.572 0.149 1.185 320-325 6.790 0.080 1.178 320-325 20.055 0.264 1.316 325-330 6.560 0.090 1.372 325-330 18.065 0.227 1.257 330-335 7.090 0.110 1.551 330-335 15.910 0.164 1.031 335-340 6.850 0.100 1.460 335-340 21.414 0.199 0.929 340-345 7.100 0.130 1.831 340-345 16.041 0.119 0.742 345-350 7.040 0.090 1.278 345-350 16.200 0.151 0.932 350-355 11.500 0.150 1.304 350-355 19.631 0.193 0.983 355-360 11.640 0.180 1.546 355-360 21.253 0.161 0.758 360-365 10.810 0.140 1.295 360-365 15.355 0.092 0.599 365-370 10.850 0.090 0.829 365-370 20.555 0.115 0.559 370-375 11.180 0.090 0.805 370-375 24.605 0.147 0.597 375-380 11.050 0.070 0.633 375-380 24.302 0.140 0.576 380-385 11.030 0.080 0.725 380-385 26.563 0.157 0.591 385-390 10.970 0.060 0.547 385-390 24.047 0.157 0.653 390-395 10.740 0.060 0.559 390-395 25.202 0.162 0.643 395-400 10.740 0.050 0.466 395-400 23.846 0.145 0.608 400-405 12.660 0.050 0.395 400-405 22.671 0.113 0.498 405-410 11.850 0.060 0.506 405-410 22.242 0.104 0.468 410-415 11.980 0.050 0.417 410-415 27.907 0.138 0.494 415-420 11.740 0.060 0.511 415-420 14.543 0.185 1.272 420-425 11.830 0.060 0.507 420-425 22.540 0.111 0.492 425-430 10.490 0.040 0.381 425-430 31.475 0.155 0.492 430-435 12.430 0.030 0.241 430-435 37.687 16.334 2.980 435-440 11.810 0.050 0.423 435-440 28.339 0.162 0.572 440-445 12.360 0.050 0.405 440-445 26.322 0.133 0.505 445-450 11.820 0.040 0.338 445-450 27.784 0.149 0.536 450-455 11.670 0.040 0.343 450-455 25.759 0.150 0.582 455-460 12.360 0.050 0.405 455-460 23.858 0.109 0.457 460-465 11.420 0.040 0.350 460-465 25.404 0.112 0.441 465-470 11.670 0.040 0.343 465-470 26.074 0.156 0.598 470-475 11.820 0.040 0.338 470-475 27.792 0.128 0.461 475-480 11.790 0.040 0.339 475-480 24.941 0.109 0.437 480-485 11.950 0.050 0.418 480-485 19.582 0.321 1.639 485-490 12.900 0.050 0.388 485-490 25.397 0.116 0.457 490-500 16.160 0.050 0.309 490-495 29.315 0.153 0.522

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495-500 27.788 0.141 0.507 500-505 19.070 0.144 0.755 505-510 18.364 0.160 0.871 510-515 20.191 0.194 0.961 515-520 17.627 0.194 1.101 520-525 24.230 0.306 1.263 525-530 22.691 0.213 0.939 530-535 16.854 0.204 1.210 535-540 23.876 0.462 1.935 540-545 19.323 0.243 1.258 545-550 23.898 0.264 1.105 550-555 25.880 0.240 0.927 555-560 19.989 0.195 0.976

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CHAPTER 3

BASALTS AND VHM FROM SEAMOUNT DOMINATED AREAS

Chapter 3

3.1 Introduction Seamounts can be considered as windows to the mantle and reflect processes that occur at the crust-mantle boundary. Seamounts are distributed in space and time and occur in groups, chains or in isolation. A majority of seamounts also occur along convergent plates, in areas of vertical tectonic movement, at ridge-transform fault intersections, overlapping spreading centers and hotspots. It has been estimated that ~50 million tons/yr of basalts are utilized in the production of seamounts (Lisitizin, 1996). Several factors influence seamount production and disposition including mantle upwelling associated with superfast spreading, off-axis mantle heterogeneities, mini-plumes, local upwelling and the vulnerability of the lithosphere to magma penetration (Batiza, 2001; Iyer et al., 2012a and references therein). Seamounts that are transported into the subduction zone (e.g., West Junggar, Central Asian orogenic belt) not only add trace elements but also affect the mantle composition, chemistry of the arc and back-arc lavas and influence hydrothermal activity (Yang et al., 2015). Seamounts are generally composed of basalts and hyaloclastites (e.g., Batiza et al., 1984; Helo, 2014) and sometimes gabbros (Bideau and Hekinian, 2004) and serpentinites (Ueda et al., 2011). Non-hotspot seamounts occur on a young, thin and hot lithosphere and host tholeiites while plume-generated seamounts, on the other hand, lie on a thick, older lithosphere and are composed of either tholeiitic or alkaline basalts or more evolved basalts (Batiza, 2001). Seamounts are characterized by unique biodiversity, influence the water circulation patterns and currents, cause seismicity and are sites for hydrothermal deposits (Fe, Mn, Co), rare metals and phosphorites (Iyer et al., 2012a and references therein). Seamounts not only characterize the flanks of modern fast-spreading MOR but also areas that formed during a phase of fast spreading in the geological past. This is exemplified by the seamounts present in the Central Indain Ocean Basin (CIOB). The term ‘seamount’ was defined by the International Hydrographic Organisation for an elevation(s) greater than 1,000 m in relief occurring above the seafloor. The feature could be isolated or in groups and have a conical structure. Later, Staudigel et al. (2010) described seamount as “any geographically isolated or grouped topographic feature on the seafloor taller than 100 m, including one whose summit regions may temporarily emerge above sea level, but not including features that are located on continental shelves or that are part of other major landmasses.” But generically, any conical or steep volcanic feature could be referred to as seamounts and these may or may not be volcanically active (Iyer et al., 2012a).

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Chapter 3

In this chapter, the morphology, distribution, and petrology of seamounts have been used to understand and constrain models of seafloor spreading in the CIOB. The observations also provide evidence for temporal growth of seamounts, the role of local tectonics on magma production and their emplacement mechanism. Further, this chapter also highlights the role of seamounts in the formation of volcanogenic hydrothermal materials (vhm) in the CIOB.

3.2 Literature review on seamounts Seamounts existing in the CIOB were formed during the reorganization of the tectonic plates between 60 and 50 Ma (Fig. 3.1) (Patriat and Ségoufin, 1988; Mukhopadhyay et al. 2002; Müller et al., 2016). Several seamounts in the CIOB were emplaced in an approximate N-S linear fashion (Fig. 1.3) along eight propogative fractures that developed in the basin due to the northward movement of the Indo-Australian Plate coupled with Indo-Eurasian collision events (Das et al., 2007). As evident, between magnetic anomalies, 24 and 21 propogative fractures played a crucial role in the formation of the TJT-In and a ridge-fault-fault configuration led to the westward receding of the CIR while the SEIR advanced westwards (Dyment, 1993). Based on the disposition of the seamounts and analysis of basaltic glass Mukhopadhyay et al. (1995) opined that the seamounts were of a near-axis origin, similar to those in the EPR and MAR. This study was followed by the report concerning ferrobasalts (high Fe-Ti basalts) recovered from some of the CIOB seamounts (Iyer et al., 1999a). Using Artificial Neural Network method, Das and Iyer (2009) found that most of the CIOB basalts are N-MORB while a few have compositional signatures of E (Enriched)-MORB and OIB (Ocean Island Basalts). Das et al. (2012) discussed in detail the petrography, chemistry, and origin of the CIOB basalts, while Hemond et al. (2012) reported the preliminary results of the isotopes of a few of these basalts.

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Chapter 3

Fig. 3.1 Generalized map of the Central Indian Ocean Basin and the adjacent regions showing (a) isobaths and the tectonic elements and (b) age of the ocean floor. The thick red line represents the present-day spreading system. Continuous grey lines and black dotted lines represent isobaths (IOC-IHO-BODC, 2003) and fracture zones (Wessel et al., 2015), respectively. The triple junction trace is shown as dashed purple line. Colour-coded image of the ocean floor is plotted with the latest available age grid of the ocean floor (Müller et al., 2016). The locations of the samples used in the present study are marked with red solid stars and those used by previous studies in this region are marked with blue triangles (Details are given in Table 1). SR: Sheba Ridge; CR: Carlsberg Ridge; CIR: Central Indian Ridge; ABB: Arabian Basin; ESB: Eastern Somali Basin; CIOB: Central Indian Ocean Basin; NER: Ninetyeast Ridge; DT: Deccan Trap. (Map Courtesy Dr. Yatheesh, CSIR-NIO, Goa).

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Chapter 3

3.3 Study Area Four seamounts from the CIOB were dredged for detail investigations of their petrology and to understand their genesis. Twelve dredging operations were carried so as to recover rocks near the summit, flanks, and base of the seamounts (Fig. 3.2 and 3.3). The locations and geologic setting of the earlier and presently studied seamounts are tabulated (Table 3.1). The samples used for this study are compared with the basalts from the CIOB (Das et al., 2012), SEIR (Michard et al., 1986; Dosso et al., 1988), CIR (Ray et al., 2007) and from DSDP Sites 214, 215, 216 and 254 (all near the Ninetyeast Ridge) (Frey et al., 1977; Thompson et al., 1978) and the Afanasy-Nikitin Seamount located to the north of the CIOB (Kashintsev et al., 1987; Mahoney et al., 1996). This comparison is made because the CIOB evolved as a result of the collective effect of plate movements and formation of the CIR, SEIR, and TJT-In (Fig. 3.1) (Dyment, 1993; Das et al., 2007).

Fig 3.2 Bathymetry of the three seamounts studied. Location of the dredged samples are shown as solid stars and circles.

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Chapter 3

Fig. 3.3 Bathymetry of the fourth studied seamount and location of the dredged location (Solid star).

3.4 Results 3.4.1 Morphology of the CIOB Seamounts The seamounts have been numbered as #1, 2 and 3 (Fig. 3.2) and #4 (Fig. 3.3) and are used in further discussion. Geophysical data suggest the underlying oceanic crust in the study area to have formed due to tensional stress at a superfast (half) rate of 95 mm/yr during chrons 25 and 23 (~56-51 Ma) (Royer et al., 1989; Mukhopadhyay et al., 2002). The four studied seamounts lie on magnetic anomaly A24 and have variable morphology and dimensions. Seamounts #1, 2, and 3 are along the 75o 30’E FZ and seamount #4 to the south of the 79oE FZ (Das et al., 2007). The seamounts range in height between 625 and 1,150 m, basal width 7 to 17 km and area 40 to 236 sq km (Table 3.2). The seamounts #1, 3 and 4 are conical and have an N-S elongated base while seamount #2 has an E-W elongated base.

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Chapter 3

3.4.2 Petrography of Seamount basalts The basalts (Fig. 3.4) are fresh with a veneer of ‘fresh’ glass or the glassy rind os oxidized and overlain by ferromanganese (FeMn) oxides. An absence of significant palagonite layers indicate that the seamount basalts are not extensively altered as compared to the seafloor basalts (Iyer, 1999a). A cross-section of the seamount basalts displays three zones, with different textures and minerals, starting from the outer glassy (zone A) to an intermediate (zone B) to the interior (zone C). The petrographic features of the studied samples are shown in Table 3.3 and the photomicrographs in Fig.3.5.

Fig. 3.4. Hand specimen photographs of the basalts recovered from the seamounts. A = Pillow basalts (PD 4) with a thick rind of oxidized glass. B = Pillow basalts (PD 12) with a fresh rind of glass and unaltered interior. C (sample HRX 1) and D (sample HRX 13) = Fragments of pillow basalts with variable stages of freshness of the glassy rind and interior.

In general, plagioclase is most abundant and occurs as laths (<0.25 mm), microphenocrysts or tabular grains (0.25 to 0.50 mm), phenocrysts (0.50 to 1 mm) and megacrysts (>1.0 mm) (Mislankar and Iyer, 2001). The quenched zone A (glassy) have spherulitic texture with reddish-brown globular structure in which small plagioclase crystal form nuclei (Fig. 3.5A). There are sheaf-like radial clusters of plagioclases that formed during quenching of glass. Phenocrysts and microphenocrysts of olivines and sector zoned plagioclases occur in the interstices of glass as commonly seen in N-MORB glass (Iyer and Banerjee, 1993; Gill, 2010). The transitory zone B is hemicrystalline with skeletal crystals

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Chapter 3 of plagioclases in glass, and grades into the holocrystalline zone C. This zone has intergranular, rarely intersertal, glomeroporphyritic and flow textures with profuse plagioclases in the groundmass and sometimes as phenocrysts. In contrast, olivines occur in the groundmass only if present as phenocrysts. Occasionally, there are ‘ghost’ or skeletal plagioclase grains that show only the outline while the interior is hollow. In contrast to zoned plagioclases, twinned ones are more frequent and contain inclusions of glass that extends as embayment into the plagioclase and result in fine reaction /alteration margins. Olivine phenocrysts are few and unzoned and the groups of fresh olivines in zone A (Fig. 3.5D) indicate a cumulate origin. In the dyke samples (HRX 13) opaque grains occur in irregular forms and streaks (Fig. 3.4F). In all the samples, vesicles are scanty and are mostly empty.

Fig. 3.5. Photomicrographs of the seamount basalts. A = Radial sheaves of plagioclases in a glassy groundmass (×100). B = Submicroscopic sheaves of plagioclases with tiny elliptical blebs of glass surrounded by plagioclase microlites to form a reddish alteration. Spherulitic texture formed in altered glass. C = Transition zone between fresh glass (upper left) and fresh interior. D = Fresh euhedral olivine phenocrysts in a glassy matrix. E = Intersertal and ill-developed flow textures formed by plagioclase laths glass. F = Coarse grains of hornblende and pyroxene in a basaltic dyke (sample HRX 13). A string and mass of opaque are present near the pyroxene. Magnification for B to F ×40. Scale = 0.1 mm. A-E under plane light, F under crossed polars

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Chapter 3

3.4.3 Chemical composition of Seamount Basalts The major oxides (wt%) of the seamount basalts (Table 3.4) exhibit a narrow range of SiO2 (49-51.5), Al2O3 (13.6-18.6), Fe2O3 (2.5-3.5), FeO (8-11.5), MgO (3.5-9), significant CaO (9.5-12), moderate Na2O (2.5-3.5), moderate to slightly high TiO2 (1.33-

2.12) and low P2O5 (0.1-0.2). These oxides attest to an abundance of plagioclase and rarity of olivine and augite. Sample PD 26 has the lowest K2O (0.1) and highest MgO (~9) indicating it to be very fresh while the other samples have moderate to high K2O (0.4-1.0)

(Table 3.4). The enhanced K2O content indicates seawater alteration of the basalts but this may not be a primary reason since the samples were made free of visible alteration prior to chemical analyses. Additionally, the low Fe+3 and MnO (<0.25) contents indicate less alteration and freshness of all the studied samples. On these bases, the high K could be an inherent property of the CIOB basalts as reported for the seafloor basalts (Das et al., 2012) and those from the DSDP sites in the Indian Ocean (Table 3.5) (Frey et al., 1977; Thompson et al., 1978).

3.5 Interpretations and Discussion

3.5.1. Seamount Morphology: Based on the relations between flatness and slope angle of 200 seamounts there are four types in the CIOB seamounts (Das et al., 2007). Type 1: low HW (height-width) ratio <0.08 with low slope angle (<10°) and low flatness (<0.12), type 2: low to intermediate HW ratio of 0.08 to 0.16 with variable slope angle (6° to 15°) and flatness (0.08 to 0.3), type 3: an intermediate HW ratio 0.16 to 0.23 with high slope angle (>10°) and high flatness (>0.2) and type 4: high HW ratio >0.23 with a high slope angle (>10°) and low to moderate flatness (<0.2). Type 1 seamounts indicate point source with flow of lava along the flanks and type 2 are complex seamounts with volcanic mass added in phases. Type 3 represents seamounts with collapse summits that formed through fissure eruptions whilst type 4 indicates seamounts that had a point (dyke) source of eruption. Based on the above parameters, the presently studied seamounts #1 and 2 are complex while seamounts #3 and 4 are a combination of fissure and dyke eruptions (Table 3.2).

3.5.2 Petrography: The petrographical features and mineral assemblages of the presently studied seamount basalts are similar to those from other seamounts in the CIOB (cf. Mukhopadhyay et al., 1995). The CIOB basalts are of four types: aphyric, plagioclase dominant, plagioclase + olivine and plagioclase + olivine + augite (Mislankar and Iyer,

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Chapter 3

2001). On this basis, the presently investigated seamount basalts are plagioclase dominant variety and exhibit characteristics of the HPPB and MPPB varieties (i.e., highly- and moderately- Plagioclase Phyric Basalts) of the MAR basalts (Bougault and Hekinian, 1974). The plagioclase morphotypes in the CIOB basalts represent different stages in their cooling. The sheaf-like radial clusters and spherulitic textures (Fig. 3.5A, B) form due to rapid growth during quenching of the lava and/or resorption because of disequilibrium with the melt (Gill, 2010). In oceanic basalts, several types of twinned and zoned plagioclase have been recognized that are diagnostic recorders of their growth and of the magmatic conditions. In the CIOB basalts (seafloor and seamounts) a profusion of zoned plagioclases (Mislankar and Iyer, 2001) is absent in contrast to the MORB from the CR (Iyer and Banerjee, 1993) and CIR (Ray et al., 2007). The factors that favor the growth of unzoned crystals are: very slow cooling of the magma; rapid stirring of the magma; large supply of the magma that does not change the bulk composition; growth from a bulk composition of minimum melting temperature; interruption of zoning by settling or floating of uniform crystals that form a compact rock; and rapid eruption of a volcanic rock with inhibition of reaction of early formed phenocrysts with the host magma (Gill, 2010). For e.g., the varieties of plagioclases in the FAMOUS region (French-American Mid-Ocean Undersea Study), Iceland and CR suggest magma mixing, which is common at slow- to intermediate-spreading ridges (Iyer and Banerjee, 1993; Ray et al., 2007). The amount of zoning could be approximately proportional to ascent rate of magma and magma chamber size could affect the abundance of unzoned crystals (Fisk, 1984). On these tenets, it appears that magma for the CIOB basalts (seafloor and seamounts) upwelled during a fast rate of spreading and ascended rapidly from small chambers with little time to form zoned crystals. Alternatively, unzoned minerals may form due to removal of the residual host liquid prior to its complete crystallization and this results in glassy inclusions in the plagioclase phenocrysts (Mislankar and Iyer, 2001). The phenocrysts probably formed when the magmatic mush underwent re-melting and re- crystallization and show the effects of magmatic corrosion. Therefore, predominance of plagioclases suggests the CIOB seamount basalts are moderately evolved HPPB/MPPB that imply fractional crystallization of a plagioclase-rich magma.

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Chapter 3

3.5.3 Major oxides: The CIOB seamount basalts (major oxide in wt%) are compared with the CIOB seafloor basalts that have TiO2 <2 wt% and TiO2 >2 wt% (Das et al., 2012), the N-MORB from the CIR (Ray et al., 2007), plagioclase-olivine basalts from the Afanasy- Nikitin Seamount (Kashintsev et al., 1987; Mahoney et al., 1996) and ferrobasalts from the CIOB (Iyer et al., 1999), SEIR (Michard et al., 1986; Dosso et al., 1988) and Ninetyeast Ridge (Frey et al., 1977; Thompson et al., 1978) (Table 3.5). On the TAS diagram (Fig. 3.6), the CIOB seamount samples and from other locations fall in the field of basalt, except the samples from the Afanasy-Nikitin Seamount which lie in the field of basanite.

Fig. 3.6 TAS diagram shows the rocks from the CIOB seamounts to be basalts similar to DSDP and SEIR sites. The samples from the Afanasy-Nikitin Seamount are basanite.

Abbreviations: Ph = Phonolite; TPh = Tephriphonolite; PhT = Phonotephrite, T = Tephrite; Ba = Basanite; PBas = picro-basalt; TrBas = Trachybasalt; BaTrAnd =Basaltic trachyandesite;TrAnd = Trachyandesite; BasAnd = Basaltic andesite.

In binary plots of MgO vs. oxides two distinct clusters are noticed (Fig. 3.7).

Cluster C1 has high MgO (6-10) and low TiO2 (<2 wt%) and low FeO (~10) and consists of from the presently studied seamounts, CIOB seafloor, CIR, SEIR and DSDP Sites 215 and 254. Cluster C2, with lower MgO (2-6) and higher TiO2 (>2 wt%) and FeO (>10 wt%), contains the CIOB seafloor basalts and ferrobasalts from the Afanasy-Nikitin Seamount, SEIR, CIOB and DSDP Sites 214 and 216 near the Ninetyeast Ridge. (It needs to be noted that the CIOB seafloor basalts have TiO2 with either less than or more than 2

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Chapter 3 wt% and FeO either > 10 wt% or <10 wt%, cf. Das et al., 2012. Therefore, these basalts fall in clusters C1 or C2, respectively).

The plots show a scatter for SiO2 and Al2O3 for C1 and C2 (Figs. 3.7 A & B), with the high Al2O3 indicating an increase in plagioclase grains. The behaviour of FeO and TiO2 is similar (Figs. 3.7 C & D). TiO2 in C1 is lower (<2 wt%) and decreases with increasing MgO while in C2 it is higher (>2 wt%) and increases with decreasing MgO. These characteristics point to ferrobasaltic nature of C2 basalts. The TiO2 and FeO contents of the seamount basalts suggest the onset of Ti-magnetite crystallization but thin sections did not reveal significant Fe-Ti-minerals, unlike the CIOB ferrobasalts that have distinct and large- size opaque minerals (Iyer et al., 1999). Perhaps C1 basalts crystallized under low oxygen fugacity (fO2) condition that inhibited the formation of large-sized Fe-Ti minerals (Toplis and Carroll, 1995). These minerals occur in the glassy groundmass as minute single domain ferromagnetic grains that are 1 to 0.06 μm in size (Das et al., 2012). The positive trend of CaO with MgO (Fig. 3.7 E) of both the clusters indicates the removal of Ca-pyroxene that resulted in iron enrichment in the basalts. The K2O contents show a strong inverse relation with MgO for C1 basalts (K2O <0.5) while C2 basalts (K2O

>0.6) show a weak relation (Fig. 3.7 F). In terms of K2O, Zr/Nb and (La/Yb)N contents, C1 basalts resemble those of the SWIR segment (le Roex et al., 1982, 1992; Michard et al., 1986; Dosso et al., 1988), where K-rich basalts formed by fractionation and/or assimilation processes. The antithetic relation between Na2O and MgO (Fig. 3.7 G) can be accounted by accumulation of tiny crystals of Na-plagioclase in the glassy groundmass. The distribution of P2O5 (Fig. 3.7 H) is akin to that of K2O. The characteristics of C2 basalts with high TiO2 and a moderately high P2O5 and K2O (Table 3.4) may be caused by differences in bulk distribution coefficient of these elements during partial melting of the sub-oceanic mantle i.e., DK< DP< DTi (cf. Sun and McDonough, 1989).

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Chapter 3

Fig. 3.7 Interrelationship between MgO and the major oxides. The groupings of basalts in cluster C1 are shown as full oval and those of cluster C2 as dashed oval.

3.5.4 Minor, trace and rare earth elements: The average contents of minor, trace and RE elements of the CIOB seamount basalts are compared with those from the CIOB seafloor, CIR, SEIR, and Afanasy-Nikitin Seamount and ferrobasalts of the CIOB, SEIR and DSDP Sites near the Ninetyeast Ridge (Table 3.5). The Co and Cu values are akin to N-MORB and indicate the rarity of olivine and augite that host these elements. In the plots of V vs.

FeO (Fig. 3.8 A) and V vs. TiO2 (Fig. 3.8 B), C1 basalts have lower FeO (<10) and V (200-350 ppm) while those of C2 have higher FeO (>10) and V (300-400 ppm). The

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Chapter 3

positive trends indicate accumulation of FeO and TiO2 with progressive crystallization and incorporation of V in the opaque minerals and glassy phase. Fig. 3.8 The relation between V vs. FeO (Fig.

3.8A), V vs. TiO2 (Fig. 3.8B) Ba/Nb vs. MgO (Fig. 3.8C) and

CaO/Al2O3 vs. MgO (Fig. 3.38D). Legends as in Fig. 3.7.

The basalts of both the clusters have a negative trend on the plot of Ba/Nb vs. MgO (Fig. 3.8 C). The enhanced Ba/Nb ratio (13-59), in contrast to N-MORB (~5.7) and Primitive Mantle (PM 9.56), and its decrease with increasing MgO suggest partial melting of the source region that resulted in the CIOB seamount basalts. The reverse trend of increasing Ba/Nb ratio with decreasing MgO indicates that distribution of incompatible elements was controlled by fractional crystallization of the magma. The

CaO/Al2O3 ratio (~0.5 to 0.9) points to the presence of Ca-rich plagioclase (Fig. 3.8 D). This is in contrast to the CIOB

ferrobasalts that have very low CaO/Al2O3 ratio (0.1 to 0.2) (Iyer et al., 1999).

Fig. 3.9 Plots of Zr v/s FeO and TiO2. Both the clusters depict a positive trend that point to partial melting of the source magma. Legends as in Fig. 3.7.

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Chapter 3

Fig. 3.10 Plots of Y v/s Zr (A) Cluster C1 basalts have Rb <20 ppm and Sc <50 ppm for Zr between <50 and 150 ppm while cluster C2 basalts have Rb >20 ppm and Sc >50 ppm for Zr >100 ppm and both these elements depict a flat distribution (Fig. 3.10 B, C). The plot of (La/Sm)N (normalized with chondrite values of McDonough and Sun, 1995) vs. Zr/Nb (Fig. 3.10D). Legends as in Fig. 3.7.

On plots of Zr vs. FeO

(Fig. 3.9A) Zr vs. TiO2 (Fig. 3.9B) clusters C1 and C2 depict a positive trend. The Zr values are slightly low in basalts of C1 (50- 150 ppm) as compared to C2 (75-200 ppm). The C1 basalts show a positive trend between Y and Zr (Fig. 3.10A) and have lower Y (20- 40 ppm) relative to the higher Y (40-80 ppm) in C2 basalts. In case of Rb and Sc contents, C1 basalts have Rb <20 ppm and Sc <50 ppm for Zr between <50 and 150 ppm while C2 basalts have Rb >20 ppm and Sc >50 ppm for Zr >100 ppm and both these elements depict a flat distribution (Fig. 3.10 B, C).

Fig. 3.11 Plot of (La/Sm)N against TiO2 shows the clusters C1 and C2 (Fig. 3.11A). Ba/Nb vs. V (Fig. 3.11B), Ce/Y vs. Zr/Nb (Fig. 3.11C).

The plot (La/Sm)N (normalized with chondrite values of McDonough and Sun,

1995) vs. Zr/Nb (Fig. 3.10 D) shows C1 basalts to have lower (La/Sm)N (<1 to 2) and C2 has (>1.5 to 4.5) for a similar spread of Zr/Nb (30 to 150). The above observations together

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Chapter 3 with small range of Ce/Y (0.15-0.34) and Nb/Zr (0.01-0.02) ratios indicate that partial melting of the source region produced the seamount basalts. The relative enrichment of Pb, Th and U (Table 3.4) in the CIOB seamount basalts relative to the N-MORB are enigmatic. The U/Pb ratio (0.03-0.58) of the CIOB seamount basalts is similar to N-MORB whereas Ce/Pb and Ce/U ratios (0.64 to 5.75 and 8 to 31, respectively) are remarkably low vis-à-vis the N-MORB (~25, >150; Hofmann et al., 1986) and PM (28, 97; McDonough and Sun, 1995; Hannigan et al., 2001). It appears that U is more incompatible than Pb during mantle melting in the CIOB basalts as compared to the other oceanic basalts (Halliday et al., 1990). This may be reconciled as due to the recycling of the oceanic crust and sediment in the Indian Ocean (Rehkamper and Hofmann, 1997; Hemond et al., 2012; Mukhopadhyay et al., 2016).

The plot of (La/Sm)N against TiO2 (Fig. 3.11 A) shows a lateral spread for C1 basalts for a range of (La/Sm)N (<1 to 3) and suggests a larger extent of partial melting of a near homogeneous source magma that had a low TiO2 content (<2). This is in contrast to

C2 basalts that have a narrow range of (La/Sm)N (1-2) and higher TiO2 content (>2 wt%) and perhaps formed due to a smaller extent of partial melting. The plot of Ba/Nb vs.V (Fig. 3.11 B) portrays a flat distribution for C1 than C2 basalts. The plot of Ce/Y vs. Zr/Nb (Fig. 3.11C) shows C1 to have a higher range of Zr/Nb (>80) similar to N-MORB (>30; Wilson, 1989). This is analogous to the SEIR basalts (Michard et al., 1986; Dosso et al., 1988) and points to a genetic relation between the basalts of the CIOB seamounts and SEIR. The La/Yb and Ce/Y ratios (~0.8-2 and ~0.15-0.34, respectively) of the CIOB seamount basalts are close to the chondrite values (La/Yb ≈1.39 and Ce/Y ≈ 0.39; Sun and McDonough, 1989) and were probably affected by olivine and pyroxene crystallization. The concentrations of incompatible and RE elements of the CIOB seamount basalts were plotted after normalizing to N-MORB and PM compositions (after McDonough and Sun, 1995) (Figs. 3.12-3.14). The values of N-MORB, E-MORB and OIB (Sun, 1980) are also shown. In general, the Ba, U and Pb contents are higher and Th and Nb are relatively lower than N-MORB whereas the less incompatible elements (e.g., Nd, Zr, Sm, and Yb) show a flat N-MORB normalized distribution. The basalts from the CIOB seafloor, CIR and SEIR (Fig. 3.12), also reflect this pattern. The CIOB seamount basalts are slightly depleted in Light REE

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Chapter 3

(LREE) (Fig. 3.13) with the exception of sample HRX 13 that shows a prominent positive Ce anomaly while the High REE (HREE) show a near flat pattern. The average ratio of LREE/HREE is quite alike being 1.63, 1.75 and 1.63 for the seamount basalts, CIOB seafloor

basalts (TiO2 < 2) and CIOB

seafloor basalts (TiO2 > 2), respectively.

Fig. 3.12 Spidergrams of incompatible trace elements of the CIOB seamounts. A: sample /N- MORB. B: sample /primitive mantle. Also plotted are the values for N-MORB and E-MORB (McDonough and Sun, 1995) and OIB (Ocean Island Basalts; Sun, 1980).

The above observations are on par with the N-MORB, DSDP Site 215 basalts (Thompson et al., 1974; 1978; Reddy et al., 1978; Frey et al., 1977), SEIR basalts and “normal” MAR and EPR basalts (Dosso et al., 1988). The behavior of the trace and RE elements and similar LREE/HREE ratio suggest that the variations and trends for the CIOB seamount and seafloor basalts are comparable irrespective of the location of the samples, and suggest a similar source and mode of evolution.

Fig. 3.13 Spidergrams of incompatible trace elements of basalts from the CIOB seamounts and other areas (See Table 3.5). A: average sample /N-MORB. B: average sample / primitive mantle.

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Chapter 3

Considering the seamount morphology, and petrography and composition of the basalts, I now discuss their origin, mode of formation and relation with the seafloor basalts. As mentioned earlier, the CIOB seamount basalts are geochemically comparable to those from the seafloor (with TiO2<2), CIR (Ray et al., 2007) and SEIR (Michard et al.,

1986) but for high K2O (0.63) and low P2O5 (0.11) contents that are akin to the SEIR basalts (K2O 0.66; Dosso et al., 1988) (Table 3.5). K-enrichment is also noted in the ferrobasalts at DSDP Site 216 (K2O 0.90), Spiess Ridge (K2O 0.5-1.11), Chain Ridge (K2O

0.77-0.83), Iceland (K2O 0.74), Broken Ridge (K2O 0.37-2.06), Kerguelen Plateau (K2O 0.72-1.95) and the CIOB ferrobasalts (le Roex et al., 1982; Dosso et al., 1988; Mahoney et al., 1995; Iyer et al., 1999). It is noteworthy that all the above sites with enhanced K2O have a ferrobasaltic affinity, except DSDP Site 215 which lies to the east of the CIOB.

Similar to the high K2O values as the CIOB seamount basalts are the K-P-rich basalts from the DSDP Site 215 that have enhanced K2O (0.90) and P2O5 (0.31) and low TiO2 (1.71) and FeO* (8.43) (Table 3.5). This fact indicates a common source for basalts of DSDP

Site 215 and CIOB seamounts and seafloor (TiO2 <2). The high-K content could be an inherent property (Frey et al., 1977; Thompson et al., 1978) and indicate the mildly alkalic nature of the CIOB seamounts and seafloor basalts. Such basalts occur in the rift zones e.g. Iceland (cf. Thy, 1989) and off the west coast of India (Karisiddaiah and Iyer, 1991).

Fig. 3.14 Plots of rare earth elements of the CIOB seamounts. A: sample /N-MORB. B: sample /primitive mantle.

The range of Zr/Nb ratio of the studied basalts is high (40-143) (Table 3.4) and average Zr/Nb ratio of the CIOB seamounts is 54.53; CIOB seafloor (<2 wt% TiO2; 48.6);

CIOB seafloor (> 2 wt% TiO2; 56.8), CIR (23.4) and SEIR (28.5). High Zr/Nb ratio (30- 110) has been reported for the basalts present along the different segments of the MAR and

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Chapter 3 for the abyssal basalts from the Indian and Pacific oceans (Erlank and Kable, 1976). Considering the similar Zr/Nb ratio of the CIOB basalts (seamounts and seafloor) it appears that the source areas were uniformly depleted. In the world ocean, ferrobasalts have been dredged not only from propagating spreading centre (Christie and Sinton, 1981) but also from fast and intermediate spreading ridges, such as Juan De Fuca Ridge (half-rate 30 mm/yr; Vogt and Byerly, 1976); the Galapagos Spreading Center (GSC; 30 mm/yr; Byerly, 1980), Reykjanes Ridge (23 mm/yr; Rona and Scott, 1993), and the Nazca Plate (DSDP Leg 34; 85 mm/yr; Mazzullo and Bence, 1976). Therefore, it is apparent that ferrobasalts are not only associated with propogative rifts but are also found in high amplitude magnetic (HAM) zones, fast- spreading ridges, or a combination of such sites. The CIOB seafloor (TiO2 >2) and some seamounts have ferrobasalts that formed near HAM zone which represents the TJT-In (Iyer et al., 1999). This observation points to the occurrence of two types of basaltic volcanism in the CIOB: ferrobasaltic and N-MORB. The CIOB seamount basalts with moderate FeO and MgO contents indicate significant fractionation of the magma. Furthermore, the narrow CaO/Al2O3 ratio, moderate to high abundances of K2O and P2O5 and comparable ratios of highly incompatible elements, point to a single or uniform magmatic source. The fairly enriched LREE of the CIOB seamount basalts (Fig. 3.14) suggests an absence of extensive partial melting of the source region as this would have resulted in lower LREE or a flat REE pattern (Wilson, 1989; Feigenson et al., 1996; Das et al., 2012). Hence, it is evident that the CIOB basalts formed by fractional crystallization of a ferrobasaltic melt that had undergone low partial melting. A similar process was proposed for the EPR ferrobasalts (Sinton and Detrick, 1992). The formation of ferrobasalts has been suggested to be due to fractional crystallization of melts occurring at shallow-level (Vogt and Byerly, 1976; Christie and Sinton, 1981; Anderson et al., 1980). This mechanism has been considered to account for the ferrobasalts of the DSDP sites 214 and 216 (Thompson et al., 1978), GSC (Byerly, 1980), the Conrad FZ (le Roex and Dick, 1981) and Spiess Ridge (le Roex et al., 1982). Ferrobasalts could also form from a Fe-Ti rich parent (Klein et al., 1991), magma mixing (Wilson et al., 1988) or from a large molten magma chamber (e.g. 13o and 23oS in the EPR with a spreading rate of 75 mm/year) (Sinton and Detrick, 1992). The along-axis petrologic variations of fast-spreading ridges, such as the EPR, the GSC and moderate to fast-spreading SEIR are suggested to be due to variable degrees of

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Chapter 3 shallow crystallization or by differences in the depths of melt generation and residence (Byerly, 1980; Langmuir et al., 1986; Klein et al., 1991). The mechanism of ‘neutral buoyancy zonation’ structure of magma reservoirs coupled with fractional crystallization could explain the retention of magma at shallow depths for long duration of time (Ryan, 1994). The horizon of neutral buoyancy (HNB), defined as that ‘depth interval within which the magma density and aggregate country rock density are equal occurs sub- lithospherically and has a narrow vertical and a wide lateral extent. Beneath the HNB zone, magma ascends due to positive buoyancy and is stabilized at a shallow depth (2–4 km), while above this region the magma descends by negative buoyancy. The HNB of tholeiitic melts thus provides congenial conditions for the long-term stability of magma reservoirs and the nearly constant magmatic compositions over millions of years (Ryan, 1994; Thompson and Petford, 2008). The CIOB seafloor formed during a phase of superfast spreading (half) rate (95 mm/yr) between ~56 and 51 Ma (Mukhopadhyay et al., 2002). The similarity in compositions of the basalts from the CIOB seafloor and the seamounts suggest sharing of a common magmatic source. This may be because of episodic eruptions due to changes in relative plate motions that may have produced the seamounts, as at Iceland (cf. Gaina et al., 2016). Such a process could be envisaged in trapped off-axis melts in older oceanic crusts (cf. Ryan, 1994) i.e. the crusts of the CIOB and the SEIR were underlain by small magma chambers at relatively shallow depths (Mukhopadhyay et al., 1995; Das et al., 2005; Das et al., 2007). The fast-spreading rate of the plates perhaps restricted the residence of the melt for a long time. Under this circumstance, the melt may not have differentiated extensively and resulted in uniform basaltic magma compositions both, at the seafloor and seamounts. Based on clinopyroxene–melt barometry it was suggested that an underplated zone at 7–14 km depth could exist (Klügel et al. 2005). In this zone, similar to the HNB, the ascending magmas are either ponded prior to eruption or are temporarily stored during eruption. Those authors’ observations supported long-term and short-term storage of magma for Cumbre Vieja volcano, La Palma (Canary Islands). Besides the spreading rate and residence time, other factors, such as rate of magma supply, low degree of partial melting or presence of small relatively stable magma chamber could also have produced basalts of similar compositions. Additionally, a lateral transport of the melts and their subsequent ascension through a thin lithosphere could result in

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Chapter 3 focusing of melts at the seamounts, similar to the observed along-axis variation at the Knipovich Ridge (Norwegian-Greenland Sea) (Schlindwein et al., 2013). The basalts from the older seamounts of the CIOB resemble those from the present- day younger, near-axis located seamounts of the MAR and EPR (Mukhopadhyay et al., 1995). Shen et al. (1995) reported that in the EPR (15o and 19oS) the production of seamounts in closely spaced (20-25 km) seamount chains could share a common source and this results in an increased magmatic flux to one chain and decrease in the adjacent chain. Based on the Na8 (extent of melting) and Fe8 (depth of melting) values, of more than 2,000 basalts from the EPR and seamounts, Zhang (2011) concluded that the fast-spreading EPR and the nearby seamounts have a common source that passed through the lithosphere with different thermal structures and underwent crystallization. In the CIOB the seamount chains are spaced between 50 and 55 km (Das et al., 2007) and therefore, it is conceivable that the seamounts and the seafloor may have tapped a common source region from the then spreading ridges and also amongst themselves.

3.5.5 Seamount emplacement and associated vhm Based on the above evidence, it could be proposed that the CIOB seafloor formed at a fast rate of spreading (95 mm/yr) and this hindered the formation of large magma chambers. On the other hand, pockets of small magma bodies were retained that were uniformly depleted, regularly tapped and replenished. This led to concurrent or penecontemporaneous production of the seafloor and associated seamounts. The seamounts were emplaced along with the N-S trending fractures that were formed as a consequence of the Indo-Eurasian collision. At a later stage the seamounts witnessed episodes of low intensity hydrothermal activity between 625 ka and 100 yr age (Iyer, 2005; Nath et al., 2008). The addition of such materials resulted in either an N-S or E-W trending bulge of the base of some of the seamounts as also seen in the present study (Fig. 3.2). The finding from the present investigation especially the emplacement of seamounts in the CIOB has opened a new avenue to understand post-emplacement changes and processes that occur at seamount sites. The ancient CIOB seafloor (60-55 Ma) has witnessed various episodes of magmatic and hydrothermal activities. These are evident from the accretion of magmatic material on the existing seamounts to occurrence of FeMn coated pumice, glass shards and volcanogenic-hydrothermal material (Iyer and Karisiddaiah, 1990; Karisiddaiah and Iyer, 1992; Mislankar and Iyer, 2001; Kalangutkar et al., 2015; Mascarenhas-Pereira et al., 2016; Mukhopadhyay et al., 2018). The occurrence

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Chapter 3 of magnetic spherules at the base of an intraplate seamount located in siliceous sediments and along the TJT-In (Iyer et al., 1997a, b; 1999b) also indicates that some of the CIOB seamounts were perhaps active in the recent past. The reported holistic view of the earlier investigations suggests the prevalence of a volcanic province in the CIOB, the possibility of hydrothermal activity, intraplate volcanism and a nexus between the volcanics and FeMn deposits (Iyer, 2005; Jauhari and Iyer, 2008). Considering the presence of hundreds of seamounts and their role in the formation of vhm, in subsequent chapters I discuss the role of seamounts in the formation of magnetic spherules (Chapter 4) and metal particles (Chapter 5) in the CIOB.

3.6. Conclusions The major findings of the study of the CIOB seamounts are as follows: 1. Seamounts of variable shapes (conical and elongated) and heights (625 to 1150 m) were produced when the CIOB experienced a half-spreading rate of 95 mm /yr between 56 and 51 Ma. 2. The basalts have similar petrography and are high to moderately plagioclase phyric.

3. Compositionally, the basalts are homogeneous with low FeO* and TiO2, elevated

K2O contents and slightly enriched LREE. 4. Seamount basalts are geochemically comparable with those from the CIOB seafloor

(TiO2 <2 wt%), CIR and SEIR but unlike those from the CIOB seafloor (TiO2 >2 wt%), and ferrobasalts from the CIOB, SEIR and DSDP Sites near the Ninetyeast Ridge. 5. The CIOB seafloor and the associated seamounts were generated from the unified crusts that formed the CIR and the SEIR and these shared a common ferrobasaltic melt source that in course of time differentiated to N-MORB. 6. Seamounts could be potential sites for volcanogenic and hydrothermal activities and their products.

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Chapter 3

Table 3.1 Locations of samples collected from four seamounts in the CIOB (#1 to 12). Seamount 1 samples #1 to 5, seamount 2 samples #6 to 10, seamount 3 sample #11 and seamount 4 sample #12. na= not available.

Sr.No. Sample (#) Latitude (S) Longitude (E) Water depth (m) Sample position at seamount 1 PD4 12o 30’ 76o 23’ 5320 at foot eastern side 2 PD9 12o 30’ 76o 20’ na at summit 3 PD15 12o 30’ 76o 24’ 5385 at western base 4 PD17 12o 28’ 76o 20’ 4800 along northern upper flank 5 HRX15 12o 28’ 76o 18’ 5100 at northwest flank 6 PD12 12o 35’ 76o 15’ 4400 along western flank 7 PD26 12o 34’ 76o 22’ 5375 at foot eastern side 8 PD29 12o 37’ 76o 23’ 5250 at southeast flank 9 PD33 12o 33’ 76o 18’ 4300 at northern flank 10 PD37 12o 32’ 76o 14’ 5250 at northwest base 11 HRX1 12o 31’ 76o 12’ 5150 at southwestern upper flank 12 HRX13 14o 05’ 79o 20’ 4820 at northeast upper flank

Table 3.2 Characteristics of the four studied seamounts wherefrom the samples were dredged from the CIOB. HW = height-width ratio, Sigma = 1-f, where f is the flatness of the seamount. For locations of seamounts 1 to 4, see Fig. 3.2a,b. Seamount Height Basal area Summit area Volume Flatness Slope angle Summit HW ratio Sigma Basal width 1-f (m) (sq km) (sq km) (cu km) (°) width (km) (km) 1 750 65.00 0.80 22.50 0.11 12.90 1.01 0.16 0.19 9.10 0.89 2 1200 236.00 1.00 101.00 0.07 8.40 1.13 0.14 0.15 17.33 0.93 3 625 40.00 0.30 9.10 0.09 10.90 0.62 0.18 0.19 7.14 0.91 4 1150 88.61 0.66 33.97 0.08 12.02 0.92 0.21 0.23 10.80 0.92

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Chapter 3

Table 3.3 Petrographic details of the seamount samples. Samples #PD 4, PD 9, HRX 15 from seamount 1; samples PD 12, PD 33, PD 37; Sample HRX#1 from seamount 3; sample HRX#13 from seamount 4. See figure 2 for sample locations. RFA = red feathery alteration.

Sample Petrographical features Seamount # morphology PD 4 Medium to fine grain, textures (crude flow, glomero-porphyritic, Medium size, intersertal), fresh to altered glass with RFA, plagioclase laths plenty, conical, steep few euhedral plagioclase microphenocrysts and phenocrysts, flanks, single twinned, stressed, sector zoned. Few olivine. summit, N-S elongated PD 9 Medium grain, plagioclase very abundant, laths, microphenocrysts, phenocrysts, sector zoned. Thick streak of opaque. HRX Medium to coarse grain, fresh to less altered glass, abundant 15 plagioclase laths, crude flow texture, euhedral plagioclase phenocrysts and microphenocrysts, stressed, fractured, twinned, cross-cutting. Few vesicles, patches of opaque. PD 12 Altered glass, reddish-brown, spherulitic, 3 fresh euhedral twinned Large,E-W plagioclase phenocrysts. elongated, gentle slope. PD 33 Glass with RFA, abundant plagioclase laths few stressed, sector zoned, microphenocrysts. PD 37 Quite fresh glass, medium to coarse grain, sub-ophitic, spherulitic, intersertal, abundant plagioclase laths, 2 megacrysts with glass inclusions, round and irregular vesicles. HRX 1 Glass is partly altered to palagonite, RFA, profuse plagioclase small, conical, phenocrysts and laths show cross-cutting and sector zones. steep slope. Megacrysts have glass inclusions. Below the glass is holocrystalline, plenty plagioclase laths, intersertal, porphyritic, ophitic and glomeroporphyritic textures. Twinned and zoned plagioclase. Fine grain olivine and augite in matrix. A few olivine microphenocrysts. Irregular empty vesicles. HRX 13 Two samples. (1) Fresh glass with abundant fresh euhedral olivine Part of a twin phenocrysts. Glass has spherulitic texture. Below glass is crystalline, seamount, intersertal, crude flow and intergranular textures. Profuse plagioclase conical, single laths, megacrysts, swallow-tailed, twinned, and stressed. Few empty summit, steep vesicles. (2) Coarse grain, plagioclase. Hornblende replaces pyroxene, slope, flank expulsion of large opaque grains. Dyke rock. cones.

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Chapter 3

Table 3.4 Whole-rock composition of basalts from four CIOB seamounts. Major oxides are in wt% and trace and rare earth elements are in ppm. nd=not detected. ------1------------2------ -- 3- -- 4-- wt % PD4 PD9 PD15 PD17 HRX15 PD12 PD26 PD29 PD33 HRX37 HRX1 HRX13

SiO2 51.41 50.25 49.62 51.45 51.05 49.14 49.55 49.49 50.27 48.78 51.1 50.22

TiO2 1.36 1.56 1.41 1.61 1.38 1.96 1.47 1.51 1.52 2.12 1.33 1.46

Al2O3 13.64 16.32 15.26 16.83 18.61 14.67 14.35 15.88 16.11 17.23 15.4 17.65

Fe2O3 2.89 3.25 3.03 3.09 2.57 3.53 2.64 3.19 3.21 2.99 2.5 2.61 FeO 9.40 10.57 9.85 10.05 8.37 11.47 8.58 10.37 10.45 9.71 8.12 8.48 MnO 0.16 0.15 0.15 0.18 0.25 0.17 0.16 0.12 0.10 0.12 0.20 0.10 MgO 6.20 3.52 5.28 3.44 3.64 5.19 8.87 4.31 3.68 5.64 6.94 5.47 CaO 11.75 10.02 11.8 9.75 9.84 10.05 11.71 11.20 10.72 9.57 10.2 10.18

Na2O 2.68 2.86 2.72 2.83 2.60 2.80 2.49 2.77 2.73 3.46 3.74 3.48

K2O 0.37 0.99 0.68 0.67 0.91 0.88 0.10 0.93 0.90 0.37 0.43 0.37

P2O5 0.16 0.14 0.21 0.11 0.19 0.15 0.10 0.22 0.18 nd nd nd Total 100.02 99.63 100.01 99.41 100.01 100.02 99.99 100.01 100.02 99.99 99.96 100.02 ppm PD4 PD9 PD15 PD17 HRX15 PD12 PD26 PD29 PD33 HRX37 HRX1 HRX13 La 3.11 3.40 5.92 5.06 6.73 3.50 3.50 2.84 7.37 9.09 2.73 7.15 Ce 7.20 7.43 6.79 6.91 10.57 10.33 10.45 6.64 6.70 18.15 7.99 44.85 Pr 1.15 1.32 1.95 1.73 1.68 1.99 1.87 1.53 1.76 2.83 1.29 1.90 Nd 8.53 9.50 10.04 9.31 11.18 9.55 8.63 8.55 11.58 17.93 9.23 10.92 Sm 2.89 3.19 3.39 3.33 3.46 3.86 3.37 3.19 3.46 5.16 3.03 2.77

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Chapter 3

Eu 1.12 1.26 1.28 1.30 1.28 1.36 1.17 1.24 1.34 1.73 1.21 1.02 Gd 4.63 5.11 4.56 4.52 5.32 5.20 4.46 4.30 5.59 7.71 4.83 4.41 Tb 0.88 0.98 0.85 0.86 1.01 0.97 0.82 0.81 1.05 1.42 0.94 0.72 Dy 5.04 5.56 5.75 5.68 5.80 6.49 5.37 5.70 6.09 7.99 5.34 3.99 Ho 1.09 1.23 1.32 1.27 1.23 1.44 1.20 1.30 1.34 1.73 1.17 0.83 Er 3.56 3.81 3.56 3.31 3.82 3.92 3.19 3.46 4.27 5.54 3.76 2.57 Tm 0.62 0.63 0.55 0.51 0.64 0.61 0.50 0.53 0.72 0.93 0.63 0.42 Yb 3.32 3.39 3.75 3.37 3.32 4.05 3.23 3.60 3.88 4.85 3.54 2.31 Lu 0.51 0.52 0.56 0.49 0.50 0.65 0.52 0.55 0.61 0.74 0.54 0.36 Y 36.47 37.72 39.90 35.37 37.54 40.19 33.89 38.28 44.53 52.65 37.57 22.62 Sc 44.24 47.53 45.68 46.04 47.46 45.96 34.67 46.87 46.56 48.97 46.69 42.69 Nb 0.95 1.02 0.63 0.67 1.33 1.53 2.56 0.73 1.01 2.81 1.76 3.87 Th 0.12 0.15 0.04 0.08 0.32 0.13 0.16 0.03 0.12 0.76 0.26 0.23 Sr 130.10 152.28 130.50 149.59 159.03 119.65 nd 138.69 140.79 nd 137.54 132.33 Rb 6.00 12.38 9.18 14.98 14.58 16.66 2.39 9.66 10.33 22.08 9.04 3.15 Ba 52.92 42.58 13.91 51.58 62.19 38.19 5.73 14.96 59.68 64.12 31.45 51.72 Co 45.48 38.49 39.52 39.42 58.34 34.74 39.37 24.76 22.84 73.63 41.31 107.72 Cu 123.68 128.31 142.53 133.13 159.15 60.22 54.82 105.73 189.52 nd 117.61 133.06 V 288.88 279.43 300.54 276.52 273.18 434.21 297.9 315.61 271.90 310.5 295.30 247.52 Ga 14.13 14.75 14.93 16.78 15.09 24.04 13.94 16.01 14.42 17.39 13.98 11.74 Zn 125.28 127.03 73.40 66.27 127.00 78.64 44.16 67.04 125.72 164.8 120.53 73.38 Zr 77.63 85.87 78.82 95.78 85.34 98.21 103.4 63.77 84.12 nd 83.71 36.46 U 0.40 0.86 0.24 0.84 0.72 0.59 0.33 0.42 0.74 1.00 0.62 0.28 Pb 6.35 2.47 1.84 1.45 4.00 2.82 1.81 3.23 10.48 3.60 5.76 10.84

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Chapter 3

Table 3.5 CIOB seamount basalts compared with those from the seafloor, CIR and SEIR. Oxides are in wt% and elements in ppm. 1

= Present study; 2 = avg of 16 CIOB basalt (<2 TiO2); 3 = avg of 25 CIOB basalts (>2 TiO2) (both from Das et al., 2012); 4 = avg of 15 CIR basalts (Ray et al., 2007), 5 = average of 10 SEIR basalts (Dosso et al., 1988); 6 = avg of 2 basalts from Afanasy-Nikitin (Mahoney et al.,1996); 7 = avg of 3 basalts from Afanasy-Nikitin Seamount (Kashintsev et al.,1987); 8 = avg of 10 CIOB ferrobasalts (Iyer et al., 1999); 9 = ferrobasalts of SEIR (Dosso et al., 1986); 10 = Ferrobasalt from site 214; 11 = DSDP Site 215 basalt; 12 = DSDP Site 216 ferrobasalt; 13 = DSDP Site 254 ferrobasalts (Thompson et al., 1978); 14 = avg of 7 ferrobasalts from SEIR (Michard et al.,1986). na = not available, nd=not determined

wt % 1 2 3 4 5 6 7 8 9 10 11 12 13

SiO2 50.2 49.70 49.41 49.13 49.62 48.65 44.62 47.75 50.17 48.1 50.4 49.5 47.66

TiO2 1.56 1.56 2.28 1.37 1.19 2.01 3.26 2.58 3.05 2.35 1.71 2.75 2.12

Al2O3 16 15.54 15.41 16.03 16 17.47 17.04 15.91 13.21 14.9 16.8 13.5 15.37

T Fe2O3 na na na na 10.05 na na na 15.26 na na na na

Fe2O3 2.96 1.84 2.09 9.98 nd 7.45 nd nd nd nd nd nd nd FeO 9.62 9.38 10.66 nd nd nd 12.04 14.88 nd 14.6 8.43 13.8 12.9 MnO 0.16 0.17 0.22 0.20 0.17 0.14 0.15 3.75 0.21 nd nd nd nd MgO 5.18 5.76 5.08 7.28 8.14 1.38 1.71 9.07 4.22 6.45 6.48 6.57 8.92 CaO 10.57 11.15 9.80 10.91 11.91 8.78 6.37 3.18 9.03 9.04 10.95 8.79 8.53

Na2O 2.93 3.07 2.97 3.03 2.45 4.13 3.12 0.89 3.15 2.75 3.17 2.57 2.69

K2O 0.63 0.53 0.70 0.34 0.17 3.61 2.12 0.43 0.66 0.37 0.9 0.9 0.32

P2O5 0.16 0.15 0.18 0.17 0.11 1.37 1.15 0.31 0.37 0.19 0.31 0.22 0.24 LOI nd 1.09 1.22 0.74 nd 4.50 nd nd nd nd nd nd nd

H2O na na na na na na 6.07 na na na na

Total 99.97 100.95 101.05 99.19 99.81 99.45 97.65 98.76 99.33 98.75 99.15 98.6 98.75

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Chapter 3 ppm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 La 5.03 7.46 8.34 na 3.5 47.5 37 na 19 8.4 16.2 13.2 9.9 3.43 Ce 12 12.29 15.41 na na 89.85 72 na na na 36 na na 9.67 Pr 1.75 2.18 2.62 na na 9.9 na na na na na na na na Nd 10.41 12.68 15.79 na 7.99 41.05 na na 23 na 20.1 na na 7.45 Sm 3.43 3.68 4.8 na 2.66 8.59 6.7 na 5.9 5.1 4.5 5 4.8 2.62 Eu 1.28 1.31 1.64 na 1.07 2.54 na na 2.35 1.7 1.5 1.45 1.63 0.99 Gd 5.05 5.39 7.02 na na 8.21 na na na na na na na 3.64 Tb 0.94 1 1.31 na 0.64 1.105 na na 1.14 na 0.8 na na na Dy 5.73 5.78 7.7 na na 6.15 na na na na na na na 4.02 Ho 1.26 1.23 1.67 na na 1.2 na na na na na na na na Er 3.73 3.71 5.1 na na 3.18 na na na na na na na 2.55 Tm 0.61 0.61 0.84 na na 0.445 na na na na na na na na Yb 3.55 3.45 4.74 na na 2.585 na na na na na 3.6 3.3 2.53 Lu 0.55 0.53 0.72 na na 0.375 na na na na 0.51 na na na Y 38.06 37.54 50.21 35.48 29.29 36.5 na 60.13 47.3 26 36 31 50 na Sc 45.28 43.29 47.46 37.27 na 13.5 na na na 46 47 42 42 na Nb 1.57 2.09 2.56 4.59 2.97 35 na na 34 na na na na na Th 0.2 0.45 0.51 1.13 0.26 4.43 na na 2.22 na na na na na Sr 135.73 139.56 139.76 141.11 120.8 644.5 959 na 245.2 265 390 235 115 140 Rb 10.87 15.3 16.39 6.09 2.2 62.5 na na 8.27 na na na na 2.67 Ba 40.75 45.12 68.1 52.31 na 1145.5 2530 59.55 na 45 450 140 56 na Co 47.14 46.71 53.09 92.33 na na na 72. 93 na 65 50 53 39 na Cu 124.28 160.72 149.91 73.89 na na na 114.32 na na 65 na na na

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Chapter 3

V 299.29 280.44 323.2 264.98 249 159.5 na 370.73 365 525 245 445 342 na Ga 15.6 14.93 16.83 16.56 na na na na na na na na na na Zn 99.44 122.53 142.69 81.58 na na na 188.85 na na na na na na Zr 85.62 101.64 133.55 107.17 84.6 279.5 331 191.35 197 120 160 159 156 na U 0.59 0.57 0.68 0.52 na 1.145 na na na na na na na na Pb 4.56 4.44 5.27 3.57 na 9.7 na na na na na na na na Ni na na na na 129 47.5 27 116.4 24 60 100 44 194 na Cr na na na na 328 na na 118.4 6 38 250 45 469 na Li na na na na na na na na na na 13 na na na B na na na na na na na na na na 4 na na na

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CHAPTER 4

IRON-RICH MAGNETIC SPHERULES

Chapter 4

4.1 Introduction The first report of black magnetic spherules from deep-sea sediments was of the Pacific Ocean by Sir John Murray during the HMS Challenger Expedition in 1876. Later, opaque magnetic and metallic spherules were reported from the Pacific Ocean manganese nodules (Murray and Renard, 1891). After a lull of tens of years a series of reports concerning magnetic spherules were published. For example, spherules in FeMn crusts at 45⁰ N on the MAR (Aumento and Mitchell, 1975), limestones and carbonates (Freeman, 1986; Suk et al., 1990), lunar soils (Heiken et al., 1974), and in extraterrestrial materials (El Goresy and Fechtig, 1967; Szoor et al., 2001). Till the last few decades, the source of metallic spherules in the deep-sea sediments of similar morphology and chemical composition to that of the cosmic spherules were misleading. The origin of metallic spherules with a lesser meteoritic abundance of Ni and an appreciable Mn were enigmatic to explain (El Goresy and Fechtig, 1967; El Goresy, 1968). Later, spherules with an absence of Ni core and enhanced Si were suggested to be of volcanic origin (Finkelman, 1970). To date, spherules have been reported to be composed of single or composite minerals such as silicate (Lefevre et al., 1986), K-feldspar and glauconite (Nashlund et al., 1986), phosphatic conodont pearls (Wang and Chatterton, 1993); basaltic glass (Melson et al., 1988; Robin et al., 1996), and aluminium-rich (Shterenberg et al., 1986; Dekov et al., 1995; Iyer et al., 2007). Different sources have been proposed for their formation: volcanism (dominant source), while other sources are anthropogenic, biogenic activity, sedimentary processes, cosmic dust, extra-terrestrial impacts and modern industrial processes (Huang and Xu, 2012; Zagurskii et al., 2009; Shetye et al., 2019). The occurrence of spherules in diverse geological environments have led to a wide application and could be used as stratigraphic markers to infer palaeo-environmental conditions and geological environments (Wang and Chatterton, 1993; Ma and Bai, 2002). This is evident from the occurrence of spherules between strata of major geological events such as the extra-terrestrial impact during Permian–Triassic and K-T times (Alvarez et al., 1992; Parthasarathy et al., 2019), Precambrian-Cambrian, Early-Middle Cambrian, Frasnian-Famennian, Permian-Triassic, Callovia-Oxfordian and Maastrichtian-Danian boundary intervals (Glass and Simonson, 2012; Huang and Gong, 2014; Zhang et al., 2014; Zhang et al., 2010; Chang et al., 2017) and in Mesozoic-Cenozoic deposits from the Atlantic Ocean (Lozovaya, 1981). Magnetite spherules have also been reported from a thin

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Chapter 4 ferruginous layer of Fatehgarh Formation (Cretaceous), Barmer Basin, India (Mathur et al., 2019).

4.2 Earlier studies on Fe-rich spherules in the Indian Ocean Harrison and Peterson (1965) reported the occurrence of an unusual abundance of a highly magnetic mineral in a sediment core recovered at 23º 56’ S and 73º 53’ E (water depth 3,676 m) and was identified to be maghemite with some aspects of magnetite. Perhaps this was the first report of spherules in the Indian Ocean. Later, Iyer et al. (1997a,b; 1999b) reported the occurrence of iron (Fe) -rich magnetic spherules from two different sites in the Central Indian Ocean Basin (CIOB): one at the base of an intraplate seamount located in siliceous sediments and the other along the TJT-In. These are the only reports concerning the deep-sea spherules of the CIOB. In light of the above findings, this chapter focuses on the spatial distribution and classification of Fe-rich spherules based on their occurrence, physical parameters, and chemical compositions. The data generated from this study have been compared with the spherules of the Pacific and Atlantic oceans and to those of terrestrial volcanoes and experimental products. Considering all these facts, the possible mechanisms for the formation of spherules in the CIOB are proposed.

4.3 Study area

Preliminary observations were made based on the spatial distribution of the Fe-rich magnetic spherules. Further, to understand their genesis in the deep-sea environment, different tectonic settings (i.e., seamounts and FZ) and sediment domains (i.e., siliceous and pelagic) were targeted for the study.

4.3.1 Sampling strategy Site-1 (Seamounts clustered area): A total of 95 stations (55 stations from AAS-38 cruise; 40 stations from SSD-48 cruise) were examined in this study. The cruise details are provided in Chapter 2 and location details of samples are in Tables 4.1A &B. Site-2 (Near 79⁰ E FZ): Two cores: AAS-22/7 from siliceous ooze and AAS-22/3 from pelagic domain are investigated. The details are provided in Table 2.1 and Fig. 4.1.

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Chapter 4

Fig. 4.1 Left panel: General map of southern part of Indian Ocean (From Google earth). Centre panel: Locations of the study area. The yellow solid circles represent 55 locations and the blue diamonds inside a black outline box represent 40 locations. These 40 locations are referred to as Site-1. The red star symbol is the location of the gravity cores AAS-22/7 (north) and AAS-22/3 (south) referred to as Site-2. The pink solid triangle represents SS2/89 (Iyer et al., 1997a), Purple solid triangle represents SS10/657 (Iyer et al., 1997a), the black solid triangle represents ODP Site 717-719 (Boulegue and Mariotti, 1990) and the red solid triangle represents SK-212 (Sarma et al., 2018). Right panel: The map shows detail bathymetry of the study area (Site-1) along with depth contour, and FZ crossing the sampling site.

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Chapter 4

4.4 Results and Interpretations Preliminary observations on distribution of spherules were made for the 55 stations from AAS-38 cruise located in Site-1. Sampling was carried out in a close grid pattern to understand spatial distribution of the spherules. The data generated using these samples are depicted in Fig. 4.2.

Fig 4.2 Spatial distribution of spherules from 55 locations (AAS- 38) in the CIOB. The vertical scale shows number of spherules.

4.4.1 Sediment characteristics and coarse fraction descriptions The 40 surface samples from SSD-48 cruise within Site-1, are pale yellow to light brown in colour and have a silty texture, while the northern core AAS-22/7, retrieved from the siliceous domain has yellowish-brown colour sediment with a silty-clayey texture. This core has also been referred in Chapters 6 (baked sediments) and 8 (glass shards). The southern core AAS-22/3 lifted from pelagic clay has dark brown sediment with a clayey appearance and is slightly compact. This core is detailed in Chapters 5 (metal grains), 7 (palagonite) and 8 (glass shards). It was noted that in the core AAS-22/7, the sediments associated with the Fe-rich magnetic spherules had a slightly darker color and appeared to be baked due to the presence of an entrapped basaltic material (Fig. 4.3).

Fig 4.3 Photograph of baked sediment from depth 280-355 cm-bsf, AAS-22/7.

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Chapter 4

The graph of CF% vs core depth (Fig. 4.4) shows considerable variations in both the cores. In the northern core AAS-22/7, CF% show an abrupt change between 5 and 20 cm with prominent peaks (nearly 13%) occurring between 285 and 355 cm. In the southern core AAS-22/3, CF% show higher value in the top 35 cm of the core, while below 35 cm there is a relative decrease in the CF% (2-3%). In general, the CF from Site-2 revealed an assemblage of components mainly radiolarians, FeMn micronodules, basaltic glass, palagonite grains, volcanic glass shards, microtektites, pumice pieces, bread crust (oxidized basaltic glass), magnetic spherules, mica flakes and phillipsite crystals (Amonkar and Iyer, 2017; see Chapter 7). An examination of CF from Site-1 revealed the presence of FeMn micronodules, diatoms and radiolarians as dominant phases together with spherules and volcanic glass shards (Amonkar and Iyer, 2019).

Fig. 4.4 Down-core variation of CF% in two studied cores from Site-2 of the CIOB. AAS-22/7 is the northern core while AAS-22/3 is in the southern part of the basin. The yellow highlighted region indicates the depth of occurrence of iron-rich magnetic spherules and grains.

4.4.2 Iron-rich magnetic spherules Though volcanogenic hydrothermal material (vhm) is a package of different components such Fe-rich spherules, Fe particles, metal particles, palagonite and bread crust and glass shards, in this chapter emphasis is on Fe-rich magnetic spherules and grains. The spherules were seen to be nearly uniform in size in all the surface sediments collected from Site-1 while their abundance varied in a few hundreds. Spherules occur in the top section i.e., up to 5 cm in the core AAS-22/7 and in the top 10 cm in the core AAS-22/3. Apart from these, in the core AAS-22/7 spherules are observed at two distinct depths i.e., 280-355, and 460-475 cm (Fig. 4.4). In the top sections

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Chapter 4 of both the cores, the spherules range in number from 180 to 200/100 g of sediment. Relatively higher abundance is noted between 280 and 355 cm i.e. 280-440/100 g of sediment, while in the lowermost depth 470-475 cm, spherules are 280-320/100 g of sediment. It is noteworthy that highest number of spherules occur at an intermediate depth and these are associated with baked sediments (Fig. 4.3) and vhm. The occurrence and significance of baked sediments in the CIOB are discussed in Chapter 6.

4.4.2.1 Spatial distribution The data on 55 sediment locations (Table 4.1) indicate the bulk Fe-rich spherules present in the CF. The area between 10.3°S and 11°S has very high number of spherules ranging from 400 to 996 (Fig. 4.2). These locations when plotted on a bathymetry map (Fig. 4.1B) showed their presence near a seamount and FZ adjacent to the sampled Site-1.

4.4.2.2 Physical characteristics (Shape, Size and Texture)

In general, the spherules under the binocular microscope appear as tiny spheres adhered to one another by weak magnetic forces. The spherules are dark grey to mostly black while a few are reddish-brown in colour and have a metallic lustre. Most of the spherules appeared to be smooth while a few had an uneven surface. The uneven surface was mainly because of additional material in the form of fragile aggregates. A total of 77 spherules were observed under an SEM of which 38 spherules from Site-1 (Table 4.2A and 39 spherules from Site-2 (Table 4.2B) are described with respect to shape, size, texture and chemical composition.

Though the spherules appeared to be a sphere under a binocular microscope but detailed examination under SEM revealed five major shapes. These are spherical, broken, teardrop, unevenly spherical and oval. Based on the shapes the CIOB spherules at both sites are classified (Table 4.2). At site-1, the spherules are mainly spherical (34) with a few broken (3) and teardrop (1) while those at Site-2 are spherical (25), non-spherical (9) and oval (5).

Out of the total 77 spherules studied from both the locations, 59 spherules (77%) were completely spherical, 9 spherules (12%) were non-spherical, 3 were broken (4%), 1 had a teardrop morphology (1%) while the remaining 5 spherules (6%) were oval in shape (Fig. 4.5A). Though, most of the spherules are spherical in nature, a few had a bulging and

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Chapter 4 mould-like protrusion with an incipient tail-like feature. Metal aggregates were observed on the surface of some of the spherules.

Fig. 4.5A Classification of Fe-rich magnetic spherules based on shape. The data are shown in Table 4.2.

Fig. 4.5B Classification of Fe-rich magnetic spherules based on size. The data are in Table 4.2.

The spherules were classified based on three major size range (63- 125, 126-220, and >220 µm). In general, the size of the spherules ranges from 63 to 390 µm. The size classification data indicate that 30 spherules (41%) fall in the range of 63-125 µm and 35 (47%) are 126-220 µm while 9 (12%) are >220 µm. Based on texture, the CIOB spherules are classified into two types: smooth spherules with no crystallinity (Fig. 4.6) and spherules with well-defined crystals that are arranged in different geometrical patterns (Fig. 4.7). Most of the spherules appear smooth (Fig. 4.6a & b) while on some smooth textured spherules, secondary mineral aggregate is observed giving a slightly uneven texture (Fig. 4.6c &d). Apart from these, some spherules have a protruded end (Fig. 4.6e) while a few are fused (Fig. 4.6f). Broken spherules(Fig. 4.6g & h) are quite uncommon but are observed in a few CF.

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Chapter 4

Fig. 4.6 Scanning electron micrographs of type 1: Smooth spherules with no crystallinity (a, b), smooth spherules with surface aggregates (c, d), protruding tail (e), fused spherule (f) and broken spherule (g, h).

Furthermore, the crystalline spherules are classified into brickwork (Fig.4.7a&b), corkscrew (Fig. 4.7c), interlocking (Fig. 4.7d) and dendritic (Fig. 4.7e &f). Blowholes (Fig. 4.7g) on the surface of the spherules are circular to lenticular in shapes and are mainly observed between two larger crystals. Apart from all these textural varieties, I observed few grains of titanomagnetite (Fig. 4.7h).

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Chapter 4

Fig 4.7 Scanning electron micrographs of type 2: textured spherules.

Brickwork (a,b), corkscrew (c), interlocking (d) and dendritic (e,f). The blowholes are seen occurring between spaces of the crystal (g) and well- crystallized titano- magnetite grain (h).

The presence of surface textures, blowholes and mineral aggregates as described above, led to the inquisitiveness about the internal structure of the spherules. Hence, several polished spherules were examined to understand the continuity of surface textures and presence of mineral inclusions if any, inside the spherules. It was noted that the textures were mostly restricted to the surface but sometimes continued a few microns inwards. The internal surface showed massive aggregate (Fig. 4.8A) and in a few spherules, small crystals are embedded in a smooth matrix (Fig. 4.8B). Continuity of the blowholes was noted in the interior of the spherules (Fig. 4.8C, D). A few spherules show widening of blowholes towards the interior while some were completely hollow (Fig. 4.8E). Apart from these, some spherules show a dense texture (Fig. 4.8F-H)

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Chapter 4

Fig. 4.8 Scanning electron micrographs of polished spherules (A-H). See text for explanation. White box is the micro probe spot.

4.4.2.3 Chemical composition of spherules

The major oxides data for 57 non-polished spherules and 14 polished spherules from the two sampled sites are shown in Tables 4.3A, B & C. The 31 spherules from Site- 1 show high amount of Fe along with insignificant amount of Si, Al, Mn, Mg, Ca, K, N, and Ti (Table 4.3A). The Fe content (all in wt.%) ranges from 62 to 87 with spherules (sample S-S4) having highest Fe (87). Si ranges from 0.2-2.5, Al 0.3-4.6, and Mn 0.4-1.5. K is present in spherules S-S5 (2.6) and in S-KS2 (4.6). Ca, Mg and Na are very low (< 3). Ti is noted only in spherule S-K3S5 (0.8) and was below detection limit in all others. The 26 spherules analyzed from Site-2 show a similar elemental distribution. Fe ranges between 68 and 77 with lowest content in F-S47-2 (60). Si (0.09-2), Mn (0.3-1.8) and Al (0.2-0.8) while K, Mg, Ca and Na are low (<5). Though, the concentration of Ti is very

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Chapter 4 low (0.2 to 0.4) yet it is noted in more number of spherules from Site-2 as compared to those at Site-1. Spherule F-S46 has the highest concentration of Ti (0.95) while F-S47-2 has an exceptionally high Na (4.7). The averages calculated for spherules occurring in both the studied sites indicate that spherules near the Indrani FZ (Site-2) have a higher amount of Fe and Na while those near seamounts (Site-1) are enriched in Si, Al, Mn, K, and Ca with values nearly two times more than Site-2 spherules.

In addition to the major elements, Cr, S, Zn, and Ni are present in a few spherules (Table 4.3C). Cr shows a range from 0.17 to 0.99, S (0.11 to 1.68), Zn (0.31 to 0.56) and Ni (2.67 and 7.79). The spherules from Site-1 have notably higher Ni (7.79) along with Zn (3.57) and S (1.68), while those at Site-2 show more Cr (0.92). Cr, Zn, and Ni mostly occur in the FeMn oxides that partially coat the spherules. Based on the detailed physical and chemical observations, I infer that the CIOB spherules, despite their variable sizes (Fig. 4.5) and wide range of textures (Figs. 4.6 & 4.7) and a narrow range in elemental contents (Tables 4.3A & B), perhaps were formed by a similar process.

Analysis of 14 polished spherules for major and trace element distribution within interior of the spherules (Tables 4.4A & B) shows a strong variance compared to unpolished ones. Fe is noted to be extremely high (>70) on surface of spherules compared to interior (15-34). EDS shows a phase change and aggregate of silicate minerals inside the polished spherules while some probe spots have high Fe (62 to 77) that is accreted in the form of granules. Si (8-19) is relatively higher at a few spots where Fe wt.% was low. The elements Ti, Al, Mn, K, Mg, Ca and Na were significantly higher in the probe spots associated with Si. Al varies between ~2 and 5 and Mg from 0.41 to 4 in the Si-rich matrix. Mn values range between 0.4 and 0.9, while P18-1 polished spherule shows extremely high amount (27). Ca ranges from 0.2 to 11, while K does not show much variation in values over the unpolished spherules. Ti (0.5-1.7) is associated with Si. It was enigmatic to note presence of sulphur in an appreciable amount (0.2 to ~8). Cu is observed only in spherule P15-1 (0.6). The interior of the spherules P9-1 and P18-1 shows an appreciable amount of Pb (1.2- 1.5) and Cr (0.2-1) (Table 4.4B).

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Chapter 4

4.4.3 Fe-rich grains: Physical and chemical parameters

Within the CF, there are fine magnetic dusts and slivers of basaltic glass that are cm-sized, co-occur with Fe-rich spherules and sometimes attached by a weak magnetic force. These fragile and uneven grains are reddish-brown to dark red in colour and some are nearly black. Under the SEM, these particles portray a fine granular texture (Fig. 4.9).

Fig. 4.9 Scanning electron micrographs of Fe-rich grains associated with Fe-rich spherules. The grains are stubby and have a fine granular texture.

A few Fe-rich grains were analyzed under SEM-EDS (Table 4.5). In these grains, Fe ranges from 47 to 74, Al (0.4-2), Ti (7 to 12), Mg (1-1.8), Ca (0.2-0.4) and Mn (0.6-0.8) while Cr, Zn, Cu, V, and Pb are minor amounts. The Fe content (47 – 74) of these grains is relatively lesser than those in the spherules (62 to 87). Si is slightly low (0.19-1.06) and shows an inverse relation with Fe. Ti is exception low in grains 28-1 and 32-2 that have about 2 wt.%. Along with these grains, there are others with moderate Fe (47) and high Si (4.8) and Ti (12) and well-developed cubic form (Fig. 4.7H). These were identified to be a titano-magnetite crystal. Overall, the fine magnetic grains are mainly Fe-Ti rich with low silica.

4.4.4 Comparison with other deep-sea Fe-rich magnetic spherules

The present data show a remarkable similarity with the spherules recovered at stations SS2/94, SS2/89, and SS10/657 from the CIOB (Table 4.6) (Iyer et al., 1997a, b; 1999). The studied spherules show relative enrichment in Mg, Mn, Ca, K, and Na along with Cr and Ni. The Ti value of Site-2 and those of SS2/89 and SS10/657 (Iyer et al.,

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Chapter 4

1997a) are low (0.14-0.4), while the Site-1 has significantly higher Ti (0.80). Si, Al and Ca values from Site-2 are akin to those reported earlier unlike the spherules at Site-1.

When compared with the Fe-rich spherules from Mt. Etna, Mt. Lipari, Mt. Vesuvius, and Mt. Bracciano (Table 4.6, Sr No. 7-10) (Del Monte et al., 1975) it was noted that Fe content is similar but not for the other elements that co-occur in the spherules. The spherules from the MAR also resemble the CIOB spherules but the former have less Fe content. The CIOB Fe-rich magnetic spherules do not compare well with basaltic spherules from DSDP Site 32, NE Pacific Ocean (Melson et al., 1988) and neither with experimentally produced large Fe-Al, small Fe-Al, and blocky Fe particles (Heiken and Wohletz, 1985) and basaltic microlapilli from the Vityaz FZ, Indian Ocean (Nath and Iyer, 1989). The Fe content is very low in all of these reported materials and thus indicate that the process of formation could be quite different for the CIOB spherules.

4.5 Discussion Since ages, the origin of magnetic spherules in the deep-sea sediments has been a subject that is less explored and debated. Deep-sea spherules for a very long time was considered to be melted terrestrial products that were ejected during the impact of an extra- terrestrial body on Earth (Glass, 1990). However, with experimental methods of spherule formation it was clear that these are distinct products of volcanism (Peckover et al., 1973; Sheridan and Wohletz 1981; Wohletz et al., 1995; White 1996). Volcanic Fe-rich spherules are easily identified by their surface appearance and presence of magmaphile elements. In contrast, cosmic spherules have an etched surface, barred texture and an Ni-rich core (Finkelman, 1970; Heiken and Wohletz, 1985; Iyer et al., 1997a, b). Studies have shown that the abundance of cosmic spherules in the CIOB is relatively very low as compared to volcanic spherules but their occurrence cannot be neglected (Reshma et al., 2013). Considering the surface textures and compositions, I have discarded the studied spherules to be of cosmic origin and suggest their source to be from volcanic activities. To understand the mechanism of formation and distribution of spherules in the CIOB it was important to know the nature of spherules occurring in both the tectonic domains (seamounts and FZ). Hence, the physical and chemical parameters were used to classify the spherules occurring in two different geological settings in the CIOB. It was observed that out of the total 77 studied spherules from both the sites, most of them are spherical, while a few are uneven or oval. Further, based on the pie chart it was clear that the spherical spherules dominate at both the sites (Fig. 4.5A) with 34 occurring

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Chapter 4 at Site-1 and 25 occurring at Site-2 (Table 4.2A& B). Five major shapes were observed mainly spherical, broken, teardrop, unevenly spherical and oval. The size classification of the spherules was made based on the three main categories (Fig. 4.5B) and it is seen that most of them range from 126 to 220 µm. But overall the size of the studied spherules varies from 63 to 390 µm. Our observed statistics corroborate well with the earlier reported sizes (upto 475 µm) (Iyer et al., 1999b). The volcanic spherules from different areas such as Hawaii, Viti Levu and Tahiti (Fredrickson and Martin, 1963), Etna, Lipari, Vesuvius and Bracciano (Del Monte et al., 1975), Pacific coast volcanoes, Irazu, Kilauea Iki, Ubinas and Huainaputina (Wright and Hodge, 1964, 1965) are relatively smaller in size. The CIOB spherules show a wide range of surface textures. The classification made here helped to distinguish crypto-crystalline (smooth-surfaced) and crystalline spherules. Spherules with smooth surface texture would have probably formed by a process of quenching of hot lava when it contacted cold seawater. Therefore, the spherules must have got very little time for the growth of crystals and therefore crystals are either absent or minute in size (cryptocrystalline) (Fig. 4.6). On the other hand, the well-textured spherules suggest a slow cooling of the lava (Fig. 4.7). Vacuoles/blow holes are quite common in most of the spherules and these possibly developed during the escape of volatiles (such as sulphur) from within the spherules (cf. Iyer et al., 1997a). I have compared the chemical compositions of the spherules from Sites-1 and 2 with those reported by other workers (Table 4.6). Overall, the chemical data are in agreement with those reported from site SS2/94, SS2/89, and SS10/657 (Iyer 1997a,b; 1999b). The CIOB spherules compositionally differ from volcanic spheres and droplets of basaltic glass of Kilauea Iki, Hawaii (Heiken and Wohletz, 1985), basaltic hydromagnetic ash (Wohletz and McQueen, 1984a,b), basaltic spherules from the eastern Pacific Ocean (Melson et al., 1988) and those near the Vityaz FZ, Indian Ocean (Nath and Iyer, 1989) (Table 4.5). In the polished sections, the interior had a composition different from surface of the spherules. The contents of Al, Ti, Mn, K, and Mg were relatively high along with significant amount of silica (Table 4.3). Further, there are discrete areas (Fig. 4.8) with high Fe values (65-74) in a silicate matrix. The presence of Si and other elements together with S in appreciable amounts reaffirms the CIOB spherules to be volcanic while the Fe- rich particles associated with these spherules were enriched in Ti (7) along with moderate

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Chapter 4

Fe (65). The association of these particles along with the spherules suggest that they must have formed together by a common process. The spatial distribution map indicate higher abundance of spherules in the areas associated with seamounts and FZ. This implies that morphological parameters at Site-1 i.e., seamounts and FZ (Fig. 4.1B) may be an active regime for localized volcanic activity, which may have produced these spherules. The possibility of these Fe-rich magnetic spherules to be of recent origin cannot be ruled out as these are associated with surface sediments. The temporal distribution of spherules derived based on the gravity cores retrieved from the sediment adjacent to the 79⁰ E FZ also shows surface accumulation of spherules. The abundance was calculated to be 180-200 spherules/100 g of the dry sediment. The spherule abundance was relatively higher at depth 280-355 cm (280-440/100 g) and moderate at depth 460-475 cm (280-320/100 g). The spherules occurring at these depths and associated with basaltic material and vhm again indicates their volcanic origin. The morphology and composition of spherules from Sites-1 and 2 do not display much difference. This fact implies that the process of formation of the CIOB spherules could be similar irrespective of the sites at which these formed.

4.5.1 Formational Process of Fe-rich magnetic spherules The investigations of the CIOB spherules by Iyer et al. (1997a,b; 1999b) ruled out several sources that could probably account for their occurrence in different sediment types and near morpho-tectonic features. The sources are sampling artefacts, industrial or anthropogenic contributions (Puffer et al., 1980), terrestrial volcanic eruptions (Fredrickson and Martin, 1963; Hodge and Wright, 1964, Wu et al., 2013), extra-terrestrial inputs (Schmidt and Keil, 1966; Mutch, 1966), diagenetic (McCabe et al., 1983) and biological (Wang and Chatterton, 1993; Ma and Bai, 2002). Deep-sea spherules of cosmic origin have textures akin to volcanic spherules but the former have Ni-rich core (e.g. Finkelman, 1970, Genge et al., 2017) while the volcanically derived spherules have magmaphile elements (Heiken and Wohletz, 1985). Based on such evidence Iyer et al. (1997a,b) opined that the CIOB spherules were formed volcanically in an intraplate environment. Considering several factors, the likely processes that could result in the production of spherules are: (i) submarine hydrothermal exhalations, (ii) fuel-coolant interaction (FCI)

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Chapter 4 and (iii) molten fuel-coolant interaction (MFCI). The mechanism related to each of these processes and their application to the present study is now discussed. The first proposed mechanism for the formation of spherules was submarine hydrothermal exhalations. Harrison and Peterson (1965) suggested the role of hydrothermal exhalations and release of CO2 that could bring out a change in temperature and pressure, thereby affecting the pH of the water column and enhancing the formation the Fe-rich mineral phase in the deep-sea sediments. This process was used to explain the unusual abundance of iron in the EPR (Goldberg and Arrhenius, 1958), in-situ origin of a magnetic mineral near abyssal hills in the Indian Ocean (Harrison and Peterson, 1965), the Amorphous Goethite Facies of the Red Sea metalliferous sediments (Bischoff, 1969), microscopic amorphous iron-manganese oxide precipitates (0.5 to 25 µm) in sediment columns near the Mendocino and Murray FZ (von der Borch and Rex, 1970) and in formation of modern iron ooids from Panarea volcanic island, Aeolian Arc (Bella et al., 2019). Later, Peckover et al. (1973), Sheridan and Wohletz (1981), Wohletz and McQueen (1984a,b) and Wohletz et al. (1995) have experimentally demonstrated that during hydrovolcanic eruptions a mechanism that resembles FCI takes place. During such a process an explosion is caused by rapid vaporization of water during its contact with molten material. This phenomenon of rapid vaporization of water is also noted in the metal industry. Wohletz (1983) reported that when a hot magma is exposed to cold seawater, it results in mixing of the two melts before the explosion. Fragmentations in such case have been reported to increase the contact area of melt and water resulting in an increase in conductive heat exchange at the site of explosion. While the third mechanism (MFCI), is a refinement of the FCI with molten melt as an additional parameter. Zimanowski et al. (1997) opined that MFCI takes place in four phases: (1) a hydrodynamic mixing phase, (2) followed by the trigger phase, (3) fine fragmentation phase, and finally (4) vaporization and expansion phase. In the early stage, the process is initiated with hydrodynamic mixing of water and magma over a considerable time period (Zimanowski et al., 1995). The author further states that even in a reduced state, magmatic melt dispersed within the water will effectively allow the solidification of melt with less than 1 second. Those authors conducted an experiment on a basaltic melt with ideal close conditions and this resulted in particles of three major shapes: (a) most abundant spherical particles that were perfectly rounded, (b) elongated and cylindrical particles, and (c) angular particles with pitted and cracked surface.

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Chapter 4

Wohletz and McQueen (1984a,b) mixed thermite (Al + Fe2O3) (equivalent to basalt) and quartzo-feldspathic material (akin to siliceous sediment) and water in a steel container and this mixture was blown at high pressure. The resultant products were spherules of different shapes and sizes and with variable Fe, Si and Al contents. In the CIOB the presence of siliceous and pelagic sediments, occurrence of MORB (on the seafloor and at the seamounts) (Das et al., 2007; Iyer et al., 2018), and hydrothermal activity at Sites SS2/89, SS10/657 and SS2/94 (Fig. 4.1) (Iyer et al., 1997a, b; 1999); all indicate a condition similar to the above discussed field experiments. During MFCI a hot Fe-rich melt seeped from fractures and fissures, present either near seamounts and/or FZ. This molten material was rapidly quenched due to interaction with cold seawater and as a consequence Fe-rich spherules of variable shapes, sizes, and textures formed in the CIOB sediments. The sediments and MORB occurring on the seafloor may have been additionally responsible in contributing other metals such as Ti, Cu, Zn, Pb, and S that co-exist with the Fe-rich spherules. The process of MFCI, together with submarine hydrothermal exhalations, is presented in a schematic model (Fig. 4.10). One constraint in the model may be the great water depth (5,000 m) (cf. Zimmerman et al., 1997) from where the spherules were recovered from the CIOB. Recently, Bella et al. (2019) have reported that hydrovolcanic activity at abyssal depth could be enhanced by the release of CO2 and the formation of a vapor-phase. And such a condition prevails when the hydrostatic pressure is higher than the critical pressure of the water column.

Fig. 4.10 A schematic diagram of magmatic activity associated with fracture zone and seamounts and formation of spherules. Magmatic fluids of high temperature that are channelizedalong fractures and fissures undergo quenching on contaxct with cold seawater. Processes such as MFCI and

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Chapter 4 hydrothermal exhalations are at play to produce Fe-rich spherules, basaltic clast and Fe- rich grains on the seafloor as well as within the sediments. The occurrence of such a mixture of pyroclast within the sediments is perhaps due to focused lateral movement of magmatic fluids.

I postulate that the present study area, composed of fissures and fractures that developed at the base of the seamount (Site-1) and FZ on the abyssal plain (Site-2), could be sites for hydrovolcanic activity (Fig. 4.10). The association of basaltic material along with spherules further confirms the finding. Further, the occurrence of spherules at two different depths (280-355, and 460-475 cm) in the core AAS-22/7 indicates lateral movement of the magmatic fluid. Apparently, there are also other sites in the CIOB where active Fe-rich fluids emanations are common, as proposed earlier by (cf. Iyer et al., 1997a, b, 1999b).

4.5.2 Volcanic-hydrothermal activities in the CIOB Volcanic activities in the CIOB are attested by the extensive occurrence of basalts, spilites, pumice and glass shards (Iyer and Sudhakar, 1995). In addition, there are hydrothermal signatures on ferromanganese crusts occurring near 76º30’ E FZ (Iyer, 1991) and on the FeMn coated pumice (Kalangutkar et al., 2015). Furthermore, such activities ranging in age from 625 to 10 ka have also been reported near seamounts (Iyer et al., 1997a, 1999b). A younger hydrothermal event of ~100 yrs was reported by Nath et al. (2008) based on 210Pb, 238U-230Th, 10Be, major and trace elements, and micromorphological and microchemical data of sediments from the flank of a seamount located at 75.30°E FZ. In addition to these, hydrothermal events corresponding to ~7.5-9 Ma and 0.5 Ma have been reported from ODP Site 717-719 (Boulegue and Mariotti, 1990) (Fig. 4.1) while the formation of humic acid coupled with hydro-thermogenesis at1°59.8’S/80°E, just below the ODP Site (Sarma et al., 2017). These observations corroborate my findings and suggest that channelized hydrothermal-volcanogenic activities may not be uncommon in the CIOB. The present study also supports the observations that seamounts and FZ play vital roles in controlling the volcanic-hydrothermal activities in the CIOB. These sites could be potential areas for the flow of magmatic-hydrothermal material and result in the formation of sub-seafloor vhm. In addition, such events, albeit sporadic in nature, tend to produce metal-riched particles in the seafloor sediments which will be discussed in the next chapter.

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Chapter 4

4.6 Conclusions This study is a comprehensive record of occurrence of Fe-rich magnetic spherules from the CIOB. The detailed sediment sampling from two sediment types and tectonic settings offer a first-hand understanding of the genesis of Fe-rich magnetic spherules and associated Fe-rich grains from a water depth of 5,000 m. The Fe-rich spherules were categorized into five major shapes: mainly spherical, broken, teardrop, unevenly spherical and oval. While textural studies indicate two major forms i.e. crypto-crystalline with smooth texture or well crystallized with different surface textures such as brickwork, corkscrew, interlocking, and dendritic. The similarity in the composition of Fe-rich spherules and associated Fe-rich grains from both the sites suggested that they are formed by a similar process of hydrovolcanic activities. Based on the observations and conditions prevailing in the basin, I conclude that MFCI coupled with submarine hydrothermal exhalations could be plausible processes for the production of spherules. The morpho-tectonic features such as FZ and seamounts where spherules are abundant may be favourable sites where MCFI could be active. The fluid-driven hydrovolcanic activity at a water depth of 5,000 m was perhaps enhanced by the release of CO2 and formation of a vapor-phase, allowing the melt to get dispersed with a simultaneous lowering of the temperature. The dispersed melt either resulted in their quenching or produced well-crystallized Fe-rich spherules. The occurrence of spherules within the surface sediments of Site-1 and in top 10 cm in both the cores (Site-2) suggests that hydrovolcanic events are of recent age. While the presence of spherules at two different depths (280-355, and 460-475 cm) in the core AAS-22/7 indicates lateral seepage of the magmatic fluid. Based on the observations and regional scenario, I conclude that certain geologic settings in the CIOB, such as seamounts and FZ, are potential sites for volcanic and hydrothermal activities.

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Chapter 4

Table 4.1A Details of 40 sediment sampling locations from Site-1 (OG = Okean Grab).

Sr. No Station Latitude ⁰S Longitude ⁰E Water depth (m) 1 OG-001 13.74989 74.68777 5329 2 OG-002 13.74992 74.81276 5195 3 OG-003 13.74994 74.93778 5261 4 OG-005 13.68751 74.68777 5131 5 OG-006 13.6875 74.75024 5200 6 OG-007 13.68742 74.81275 5168 7 OG-008 13.68749 74.87526 5279 8 OG-010 13.68748 75.00024 5167 9 OG-011 13.62501 74.93779 5260 10 OG-012 13.62498 74.81278 5260 11 OG-013 13.62493 74.68780 5193 12 OG-014 13.62492 74.56274 5204 13 OG-015 13.56254 74.50024 5095 14 OG-016 13.56241 74.56276 5230 15 OG-018 13.56250 74.75025 5168 16 OG-019 13.56224 74.81278 5167 17 OG-020 13.56226 74.87536 5204 18 OG-021 13.49994 74.56275 5170 19 OG-022 13.43749 74.50027 5204 20 OG-023 13.43741 74.56260 5192 21 OG-024 13.43742 74.62510 5141 22 OG-025 13.43741 74.75009 5221 23 OG-026 13.43741 74.81259 5190 24 OG-027 13.43741 74.87510 5192 25 OG-028 13.37490 74.56259 5190 26 OG-029 13.37491 74.68760 5153 27 OG-030 13.37497 74.81264 5152 28 OG-032 13.31253 74.62528 5232 29 OG-033 13.31262 74.68777 5220 30 OG-034 13.31257 74.75027 4750 31 OG-036 13.24993 74.68783 5165 32 OG-037 13.2500 74.81273 5123 33 OG-039 13.12495 74.06271 5249 34 OG-040 13.18748 75.00027 5229 35 OG-041 13.18743 75.06276 5064 36 OG-043 13.50001 75.56270 5217 37 OG-044 13.56245 75.50024 5280 38 OG-045 13.56251 75.56276 5156 39 OG-047 13.62495 74.56276 5230 40 OG-048 13.50005 74.81275 5140

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Chapter 4

Table 4.1B Number of iron-rich magnetic spherules occurring at 55 sediment sampling locations from Site -1.

Sr. No Station Latitude ⁰S Longitude ⁰E Spherules 1 134 10.5003 75.9284 262 2 135 10.5552 75.9927 121 3 137 10.6123 75.9112 231 4 138 10.5622 75.8764 485 5 139 10.6852 75.8697 321 6 140 10.3758 75.6883 561 7 142 10.3667 75.7459 261 8 143 10.4921 75.6795 268 9 146 10.3796 75.4456 491 10 147 10.4353 75.3807 605 11 148 10.6299 75.4377 389 12 149 10.6866 75.4992 727 13 151 10.6862 75.3788 751 14 153 10.6255 75.1908 460 15 154 10.74 75.1847 672 16 155 10.6977 75.1336 388 17 156 10.625 74.9361 393 18 157 10.7425 74.9427 388 19 159 10.4323 74.7255 319 20 163 10.3759 7462.72 996 21 164 10.4356 74.6286 172 22 165 10.5494 74.6201 271 23 166 10.6208 74.5628 552 24 167 10.556 74.4393 241 25 168 10.8781 74.0625 223 26 169 10.9396 74.0066 167 27 170 10.9946 74.0591 186 28 172 10.938 74.506 86 29 173 10.8847 74.8972 218 30 175 10.9346 74.6224 145 31 177 11.4415 75.0089 533 32 178 11.4936 75.0507 85 33 179 11.4313 75.1239 227 34 180 11.8097 75.6323 126 35 182 11.8748 75.7001 224 36 185 11.6265 75.8103 242 37 186 11.7434 75.8056 165 38 187 11.6818 75.8696 156 39 188 11.5623 75.8808 161 40 189 11.5064 75.9359 541 41 191 12.1335 75.803 604

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Chapter 4

42 192 12.0806 75.8037 215 43 193 12.3684 75.8021 139 44 194 12.311 75.8671 29 45 195 12.1794 75.8745 184 46 196 12.1876 75.755 168 47 197 12.3035 75.7561 213 48 198 12.4261 75.616 305 49 201 12.5622 75.481 241 50 202 12.6251 75.4411 156 51 203 12.5646 75.3315 154 52 204 12.563 75.2481 129 53 205 12.5053 75.1848 209 54 206 12.5662 75.1321 201 55 207 12.6216 75.1894 141

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Chapter 4

Table 4.2A Morphological classification of iron-rich magnetic spherules (Site-1).

Sr Sample Size Shape Description Texture No Id (µm) 1 S1_0003 Spherical Spherical with Large crystals 170 Brickwork 2 S1_0005 Spherical Large and small crystals of magnetite 250 Brickwork 3 S1_0013 Spherical Large crystals well arranged with blow holes 267 Broken Tile 4 S1_0014 Spherical Large crystals arranged in an interlocking pattern 97 Corkscrew 5 S1_0018 Spherical Large crystals on the surface, smooth 80 Corkscrew 6 S1_0021 Spherical Large crystals on the surface 163 Brickwork 7 S1_0023 Spherical Small crystals on the surface 196 Smooth 8 S1_0024 Spherical Large crystals arranged in an interlocking pattern 85 Corkscrew 9 S1_0026 Spherical Large crystals arranged in an interlocking pattern 136 Corkscrew 10 S1_0027 Spherical Small crystals closely arranged 86 Smooth 11 S1_0028 Spherical Small crystals well arranged 103 Broken Tile 12 S1_0029 Spherical Large crystals on the surface well arranged 245 Brickwork 13 S1_0030 Spherical Large crystals on the surface 275 Brickwork 14 S1_0031 Spherical Small crystals covering the whole spherule 116 Smooth 15 S1_0032 Spherical Large crystals of magnetite, well arranged 101 Corkscrew 16 S1_0034 Spherical Small crystals covering the whole surface 215 Brickwork 17 S5_0081 Spherical Small crystals aggregate was seen on the surface 213 Corkscrew 18 S5_0082 Spherical Small size spherule 120 Smooth 19 S3 Spherical Surface is a smooth s with few aggregates 136 Smooth 20 S3-1 Spherical Metal aggregates on spherule 136 Smooth 21 S4 Spherical Platelets of magnetite crystals 185 Dendritic 22 S5 Spherical Brick-work arrangement of magnetite crystals 224 Brickwork 23 S6 Broken Spherule was partly broken Nd Dendritic The broken part has Fe rich protrusions coming out S6-1 Broken Nd Smooth 24 of it 25 S7 Broken Spherule has a smooth surface with blowhole Nd Smooth 26 S8 Spherical Rough surface with attached aggregates on it 262 Smooth 27 S14 Teardrop Smooth surface with few small blob protruding out 192 Brickwork 28 S15 Spherical Quench texture on the surface of a spherule 184 Dendritic 29 S16 Spherical Smooth surface with aggregates on it 177 Smooth 30 S16-1 Spherical Metal aggregate on spherules 177 Smooth 31 S17 Spherical Contains dendritic texture on the surface 73 Dendritic Platelets of magnetite crystals with cork-screw-like S19 Spherical 87 Cork-screw 32 distortions 33 S20 Spherical Smooth surface with very slight cracks 150 Smooth 34 S21 Spherical Euhedral magnetite crystals are visible 120 Dendritic 35 S22 Spherical Blowholes are abundant with moss- like texture 71 Moss like 36 S23 Spherical Two spherules are fused together 82 Cork-screw 37 S24 Spherical Spherule has blowholes and brick-work like texture. 97 Brickwork 38 S1_0103 Spherical Large magnetite crystals on the surface 54 Brickwork

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Chapter 4

Table 4.2B Morphological classification of iron-rich magnetitc spherules (Site-2).

Sr Sample Size Shape Description Texture No Id (µm) 39 F_S251 Spherical Smooth surface with few blowholes 125 Smooth 40 F_S27 Spherical Surface contains platelets of larger crystals 207 Brickwork 41 F_S28 Spherical Small crystals well arranged 189 Broken tile 42 F_S29 Spherical Well-developed euhedral crystals occurring on the surface 207 Brickwork 43 F_S30 Spherical Surface is smooth with few blowholes 85 Smooth 44 F_S311 Spherical Surface is smooth, contains many blowholes and vesicles 234 Smooth 45 F_S32 Spherical Euhedral crystals with few blowholes 136 Brickwork 46 F_S33 Spherical Subhedral crystals with blowholes 72 Smooth 47 F_S34 Spherical Rough surface and has many blowholes and vesicles 98 Smooth Unevenly F_S35 Smooth surface with a few blowholes and cracks 79 Smooth 48 Spherical 49 F_S361 Spherical Tiny magnetite crystals with interlocking texture 219 Quenched Unevenly F_S37 Surface contains blowholes and some accretion of material 170 Brickwork 50 Spherical 51 F_S38 Oval Smooth surface with a few blowholes 71 Smooth 52 F_S39 Spherical Platelets of magnetite crystals with blowholes and cracks 167 Corkscrew 53 F_S40 Oval Smooth surface with few blowholes 74 Smooth 54 F_S41 Oval Oval in shape with smooth surface 62 Smooth Unevenly F_S42 Smooth surface with a few blowholes 72 Smooth 55 Spherical 56 F_S43 Spherical Spherical with a few blowholes 110 Smooth Unevenly F_S44 Unevenly spherical with a few blowholes 140 Corkscrew 57 Spherical Unevenly Unevenly spherical with subhedral crystals and contains a lot F_S451 255 Dendritic 58 Spherical of blowholes 59 F_S46 Spherical Platelets of magnetite crystals can be seen 216 Brickwork Unevenly F_S471 Contains few aggregates and blowholes on the surface 139 corkscrew 60 Spherical Unevenly F_S472 Spherical with a few blowholes 139 corkscrew 61 Spherical Unevenly F_S48 Rough surface with a few blowholes. 261 Dendritic 62 Spherical 63 F_S50 Spherical Smooth surface with a few aggregates. 105 Smooth Euhedral magnetite crystals, smooth surface with a few F_S51 Spherical 179 corkscrew 64 blowholes 65 F_P21 Spherical Contains blowholes and crystal growth 118 Brickwork 66 F_P71 Spherical Spherule with a sub spherule attached to it 390 Corkscrew Unevenly F_P81 Uneven spherule. Crystals are clearly visible. Rough surface 138 Dendritic 67 spherical 68 F_P91 Oval Rough surface with uneven bands of grey and white colour 137 Corkscrew 69 F_P92 Oval Oval in shape with high Fe 137 Corkscrew 70 F_P121 Spherical Surface contains blowholes and some accretion of material 190 Broken tile 71 F_P131 Spherical Surface are present on the spherule 101 Brickwork 72 F_P141 Spherical Spherule contains an extra coating of Fe 101 Smooth 73 F_P151 Spherical The aggregate of high Si and Fe was observed on the surface 285 Brickwork 74 F_P182 Spherical Spectrum with large crystals on the surface 210 Brickwork Spherule has blowholes and few aggregates with high Fe and F_P201 Spherical 141 Brickwork 75 high Na. 76 F_P322 Spherical Bands on the surface were observed 111 Smooth 77 F_P481 Spherical Contains small crystals of magnetite 155 Smooth

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Chapter 4

Table 4.3A Major element composition (wt%) of Fe-rich magnetic spherules from Site-1. nd = not detected.

Sr No. Sample Id Si Ti Al Fe Mn K Mg Ca Na O Total 1 S3 nd nd nd 74.01 nd nd nd nd 0.1 25.12 99.22 2 S3-1 11.39 nd 4.63 80.73 nd nd nd 3.25 nd nd 100 3 S4 nd nd nd 86.94 nd nd nd nd nd 13.06 100 4 S5 nd nd nd 62.04 0.4 2.6 1.1 nd 0.8 32.46 99.4 5 S6 nd nd nd 61.76 nd nd nd nd nd 33.52 95.28 6 S6-1 nd nd 2.6 62.97 1.2 nd 1.6 0.2 0.5 29.43 98.5 7 S7 nd nd nd 65.69 1.51 nd nd nd nd 31.23 98.43 8 S8 2.58 nd 2.27 77.11 nd 0.1 nd nd nd 17.72 99.78 9 S14 0.35 nd nd 80.05 nd nd nd nd nd 19.6 100 10 S15 0.45 nd nd 71.60 nd nd nd nd nd 27.95 100 11 S16 1.33 nd nd 65.24 nd nd nd nd 0.21 31.75 98.53 12 S16-1 0.41 nd 1.29 65.24 1.83 nd nd nd nd 31.03 99.8 13 S17 0.4 nd nd 61.08 nd nd nd 0.31 nd 33.34 95.13 14 S19 nd nd nd 72.11 nd nd nd nd nd 27.89 100 15 S20 nd nd nd 72.24 nd nd nd nd nd 27.76 100 16 S21 0.51 nd nd 75.94 nd nd nd nd nd 23.55 100 17 S23 0.39 nd nd 68.6 0.53 nd nd nd nd 30.47 99.99 18 S24 nd nd nd 69.56 nd nd nd nd nd 30.44 100 19 S-KS2 nd nd nd 76.87 0.86 nd nd nd nd 22.27 100 20 S-KS8 0.3 nd nd 77.23 nd nd nd nd nd 22.47 100 21 S-KS2 nd nd 0.4 69.43 nd 4.6 2.4 nd 0.9 22.25 99.98 22 S-KS3 0.86 nd nd 70.56 0.65 nd nd nd 1.57 22.8 96.44 23 S-KS4 1.07 nd 0.49 75.24 nd nd nd nd nd 23.2 100 24 S-KS5 0.35 nd 0.55 76.32 nd nd nd nd nd 22.76 99.98 25 S-KS1 nd nd 0.39 77.16 nd nd nd nd nd 22.45 100 26 S-K6S2 0.43 nd nd 77.17 nd nd nd nd nd 22.4 100 27 S-K3S5 1.64 0.8 0.44 71.72 1.39 0.81 nd nd nd 23.2 100 28 S-K3S4 0.34 nd nd 76.44 nd nd nd nd nd 22.47 99.25 29 S-K3S1 0.28 nd nd 75.87 1.4 nd nd nd nd 22.44 99.99 30 S-K6S1 0.4 nd 0.43 76.44 nd nd nd 0.5 nd 22.73 100.5 31 S-KA25 0.36 nd 0.3 75.31 0.4 nd 1.1 nd nd 22.6 100.07 Avg. 31 1.25 0.80 1.25 72.54 1.02 2.03 1.55 1.07 0.68 25.35 99.36 Std Dev 2.53 - 1.38 6.22 0.51 2.01 0.61 1.46 0.54 4.98 1.35

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Chapter 4

Table 4.3B Major element composition (wt%) of Fe-rich magnetic spherules from Site-2. nd = not detected.

Sr No. Sample Id Si Ti Al Fe Mn K Mg Ca Na O Total

32 F-S25-1 nd nd 0.72 76.48 nd nd nd nd 0.1 22.63 99.93 33 F-S27 nd nd nd 77.73 nd nd nd nd nd 22.27 100 34 F-S28 0.64 nd 0.33 76.19 nd nd nd nd nd 22.84 100 35 F-S29 0.38 nd nd 75.26 1.84 nd nd nd nd 22.52 100 36 F-S30 0.41 nd nd 75.22 1.01 nd nd 0.08 nd 22.64 99.36 37 F-S31-1 nd 0.41 nd 71.06 nd 0.52 2.7 nd 2.1 22.19 98.98 38 F-S32 nd nd nd 77.19 0.54 nd nd nd nd 22.27 100 39 F-S33 0.17 nd nd 76.98 0.46 nd nd nd nd 22.38 99.99 40 F-S34 nd nd 0.77 76.6 nd nd nd nd nd 22.63 100 41 F-S35 nd nd 0.15 77.31 nd nd nd nd nd 22.36 99.82 42 F-S36-1 1.92 nd 0.32 68.46 nd nd 2.29 0.85 nd 24.33 98.17 43 F-S37 0.23 nd nd 76.67 0.67 nd nd nd nd 22.4 99.97 44 F-S38 0.24 nd nd 76.3 0.63 nd nd nd nd 22.48 99.65 45 F-S39 0.37 nd nd 76.14 0.97 nd nd nd nd 22.52 100 46 F-S40 0.26 nd 0.15 76.1 0.6 nd 0.1 0.05 nd 22.56 99.82 47 F-S41 nd nd nd 77.73 nd nd nd nd nd 22.27 100 48 F-S42 0.11 nd nd 77.05 0.49 nd nd nd nd 22.35 100 49 F-S43 0.28 nd nd 76.66 0.61 nd nd nd nd 22.45 100 50 F-S44 0.15 0.24 nd 76.06 nd nd nd nd nd 22.57 99.02 51 F-S45-1 nd nd 0.52 76.50 0.47 nd nd nd nd 22.51 100 52 F-S46 0.12 0.95 0.78 74.70 0.37 nd nd nd nd 22.97 99.89 53 F-S47-1 0.09 0.20 0.3 75.30 0.30 nd nd nd 0.12 22.6 98.91 54 F-S47-2 1.06 0.23 1.47 60.47 nd 1.14 0.98 0.88 4.66 24.52 95.41 55 F-S48 0.24 nd 0.51 75.92 0.52 nd nd nd nd 22.63 99.82 56 F-S50 0.15 0.27 0.48 75.26 nd nd nd nd nd 22.77 98.93 57 F-S51 0.6 nd 0.22 74.97 1.21 nd nd nd nd 22.77 99.77 Avg. 26 0.41 0.38 0.52 75.17 0.71 0.83 1.52 0.47 1.75 22.67 99.52 Std Dev 0.45 0.29 0.36 3.59 0.40 0.44 1.20 0.46 2.16 0.55 0.97

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Table 4.3C Minor elements occurring in Fe-rich magnetic spherules from the study area, S= (S16-1=1.68), (S47-2=0.82), Cu= (S-K3S3=0.79, S-K3S4= 0.73). (nd= not detected).

Sr No Sample Cr S Zn Ni Id 1 S3 nd nd nd 0.88 2 S6 nd nd nd 4.73 3 S6-1 nd nd nd 7.59 4 S7 nd nd nd 3.08 5 S8 nd nd 0.31 nd 6 S16* 0.43 nd nd nd 7 S16-1 nd 1.68 nd nd 8 S17 * nd nd nd 4.52 9 S-KS2 nd 0.3 nd 7.79 10 S-KS3 nd nd 3.57 nd 11 S-K3S4 nd nd nd nd 12 S-K3S3 nd nd 0.64 nd 13 F-S25-1 0.17 nd nd nd 14 F-S30 0.72 nd nd nd 15 F-S31-1 nd nd 6.34 nd 16 F-S35 0.18 nd nd nd 17 F-S36-1 nd 0.11 nd 2.67 18 F-S38 0.33 nd nd nd 19 F-S40 0.32 nd nd nd 20 F-S44 0.99 nd nd nd 21 F-S47-1 0.92 nd nd nd 22 F-S47-2 0.68 0.82 0.56 nd 23 F-S50 0.92 nd nd nd 24 F-S51 0.13 nd nd nd

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Table 4.4A Elemental composition (wt.%) of major elements occurring in polished Fe-rich magnetic spherules. The grey highlighted data reflects the composition of the matrix.

Sr No # Si Ti Al Fe Mn K Mg Ca Na O Total 1 P2-1 8.11 nd 1.84 46.60 nd 0.46 0.41 10.75 1.10 30.15 99.42 2 P7-1 na nd nd 77.73 nd nd nd nd nd 22.27 100.00 3 P7-2 0.29 nd 0.27 76.85 nd nd nd nd nd 22.59 100.00 4 P8-1 0.40 nd nd 75.98 0.98 nd 0.04 nd 0.01 22.58 99.99 5 P9-1 7.58 0.10 nd 62.43 nd 0.20 nd 0.47 nd 27.28 98.06 6 P9-2 4.27 nd nd 70.12 nd nd nd 0.21 nd 25.04 99.64 7 P12-1 1.97 1.76 0.91 66.21 2.41 nd nd 0.25 1.66 24.58 99.75 8 P12-2 16.23 1.21 4.54 28.32 0.73 1.60 1.94 2.44 5.00 36.53 98.54 9 P13-1 17.00 0.52 5.03 34.14 0.49 0.72 3.92 0.52 0.10 37.03 99.47 10 P14.-1 0.43 nd 0.26 71.18 0.81 nd nd 3.93 nd 23.20 99.81 11 P15-1* 19.00 1.00 4.40 15.80 4.79 1.00 3.10 0.90 7.53 38.50 96.02 12 P18-1 12.40 0.80 1.85 17.00 26.80 2.00 0.60 0.92 2.90 31.80 97.07 13 P18-2 3.34 nd 0.70 66.20 nd nd 0.34 nd 3.62 25.08 99.28 14 P20-1 1.98 nd nd 27.47 nd 1.46 nd 3.09 25.00 32.00 91.00 Avg. 14 7.15 0.90 2.20 52.57 5.29 1.06 1.48 2.35 5.21 28.47 98.43 Std Dev 6.85 0.57 1.94 23.38 9.61 0.65 1.53 3.23 7.81 5.82 2.45

Table 4.4B Elemental composition of minor elements (wt%) occurring in polished Fe-rich magnetite spherules. Sample P15-15* contains 0.6 wt % Cu. The grey highlighted data reflects the composition of the matrix.

Sample Id Cr S Pb P2-1 nd 0.61 nd P9-1 0.2 0.29 1.25 P12-2 nd 0.36 nd P13-1 nd 0.24 nd P14-1 nd 0.19 nd P15-1* 1.02 nd nd P18-1 1.20 0.41 1.50 P18-2 0.80 0.15 nd P20-1 nd 7.93 nd

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Table 4.5 Elemental composition of Fe-rich grains associated with spherules. Zn= 2.47 wt% in 32-1*, V= 0.63 wt% in 31-1 and 0.70 wt% in 16-1, Pb= 0.96 wt% in 25-1, Cr and Cu is 0.10 wt% in 16-1. nd = not detected.

Sr Sample No Id Si Ti Al Fe Mn K Mg Ca Na O Total 1 16-1 4.77 12 2.2 47 0.82 0.2 1 0.4 0.76 30.47 99.62 2 17-1 0.2 7.27 0.4 67 0.6 nd nd nd nd 23.8 99.27 3 23-1 1.06 7.82 1.85 59.98 0.6 nd 1.84 nd nd 26.63 99.78 4 25-1 0.75 nd 0.39 74.73 nd 0.1 nd 0.18 0.3 22.76 99.21 5 28-1 0.19 2.24 0.62 73.59 nd nd nd nd nd 23.35 99.99 6 31-1* 0.43 7.32 2.08 61.44 nd nd 1.66 nd nd 26.43 99.36 7 32-2 0.26 2.02 0.66 73.42 nd nd nd 0.26 0.01 23.37 100.0 Avg. 1.09 6.45 1.17 65.31 0.67 0.15 1.5 0.28 0.36 25.26 99.60 Std Dev 1.65 3.78 0.83 10.04 0.13 0.07 0.44 0.11 0.38 2.77 0.33

Table 4.6 Chemical data of Iron rich spherules from world oceans. Compared with Sr. No. 1= Average of 31 Seamount (contains Avg. Zn= 1.51 wt%), 2= Average of 26 FZ (contains Avg. Zn= 3.45 wt%), 3= Average of 7 Fe particles, 4= Average of 17 volcanic magnetite spherules from SS2/94 (Iyer et al., 1999), 5= Average of 24 volcanic magnetite spherules from SS2/89 and SS10/657 (Iyer et al., 1997a), 6= Average of 5 Ti-rich areas in SS2/89 spherules (Iyer et al., 1997a), 7= Magnetite spherules, Mt. Etna, 8= magnetite spherules, Mt. Lipari, 9= magnetite spherules, Mt. Vesuvius, 10= magnetite spherules, Mt. Bracciano (Del Monte et al., 1975), 11= Volcanic spherule from MAR, 12= Average of 57 basaltic spherules from DSDP Site 32, NE Pacific Ocean (Melson et al.,1988), 13= Large Fe-Al Sphere, experimental Products (Heiken and Wohletz, 1985), 14= Small Fe-Al Sphere, experimental Products (Heiken and Wohletz, 1985), 15= Blocky Fe Particle, experimental Products (Heiken and Wohletz, 1985), 16= VFZ= Vityaz fracture zone basaltic microlapilli (Nath and Iyer, 1989. (nd = not detected; na = not available).

Sr. No Si Ti Al Fe Mn Mg Ca Na K Cr Ba Ni S 1 1.25 0.80 1.25 72.54 1.02 1.55 1.07 0.68 2.03 0.43 nd 4.77 0.99 2 0.41 0.38 0.52 75.17 0.71 1.52 0.47 1.75 0.83 0.54 nd 2.67 0.47 3 1.09 6.45 1.17 65.31 0.67 1.5 0.28 0.36 0.15 nd nd nd nd 4 0.234 0.14 1.46 75.98 0.32 0.62 0.08 0.05 0.06 0.13 0.13 1.14 0.07 5 0.89 0.36 0.36 73.8 0.36 0.63 0.32 0.29 0.07 0.11 0.12 0.14 1.55 6 7.35 31.25 2.8 9.6 4.96 0.86 1.75 0.37 2.21 0.43 0.68 na na 7 0.23 na 0.1 73.05 na na na na na na na na na 8 0.15 na 0.09 71.91 na na na na na na na na na 9 0.07 na 0.05 72.1 na na na na na na na na na 10 0.25 na 0.16 71.01 0.05 na na na na na na na na 11 na na na 71.5 0.37 na na na na na na na na 12 23.67 0.65 7.33 10.04 na 3.9 6.85 1.97 0.27 na na na na 13 4.71 na 16.63 45.04 na na 0.32 na 0.26 na na na na 14 5.02 na 13.28 49.89 na na na na na na na na na 15 6.7 1.35 6.01 44.31 1.22 3.88 1.46 2.28 1.59 na na na na 16 21.86 0.7 7.41 7.15 0.15 5.4 9.39 na 0.36 na na na na

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CHAPTER 5

METAL-RICH AND NATIVE GRAINS

Chapter 5

5.1 Introduction The first discovery of metalliferous sediments was reported from the Galapagos Rift (Corliss et al., 1978; Honnorez et al., 1981), EPR, Bauer Deep and Central Basin (Dymond et al., 1973; Heath and Dymond, 1977) and from the Lau Basin (Bertine, 1974). The early research on metalliferous deposits mainly concerned the hydrothermal vents at the MOR where the first discovery of hydrothermal activities were confirmed from black and white smokers, Fe-Mn rich sediments, and active methane vents (Fouquet et al., 1988; Rona and Scott, 1993). The 70,000 km long MOR consists of 280 active hydrothermal sites and among these Indian Ocean ridges contribute to ~10% (Baker and German, 2004). In the oceanic setting, apart from the MOR, hydrothermal metal grains occur in different domains such as fracture zones (FZ) (Bonatti, 1981), in intraplate volcanic areas (such as Aegean and Tyrrhenian sea), basalt-sediment contact (discussed in Chapter 6), and in marginal or back-arc basins (Mariana Trough) (Pirajno, 1992). Seamount related hydrothermal activity is also quite common in the world ocean and has gained attention of researchers in recent years. At seamount sites the associated FZ and fissures associated with seamounts play a major role in hydrothermal precipitation of metal grains (Alt et al., 1987). The Green Volcano occurring at 21°N in the EPR hosts sulphide chimneys, and massive deposit of barite-opal along with Fe-Mn sediments. The Red Volcano consists of red-orange mud along with nontronite and talc. The Axial Seamount occurring on the Juan de Fuca Ridge consist of low temperatures active fluids that are discharged through several fissures. The well known Loihi Seamount (Hawaii) also contributes to yellow brown goethite-Fe-montmorillonite-nontronite (Pirajno, 1992). Apart from these, the other known seamounts that have hydrothermal deposits are: Palinuro Seamount occurring in the Tyrrhenian Sea (stibnite, barite, bismuthinite, and pyrite - Minniti and Bonavia, 1984) and Manji Seamount occurring near Philippine Sea (porphyry copper, Ishizuka et al., 2002). Banakar et al. (2007) reported gold and platinum group elements from the Co-rich ferromanganese crust occurring at the Afanasy–Nikitin Seamount. Rao et al. (1992) reported phosphorites (laminated crusts and massive form) from Error Seamount (NW Arabian Sea). Iyer et al. (2012 a,b) provide a synthesis of hydrothermal deposits found at/near seamounts in the world ocean and possibility of similar ones in seamounts that are in offshore areas of India. In the Indian Ocean, different sites have been identified for hydrothermal mineralization such as the SONNE hydrothermal field, CIR (Pluger et al., 1990), CR

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Chapter 5

(Murton et al., 2006; Popoola et al., 2019), and Rodriguez Triple Junction (Gamo et al., 2001, Meike et al., 2019) (Fig. 5.1). Apart from these, there are reported areas of hydrothermally precipitated sulphide deposits in the SWIR (Liao et al., 2018, 2019). Hydrothermal activity is evident in the CIOB from the occurrence of volcanic magnetite spherules and grains of native aluminium and sporadic hydrothermal events are dated to be 625 ka (Iyer et al., 1997a,b; 1999b; 2007; Iyer, 2005) to as recent as 100 yr (Nath et al., 2008). But a lacuna in knowledge still exists concerning the aspect as to whether there is any intraplate hydrothermal activity in the Indian Ocean that could result in metalliferous deposits.

Fig. 5.1 Location of hydrothermal sites occurring in the Indian Ocean. Red filled box = Present study site of surface sediments, CIOB; White solid star = Core location (AAS- 22/3), CIOB, present study. Red star = Hydrothermal site on SEIR; Yellow solid rectangle = SONNE hydrothermal site; Red outline box = Hydrothermal site on Carlsberg Ridge (base map after Cao et al., 2016).

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Chapter 5

The distribution of metal-rich grains in the oceanic sediments needs special attention not only regarding their formation and indicators of hydrothermally-affected areas but also their viability as future resources (Amonkar, 2019). In the present chapter, I detail a study of metallic minerals (both native and non-native forms) that occur in the CIOB sediments. The data presented here are the first to support possible metallogenesis in the CIOB.

5.2 Study Area Unusually shaped and shiny grains were noted to occur along with Fe-rich magnetic spherules in surface sediments collected from 12 locations in the vicinity of seamounts (Site-1) and in the top 10 cm of the core AAS-22/3 (Site-2). The Fe-rich magnetic spherules at both these sites have been discussed (see Chapter 4).

5.3 Results and Interpretations As mentioned in the previous chapter, the multi-component coarse fractions (CF) are composed of biota (radiolarians, sharks teeth, and calcareous shells), volcanic glass shards, manganese micronodules, tektites, and a few pumice clasts. Apart from these, the CF are dominated by basaltic rock fragments and glasses, Fe-rich spherules, orange and blue mineral flakes, and well-crystallized mineral grains (titano-magnetite).

The composition of 39 shiny and lustrous grains of different morphology and colours are tabulated (Tables 5.1. to 5.5). The chemical data indicate that these grains are metals and/or metal complexes while some are in native form.

5.3.1 Titano-magnetite grains

The magnetic fractions are composed of two forms: (i) abundant magnetic spherules and (ii) grains (Fig. 5.2A and B). The spherules are generally light grey to black in colour with a metallic luster and range in size from a few microns to 450 µm. Besides spherical shapes, there are also oval or partially broken spherules.

The other magnetic grains are mainly unevenly shaped (Fig. 5.2A), rarely spherical and with very few showing well-crystallized forms. These grains are dark black to reddish- brown in colour and a few microns in size. Unlike the magnetic grains, these grains are enriched in Ti (23-28 wt. %, avg 22 wt. %) along with high Fe (32-65 wt. %, avg 41wt. %) (Table 5.1). These Ti-Fe-rich grains with low contents of Al, Si, and Mn, point to the

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Chapter 5 presence of well-crystallized titano-magnetite grains (Fig. 5.2 B,C) in the CIOB sediments. The association of these grains along with Fe-rich spherules indicate a similar process of their formation by MFCI (see Chapter 4).

5.3.2 Metallic grains: Zn-Cu-S, Zn-Cu

The surface sediment samples consist of orange to orange-yellow colour grains. These are stubby, uneven in shape and a few have a coat of silvery veneer on the surface. The grains range in size between 200 and 600 µm (Fig. 5.2 D).

Eleven grains with a flaky appearance were analyzed from the surface sediments collected from Site-1. The colour of these grains is well reflected in the chemical composition (Tables 5.2A-C). For example, it is observed that grains of buff dark orange colour show high percentage of Zn (28-38 wt. %), and low Cu (1.9-2.2 wt. %) and S (0.98- 3.4 wt. %). The second variety of grains is the slightly lighter orange colour grains, compared to the one mentioned above (Fig. 5.2 E), and these have high S (14-28 wt. %) and Cu (3-7 wt. %) and Zn (3-10 wt. %). The former grains with high Zn have relatively high values of Al (10-28wt. %) compared to light orange grains with low Al (1-4wt. %).

Apart from these, a few grains are very pale orange in colour (Fig. 5.2 F). These grains have an extremely high content of Zn (52-62 wt. %) and low Cu (1-3 wt. %), and S (1.5-3 wt. %) (Table 5.2C). Some grains with uneven form and massive appearance have very high amount of Cu (64 wt. %) and with Al (13 wt. %) and Zn (18 wt. %) (Table 5.2D). These appear to be grains of native copper (Fig. 5.2 G).

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Fig. 5.2 Electron micrographs of metal-rich grains from the CIOB sediments. A-Titano-magnetite grains; B and C-Well-crystallized titano- magnetite grains; D- buff orange Zn-Cu grain; E- lighter orange Zn-Cu-S; F-pale orange rich in Zn; G- Native Cu grains.

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Fig. 5.2 Electron micrographs of metal-rich grains from the CIOB sediments. H-J = Barium-Sulphur grains; K-M = Native Pb grains in a titaniferous iron oxide matrix.

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Chapter 5

5.3.3 Metallic grains: Ba-S, Ba-Pb-S

Out of the 39 grains that were analyzed, 8 of them are irregular, stubby and large grains with white pitted patches (Fig. 5.2 H-J). The size ranges from 250 to 500 µm. The chemical composition of these grains revealed enrichment in Ba. The Ba-rich grains are of three distinct varieties: Ba-S, Ba-S-Pb, and Pb (Tables 5.3A, B & C).

The Ba-S grains have a narrow range of Ba (50-54wt. %) and with a significant content of S(12-14wt.%) while, Fe, Al, and Si were present in minor amounts (Tables 5.3A). The second variety is Ba-S-Pb grains with less Ba (23-25wt. %), S (6-9wt. %), significant Pb (13-17 wt.%), moderate Fe (11-12 wt.%) with slightly higher Si, Al, Mg and Ca as compared to Ba-S grains. Cr also occurs in these grains with values ranging from 2 to 25 wt. % (Tables 5.3 B).

The third variety is an elongated, large grain with size ranging from 80 to 125 µm (Fig. 5.2 K-M). The high reflective area of the grain has significant amount of Pb (~60 wt. %) while the matrix is composed of titaniferous iron oxide. Such grains are very few in comparison to the Ba-S and Ba-S-Pb grains. The native Pb grain shows insignificant amounts of As, V, and Fe (Tables 5.3 C).

5.3.4 Aluminium-rich spherules

Nine translucent spherules of size 70 to 210 µm and of buff white to pale colour were analysed (Fig. 5.2 N). The spherules either are found in a matrix of Ba-Cu-Pb-S or as individual entities.

SEM-EDS data (Table 5.4) show high amount of aluminium Al (41-53 wt. %; average 48 wt. %), a significant amount of chlorine (1-12 wt. %, avg. 7.6 wt. % and a relatively small amount of sulphur (2 wt. %). Lithogeneous elements such as Fe, Mg, and Si are nearly absent in these grains. The Al-rich grains occur only in the sediment core AAS-222/3, which is from the southern part of the CIOB. The significance of this would be discussed later.

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Fig.5.2N Electron micrographs of an aluminium-richspherule from the CIOB sediments. The spherule occurs in a matrix of Ba-Cu-Pb-S.

5.3.5 Native silver grains

Five tiny grains (30-60 µm) with a silvery luster (Fig. 5.2 O) were retrieved and analyzed from the top 10 cm of the sediment core AAS-22/3. SEM-EDS analysis (Table 5.5) of these grains show high amount of silver (Ag) that occur in a native form. Ag values range from 91 to 93 wt. % (average. 92 wt. %). Cu co-occurs with Ag and has a very low concentration (0.2 to 0.66 wt. %). The silver grain is embedded in a Cu-S matrix.

Fig.5.2O Electron micrographs of a native silver grain in a Cu-S grain.

5.4 Discussion

In the CIOB, rich deposits of ferromanganese nodules are found on the surface sediments and these are considered to be viable for exploitation and economically potential for mining (Mukhopadhyay et al., 2018). Extensive work pertaining to these nodules has revealed that elements such as Fe, Mn, Cu, Ni, and Co are either deposited by hydrogenous and/or diagenetic processes and occur in the form of oxides and

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Chapter 5 hydroxides. Pattan et al. (2017) reported trace and ultra-trace elements such as Mo, Ti, Li, REE, U, Mo, Hf, As, Bi, Be, Se and Sn and suggested that these are derived from the seawater and deposited on the ferromanganese oxides. Another possible process for these elements in the seawater (and subsequent deposition on nodules) could be due to in-situ/intraplate volcanic-hydrothermal activities. Besides the nodules, the basin hosts metal-rich grains in the sediments. These grains are either in native or oxide or sulphate forms and are of variable sizes. In general, the CF components occurring in the CIOB sediments can be described in terms of a mixture of hydrothermal, detrital, hydrogenous, and biogenous origin (cf. Bonatti et al., 1972). It is further noted that the location of the sampled sites is important because seamounts and FZ play major roles in influencing the source and input of metallic grains to the surrounding sediments. Bonatti et al. (1981) stated that these grains on the seafloor could result by a combination of different processes or be dominated by a single process. There are several possible sources (anthropogenic and natural) to account for the different metal-rich grains that occur in the oceanic sediments. The metal grains may have been derived from: (i) on-board sampling gears, industries/ anthropogenic (Dekov et al., 1995), (ii) biologically derived (Wang and Chatterton, 1993), (iii) diagenetically formed (McCabe et al., 1983), (iv) extra-terrestrial input (Schmidt and Keil, 1966), (v) volcanic sources, and (vi) from hydrothermal events/episodes. Now I examine each of the above processes to account for the metal-rich grains in the CIOB sediments. The possibility of contaminations of the collected sediments from Okean grabs is ruled out because these do not contain the type of metal-rich grains that I have investigated. Anthropogenic inputs are not possible since the study area is thousands of kilometres away from any industrialized country. Although biological organisms have the capacity to absorb certain trace elements and metals, but these cannot contribute the kinds of metal grains that are presently observed in the CIOB. Moreover, SEM observations did not reveal the presence of bacteria on the metal-rich grains. Extra- terrestrial spherules are Fe-rich and have similar textures (Freeman, 1986) as the CIOB spherules (see Chapter 4) but being derived from meteorites, the former have substantial amount of nickel content. Additionally, insignificant minor amounts of metallic grains (nickel- and platinum-rich) are present in the CIOB sediments but these are proven to be of meteoritic origin (Prasad et al., 2017).

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Chapter 5

Hydrothermal metalliferous sediments on the ocean floor are classified into two types: a) concentrated deposits – where the concentration of the grains is >50% in the sediments and b) diluted deposits with concentration <50%. The occurrence of concentrated deposits on the seafloor signifies that the source is in the vicinity of the sampling site (Pirajno, 1992). Such concentrated metal deposits are seen in the sediments of the EPR (Bonatti and Joensuu, 1966; Francheteau et al., 1979), MAR (Scott et al., 1974), Afar Rift (Bonatti et al., 1972) and Red Sea (Degens and Ross, 1969). Based on the mode of formation, metalliferous sediments have been distinguished into four major types: (i) The most commonly known process to account for metal-rich grains are hydrothermal emanations (Zelenov, 1965); (ii) hydrothermal precipitates (Hekinian et al.,1978, Hekinian et al., 1993); (iii) the reaction of hot lava with cold seawater (Corliss, 1971; Dymond et al., 1973; Honnorez et al., 1981) and, (iv) deep-sea bacterial processes (Alt et al., 1987; Alt, 1988). These processes can yield deposits within the sediment (intra-sedimentary) or can overlie the ocean floor sediments (Bonatti, 1983). Based on the occurrence of metal grains in the world ocean two theories were postulated for their origin: (i) slow precipitation from the seawater (Goldberg, 1954; Goldberg and Arrhenius, 1959) and (ii) ocean floor basalt as major contributors of metallic grains (Petterson, 1959; Bonatti and Nayudu, 1965; Dekov et al., 1996). The slow precipitation of metals was mainly seen in the form of hydrogenous deposit – polymetallic nodules in the Pacific and Indian oceans. These are out of the purview of the present work. Metal-rich grains occur in the hemipelagic clays of the Atlantic and Pacific oceans (German et al., 1993; Dekov, 2006 and papers therein). One of the suggested processes for the derivation of these grains is due to their neutral buoyancy after being derived from active vent fluids that rise some few km away and get mixed with seawater and once under buoyant condition get deposited (e.g., Klinkhammer et al., 1986; Nelsen and Forde, 1991). It has been further reported that such areas of plume activity are characterized by abundant fine-grained Fe-oxyhydroxide grains which have high contents of P, Cd, Zn, Cu, V, Cr, As, Ni, and Pb (German et al., 1991; Feely et al., 1991). Dekov et al. (1996) reported occurrence of small shiny grains of native tin and tin alloys in the EPR sediment caused by bedrock degradation. Those authors suggested that the MOR rocks could be viable sources for the presence of metallic grains in the

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Chapter 5 seafloor sediments. The occurrence of native particles of Al, Fe, Cu, Zn, Au, Ag, and Pb in the Red Sea sediments was suggested to be a result of hydrothermal activity associated with basaltic magma (Butuzova et al., 1987; Shterenberg et al., 1988). The authors opined that reduction condition for the formation of basaltic melt from the existing rocks on the seafloor could enhance the differentiation process along with the expansion of hydrothermal activity and result in precipitation of metallic grains. In the present study, I observed five different types of metal complexes and native grains. The occurrence of Fe-Ti grains along with Fe-rich spherules suggests direct precipitation from magmatic fluids. Such grains are present at different depth within the CIOB sediments and can be accounted by a lateral seepage of magmatic fluids (discussed in Chapter 4). The occurrence of Ba-rich grains in the surface sediments along with S (6-9%) and Sr (4%) are diagnostic of their hydrothermal origin. The Ba-rich grains occur in two different varieties such as Ba-S and Ba-S-Pb in the CIOB sediments (Tables 5.3 B). Ba- grains were reported to occur in the Afar Rift, located near East Africa with SrSO4 of nearly 14 mol% (Bonatti et al., 1972). The mineralization of Ba from the hydrothermal solution occurs at a temperature of over 100°C (Bertine and Keene, 1975). Although such Ba-rich grains are sporadic and so far unreported in the CIOB, their occurrence in the siliceous sediments indicates a white smoker-like condition for their formation in an intraplate region. The variation in the colour of Zn-Cu grains in the CIOB, from buff orange colour Zn (22-38), Cu (1.9-2.2%), and S (up to 3.4%), lighter orange S (24-28 %), and with Cu and Zn (3-9 %) to pale orange Zn (52-62%), Cu (1.9-3 %), S (1.5-3 %) could be mainly due to differentiation processes in metal precipitation. Among all the native metals occurring in the hydrothermal system, copper is seen as a most common metal derived from the altered basalt or from the second oceanic layer (L-2 i.e, basaltic layer) (Leinen et al., 1986). The grains rich in Ni, Co, Cu, Pb, and Zn, could have been derived from sub-seafloor hydrothermal circulation of seawater mainly through the leaching of the crustal rocks and subsequent deposition of metallic grains (Bonatti, 1981). The significant presence of the metal-rich grains on the CIOB seafloor supports the probability of black smoker-kind of condition in the CIOB. Considering the presence of native metals of Al, Ag and Cu and associated Ba- S, Ba-S-Pb, Cu, Cu-Zn complexes, especially in core AAS-22/3 (Fig.5.1), it appears that hydrothermal activity was more common in the southern part of the basin which is

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Chapter 5 predominantly of pelagic (red-brown) clay. Furthermore, the nearness of this sedimentary environment to the SEIR (where hydrothermal mineralization is reported) could be an additional factor for hydrothermal activities in the southern part of CIOB. The aluminium spherules in the CIOB sediment consist of high amount of Al (41-53 wt. %), a significant amount of chlorine (10-12 wt. %) and a relatively small amount of sulphur (1-2 wt. %). Similar occurrence of aluminium spherules was previously noted in the CIOB sediments (Iyer et al., 2007), Siberian Trap consisting of basaltic rocks (Oleynikov et al., 1978) and in non-altered serpentinized basalts and ultramafic rocks (Kovalskii and Oleynikov 1985). Nekrassov and Gorbachov (1977) used a thermodynamic model to explain the presence of native Al (Alo) and suggested its formation under low oxygen fugacity and extremely high temperature. Later, Osadchii and Alekhin (1984) proposed a mechanism for the formation of these particles in response to disproportionation of gaseous compounds such as AlCl, AlF, and Al2O3. The AlCl is suggested to have formed due to an interaction of basic and ultrabasic melt that is undersaturated with silica and a host rock or liquid containing

NaCl and SiO2. Dekov et al. (1995) suggested that the AlCl must have been disproportionating from a source that is deep seated and rose to the surface with other gases. The occurrence of Al along with significant of Cl suggest that similar process must have been occurring in the CIOB. Based on all the observations, it could be inferred that the above described mechanism for CIOB aluminium spherules holds true. The occurrence of silver grains embedded in a Cu-S matrix was also reported from metalliferous sediments of the EPR (20°30’-20°10’S; Dekov and Damyanon, 1997). Experimental studies carried out by those authors on the formation of Ag in association with Cu suggested sequential changes with lowering of temperature. These changes are:

i) Crystallization β of crystals at T = (~758 °C) - - > ii) Crystallization of α + β eutectic mixture at T = 779.4 °C - - > α and β exsolution at T < 779.4 °C to end compositions Cu + Ag-Cu eutectic

+ αii(Agii). The presence of Ag embedded in a Cu matrix indicates density liquation resulting from density differentiation of Cu and Ag smelt, wherein, the low density Cu crystals occur adjacent to Ag grain. A similar grain exhibiting such a clear phase change is observed in my study area (Fig 5.4).

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Chapter 5

Based on the observation of distribution of metalliferous sediments (cf. Iyer et al., 1997a,b) and the present observations of metal-rich grains of different compositions it is evident that these are products of hydrothermal precipitations. The occurrence of these grains in the CIOB sediments suggests that they must have formed from submarine exhalations of hot water and deposited in the surrounding sediments. Though the deposition appears to be a diluted type but perhaps such an ongoing process in the CIOB cannot be neglected. Earlier reports on hydrothermal activity occurring at the base of a seamount (12°34’-12°39’E and 76°03’-76°30’E) from the CIOB (Iyer, 1991), occurrence of spilites from 79°E FZ (Karisiddaiah and Iyer, 1992), and zeolites (Iyer and Sudhakar, 1993b) also provide indication of low-temperature fluids that were channelized along FZ and fissures. This is also supported by presence of extensive formation of palagonites within the sediment cores that I have studied (discussed in Chapter 7).

5.4.1 Model for the formation of metal grains in the CIOB It is well known that tectonic settings play major roles in the formation of metal grains in an oceanic environment. The ridge discontinuities FZ in the CIOB could be wide and deep enough to channelize hydrothermal fluids. Although in the present study only a few sampling sites bear metal-rich and native grains, yet the possibility of their occurrence at many more sites within the CIOB cannot be ruled out. The laboratory experiments conducted by Bishoff and Dickson (1975), Hajash (1975), Seyfried and Bishoff (1977) and Mottl et al. (1979) indicated leaching of elements from oceanic basalts during movement of hot waters. Rona (1984) listed elements like Li, K, Rb, Ca, Ba, Cu, Fe, Mn, Zn and Si that go into solution from the oceanic crust at a temperature of 200-400°C. I propose that similar to FCI/MFCI process (see Chapter 4), wherein a hot Fe-rich melt could rapidly quench due to its interaction with cold seawater resulting in Fe-rich spherule, the seafloor basalts along with siliceous and pelagic sediments could be contributing other metals such as Ti, Cu, Zn, Pb, and S by leaching and re-depositing these elements during a hydrothermal phase (Fig. 5.3).

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Chapter 5

Fig. 5.3 Schematic diagram of hydrothermal deposits associated with seamounts in the CIOB. Figure not to scale.

Sulphur in the pelagic sediments of the CIOB could be from two sources. The first one could be from the leaching of basaltic crust and release of sulphur in the hydrothermal water while, the second way could be by means of reduction of dissolved sulphur oxide during sub-seafloor circulation of hydrothermal fluids (Bonatti, 1981). Submarine exhalations of active fluids along the FZ and fissures could also result in such type of metalliferous sediments. In the submarine hydrothermal system, in addition to the hydrothermal fluids, magmatic fluids also play a major role in transporting metals (Kamenetsky et al., 2002; Yang and Scott, 2005). Rubin (1997) reported degassing flux of different elements that occur in the MOR or seamount related hydrothermal active areas. The sequence listed by him is As> Zn > Sb > Se > W > Cd > Bi > Ti > Au > Pb > Hg >> Sn >In and this is related to the formation of hydrothermal activity in the submarine environment and subsequent formation of hydrothermal sulphide deposits. The FZ and fissures that were discussed in chapter 4, in relation to the formation of Fe-rich magnetic spherules, may have been congenial conduits through which metal- rich solutions were channelized. Subsequently, various metallic and native grains could have been precipitated into the seafloor sediments. An important evidence to support such a process is the occurrence of barium grains in the sediments. Usually, barium content in the seawater is very low to precipitate as barium grains but the presence of

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Chapter 5 significant presence of barium grains (Fig. 5.3 A) in the CIOB sediments points to an in-situ fluid-driven activity. Based on the 238U/234U ratio, Bostrom and Fisher, (1971) suggested a submarine volcanogenic source for the enrichment of U, V, and Fe in the pelagic sediments of the Indian Ocean. Such a source was attested by the findings of volcanogenic-hydrothermal episodes in the CIOB that occurred at 10 ka and 625 ka (Iyer, 2005 and references therein) and as recent as ~100 yr (Nath et al., 2008). Boulegue and Mariotti (1990) reported two hydrothermal events of ~7.5-9 Ma and 0.5 Ma from ODP Site 717-719. Chakraborty et al. (2014) based on the experimental study of Cu speciation of sediments also reported the influence of hydrothermal activity in the CIOB. In addition to these studies, Sarma et al. (2017) reported hydro-thermogenesis below the ODP Site717- 719(Fig. 4.1). The precipitation of the metal-rich and native grains on the CIOB seafloor may have been mainly due to change in the pressure-temperature gradient of the circulating hydrothermal waters (Fig. 5.4). The reaction between the heated circulating water could result in excessive leaching of the elements, from the basaltic crust, especially Ca, Fe, Mn, and Cu. Movement of superheated water/ magmatic waters could have a diverse effect on the adjoin sediments along the wall flanks of the FZ or fissures and subsequent formation of the grains.

Fig. 5.4Schematic diagram of hydrothermal deposits associated with the fracture zone in the basin. Figure not to scale.

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Chapter 5

The SEIR near the southern part of the basin could be a major source of hydrothermal fluids that were perhaps channelized into the CIOB through the 79°E FZ. These source sites in the present study area may have been tectonically-volcanically- hydrothermally active in the past as evident from spilites in the vicinity of the 79°E FZ (Karisiddaiah and Iyer, 1992) and the widespread zeolitization of sediments in the basin (Iyer and Sudhakar, 1993b).The volcanic and hydrothermal activities could also result in the baking of the seafloor sediments (see Chapters 6 and 7). The 79°E FZ may have had a condition of high P-T and low fO2 and fS2 that were conducive for the formation of Al spherules, Ba-S, Zn-Cu, Pb and Ag grains in the CIOB.

5. 5 Conclusions

 This chapter provides the first information of occurrence of metaliferous sediments in the CIOB.  Based on the tectonic framework from which the studied samples were retrieved, it is evident that the morphological features such as seamounts, mounds (< 50 m) and the major FZ and associated fractures and cracks that traverse the basin are likely contributors for the metal-rich grains.  The source sites may have been tectonically-volcanically-hydrothermally active in the past and/or such activities may be ongoing.  The metal-rich grains are clear evidence of hydrothermal activity that occurred post-formation of the CIOB.  Further study could help to quantify these deposits once the types and aerial extent are mapped.

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Chapter 5

Table 5.1 Chemical composition of Fe-Ti grains. nd- not detected. Values in wt. %.

Grain No. O Mg Al Si Fe Ti Mn Total 16-1 30.47 1 2.2 4.77 47 12 0.82 98.26 44-1 30.61 nd nd Nd 40.2 28.25 0.56 99.62 45-2 33.53 1.63 1.4 4.55 32.82 23.66 0.54 98.13 17 29.93 1.86 0.63 Nd 43.16 23.3 0.54 99.42 Avg. 31.14 1.5 1.41 4.66 40.8 21.8 0.62 101.91 Std Dev. 1.62 0.45 0.79 0.16 6 6.91 0.14 16.06

Table 5.2AChemical composition of buff orange grains rich in Zn. nd- not detected. Values in wt. %.

Grain No. Si Al Fe Mg Ca Na Cr S Cu O Zn Total 51 9.07 3.28 5.18 3.31 0.84 6.62 nd 3.41 2.22 33.25 32.82 100 12 nd 28.22 1.2 nd 0.41 Nd 0.71 nd 1.51 33.91 31.05 97.01 13 3.23 26.52 1.3 nd 0.68 3.95 0.7 0.98 1.19 36.78 22.38 97.71 4 4.6 10.78 0.39 1.03 nd 7.06 0.3 2.9 1.9 32.47 38.57 100 Avg. 5.63 17.20 2.02 2.17 0.64 5.88 0.57 2.43 1.71 31.00 31.21 100.45 Std Dev. 3.05 12.16 2.15 1.61 0.22 1.68 0.23 1.28 0.45 1.66 6.70 1.55

Table 5.2BChemical composition of light orange grains rich in S. Grain 5 containsMg (1.83%), and Ca (1.12%). nd =not detected. Values in wt. %.

Grain No. Si Al Fe S Cu O Zn Total 3 4.58 3.00 nd 28.16 5.57 52.87 5.82 100.00 5 9.04 4.29 17.74 13.90 3.22 43.23 3.14 94.56 14 5.22 nd nd 24.41 7.02 46.66 9.84 93.15 16 4.78 1.73 5.71 24.85 3.86 48.03 5.07 98.15 Avg. 5.91 1.05 11.73 22.83 4.92 47.70 5.97 100.09 Std Dev. 2.11 1.28 Nd 6.18 1.72 4.00 2.82

Table 5.2CChemical composition of pale orange grains rich in Zn. Grain 11 contains Fe (0.68) and Na (8.41). nd = not detected. Values in wt. %.

Grain No. Si Al Ca S Cu O Zn Total 1 5.34 nd 0.42 2.86 2.53 26.44 62.41 100 2 4.93 6.34 0.53 3.13 2.94 29.72 52.41 100 11 4 0.77 0.28 1.54 1.38 24.2 53.41 85.58 Avg. 4.76 3.56 0.41 2.51 2.28 26.79 56.08 - Std Dev. 0.69 3.94 0.13 0.85 0.81 2.78 5.51 -

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Chapter 5

Table 5.2DChemical composition of Cu grains. nd- not detected. Values in wt. %.

Grain No. O Al Cu Zn Total 35 -2 20.02 Nd 63.89 16.09 100 4 -2 26.68 13.13 38.84 21.35 100 Avg. 23.4 13.1 51.4 18.7 100 Std Dev. 4.7 - 17.7 3.7 -

Table 5.3AChemical composition of metallic grains rich in barium. Grain27-1* contains 3.38% Pb. nd- not detected. Values in wt.

Grain No. O Al Si S Fe Ba Total 1 -1 29.78 0.82 1.91 13.52 1.64 52.03 99.7 24 -1 29.27 nd 0.62 14.65 1.66 52.83 99.03 25 -2 28.39 nd nd 14.27 2.3 54.66 99.62 27 -1 27.22 0.59 0.63 12.26 4.98 50.95 100.01 Avg. 28.67 0.71 1.05 13.68 2.65 52.62 99.59 Std Dev. 1.12 0.16 0.74 1.05 1.59 1.56 0.41

Table 5.3BChemical composition of grains rich in Ba-S-Pb. Grain 27.3 contains P (0.31 %) and Ti (2.66). Grain 11-2 contains 4.6 % Sr. nd- not detected. Values in wt. %.

Grain No. O Mg Al Si S Ca Fe Ba Pb Cr Total 11 -2 31.13 1.39 1.54 3.69 9.29 2.4 3.01 25.04 17.33 na 99.42 27 -2 27.99 0.76 2.45 5.86 6.27 1.51 11.87 24.99 14.57 2.48 98.75 27 -3 29 nd 1.84 5.73 6.46 1.73 12.65 23.45 13.39 1.95 99.17 Avg. 29.37 1.08 1.94 5.09 7.34 1.88 9.18 24.49 15.1 2.22 99.11 S.D 1.6 0.45 0.46 1.22 1.69 0.46 5.35 0.9 2.02 0.37 0.34

Table 5.3C Chemical composition of lead grain. Values in wt. %.

Grain O Mg Al Si Ca Fe Pb Na Cl P Ti As V Total No.

4.-1 18.35 0.18 1.09 2.25 3.45 6.44 59.37 0.52 2.66 4.79 0.53 0.24 0.12 100

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Chapter 5

Table 5.4 Chemical composition of native aluminium spherules. Grain Y2 contains S (1.85%), Y6 contains Fe (10.22%).nd = not detected. Values in wt. %.

Grain No. O Al Cl Si Total Y1-1 47.07 52.93 nd nd 100 Y1-2 47.07 52.93 nd nd 100 Y2 42.96 45.18 10.01 nd 98.15 Y3 41.45 46.61 11.94 nd 100 Y5 42.28 45.33 10.67 1.72 100 Y6 43.44 41.84 1.61 2.89 99.78 Y7 47.07 52.93 nd nd 100 Y8 46.78 52.6 0.62 nd 100 Y9 42.35 43.18 11.01 3.46 100 Avg. 44.5 48.17 7.64 2.69 100 Std Dev. 2.43 4.64 5.1 0.89 -

Table 5.5 Chemical composition of native silver grains. Grain X4 contains Cl (0.46%), and Al (0.24). nd = not detected. Values in wt. %.

Grain No. 0 Ag Cu Total X1-1 6.9 93.1 nd 100 X1-2 6.95 92.78 0.27 99.73 X2 6.9 93.1 nd 100 X3 6.9 93.1 nd 100 X4 7.16 91.49 0.66 99.31 Avg. 6.96 92.71 0.47 100 Std Dev. 0.11 0.7 0.28 -

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CHAPTER 6

BASALT EMPLACEMENT: FORMATION OF BAKED SEDIMENTS AND VHM

Chapter 6

6.1 Introduction The term “baking” in geology has been used to define the effect of magmatic body on the country rock. This is mainly seen in the form of contact zone of the country rock and an intrusive such as around a large pluton or on the side edges of a dyke. The magmatic heat of the intrusive gets dissipated in the adjoining rock/sediments thereby making it appear more dense and compact and results in a “baked” effect. Such phenomena are common on terrestrial outcrops as compared to those occurrences in the sub-seafloor environment.

Although, most of the existing reports of hydrothermal and magmatic activity are concerned with the occurrence at the MOR, subduction zones and seamounts (Dymond et al., 1973; Heath and Dymond, 1977; Alt et al., 1987; Iyer et al., 2012a), but there are only a few reports that pertain to magmatic intrusion in the soft seafloor sediments. Einsele et al. (1980) reported the first occurrence of intrusion of basaltic sill in the porous sediments of Guaymas Basin, Gulf of California. This intrusion of basaltic sill resulted in low grade metamorphism, thermal alteration and migration of organic compounds. It also changed the interstitial water chemistry and led to large scale expulsion of heated pore fluids in the sediments. Further, those authors reported baked sediments in areas with high heat flow associated with seamounts.

The volcanics of the CIOB have been subjected to compositional changes during alteration and these in many instances are accompanied by formation of zeolites (Iyer et al., 1999, 2007, 2018). Yet, so far there are no reports of magmatic activity affecting the sediments in the CIOB. My present study is the first record of an intrusive rock within a sediment core that resulted in the formation of baked sediments and associated volcanogenic hydrothermal materials (vhm) in the CIOB.

6.2 Sediment core Of the four gravity cores that were investigated during the course of this work, it was observed that only in the sediment core AAS-22/7 there was an entrapped basaltic rock and sediments in the vicinity showed baked effects. The location and details of this core are provided in chapter 2 (Fig 2.1). I studied the effect of the basaltic intrusive on the sediments by using magnetic susceptibility. Geochemical studies were carried out for the sediments to help understand the down-core distribution/behaviour of major oxides in the affected and unaffected sediments. Further, the basaltic rocks were also analysed.

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6.3 Results and Discussion 6.3.1 Physical description and coarse fraction of the sediment core AAS-22/7

The core AAS-22/7, retrieved from the siliceous domain adjacent to 79º FZ (referred as Site-2 in Chapter 4), has yellowish-brown colour sediment with a silty-clayey texture. Basaltic glass pieces along with small rock fragments and Fe-rich magnetic spherules occur between depth 280 and 355 cm (refer Chapter 4). Entrapped Basaltic rocks are seen at two different depths in this sediment core.

(1) The first basaltic rock entrapment layer occurs between 345 and 355 cm. The rock pieces here are fine grain basalts that were fragile along the cracks while the inner side was fresh (Fig. 6.1A). The cracks were filled with sediments (Fig.6.1B) and these suggest that the entrapped rock pieces must have undergone alteration. (2) The second layer associated with entrapped basalt was observed at depth 470 to 490 cm. The rock fragments here are highly altered as compared to the first layer (Fig.6.1C).

Fig. 6.1 Hand specimen of basaltic pieces in the sediment core AAS-22/7. A = Basaltic pieces at depth 345-355 cm; B = A specimen of basalt in which the cracks are filled with sediments; C = highly altered rock pieces at depth 470-490 cm; D = Down-core variation of coarse fractions. The yellow region is the depth that coincided with occurrence of iron-rich magnetic spherules. (G = Glass shards; N = FeMn micronodules; V = vhm.) The

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Chapter 6 red region is the first layer where ‘fresh’ basalts occur. The blue region is the second layer where highly altered basaltic chunks were present.

The CF% of the core AAS-22/7 show considerable down-core variations and the values range between 13 and 1.2% (Fig. 6.1D). As discussed in Chapter 4, radiolarians constitute more than 90% of the CF while the remaining are mainly glass shards, Fe-rich magnetic spherules, basaltic material and FeMn micronodules. The peaks occurring in the down-core plot (Fig. 4.4) were identified and discussed (see Chapter 4). The high CF% in the top 10 cm of the core is mainly due to a high abundance of volcanic glass shards, basaltic pieces and Fe-rich magnetic spherules (Figs. 4.6 and 8.3A). The prominent peaks in the top 200 cm of the core are ascribed to the presence of FeMn micronodules (Fig. 6.1D). Glass shards are prolific at depths 0-5 and 220-225 cm along with microtektites (Fig. 2.2) in the lower depth. These two glass shard layers and significance of microtektites are detailed in Chapter 8. The Fe-rich magnetic spherules in this core occur at three different depths: 0-5, 280- 355 and 460-475 cm. The last two depths coincide with the entrapped basalts. The CF% (upto 13%) at depth 280-355 cm and 5% between 460 and 475 cm are due to high abundance of Fe- rich magnetic spherules and basaltic glass pieces. Since the basaltic rock pieces occurring at both these depths were removed prior to wet sieving, therefore the effect of these rocks pieces on the CF% is not reflected. The number of Fe-rich magnetic spherules was calculated to be 280-440/100 gm between 280 and 355 cm and 280-320/100 gm between 460 and 475 cm (refer Chapter 4). The occurrence of Fe-rich magnetic spherules along with basaltic rock pieces in both the layers signifies the effect of magmatic activity on the sediment above the entrapped/intruded basalt.

6.3.2 Magnetic Susceptibility measurements

Magnetic susceptibility data of the core AAS-22/7 (Table 6.1A) show a slightly low χlf in the top 260 cm with values between 3.9 and 6.4 x 10-6 m3 kg-1 (average 5 x 10-6 m3 kg-1) while values are slightly higher below 260 cm (4-7 x 10-6 m3 kg-1; average 6.5 x 10-6 m3 kg-1). The down-core plot of χlf (Fig. 6.2A) indicates maximum concentration of magnetic minerals below 260 cm.

The extremely high value of χlf (79.209 x 10-6 m3 kg-1) at depth 345-355 cm could be attributed to the entrapped basaltic chunks. On the other hand, the very low values of χlf (~4 x

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Chapter 6

10-6 m3 kg-1) at depth 470-490 cm could be mainly due to complete alteration of the basaltic pieces (Fig. 6.1C) thereby diluting the magnetic signal.

The magnetic susceptibility measurements carried out for the basaltic rocks at depth 345-355 cm show high ARM values (0.695x10-5Am2kg-1). Magnetite is about 12% and S- ratio was around 0.966 (Table. 6.1A).

SIRM has low values compared to the other studied cores (see Chapter 7) and range from 0.9 to 1.4 x10-5Am2 kg-1, with high peaks at core depths 155-160, 295-300 and 480-490 cm (Table 6.1A). Overall, it appeared that the SIRM values are slightly higher with broad peaks in the lower part of the core. This indicates that the concentration of magnetic minerals is more in the lower part of the core i.e., below 260 cm.

The SIRM/χlf ratio is used to understand the grain size distribution within the core. This ratio is high in the top 260 cm and has a positive trend (Fig. 6.2A). The values of SIRM/χlf range between 1.8 and 2.26 which indicate fining of the sediments with depth. While below 260 cm the values show a negative trend and range from 0.24 to 0.13 and point to coarsening of the sediments. Below 440 cm the values are between 0.18 and 0.24 and again indicate fining of sediments (Fig. 6.2A). The variability in the grain size could be ascribed due to diatoms and radiolarians that constitute more than 90% of the CF in the top 260 cm. The magnetite concentration is <1% throughout the core except for a high value (12%) at depths coinciding with the entrapment of the basalt (Table 6.1).

Overall, a distinction can be drawn based on the magnetic susceptibility data of the core AAS-22/7 and behaviour of the magnetic minerals (Fig. 6.2A). The lower part of the core i.e. below 260 cm shows significant fluctuations compared to the upper 260 cm. Also, the χfd and SIRM show high values at depths that are associated with the mingling of the basaltic material.

Magnetic susceptibility measurement was carried on another core located three degree north (Core AAS-22/8, Table. 6.1B). The main reason to analyse this core was to understand if traces of the magmatic activity that affected the core AAS22-7 is also preserved in the adjacent core. Magnetic susceptibility data of the core AAS-22/8 show higher χlf values (3.5 and 9 x 10-6 m3 kg-1) as compared to AAS-22/7. The ARM values range from 0.8 to 2 x 10- 5Am2 kg-1, and SIRM/χlf from 0.12 to 0.28.

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Chapter 6

The χlf, ARM and SIRM parameters were used to understand the down-core concentration of magnetic minerals. These parameters show a positive trend in the top 150 cm of the core indicating an abundance of magnetic minerals while below 150 cm, there is a drastic change in the magnetic mineral concentration (Fig. 6.2 B). The values of χlf, ARM and SIRM decrease indicating a low magnetic mineral concentration between 150 and 450 cm. Based on the plot of SIRM/χlf, it was observed that there is coarsening of grain size upto 285 cm while a reverse trend is noted from 285 to 440 cm. The high values of SIRM/χlf between 285 and 440 indicate fining of sediments. Below 440 cm there is another reversal in the magnetic grain size suggesting coarsening of sediments.

Between 270 and 350 cm depth, the high values of χlf, ARM and SIRM along with SIRM/χlf could be due to an effect of the magmatic activity. However, this is not as prominent as seen in the core AAS-22/7 (Fig. 6.2A, B).

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Chapter 6

Fig 6.2 Magnetic susceptibility plots for the cores A=AAS-22/7 and B=AAS-22/8.

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Chapter 6

6.3.3 Geochemistry of core AAS-22/7 sediments

Based on the observation of CF and magnetic susceptibility parameters, the core sections were grouped into two major zones: (i) Siliceous sediment unaffected by magmatic activity/absence of intrusion, (ii) Siliceous sediments affected by magmatic activity and associated with Fe-rich magnetic spherules and entrapped basalt.

(i) Siliceous sediment unaffected by magmatic activity/absence of intrusion: The major oxide data (in wt. %) of the top 250 cm (Fig. 6.3) of the core show a narrow

range of SiO2 (56-60), Al2O3 (8.5-10), Fe2O3 (4-5), TiO2 (0.2-0.3), MgO (2-2.5)

along with CaO (0.6-0.65), Na2O (5.6-7), K2O (1.6-2), P2O5 (0.09-0.1) and MnO (0.3-0.5) (Table 6.2A). The top 250 cm of the core does not show a variation in the major oxide data suggesting that the sediments are unaffected by the basalt intrusion.

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Chapter 6

Fig. 6.3 Down-core variations in major oxides. The green field indicates sediment depth affected by the magmatic activity while the red bars are the depth where basaltic rocks are entrapped.

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Chapter 6

(ii) Siliceous sediments affected by magmatic activity and associated with Fe-rich magnetic spherules and entrapped basalt: The major oxide data of the sediment occurring between 280 and 360 cm (i.e., sediment associated with first

entrapped rock layer) show reducing trends in SiO2 i.e., 59 at 285 cm to 53 at

360 cm while Al2O3 shows an increasing trend from 8.5 to 11.5 (Fig. 6.3). The

values of TiO2 range from (0.2-0.6), Fe2O3 (4-7), MnO (0.09-0.7), MgO (2-2.4),

CaO (0.5-0.85), K2O (1.5-2.0) P2O5 (0.1-0.2) and Na2O (5-6). Except SiO2,

Al2O3 and Na2O, it was interesting to see the increase of other major oxides

towards the entrapped layer while Na2O shows a reverse trend. Trace elements data (in ppm) for the first layer shows increasing trends in Li (29-45), Sc (18-25), Ti (0.25-0.46), and Nb (5-8) with depth. While V (62- 95), Rb (27-55), Sr (150-215), Y (36-54), and Sb (0.9-1.7) shows higher value towards the entrapped rock layer with diluted concentration away from this layer (Table. 6.2B).

The major oxide data of the sediment between 445 and 560 cm (i.e.,

sediment associated with second entrapped rock layer) show values of SiO2

range from (52-54), Al2O3 (10-13), TiO2 (0.3-0.5), Fe2O3 (5-7), MnO (0.5-0.8),

MgO (2.2-2.6), Cao (0.6-0.9), K2O (1.9-2.0) P2O5 (0.12-0.18) and Na2O (4.5- 5.5). Here also the major oxides show higher concentration towards the entrapped layer. The concentration is seen diluting away from the entrapment.

Similar to the first layer, Na2O shows increasing trend away from the entrapment (Table 6.2A).

Trace elements data for the second layer show an increasing trends in Li (49-52), Cr (55-60), V (91-107) and Ti (0.4-0.5) with depth. On the other hand, Rb (37-57), Sr (174-198), Y (53-118), Nb (7.4-8.5), Mo (19-34), and Sb (1.8- 2.1) show high values in the entrapped rock layer but decreases away from this layer.

The higher value of major oxides and trace elements in the sediments at both these depths associated with entrapped rocks clearly signifies that there is baking effect on the sediment.

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The plot of Co, Ni, Cu, and Zn for sediments below 280 to 500 cm shows increasing trends in the trace metal concentration within the sediment at depth 280-300 cm (Fig. 6.4). A decreasing trend of Ni, Cu and Co from 335 to 350 cm along with an increase in the concentration upto depth 360 cm is seen Fig. 6.4. The gradual decrease of Ni, Cu and Co towards the entrapped rock zone indicates removal of these elements from the sediment phase. While a high peak of Zn at the same depth (345-355 cm) suggests its enrichment due to entrapped rock.

Fig. 6.4 Plot of Co, Ni, Cu, and Zn against depth

Below 480 to 500 cm the Ni, Cu and Co contents increase while Zn decreases. The inverse relation between Ni, Cu and Co and Zn because of the basaltic chunk at this depth suggests that there is alteration of sediments.

REE data for the sediment normalised with PAAS (Taylor and Mc Klennan, 1985) show a positive europium (Eu) and negative samarium (Sm) anomalies (Fig. 6.5A). The Light (LREE) shows significant variation in the sediments while the Heavy (HREE) has remained uniform. This could be mainly due to the redistribution of REE within the sediments due to magmatic activity.

Fig. 6.5A Plot of rare earth elements of the sediment from core AAS-22/7. The values were normalised with PAAS (Taylor and McKlennan, 1985).

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Nath et al. (1992) reported REE concentration in bulk sediments occurring in the CIOB and found flat shale normalized pattern along with positive cerium (Ce) anomaly. In the present study, the siliceous sediments also show similar behavioural pattern (Fig. 6.5A). Though, the Ce peak is not as prominent as compared to Eu anomaly, the significantly low values in the Ce could be due to effect of magmatic activity on the sediments.

REE data for the sediment core normalised with N-MORB show a positive Eu and negative Sm (Fig. 6.5B). Similar to the PASS normalized plot, the sediments show a linear trend in the HREE pattern.

Fig. 6.5 B Plot of rare earth elements of the sediment from core AAS-22/7. The values were normalised with N MORB (McDonough and Sun, 1995).

6.3.4 Geochemistry of the entrapped rock

The analysis of the rock piece occurring within the sediment at depth 345-355 cm shows SiO2 (45.88), Al2O3 (13.16), TiO2 (2.6), Fe2O3 (13.51), MnO (0.69), MgO (2.76),

CaO (1.07), Na2O (2.79), K2O (2.3) and P2O5 (0.133) (Table 6.2A). The SiO2 v/s Na2O +

K2O plot used for classification of the entrapped chunk signifies basaltic composition (Fig. 6.6). The high value

of TiO2 (2.6) and Fe2O3 (13.51) suggest a composition similar to the ferrobasalts.

Fig. 6.6 Classification of entrapped basalt based on TAS. The nomenclature fields are after LeBas et al. (1986).

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Similarly, the analysis of two rock pieces occurring in second layer (depth 480490 cm) shows SiO2 (44.1- 45.07 with avg. 44.59), Al2O3 (9.9 to 10.42, avg. 10.16), TiO2

(0.56-0.55, avg. 0.56), Fe2O3 (9.7-10.11, avg. 9.95), MnO (1.9-1.8, avg. 1.86), MgO (4.8-

4.7, avg. 4.8), CaO (1.44-1.39, avg. 1.4), Na2O (4.7-4.5, avg. 4.63), K2O (2.5-2.4, avg.

2.47) and P2O5 (0.76-0.74, avg. 0.75). A depletion in the TiO2, Fe2O3, and Al2O3 along with enrichment of MnO, MgO, CaO, Na2O, and K2O was seen compared to the basalt occurring in the first layer. This further suggests that the rock occurring in second layer is altered (Table 6.2A).

REE data for the entrapped rock piece normalised with N-MORB show a positive Eu and negative Sm for basalt in the upper layer (Fig. 6.5B). The REE value in the entrapped rock pieces in the lower layer is higher than the average sediment REE and the basalt from the upper layer. Overall in the entrapped rock the LREE show higher concentration compared to the HREE. The entrapped rock piece occurring in first layer shows positive Ce while the altered rock piece occurring in the second layer shows negative Ce.

6.3.5 Effect of magmatic activity on the sediments Although the effect of magmatic activity is common at the MOR such a process could also affect the abyssal sediments but reports on such activity are infrequent. During magmatic activity, the magmatic fluid traverses through the oceanic layers to form a dyke or sill. It is observed that the heat released during such an interaction is much higher at the MOR system. However, in the intra basinal environment during emplacement of a dyke the heat gets dissipated into the sediments resulting in a baked effect as seen in the present study. This is attested by the composition of the affected sediments (345-355 and 470-490 cm) and those above and below these sections and by the presence of basaltic rocks. The sediment occurring in the core was used in discriminatory plot to understand the mineralogy composition. The plot of Fe2O3–Al2O3–MgO for the characterization of clay (after Chamley, 1997) shows that the studied sediments of core AAS-22/7 are Fe-Al smectites clays (Fig. 6.7A) similar to the other cores discussed in this thesis.

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Fig. 6.7A The plot of Fe2O3–

Al2O3–MgO for the characterization of clay (fields after Chamley, 1997). Red solid dots = Bulk sediment from the core AAS 22/7.

The scatter diagram of

Al2O3 versus Fe2O3 was also used to infer the mineralogy of the core sediment. The plot is created using literature values of clay minerals (Fig. 6.7B) such as kaolinite, chlorite, smectite, and illite from Grim (1968); plagioclase and zeolites from Deer et al. (1967); Al-Bediellite and nontronite from Aoki et al. (1996) and authigenic Fe-montmorillonite from McMurtry and Yeh (1981). The presently studied sediments showed composition close to Fe-montmorillonite and this is in agreement with the CIOB sediments reported by Mascarenhas-Pereira et al. (2010).

Fig.6.7B Plot of Al2O3 versus Fe2O3 indicating mineralogy of the studied core sediment. The red dots are sediments from core AAS-22/7. The black solid dots indicate sediments from the CIOB (Mascarenhas-Pereira et al., 2010).

A ternary diagram of K2O -

Al2O3 – CaO + Na2O (Fig. 6.7C) helped to infer the mineralogy of the studied core sediments. The analysed samples closely resemble Fe-montmorillonite clays with a mixed signature of phillipsite. This could be mainly due to the alteration of sediments during the basalt intrusion (Fig 6.1A-C).

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Fig. 6.7C Ternary diagram of

K2O - Al2O3 – CaO + Na2O. Pink solid dots are samples from the present study. Black solid dots are data of bulk sediments from the CIOB (after Mascarenhas-Pereira et al., 2010).

Based on the discriminatory plots for core AAS-22/7, it appears that the sediments have been affected by the magmatic activity and alteration resulting in the formation of Fe-montmorillonite. My data do not fall in the field of nontronite (Fig. 6.4B) thereby indicating that this core was not affected by hydrothermal activity. The occurrence of two basaltic layers within the core could be either due to: (i) a vertical emplacement (i.e., along the core length) by a dyke or (ii) by a crosscutting (inclined) dyke that cut across the core site. The chemical composition of sediments below the second layer (i.e., between 545 and 560) shows low values of Fe2O3, MgO, MnO, K2O, and TiO2 compared to the upper magmatically affected sediments. This suggests that effect of magmatic activity below 545 cm is significantly low in the sediments thereby ruling out the possibility of a vertical emplacement of a dyke from the oceanic crust.

The second explanation for the entrapment of basaltic pieces at two layers within the sediment core could be due to two crosscutting dykes at the core site. The second layer (470-490 cm) may have been the first magmatic event that resulted in basalts which were later altered as seen from their composition and hand specimens. While the first layer (345- 355 cm) could have formed by another later magmatic activity which led to the formation of basalts that are relatively fresh. The association of Fe-rich magnetic spherules in the vicinity of these two layers suggests their contemporaneous formation. The lateral seepage

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Chapter 6 of the magmatic fluids must have led to the production of spherules by a process of MFCI. This mechanism corroborates the earlier discussion (see Chapter 4, Fig. 4.10). However, the magnetic susceptibility measurements carried out for the core AAS- 22/8 (three degrees to the north) and absence of rock pieces also do not show any evidence of the effect of these two dykes. Thus, the findings indicate that the observed crosscutting dykes in the core AAS-22/7 could be a localized event.

6.4 Conclusion The occurrence of basaltic rock and baked sediments at two different depths 345 to 355 and 470 to 490 cm within a sediment core (length 5.6 m) suggest a lateral emplacement of a magmatic body in the form of a dyke. This is attested by the compositional changes within the core and the presence of iron-rich magnetic spherules and vhm that are present in the vicinity of the basaltic rock. The basaltic rocks could have been emplaced by lateral seepage of magma, with the rocks in the first layer (345 to 355 cm) retaining its original basaltic composition, while those in the second layer (470 and 490 cm) are significantly altered. Further, the magmatic activity has resulted in baking of the sediments below 250 cm till end of the core. This new finding of baking of deep-sea sediments due to volcanic activity could help to understand similar processes in the world ocean.

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Table 6.1A Magnetic susceptibility data for the core AAS-22/7. Core 22/7 LF kfd S-ratio ARM SIRM ARM /SIRM SIRM/klf Magnetite depth cm % (1T) % 0-5 5.4110 10.3306 0.9796 0.1399 1.1346 0.1233 0.2097 0.8198 5-10 4.6945 4.7847 0.9755 0.1402 1.1249 0.1247 0.2396 0.7113 10-15 5.2503 0.7519 0.9668 0.1152 0.9997 0.1152 0.1904 0.7955 15-20 5.6790 16.1290 0.9950 0.1334 1.0375 0.1286 0.1827 0.8605 20-25 4.5843 8.6538 0.9575 0.1413 1.0486 0.1347 0.2287 0.6946 25-30 4.8334 11.0553 0.9827 0.1466 1.1209 0.1308 0.2319 0.7323 30-35 4.5039 6.3415 0.9602 0.1422 1.1216 0.1268 0.2490 0.6824 35-40 4.3551 10.8808 0.9678 0.1401 1.0410 0.1346 0.2390 0.6599 40-45 4.3942 9.4675 0.9421 0.1386 1.0084 0.1375 0.2295 0.6658 45-50 5.1691 6.7961 0.9443 0.0558 1.0215 0.0546 0.1976 0.7832 50-55 5.1971 12.8713 0.9652 0.1578 1.1242 0.1404 0.2163 0.7874 55-60 5.1271 10.0478 0.9644 0.1498 1.1156 0.1343 0.2176 0.7768 60-65 5.2069 8.1081 1.0047 0.1558 1.1154 0.1397 0.2142 0.7889 65-70 5.3756 12.2271 0.9649 0.1556 1.1553 0.1347 0.2149 0.8145 70-75 4.8991 4.0000 0.9862 0.1569 1.1604 0.1352 0.2369 0.7423 75-80 4.9756 7.2848 0.9338 0.1579 1.2145 0.1300 0.2441 0.7539 80-85 3.8864 -1.8182 0.9706 0.1368 1.0357 0.1320 0.2665 0.5888 85-90 4.5346 1.6393 0.9731 0.1489 1.1216 0.1328 0.2473 0.6871 90-95 4.4770 4.7619 0.9362 0.1413 1.0845 0.1302 0.2422 0.6783 95-100 5.1467 15.0000 0.9891 0.1495 1.1889 0.1258 0.2310 0.7798 100-105 5.1408 7.5630 0.9680 0.1486 1.1515 0.1291 0.2240 0.7789 105-110 5.8930 11.0429 0.9639 0.1562 1.2288 0.1271 0.2085 0.8929 110-115 5.5138 2.1834 0.9803 0.1570 1.1972 0.1311 0.2171 0.8354 115-120 4.5000 11.5183 0.9892 0.1215 0.9446 0.1286 0.2099 0.6818 120-125 4.4938 9.8039 0.9584 0.1310 1.0284 0.1274 0.2289 0.6809 125-130 4.3772 8.6957 0.9713 0.1330 1.0140 0.1312 0.2317 0.6632 130-135 5.2987 6.9264 0.9512 0.1532 1.1837 0.1294 0.2234 0.8028 135-140 5.0281 11.9266 0.9506 0.1375 1.0678 0.1288 0.2124 0.7618 140-145 4.5164 6.1538 0.9446 0.1331 1.0271 0.1296 0.2274 0.6843 145-150 4.0984 1.7241 0.9878 0.1291 1.0075 0.1282 0.2458 0.6210 150-155 5.2680 13.9918 0.9793 0.1444 1.0959 0.1317 0.2080 0.7982 155-160 6.1473 11.9691 0.9266 0.1758 1.3802 0.1274 0.2245 0.9314 160-165 6.3903 14.7208 0.9284 0.1702 1.3690 0.1244 0.2142 0.9682 165-170 5.6420 8.0460 0.9658 0.1580 1.1865 0.1331 0.2103 0.8549 170-175 5.4134 9.2920 0.9678 0.0010 1.1274 0.0009 0.2083 0.8202 175-180 5.8001 12.0482 0.9899 0.0009 1.2069 0.0007 0.2081 0.8788 180-185 4.9326 4.4053 0.9481 0.0008 1.0934 0.0007 0.2217 0.7474 185-190 5.3292 7.9646 0.9472 0.0008 1.1026 0.0008 0.2069 0.8075 190-195 4.9150 2.7027 0.9853 0.0009 1.0819 0.0009 0.2201 0.7447 195-200 6.1153 2.7027 0.9586 0.0013 1.3078 0.0010 0.2139 0.9266 200-205 5.1388 1.2987 0.9528 0.0009 1.1832 0.0008 0.2302 0.7786 205-210 5.3343 11.5207 0.9445 0.0007 1.1285 0.0007 0.2116 0.8082 210-215 5.1099 14.0969 0.9502 0.0007 1.0779 0.0006 0.2109 0.7742 215-220 4.3860 7.9602 0.9433 0.0005 0.9585 0.0006 0.2185 0.6645 220-225 4.5732 7.6923 0.9292 0.0006 0.9802 0.0006 0.2143 0.6929 225-230 4.6047 0.0000 0.9371 0.0008 1.0416 0.0007 0.2262 0.6977 230-235 5.1632 4.0000 0.9615 0.0008 1.0989 0.0007 0.2128 0.7823 235-240 4.4730 4.8913 0.9641 0.0007 0.9821 0.0007 0.2196 0.6777 240-245 5.5958 8.5227 0.9571 0.0009 1.1966 0.0008 0.2138 0.8479

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245-250 4.8324 6.9869 0.9508 0.0008 1.0581 0.0007 0.2190 0.7322 250-255 4.9400 -1.0471 0.9353 0.0007 1.1129 0.0006 0.2253 0.7485 255-260 4.8139 11.1111 0.9722 0.0007 1.1244 0.0006 0.2336 0.7294 260-265 4.2208 -0.5128 0.9541 0.0007 1.0118 0.0007 0.2397 0.6395 265-270 4.7972 10.8911 0.9709 0.0006 0.9799 0.0007 0.2043 0.7268 270-275 4.4129 14.8387 0.9829 0.0007 0.9165 0.0008 0.2077 0.6686 275-280 5.1084 -0.5587 0.9634 0.0712 0.9862 0.0722 0.1930 0.7740 280-285 5.2677 5.6701 0.9408 0.0006 0.9490 0.0006 0.1802 0.7981 285-290 4.7895 8.4507 0.9477 0.0007 1.0706 0.0007 0.2235 0.7257 290-295 4.8313 3.5533 0.9698 0.0009 1.0920 0.0008 0.2260 0.7320 295-300 6.9601 6.7568 0.9569 0.0694 1.4193 0.0489 0.2039 1.0546 300-305 5.7095 10.2941 0.9340 0.0007 1.2709 0.0006 0.2226 0.8651 305-310 5.6125 5.9603 0.9474 0.0007 1.2020 0.0006 0.2142 0.8504 310-315 4.8739 8.0000 0.9459 0.0007 1.0801 0.0006 0.2216 0.7385 315-320 5.7599 7.2961 0.9226 0.0008 1.2366 0.0006 0.2147 0.8727 320-325 5.3130 10.9244 0.9583 0.0007 1.1229 0.0007 0.2114 0.8050 325-330 5.0963 11.1111 0.9353 0.0007 1.0982 0.0006 0.2155 0.7722 330-335 5.1965 10.5691 0.9295 0.0006 1.1134 0.0006 0.2143 0.7873 335-340 5.0699 5.8824 0.9648 0.0007 1.0410 0.0007 0.2053 0.7682 340-345 5.4208 11.1111 0.9266 0.0006 1.1530 0.0006 0.2127 0.8213 345-350 79.2091 7.0749 0.9664 0.6952 13.0187 0.0534 0.1644 12.0014 350-355 5.5707 10.7407 0.9476 0.0006 1.1907 0.0005 0.2137 0.8440 355-360 4.6085 6.4220 0.9582 0.0006 1.0069 0.0006 0.2185 0.6983 360-365 4.7477 4.2802 0.9438 0.0007 1.0962 0.0006 0.2309 0.7193 365-370 5.6984 12.4088 0.9326 0.0007 1.2536 0.0006 0.2200 0.8634 370-375 4.7498 8.0357 0.9094 0.0007 1.0868 0.0006 0.2288 0.7197 375-380 5.3526 15.6134 0.9568 0.0007 1.0753 0.0007 0.2009 0.8110 380-385 4.3046 10.7692 0.9583 0.0006 0.9105 0.0007 0.2115 0.6522 385-390 4.4796 12.3288 0.9378 0.0006 0.9453 0.0006 0.2110 0.6787 390-395 6.9007 8.6957 0.9438 0.0009 1.0535 0.0008 0.1527 1.0456 395-400 6.1443 8.6817 0.9633 0.0007 1.2122 0.0006 0.1973 0.9310 400-405 4.6456 13.1783 0.9539 0.0007 1.0018 0.0007 0.2157 0.7039 405-410 5.4459 7.2917 0.9434 0.0009 1.1242 0.0008 0.2064 0.8251 410-415 6.4235 9.9196 0.9422 0.0006 1.1530 0.0005 0.1795 0.9733 415-420 5.8343 7.5085 0.9936 0.0007 1.1506 0.0006 0.1972 0.8840 420-425 5.7161 3.6789 0.9401 0.0007 1.2400 0.0006 0.2169 0.8661 425-430 5.0727 4.6610 0.9647 0.0007 1.0466 0.0007 0.2063 0.7686 430-435 5.1412 4.5082 0.9544 0.0007 1.1535 0.0006 0.2244 0.7790 435-440 5.7756 12.1581 0.9391 0.0005 1.0383 0.0005 0.1798 0.8751 440-445 5.0857 10.0000 0.9443 0.0006 0.9789 0.0006 0.1925 0.7706 445-450 5.1698 9.2715 0.9360 0.0006 0.9991 0.0006 0.1932 0.7833 450-455 4.3227 7.6233 0.9540 0.0006 0.9997 0.0006 0.2313 0.6550 455-460 4.0782 4.0816 0.9479 0.0006 0.9247 0.0006 0.2267 0.6179 460-465 4.0852 3.2787 0.9608 0.0006 0.9431 0.0006 0.2309 0.6190 465-470 4.7046 10.8696 0.9643 0.0005 0.9342 0.0006 0.1986 0.7128 470-475 4.3405 7.5829 0.9518 0.0006 0.9291 0.0006 0.2141 0.6577 475-480 4.2571 11.3990 1.0032 0.0006 0.9174 0.0006 0.2155 0.6450 480-485 6.1424 7.2165 0.9160 0.0750 1.3205 0.0568 0.2150 0.9307 485-490 6.5308 6.6667 0.9606 0.0685 1.3743 0.0499 0.2104 0.9895 490-495 5.2123 6.1224 0.9586 0.0007 1.1368 0.0006 0.2181 0.7897 495-500 5.0058 12.2302 0.9820 0.0006 1.0118 0.0006 0.2021 0.7584 500-505 5.0290 7.3826 0.9651 0.0723 1.0219 0.0708 0.2032 0.7620

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505-510 4.5602 0.8696 0.9879 0.0006 1.0789 0.0005 0.2366 0.6909 510-515 4.8618 2.9787 0.9505 0.0006 1.1202 0.0005 0.2304 0.7366 515-520 6.2117 10.6618 0.9482 0.0006 1.1460 0.0005 0.1845 0.9412 520-525 4.8599 4.4118 0.9511 0.0006 1.0400 0.0006 0.2140 0.7364 525-530 4.9841 9.0517 0.9529 0.0005 1.1226 0.0005 0.2252 0.7552 530-535 4.5353 10.3448 0.9603 0.0007 0.9813 0.0007 0.2164 0.6872 535-540 4.2519 2.1390 0.9660 0.0006 1.0116 0.0006 0.2379 0.6442 540-545 4.2515 9.2896 0.9455 0.0006 1.0216 0.0006 0.2403 0.6442 545-550 4.1906 7.0652 0.9132 0.0007 1.0553 0.0006 0.2518 0.6349 550-555 4.8142 11.0619 0.9627 0.0007 1.0359 0.0007 0.2152 0.7294

Table 6.1B Magnetic susceptibility data for the core AAS-22/8. Core 22/8 depth LF HF kfd S-ratio ARM SIRM ARM / SIRM SIRM/klf Magnetite cm % (1T) % 0-5 5.143 4.759 7.463 0.946 0.142 1.170 0.121 0.227 0.779 5-10 5.506 5.158 6.329 0.958 0.155 1.235 0.125 0.224 0.834 10-15 5.556 5.460 1.724 0.948 0.153 1.254 0.122 0.226 0.842 15-20 6.246 5.707 8.633 0.957 0.162 1.337 0.122 0.214 0.946 20-25 6.413 5.707 11.005 0.960 0.163 1.344 0.121 0.210 0.972 25-30 6.727 6.139 8.734 0.961 0.166 1.350 0.123 0.201 1.019 30-35 7.163 6.799 5.085 0.955 0.164 1.339 0.122 0.187 1.085 35-40 6.266 5.690 9.184 0.954 0.159 1.303 0.122 0.208 0.949 40-45 5.393 5.099 5.464 0.961 0.138 1.144 0.121 0.212 0.817 45-50 5.951 5.763 3.158 0.960 0.159 1.285 0.123 0.216 0.902 50-55 5.918 5.742 2.974 0.969 0.159 1.323 0.120 0.224 0.897 55-60 6.061 5.455 10.000 1.044 0.154 1.181 0.131 0.195 0.918 60-65 6.120 4.979 18.653 0.992 0.146 1.208 0.121 0.197 0.927 65-70 5.590 5.160 7.692 0.975 0.150 1.229 0.122 0.220 0.847 70-75 6.284 5.862 6.719 0.966 0.168 1.482 0.113 0.236 0.952 75-80 6.365 5.804 8.824 0.948 0.169 1.411 0.120 0.222 0.964 80-85 6.492 5.916 8.874 0.957 0.179 1.436 0.124 0.221 0.984 85-90 6.938 6.104 12.027 0.959 0.175 1.412 0.124 0.204 1.051 90-95 7.289 5.986 17.871 0.955 0.179 1.460 0.122 0.200 1.104 95-100 5.910 5.389 8.824 0.975 0.162 1.303 0.124 0.220 0.896 100-105 5.931 4.882 17.690 0.951 0.149 1.253 0.119 0.211 0.899 105-110 6.305 5.475 13.158 0.978 0.158 1.314 0.120 0.208 0.955 110-115 5.604 4.972 11.275 0.977 0.144 1.164 0.124 0.208 0.849 115-120 4.397 4.347 1.130 0.968 0.123 0.978 0.126 0.222 0.666 120-125 5.106 4.855 4.908 0.976 0.139 1.112 0.125 0.218 0.774 125-130 5.863 5.214 11.071 0.942 0.151 1.216 0.124 0.207 0.888 130-135 6.559 5.787 11.765 0.974 0.156 1.265 0.124 0.193 0.994 135-140 6.219 5.580 10.268 0.974 0.152 1.240 0.123 0.199 0.942 140-145 7.560 7.184 4.979 0.959 0.194 1.608 0.121 0.213 1.145 145-150 6.485 5.473 15.610 0.977 0.148 1.248 0.119 0.192 0.983 150-155 6.200 5.636 9.091 0.963 0.149 1.277 0.117 0.206 0.939 155-160 6.097 5.591 8.304 0.966 0.158 1.313 0.120 0.215 0.924 160-165 5.833 5.476 6.122 0.963 0.156 1.248 0.125 0.214 0.884 165-170 5.670 5.222 7.907 0.968 0.147 1.188 0.124 0.209 0.859 170-175 6.239 5.102 18.222 0.961 0.167 1.199 0.139 0.192 0.945

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175-180 6.014 5.241 12.857 0.969 0.143 1.197 0.120 0.199 0.911 180-185 6.107 5.699 6.689 0.971 0.155 1.277 0.121 0.209 0.925 185-190 6.103 5.446 10.769 0.971 0.152 1.272 0.119 0.208 0.925 190-195 5.424 5.043 7.018 0.963 0.133 1.170 0.114 0.216 0.822 195-200 5.260 4.632 11.940 0.972 0.134 1.157 0.116 0.220 0.797 200-205 5.242 4.505 14.054 0.961 0.125 1.055 0.119 0.201 0.794 205-210 4.001 3.415 14.634 0.965 0.108 0.878 0.123 0.219 0.606 210-215 4.616 3.760 18.543 0.959 0.102 0.834 0.122 0.181 0.699 215-220 4.049 3.550 12.338 0.970 0.089 0.756 0.117 0.187 0.614 220-225 4.318 3.701 14.286 0.973 0.089 0.790 0.113 0.183 0.654 225-230 4.286 4.260 0.610 0.981 0.101 0.891 0.114 0.208 0.649 230-235 5.517 4.546 17.588 0.970 0.135 1.075 0.125 0.195 0.836 235-240 5.506 4.834 12.209 0.977 0.142 1.062 0.133 0.193 0.834 240-245 5.209 4.645 10.828 0.978 0.139 1.010 0.138 0.194 0.789 245-250 5.152 4.817 6.494 0.976 0.139 1.029 0.135 0.200 0.781 250-255 5.428 4.954 8.734 0.960 0.142 1.112 0.128 0.205 0.822 255-260 5.298 4.859 8.290 0.982 0.144 1.102 0.130 0.208 0.803 260-265 5.536 4.920 11.111 1.006 0.143 1.048 0.137 0.189 0.839 265-270 5.756 5.274 8.374 0.976 0.144 1.154 0.124 0.201 0.872 270-275 6.081 5.240 13.839 1.004 0.144 1.080 0.133 0.178 0.921 275-280 5.688 5.000 12.088 1.387 0.154 0.770 0.200 0.135 0.862 280-285 5.681 4.881 14.070 0.688 0.138 1.501 0.092 0.264 0.861 285-290 4.755 4.537 4.575 0.986 0.133 1.009 0.132 0.212 0.720 290-295 5.329 5.117 3.965 0.974 0.146 1.177 0.124 0.221 0.807 295-300 5.510 5.060 8.173 0.965 0.149 1.177 0.126 0.214 0.835 300-305 5.430 4.541 16.374 0.981 0.134 1.086 0.123 0.200 0.823 305-310 4.274 4.131 3.333 0.983 0.127 0.935 0.136 0.219 0.648 310-315 4.859 4.314 11.215 0.961 0.135 1.003 0.135 0.206 0.736 315-320 4.662 4.206 9.790 0.973 0.127 0.948 0.134 0.203 0.706 320-325 5.067 4.066 17.722 1.008 0.129 0.930 0.139 0.184 0.768 325-330 4.203 4.112 2.158 0.976 0.126 0.927 0.136 0.221 0.637 330-335 3.537 3.468 1.942 0.974 0.127 1.002 0.126 0.283 0.536 335-340 4.305 3.721 13.571 0.969 0.122 0.964 0.126 0.224 0.652 340-345 4.514 3.895 13.699 0.988 0.117 0.867 0.135 0.192 0.684 345-350 4.864 4.367 10.219 0.985 0.116 0.879 0.132 0.181 0.737 350-355 5.047 4.348 13.846 0.985 0.138 0.997 0.138 0.198 0.765 355-360 4.353 4.095 5.926 0.979 0.122 0.894 0.137 0.205 0.659 360-365 4.672 3.989 14.615 0.965 0.124 0.935 0.133 0.200 0.708 365-370 4.834 4.501 6.897 0.986 0.135 1.010 0.133 0.209 0.732 370-375 5.048 4.547 9.914 0.976 0.144 1.037 0.139 0.205 0.765 375-380 4.954 4.695 5.229 0.989 0.138 1.041 0.132 0.210 0.751 380-385 5.156 5.078 1.523 0.977 0.137 1.085 0.126 0.210 0.781 385-390 4.741 4.711 0.641 0.991 0.145 1.084 0.134 0.229 0.718 390-395 4.710 4.469 5.109 0.988 0.141 1.060 0.133 0.225 0.714 395-400 3.950 3.799 3.817 0.980 0.117 0.882 0.132 0.223 0.598 400-405 4.712 4.668 0.926 0.954 0.137 1.052 0.130 0.223 0.714 405-410 5.124 5.050 1.456 0.948 0.155 1.205 0.129 0.235 0.776 410-415 4.673 4.638 0.746 0.977 0.127 0.977 0.130 0.209 0.708 415-420 5.216 5.089 2.439 0.964 0.139 1.061 0.131 0.203 0.790 420-425 4.751 4.047 14.815 0.983 0.149 1.038 0.144 0.218 0.720 425-430 5.261 5.082 3.409 0.971 0.150 1.132 0.133 0.215 0.797 430-435 4.010 3.573 10.891 0.980 0.068 0.771 0.089 0.192 0.608

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Chapter 6

435-440 5.261 4.783 9.091 0.979 0.141 1.125 0.126 0.214 0.797 440-445 5.695 5.190 8.861 1.001 0.148 1.190 0.124 0.209 0.863 445-450 5.509 4.940 10.326 0.978 0.153 1.203 0.127 0.218 0.835 450-455 5.863 4.942 15.714 0.998 0.149 1.175 0.127 0.200 0.888 455-460 6.019 5.127 14.815 0.984 0.140 1.171 0.119 0.194 0.912 460-465 5.161 4.603 10.811 0.992 0.132 1.102 0.120 0.213 0.782 465-470 5.509 5.236 4.969 0.983 0.142 1.182 0.120 0.215 0.835 470-475 6.868 6.169 10.182 0.982 0.167 1.360 0.123 0.198 1.041 475-480 7.106 6.081 14.423 0.984 0.160 1.305 0.123 0.184 1.077 480-485 8.465 8.042 5.000 0.960 0.215 1.815 0.118 0.214 1.283 485-490 5.467 4.910 10.204 0.976 0.139 1.158 0.120 0.212 0.828 490-495 5.442 5.091 6.452 0.978 0.138 1.154 0.120 0.212 0.825 495-500 4.993 4.793 4.000 0.982 0.125 1.072 0.117 0.215 0.757 500-505 5.667 4.993 11.881 0.971 0.133 1.145 0.116 0.202 0.859 505-510 4.062 3.455 14.925 0.962 0.105 0.848 0.124 0.209 0.615 510-515 4.868 3.912 19.632 0.996 0.104 0.813 0.127 0.167 0.738 515-520 4.520 4.413 2.367 0.972 0.138 1.075 0.129 0.238 0.685 520-525 3.917 3.648 6.878 0.975 0.105 0.970 0.108 0.248 0.593 525-530 4.557 4.350 4.545 0.987 0.134 1.106 0.121 0.243 0.690 530-535 4.025 3.539 12.081 0.992 0.096 0.867 0.111 0.215 0.610 535-540 5.228 4.781 8.537 0.995 0.144 1.182 0.122 0.226 0.792 540-545 4.136 3.887 6.015 0.975 0.116 0.984 0.117 0.238 0.627 545-550 5.754 5.435 5.556 0.998 0.162 1.309 0.124 0.227 0.872 555-560 5.610 4.736 15.578 1.010 0.152 1.131 0.135 0.202 0.850

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Chapter 6

Table 6.2A Down-depth major oxides (wt%) of core AAS-22/7. The three depths with (R) indicate the entrapped rock.

Depth (cm) SiO2 Al2O3 TiO2 Fe2O3 MnO MgO CaO Na2O K2O P2O5 0-5 57.58 9.343 0.328 4.492 0.555 2.519 0.608 6.914 1.795 0.112 45-50 57.33 9.351 0.292 4.338 0.4 2.277 0.605 5.878 1.614 0.097 95-100 57.82 8.52 0.275 4.083 0.136 2.059 0.611 6.275 1.679 0.106 145-150 59.55 8.925 0.291 4.288 0.45 2.21 0.6 5.822 2.008 0.139 195-200 55.97 10.296 0.356 4.999 0.381 2.414 0.658 5.657 1.873 0.126 245-250 59.59 9.868 0.343 4.706 1.001 2.332 0.623 6.198 1.786 0.122 280-285 59.16 8.512 0.29 4.186 0.094 2.097 0.562 5.852 1.557 0.11 285-290 57.04 9.269 0.321 4.553 0.091 2.245 0.579 6.08 1.7 0.101 290-295 55.69 9.805 0.325 4.822 0.249 2.335 0.628 6.169 1.814 0.124 295-300 52.93 9.563 0.315 4.813 0.491 2.316 0.635 5.452 2.029 0.23 330-335 52.66 11.293 0.614 6.909 0.717 2.455 0.855 5.2 2.026 0.157 335-340 53.17 11.329 0.434 5.624 0.44 2.367 0.701 5.567 1.983 0.157 340-345 54.35 11.566 0.467 5.784 0.363 2.368 0.694 5.438 1.995 0.152 345-350 54.49 11.164 0.459 6.059 0.188 2.581 0.8 5.552 2.029 0.23 350-355 53.48 11.337 0.438 5.719 0.238 2.398 0.698 5.462 1.997 0.156 355-360 53.89 11.578 0.451 5.831 0.376 2.359 0.682 5.229 2.014 0.154 445-450 52.2 13.335 0.597 5.106 0.595 2.597 0.676 5.623 2.062 0.165 470-475 52.79 11.952 0.419 5.599 0.564 2.369 0.641 5.563 2.144 0.133 475-480 52.3 11.931 0.414 5.597 0.544 2.378 0.634 5.465 2.163 0.125 480-485 46.9 10.723 0.39 7.373 0.644 2.247 0.602 4.962 2.012 0.165 485-490 50.88 11.763 0.469 6.144 0.605 2.574 0.916 5.109 2.056 0.187 490-495 54.12 12.573 0.47 6.084 0.753 2.546 0.717 5.509 2.106 0.162 495-500 50 11.687 0.453 6.002 0.805 2.538 0.723 5.232 2.043 0.161 545-550 50.74 11.847 0.455 5.953 0.785 2.473 0.73 5.843 1.976 0.122 550-560 54.97 11.314 0.439 5.514 0.368 2.43 0.661 5.632 1.924 0.125

R-345 -355 45.89 13.16 2.6 13.51 0.699 2.768 1.087 2.7 2.306 0.133 R-480-485 44.12 9.902 0.566 10.119 1.902 4.857 1.446 4.748 2.502 0.768 R-485-490 45.08 10.422 0.555 9.789 1.818 4.778 1.393 4.512 2.445 0.748

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Chapter 6

Table 6.2B Down-depth of trace and REE (ppm) of core AAS-22/7. The three depths with (R) indicate the entrapped rock.na = not available.

Depth (cm) Li Sc Ti V Cr Mn Co Ni Cu Zn Ga Rb Sr Y Nb Mo Sb Ba 280-285 29.94 18.53 0.258 61.72 41.78 721.9 17.51 47.1 124.2 55.61 14.5 55.48 150.7 36.82 5.006 1.716 0.982 3254 285-290 33.18 19.9 0.294 65.56 44.91 748.1 20.67 52.42 132.9 61.25 16.21 26.65 161.8 36.37 5.697 1.426 1.149 3475 290-295 34.07 21.61 0.308 71.65 45.28 1944 39.59 69.2 165.7 66.06 17.64 27.4 192.7 46.98 5.688 6.208 1.263 4161 295-300 35.42 21.34 0.329 77.03 45.97 4116 54.25 118.2 231.7 69.98 17.87 27.74 215.5 49.74 6.299 16.96 1.731 4676 335-340 44.05 22.93 0.414 91.46 57.05 3685 48.13 135.1 207.1 72.03 20.67 47.27 182.8 53.16 7.245 14.37 1.64 3801 340-345 44.39 23.48 0.435 91.48 57 3092 49.03 116.2 202.7 70.99 21.21 31.95 181.9 50.71 7.572 10.45 1.49 3807 345-350 45.39 25.01 0.433 88.89 58.59 1652 32.09 79.31 185 81.83 20.93 32.25 176.2 110 7.686 4.44 1.365 3164 350-355 45.67 23.81 0.436 87.93 56.75 1963 36.24 80.88 187.7 72.22 21.3 47.35 177.9 54.33 7.37 5.114 1.329 3480 355-360 44.99 22.76 0.469 95.31 60.54 3224 60.51 103 196.9 61.14 21.45 33.01 174.4 50.7 7.71 10.39 1.3 3652 475-480 49.29 23.04 0.435 92.84 57.97 4593 56 137 247.8 60.76 22 57.37 194.5 53 7.652 19.52 1.877 3664 480-485 49.59 23.95 0.451 91.77 55.58 5499 49.66 153.4 264.5 75.84 21.38 40.06 198.2 56.06 7.422 26.24 2.174 3659 485-490 50.82 25.01 0.481 102.8 59.88 5422 58.29 151.4 270.6 71.3 22.33 37.88 174.2 118.6 8.551 26.25 1.983 2544 490-495 52.26 24.19 0.499 103.8 59.29 6593 59.04 170.3 298.4 62.93 22.18 37.76 185.9 66 8.038 31.65 2.101 2967 495-500 52.08 23.81 0.537 107.2 60.57 7048 59.91 287.6 322.2 66.97 22.79 55.47 189.2 67.06 8.322 34.93 2.163 3267

R-345 -355 46.43 28.05 2.913 129.1 19.86 6307 59.65 155.7 364.1 276.5 22.31 24.5 196.1 33.54 51.62 19.89 4.638 107.6 R-480-485 39.28 30.28 0.658 174.4 30.06 17160 67.41 749.6 631.8 172.3 25.44 66.01 185.1 567.3 14.06 66.71 5.005 754.2 R-485-490 39.24 32.54 0.67 157.4 45.01 na 83.26 648.1 594 110.6 21.98 95.51 176.7 577.1 13.11 63.82 5.039 555.9

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Chapter 6

Depth (cm) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 280-285 22.37 68.65 5.941 26.46 2.537 2.928 5.063 0.969 6.309 1.451 3.815 0.542 3.396 0.571 285-290 21.87 54.66 5.808 26.52 1.857 2.998 4.923 0.872 6.248 1.322 3.616 0.468 3.238 0.504 290-295 28.74 98.04 7.43 34.19 2.797 3.491 6.125 1.198 7.714 1.671 4.479 0.623 4.029 0.635 295-300 30.75 102.1 7.694 35.55 1.935 3.648 6.533 1.256 7.941 1.746 4.743 0.662 4.228 0.671 335-340 33.97 101.3 8.152 35.98 4.049 3.366 6.818 1.344 8.905 1.99 5.244 0.768 4.714 0.755 340-345 31.1 104.9 8.064 36.22 4.043 3.601 7.271 1.373 9.122 2.025 5.288 0.771 4.711 0.73 345-350 45.64 103.8 11.41 45.22 8.894 3.91 8.615 1.805 12.75 2.925 7.582 1.18 6.661 1.085 350-355 35.47 120.2 8.913 39.13 5.316 3.378 7.554 1.49 9.636 2.119 5.59 0.848 5.045 0.809 355-360 29.2 100.6 7.418 33.22 3.598 3.258 6.69 1.301 8.672 1.962 5.109 0.78 4.655 0.733 475-480 34.46 132.9 8.945 40.28 4.692 3.409 7.738 1.493 9.484 2.05 5.242 0.784 4.703 0.736 480-485 38.23 136.4 9.349 41.95 5.017 3.448 7.707 1.536 9.597 2.123 5.468 0.813 4.735 0.745 485-490 42.5 139.1 11.17 48.48 9.89 3.785 9.728 1.913 13.01 2.972 7.474 1.155 6.58 1.028 490-495 42.53 162.3 10 43.37 7.599 3.39 9.343 1.786 11.37 2.581 6.48 1.023 5.734 0.906 495-500 41.09 142.9 9.634 41.4 6.706 3.334 8.803 1.724 11.27 2.59 6.57 1.055 5.82 0.927

R-345 -355 23.57 196.9 5.881 35.82 7.489 1.652 6.886 1.236 6.267 1.375 3.42 0.555 3.103 0.448 R-480-485 242.5 228.4 61.95 209.8 36.87 7.503 23.12 5.202 40.33 9.911 24.84 4.099 21.84 3.426 R-485-490 239.4 230.1 60.15 209.1 39.43 7.699 24.65 5.453 42.4 10.61 26.26 4.312 22.85 3.508

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CHAPTER 7

EFFECT OF LOW-TEMPERATURE ALTERATION: PALAGONITE

FORMATION AND MASS DISAPPEARANCE OF RADIOLARIANS

Chapter 7

7.1 Introduction

The term “palagonite” was first used by Sartorius von Waltershausen in the year 1846 for an altered basaltic glass from Palagonia in the Hyblean Mounts (Sicily, Italy) and he identified three different varieties of palagonite based on the colour: hyblite, notite and korite. Bunsen (1847) stated that the chemical composition of palagonite is similar to that of basalts but the definition of palagonite was yet not clear. Taylor and Peacock (1926) reported that palagonite is highly indurated basaltic glass with varying water content between 18 and 25% and low refraction index (1.46-1.49). Later, several authors attempted to redefine palagonite as hydrated, altered or oxidized basaltic glass. To avoid confusion between the actual tuff and breccia deposits, some authors referred such palagonatized material as hyaloclastite (Rittmann, 1958). Bonatti (1965) during his study of palagonite associated with altered Quaternary basalts of Iceland suggested that these rocks superficially appear to be like breccias or tuff and referred them as palagonite tuff and breccia.

Palagonite is known to occur on land as well as on the seafloor. The palagonite occurring in the submarine environment are mostly associated with authigenic minerals like phillipsite, calcite, celadonite, smectite, and Fe-Mn hydroxides. These authigenic minerals are known to crystallize under a wide range of temperatures. Hence the term “low temperature,” in terms of palagonite and associated authigenic mineral formation, is defined as temperature ranging between the bottom water up to the beginning of the zeolite metamorphic facies (Honnorez, 1981). However, the limit of low temperature in a submarine environment is arbitrary. The process is known to occur in two ways, one is by alteration of the basaltic glass after formation and second by direct interaction of water with basaltic melt. Crockram et al. (1969) suggested diffusion of OH- and H+ ions during the rock-water interface. On the other hand, during the sudden interaction of hot lava, the water gets turned into steam and lava gets fragmented into smaller portions. These small fragments react with steam to form a light- colored palagonite. A good example of such activity was noted known by Charles Darwin at Galapagos Island where palagonite occurs in the form of pyroclastic cones.

Palagonite form by diffusion of ions and results in thin, yellow-orange or buff orange rind on the surface or within the cracked surface of a rock. The palagonite grains are amorphous with size varying from a few micrometers to centimeters. The colour is indicative of the presence of iron in its +3 oxidation state. Palagonites appear as palagonite tuffs, composed of sideromelane and basaltic fragments in a palagonite matrix while the occurrence

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Chapter 7 of sideromelane in palagonite matrix is also referred to as hyaloclastite. The formation of palagonite is affected by different parameters such as temperature, reactive surface, structure of the primary material, structures, and growth rates of the precipitating secondary phases, time, and different fluid properties, e.g. pH, Eh, ionic strength and oxygen fugacity together with rock porosity and permeability (Stroncik and Schmincke, 2002).

Thompson (1991) suggested different factors that are of prime importance in the alteration of the oceanic crust. He suggested that alteration primarily depends on the availability of weak zones, composition of seafloor rock, seawater weathering, metamorphism, hydrothermal circulation and change in the deuteric composition caused by magmatic solidification. Seawater interaction results mainly in two main processes.

1) Low-temperature weathering with temperature >70 °C which is also referred to as halmyrolysis. This is a pervasive and continuous process and usually lasts for thousands of years. 2) High-temperature alteration with a temperature of 70 to 400 °C is also termed as a hydrothermal alteration. This process is known to occur in areas of active magmatism or at sites with high heat fluids occurring for a short time span.

The alteration of basaltic glass is most common in the deep-sea environment. Early understanding of glass alteration and formation of palagonite was suggested to be instantaneous phenomena occurring during the magmatic eruption (Bonatti, 1965). Subsequently, Moore (1966) proved that the formation of palagonite is a gradual process. Later, Honnorez (1981) suggested three stages of palagonite formation (palagonitisation) from the basaltic glass. These are mainly initial, matured and final stages, and these are discussed below.

Initial stage- This is the beginning stage in the alteration of basaltic glass to palagonite and is marked by the coexistence of fresh glass relics with altered residual glass and minerals. During the initial stage of basaltic glass alteration, there is loss of Ca with simultaneous enrichment of K and Mg in the altered area while the other oxides are not affected.

Matured stage- This stage is marked by complete alteration of basaltic glass to palagonite and simultaneously beginning of the in-situ replacement of palagonite to smectite and formation of phillipsite. The altered glass (palagonite) appears to be granular and sometimes

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Chapter 7 occurs with cemented zeolites. The intergranular zeolite occurring in this stage grades into phillipsite needles.

Final Stage- During the final stage, the altered glass is completely replaced by granular palagonite and authigenic minerals such as smectite and phillipsite. The hydroclastite (altered glass) in this final stage of palagonitization is represented by an assemblage of smectite, phillipsite, and Fe-Mn oxide aggregates.

Phillipsite grains were first recognized in the Pacific Ocean sediments, during the 1872-76 expedition of HMS Challenger and confirmed by an examination of associated metalliferous sediments (Dekov et al., 2010). Palagonites have been reported to occur with smectites and Fe-Mn oxides in the sediments of the Pacific, Atlantic and Indian oceans (Arrhenius, 1963; Bonatti, 1963, 1967; Peterson and Griffin, 1964; Kastner and Stonecipher, 1978, Pattan and Parthiban, 2011). Phillipsites are more frequently observed in areas with submarine basaltic volcanism and a very low sedimentation rate (<1 mm/1000 yr) (Honnorez, 1981).

In the deep-sea sediments, phillipsite generally forms single or multiple twinned crystals, and sometimes may replace plagioclase crystals (Bass et al., 1973). Besides these, phillipsite (harmotome, a barium-rich phillipsite) in consolidated slabs have been found in the Society Ridge, South Pacific (Morgenstein, 1967), as cavity filling of tuff as at the Sylvania Guyot, Pacific Ocean (Rex et al., 1967), as fracture filling in basalts of DSDP Leg 17 and replacement of the volcanic debris with nearly 50% of phillipsite (Bass et al., 1973) and as slabs in the CIOB (Iyer and Sudhakar, 1993). The biogenic structures in K-phillipsite that occur in amygdules of pillow basalts from the Ampere-Coral Patch Seamounts (eastern North Atlantic), indicated the presence of potential microbial life (Cavalazzi et al., 2011). Phillipsites are used as good indicators to understand the degree of palagonitisation which is discussed below.

7.2 Background work A large portion of the CIOB is occupied by siliceous sediments composed of radiolarians and diatoms that contribute to the global silica cycle. Radiolarians are largely non-motile organisms with discoidal, bell-, Y- and ball-shape with spines on the outer skeleton. They are widely used as tools to understand the geological and oceanographic conditions within the oceanic basin such as quality of the watermass, temperature, salinity,

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Chapter 7 productivity and availability of nutrients and also as palaeoclimatic indicators (Riley and Chester, 1971; Chester and Jickells, 2012). These siliceous organisms are highly susceptible to change in temperature conditions and dissolve under unfavorable conditions. Radiolarians in the CIOB show two major scale disappearances (Table 7.1) one at species-level (Nath et al., 1989; Banakar et al., 1993; Gupta, 1988, 1996; Iyer et al., 1997b, 1999b) and the other on a mass-scale. The above authors have assigned the action of AABW (Antarctic bottom water mass) currents for the species level disappearance. But to date, there are no reports to justify the large scale disappearance of these siliceous fauna. The objective of this chapter is to understand the disappearance of radiolarians (hiatus), low-temperature alteration and formation of palagonite in the CIOB. This chapter emphasizes the symbiotic relationship of radiolarians and palagonites. A combination of different parameters such as chemical analysis of palagonites and phillipsites along with sediment chemistry and magnetic susceptibility has been used to understand the degree of palagonitization in the CIOB.

7.3 Study Area Two gravity cores separated by a distance of approximately 300 km were selected for this study because of the conspicuous absence of radiolarians and abundant occurrence of palagonite grains. The first core was retrieved from the transitional area of siliceous and pelagic clay (AAS-22/5) while the second core was retrieved from the red clay ooze area (AAS-22/3) (Table 2.1 and Fig. 2.1).

7.4 Results and Interpretations In general, the coarse fractions (CF) of both the cores revealed the occurrence of manganese micronodules, glass shards, radiolarians and palagonite grains, as the major phases; while the minor phases are microtektites (discussed in Chapter 8), volcanic spherules (discussed in Chapter 4), biotite flakes, phillipsite crystals, and very few fish teeth.

7.4.1 Coarse fractions description Core AAS-22/5

The CF% in the core AAS-22/5 show a significant decreasing trend (10 to 0.1%) up to a depth of 300 cm, below this depth there is a broad peak from 300 to 400 cm (Fig 7.1). The CF% remain almost constant beyond 400 cm in the core. Microscopy observations revealed

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Chapter 7 an abundance of radiolarians, FeMn micronodules, glass shards and other biota upto a depth of 250 cm. This zone is referred to as zone A. Below 250 cm, the components are phillipsite, glass shards, and palagonite grains is referred to as zone B (Fig. 7.1). Component counts were performed on CF from 250 cm to 500 cm (core end) while the fractions from 0 to 250 cm were not considered for the fact that these were entirely dominated by radiolarians (full and broken). The data of the component counts are given in Table 7.2.

Fig 7.1 Down core variation in the CF% for the core AAS- 22/5 and AAS-22/3. The depth marked in yellow = radiolarian-rich zone A and orange = palagonite-rich zone B.

Fig 7.2 Down-core variation in the abundance of components in the core AAS-22/5 from a depth of 250 cm and below.

In the graph (Fig. 7.2) it is observed that palagonite grains are the most dominant phase in the CF followed (in decreasing order) by manganese micronodules, phillipsite grains, and glass shards. The abundance of palagonite grains

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Chapter 7 significantly increases from 250 to 500 cm along with micronodules. The phillipsite grains have remained fairly constant throughout the length of the core, while the abundance of biota is erratic. It is noteworthy that phillipsite grains occur only in the palagonite-rich zone and indicates a relation between these two components. Glass shards decrease with depth and completely disappear below 325 cm depth (discussed in Chapter 8).

Core AAS-22/3

CF% in the surface sediment is significantly high (5.5%) but is nearly 50% less than the previously studied core (AAS-22/5, 10%). The gap (Fig. 7.1) in CF% from 60-75 cm was due to the non-availability of the samples. The down-core plot shows a decrease in CF% upto 270 cm. The occurrence of abundant glass shards in different layers has significantly contributed to the CF% (discussed in Chapter 8).

Abundant radiolarians constitute the top 60 cm of the core along with other components such as shards and Fe-rich spherules (marked as zone A -radiolarian rich, Fig. 7.1). Below this depth, the radiolarians are completely absent upto a depth of 290 cm (marked as zone B- palagonite rich Fig. 7.1). The CF in this 240 cm of the core is composed of palagonite grains (very abundant) followed by micronodules, glass shards, basaltic glass, rock pieces, and few shark teeth. Below 290 cm and upto 325 cm, radiolarians reappear in the CF (marked as zone A- radiolarian rich). The palagonite grains, and micronodules are nearly absent also in this section. While below this depth of 325 cm upto the core end (560 cm) the radiolarians were absent and CF is wholly composed of palagonite and micronodules (marked as zone B -palagonite rich). In general, there are two layers of abundant radiolarians well preserved in the sediments (zone A) and two layers of abundant palagonite grains (zone B).

Buried macro ferromanganese nodules occur at a depth of 45-50 cm, 150-155 cm and 475-480 cm in AAS-22/3 while in AAS-22/5 the nodules occur between 20 and 30 cm. The compositional similarity of the surface and buried nodules suggests that the nodules were not affected by dissolution with time (Pattan and Parthiban, 2007).

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7.4.2 Palagonite Grains: Morphology and Composition

The palagonite grains are granular, with shades ranging from yellow to brown with buff colour being the most dominant. Orange coloured grains are also quite dominant in the observed sediment sections. In terms of morphology, the palagonite grains are sub-rounded to rounded and have a sugary appearance. Under SEM, the surface appears to be irregular while the size ranges from 100 to 500 µm (Fig. 7.3).

Fig 7.3 SEM of palagonite grains. A- C=palagonite grains; D=phillipsite crystals growing out from a palagonite grain.

Microprobe compositional analyses of palagonite grains (Fig. 7.4; Table 7.3) show

SiO2 to range from 48 to 52 wt. %, with MgO from 2.76 to 4.54 and CaO from 1.52 to 3. The palagonite grains are rich in Al2O3 (12.84 to 18.65), K2O (1.51 to 2.61) and H2O (14 to 18%).

FeO ranges from 5.2 to 11.61 while TiO2 values are very low (0.16 to 0.24), except for one grain that has a slightly higher value (0.43).

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Fig 7.4 EPMA analysis of polished palagonite grains. The red and green dots are the probe spots.

The SiO2 vs Na2O + K2O plot indicates the palagonite grains to show affinity with basaltic, basaltic andesite and basanite compositions (Fig. 7.5).

Fig.7.5 Classification of palagonite based on TAS. The nomenclature fields are after LeBas et al. (1986). Abbreviations: Ph=Phonolite; TPh=Tephriphonolite; PhT=Phonotephrite, T=Tephrite;

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Ba=Basanite; PBas=picrobasalt; TrBas=Trachybasalt; BaTrAnd=Basaltic trachyandesite; TrAnd=Trachyandesite; BasAnd=Basaltic andesite.

To understand the alteration effect within the sediment cores, binary plots

were made. The plot of TiO2 vs FeO (Fig. 7.6A) shows very low concentration compared to the previously reported palagonite from the CIOB (Iyer, 1999) and from world oceans such as DSDP Leg 37 (Scarfe and Smith, 1977), DSDP Leg 37 (Andrews, 1977), Melson (1973), Baragar et al. (1977), and Furnes (1978).

Fig. 7.6 Plot of (A) TiO2 vs FeO, (B) CaO+MgO vs K2O and (C) CaO+MgO+MnO vs K2O for palagonite. The red solid dots are palagonite from present study and blue solid dots are palagonite from CIOB (Iyer 1999), black solid rectangle = DSDP Leg 37 of Scarfe and Smith (1977), black solid diamond = DSDP Leg 37 of Andrews (1977), black solid triangle = Melson (1973), black solid circle and black solid hexagon = Baragar et al. (1977).

The low concentration could be mainly due to removal of these oxides during palagonite formation. The plot of

CaO+MgO vs K2O (after Matthews,

1971) shows lower K2O higher CaO+MgO (Fig. 7.6B). The presently studied samples show some resemblance to DSDP Leg 37 (Andrews, 1977) and two samples from CIOB (Iyer 1999). The substantially enriched values of CaO and MgO values indicate the occurrence of smectite clays.

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The plot of CaO+MgO+MnO

against K2O (Fig. 7.6C) depicts a narrow

range in CaO+MgO+MnO and K2O and appears as a cluster while the reference data show more scatter. The narrow range or the clustering of samples indicates that they all have undergone a similar degree of alteration.

Fig. 7.7 Plot of MgO, CaO, and K2O against LOI. The red solid dots are palagonite from the present study and blue solid dots are palagonite from CIOB (Iyer, 1999).

The plot of MgO, CaO, and K2O against LOI depicts that MgO values show a narrow range of 2.8 to 4.8 against LOI (Fig. 7.7A). The data appear to be highly scattered with no proper trend. However, the present value of MgO is much higher than those reported from the CIOB. This indicates that the palagonite from the study area still contains significant MgO and not gone into any altered phase. The plot of CaO (Fig. 7.7B) shows an increasing trend with decreasing LOI. The negative trend indicates leaching of lime with increasing alteration

(Iyer, 1999b). While the reverse trend in the plot of K2O vs LOI (Fig. 7.7C) is indicative of enrichment of K2O with increasing alteration.

7.4.3 Phillipsite Grains: Morphology and Chemistry

The phillipsite grains associated with palagonite are transparent, single crystals and 165 μm to 230 μm in length. The phillipsite occur as cruciform to inter-twinned to multiple- twinned crystals (Fig 7.8A). Overgrowth and outgrowth patterns are seen over some of the grains (Fig 7.8B). Phillipsites of similar texture and morphological features were reported by Iyer et al. (2012, 2018). Few phillipsite crystals are observed to be protruding out from the

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Chapter 7 palagonite grain. The growth of phillipsite crystals within palagonite suggests a close association between the two phases (cf. Bonatti, 1963)

Fig 7.8 Scanning electron micrographs of A) phillipsite crystals showing different forms of twinning. The left grain exhibits a cruciform crystal while the one on the right shows multiple-twinning. B) Overgrowth and outgrowth of phillipsite crystals on/from a grain of palagonite.

According to Kastner (1981), the nucleation and growth of phillipsite occur at the sediment/water interface and it continues to grow within the sediment column. Phillipsite crystals also undergo rapid dissolution which results in etched crystal faces or complete disappearance of the crystal form. These etched crystal faces were observed in a few crystals in the sediment cores of the present study. Chemical composition of phillipsite crystals (Fig.

7.8C) by EPMA analysis show SiO2 (57.13-59.35 wt. %), Al2O3 (19.55-21.39), K2O (4-6) and

Na2O (1.2-1.8) and low values of FeO (0.9- 2.7), CaO (1.5- 2.5), and MgO (0.3- 0.4) (Table 7.

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Fig 7.8C EPMA analysis of polished phillipsite grains. The red and green dots are the probe spots.

7.4.4 Composition of the core sediments

(i) The down-core variations in major oxides (in wt. %) of the core AAS-22/5 show

SiO2 varies from 42.06 to 50 (avg. 46.07), Al2O3 (9.37 to 14.53, avg. 12.78), TiO2 (0.3 to 0.5, avg. 0.46), MnO (0.99 to 1.56, avg. 1.41), MgO (2.32 to 4.16, avg. 3.29), P2O5 (0.31 to 0.54, avg. 0.40), CaO (1.05 to 1.20, avg. 1.18), Na2O from 5.05 to 7.57 (avg. 6.54), K20 ranges from

1.8 to 2.15 (avg. 1.9) and Fe2O3 from 4.28 to 7.08 (avg. 5.6) (Fig. 7.9; Table 7.5A).

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Fig. 7.9 Down core variation in the major oxide data for the core AAS-22/5 and AAS-22/3. In core AAS-22/5 above 250 cm is zone A (radiolarian-rich) and below 250 cm is zone B (palagonite-rich). In core AAS-22/3 0- 60 and 290-315 cm are zone A while 60-290 cm and 315-560 cm are zone B.

Down-core contents in CaO, K2O,

MnO, TiO2 and P2O5 hardly show any

fluctuation. SiO2 and Na2O show a negative trend compared to the other oxides but gradually increases with respect to depth.

Fe2O3 shows a gradual decrease upto a depth of 250 cm with values ranging from 4.28 to 7.02 while below 250 cm the values do not vary significantly (6.4-6.9). The

other major oxides such as Al2O3 and MgO show depletion in the top 250 cm of the core (i.e. zone A), while below 250 cm (i.e. zone

B) show enrichment of these oxides. The presence of lower values of SiO2 in the palagonite- rich zone suggests removal SiO2 during alteration.

(ii) The down-core variations in major oxides in the core AAS-22/3 indicates Al2O3, TiO2,

Fe2O3, and MgO to range between 10.52 and 14.80 (avg. 13.57), 0.3-0.6 (avg. 0.54), 4.93-7.6 (avg. 7.07), and 2.51- 3.43 (avg. 3.09), respectively. CaO values vary from 1.4 to 1.8 (avg. 1.59) and MnO from 2 to 2.3 (avg. 2.24) and these do not show a wide range, unlike other oxides. K2O values are from 2.18 to 2.47 (avg. 2.44), except in the lowermost core depth

(545-500 cm) that has high (3.41) value along with high P2O5 (1.05). The SiO2 ranges from

43 to 47 (avg. 45) and Na2O from 4.7 to 8.56 (avg. 6.1) (Fig. 7.9; Table 5B).

The down-core concentrations of Al2O3, TiO2, Fe2O3, and MgO are slightly higher in the palagonite-rich zone (zone B) while Na2O shows an inverse trend but K2O has remained constant (Fig. 7.9). The high concentrations of Al2O3, TiO2, Fe2O3, and MgO and low SiO2 compared to the radiolarian-rich zone (zone A) indicate alteration of the core sediment (Fig. 7.2). This process perhaps resulted in extensive formation of palagonite as observed by the abundant grains in the CF.

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Based on the bulk sediment composition, the plot of Fe2O3–Al2O3–MgO for the characterization of clay (after Chamley, 1997) shows that the studied sediments from both the cores of the CIOB have a composition of Fe-Al smectites clays (Fig. 7.10).

Fig. 7.10 The plot of Fe2O3–Al2O3–MgO for the characterization of clay (after Chamley, 1997).

Red solid dots = AAS 22/5; Green solid dots = AAS 22/3.

7.4.5 Magnetic Susceptibility Measurements

Magnetic susceptibility measurements were carried to understand the magnetic mineral concentration within these studied cores. The ratios of ARM/SIRM and SIRM/χlf have been used to understand the distribution of sediment grain size within the cores. Furthermore, magnetic susceptibility values are correlated with sediment geochemistry to understand the effect of in situ alteration and post-depositional processes within the sediment column. The importance of each of the above parameters used in the present study has been discussed in Chapter 2.

Core AAS-22/5: Magnetic susceptibility data show a high χlf for the top 250 cm (zone A) compared to the lower half of the core (zone B) (Table 7.6A; Fig. 7.11A). Overall, the values range between 6 and 9x10-6 m3 kg-1 with an average of 7.5x10-6 m3 kg-1. A gradual -6 3 -1 increase in the value of χlf is observed up to a depth of 125 cm (13x10 m kg ) which indicates a maximum concentration of magnetic minerals in this core. In contrast, a decreasing trend is noted below 125 cm and upto 250 cm. The χlf values in the lower half of the core (zone B) range from 5.5 to 7.5x10-6 m3 kg-1 up to the end of the core.

ARM values range from 0.16x10-5Am2 kg-1 (lowest at core top) to 0.34x10-5Am2 kg-1

(maximum at depth of 100 cm). The down-core trend of ARM is similar to that of χlf. ARM

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Chapter 7 values show an increasing trend upto 125 cm with two major peaks occurring at depths of 30 and 90 cm. A reverse trend is observed below 125 cm up to 250 cm of the core. The peaks occurring in the top 250 cm are broad and show a large difference within each section of the core. Throughout the core ARM values have a narrow range of 0.2 to 0.25x10-5Am2kg-1 (Fig. 7.11A).

SIRM shows high values in the top 10 cm of the core (2.5x10-5Am2kg-1) with a sudden drop in the value at 15 cm (1x10-5Am2 kg-1). Below this depth, there is a positive trend upto a depth of 125 cm except at dept 90 cm where a lower value is noted (1.2x10-5Am2 kg-1). The SIRM value shows a linear trend of up to 250 cm but decreases below 250 cm (Fig. 7.11A).

The ratios of ARM/SIRM and SIRM/χlf are used to understand the grain size of the core. It is observed that the ARM/SIRM ratio (Fig. 7.11A) is comparatively low in the top 5 cm of the core with the lowest value is observed at 30 cm (0.02). ARM/SIRM shows a positive trend in the top 125 cm of the core with two peaks of lower values. While below 125 cm and up to 250 cm, the ARM/SIRM shows a linear trend. A slight increase in the ARM/SIRM ratio is observed below 250 cm down core with values ranging from 0.14 to

0.18, while SIRM/χlf shows fluctuations in the top 125 cm with values from 1 to 2.5. The decreasing SIRM/χlf suggests coarsening of sediments in the top 125 cm of the core while below 125 to 500cm, SIRM/χlf remains nearly constant. Further indicating that the grain size must have remain constant.

Core AAS-22/3: Magnetic susceptibility data exhibit a range from 0.34 to 0.25 x10- 5 2 -1 -5 2 -1 Am kg , and 2.6 to 1.9x10 Am kg , between 60 and 290 cm, respectively. The χlf value ranges from 11x10-6 m3 kg-1 to 7x10-6 m3 kg-1 with ARM from 0.3x10-5Am2 kg-1 to 0.28x10- 5Am2 kg-1and SIRM ranging from 2.4x10-5Am2kg-1 to 1.6x10-5Am2 kg-1(Fig. 7.11B; Table 7.6B). Based on the magnetic susceptibility this core has been divided in to four sections. The top section consist of depth 0-60 cm, followed by 60-290 cm, 290 to 315 cm and the lowermost is 290 to 560 cm. Depths 0 to 60 cm and 290 to 315 cm coincide with zone A (radiolarian-rich) while 60-290 cm and 315 to 560 cm coincide with zone B (palagonite-rich).

In the radiolarain-rich zone (0-60 cm and 290-315 cm) χlf, ARM, and SIRM values (magnetic concentration parameters) increase with increasing ARM/SIRM (grain size -6 3 -1 -6 3 parameter). Between core depth 0-60 cm, the χlf increases from 5.8x10 m kg to 11x10 m kg-1, while ARM and SIRM ranges from 0.14 to 0.3 x10-5Am2 kg-1, and 1.9 to 2.2x10-5Am2 -1 -6 kg , respectively. In contrast, between core depth 290-315 cm, the χlf value range from 8x10

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Chapter 7 m3 kg-1 to 11x10-6 m3 kg-1, and ARM and SIRM ranges from 0.23 to 0.3 x10-5Am2 kg-1, and -5 2 -1 1.7 to 2.3x10 Am kg , respectively. SIRM/χlf values ranges from 0.215 to 0.195 (0-60 cm) and 0.21 to 0.19 (290-315 cm). The decreasing trends in SIRM/χlf in the radiolarian-rich zone suggest coarsening of the sediments with depth.

In the palagonite-rich zone (60-290 cm and 315 to 560 cm) χlf, ARM, and SIRM -6 3 -1 - values decreases with increasing ARM/SIRM. The χlf decreases from 11x10 m kg to 8x10 6 m3 kg-1, and ARM and SIRM range from 0.34 to 0.25 x10-5Am2 kg-1, and 2.6 to 1.9x10- 5 2 -1 -6 3 -1 -6 Am kg , between 60 and 290 cm respectively. The χlf ranges from 11x10 m kg to 7x10 m3 kg-1 with ARM from 0.3x10-5Am2 kg-1 to 0.28x10-5Am2 kg-1 and SIRM range from 2.4x10- 5 2 -1 -5 2 -1 Am kg to 1.6x10 Am kg . SIRM/χlf values for palagonite rich zone ranges from 0.195 to 0.21 (60-290 cm) and 0.19 to 0.22 (315-560 cm) suggesting an increasing trend. This implies that the sediments are fining with increasing depth in the palagonite zone

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Fig 7.11 Magnetic susceptibility plot of core A) AAS-22/5 and B) AAS-22/3

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7.5 Discussion

The examination of the studied cores revealed two significant findings. Firstly, the disappearance of siliceous biota i.e, radiolarians and diatoms. The factors contributing to such mass-scale disappearance of radiolarians in the CIOB could eventually give a clue to the geological processes occurring in the basin. Secondly, extensive formation of palagonite-rich zones within the sediment cores are antithetically related with the presence of radiolarians.

7.5.1 Mass disappearance of radiolarians

The two major factors that contribute siliceous organisms to the marine environment are the production of their tests in the overlying water column and deposition in a non- destructive environment. Radiolarians and diatoms being marine biota have a very thin shell made up of silica. Some of the viable reasons for the dissolution of radiolarians and diatoms are the sub-surface hydrographic parameters like temperature, Eh (oxygen content), pH (alkalinity) and amount of dissolved silicate in the water column (Riedel, 1959; Gupta and Jauhari, 1994). The radiolarian-rich zone (zone A) co-occur with volcanic glass shards and micronodules while the palagonite-rich zone (zone B) shows an abundance of palagonite grains, micronodules, and ichthyoliths (broken fish teeth). The abundance of radiolarians and diatoms in the surface sediment of the studied core could be attributed to higher biological productivity during the past 250 ka (Gupta, 2009). Kolla (1973) observed an absence of radiolarians at the base of two cores, V19-169 and V19-170, recovered from 81°E in the Indian Ocean. Khadge (1998) examined a sediment core of 7.5 m (Table 7.1) and reported an absence of radiolarians at depth of 4.70 m. A similar disappearance in radiolarians was reported in another 5 m long core at a depth of 100 cm (Sensarma et al., 2015). Mascarenhas-Pereira et al. (2006) also noted the disappearance of radiolarians at 6 cm depth in a short core of 20 cm. All these authors have opined the occurrence of palagonite in the radiolarians absent zone, but have failed to provide the exact reason for such large scale mass disappearance of radiolarians in the CIOB. The possible reasons for the absence of radiolarian could be attributed to several reasons such as (i) submarine volcanism in the CIOB (ii) presence/absence of volcanic glass shards that act as a source of food for radiolarians to build their test. (i) Submarine eruptions could lead to physio-chemical changes in the water and on the seabed. Lisitzin (1996) suggested that volcanic eruptions could result in

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either mass death or a mass increase of the marine biota and thus largely influence the marine biogenic sedimentation. Intra-basinal volcanism in the CIOB could be a likely reason for the mass disappearance of radiolarians. This is supported by the presence of Fe-rich spherules (Chapter 4) and evidence of basaltic piece within the sediment cores (Chapter 6). Furthermore, submarine volcanism and associated hydrothermal episodes were quite common in the CIOB as evident from the presence of volcanics such as basalts (Chapter 3), spilites, ferrobasalts, pumice, glass shards (Chapter 8) and volcanogenic- hydrothermal material (vhm) (Gupta, 1988; Mukherjee and Iyer, 1999; Iyer, 2005; Kalangutkar et al., 2011, 2015). Volcanism commenced in the CIOB at about ~60 Ma when the seafloor and the seamounts were formed due to plate movements and later was supplemented by recent volcanic and hydrothermal events (see Chapter 1). These processes may have caused toxicity at the sediment-water interface and facilitated mass death or mortality that resulted in poor preservation of siliceous fauna in the CIOB sediments. Volcanic events in the CIOB have not only produced magnetic spherules and other vhms but also affected the sediments resulting in palagonite formation and disappearance of radiolarians. The effect of alteration on the sediments is discussed in the next sub-section. (ii) Studies carried out in the Pacific Ocean revealed that the silicic glass shards act as a source of food for the radiolarians and this signature was well traced from the sediment cores retrieved from the adjoining areas of the volcanically active seamount. An enhanced presence of diatoms and algae during volcanism was reported by Kurenkon (1972) and Okada (1936). There is also reported evidence of association of organic siliceous material and volcanic deposits in the sedimentary formation in the United States (Bramlette, 1946) and in central Europe (Sarnthein, 1966). Huang et al. (1974) observed enrichment of siliceous fauna during period of larger input of tephra from the nearby seamount eruptions in the Balleny Island, Pacific Ocean. As would be detailed in the next chapter, there is an abundance of glass shards in the CIOB which have been derived from either land volcanoes or intraplate volcanism in the CIOB. These presence or absence of shards could have influenced either the prolific growth or mass extinction of radiolarians in the basin.

7.5.2 Alteration of sediments / Palagonitisation Volcanics on the seafloor are subjected to low-temperature alteration that results in authigenic minerals such as zeolites (e.g., phillipsite, clinoptilolite) and feldspars. The process

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Chapter 7 of alteration includes the need for a precursor, one or more agents of alteration and formation of the final products (clay minerals, zeolites). The CIOB basalts have been reported to have altered to clays (montmorillonite, smectite), palagonite, zeolites, and iron oxides and hydroxides. Three steps occurs during the palagonitization of basaltic glass (cf. Honnorez) and these have been noted for the CIOB basaltic glass: (i) Initial — presence of fresh glass relics, residual altered glass (palagonite), increase in K and Mg and loss of Ca from altered rocks; (ii) Mature — fresh glass is altered and is replaced by zeolites and smectites, and (iii) Final — authigenic minerals fully replace the residual glass (Iyer, 1999b). The stages of palagonitisation as outlined by Honnorez (1981) have been used in the present study to understand the degree of formation of palagonite in the CIOB sediments. The initial stage, which is characterized by crystallization of an inter-granular phillipsite has a composition of Na > K with low Ca and K and Mg-rich. The matured stage which is characterized by palagonitized glass granules is similar to the observation made in the present study (Fig. 7.3A). The co-occurrence of intra-granular phillipsite is a signature of matured stage while the final stage is defined by complete replacement of hyaloclastite by an intimate mixture of authigenic Ca-poor, K > Na phillipsite with smectites and Fe-Mn hydrous oxides.

The palagonites observed in the presently studied sediment cores from the CIOB have high contents of Al2O3 (12.84 to 18.65 wt%), K2O (1.51 to 2.61) and H2O (14 to 18%). The contents of SiO2 (47.86 to 51.89), MgO (2.76 to 4.540) and CaO (1.52 to 2.90) are moderate while TiO2 is very low (0.16 to 0.24). It has been reported that palagonites, over time, get transformed to zeolites such as phillipsite grains or zeolitic-rich indurated clay (Iyer and Sudhakar, 1993, Iyer, 1999; Iyer et al., 2007). Further, the abundance of palagonite grains points to prolonged low-temperature conditions in the CIOB that must have enhanced palagonitisation along with the growth of manganese nodules (both macro- and micronodules) in the sediments.

The phillipsite grains studied from the two cores show enrichment in Al2O3 (19.55 to

21.39 wt%) and K2O (4 to 6) and low FeO (0.9 to 2.7), CaO (1.5 to 2.5), CaO (1.5 to 2.5), and MgO (0.3 to 0.4). The occurrence of phillipsite with signatures of Ca-poor and with K more than Na attests to the continuation of early to final stages of palagonitisation. At times, the presence of FeMn oxides over the palagonite grains has arrested the progress of palagonitisation.

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A comparison of the palagonites from the study area with those reported from the world ocean by Scarfe and Smith (1977), Andrews (1977), Melson (1973), Baragar et al. (1977), and Furnes (1978) shows that the presently studied palagonite grains are significantly depleted in TiO2, FeO, and K2O while SiO2 and Al2O3 values are similar (Table 7.3). Normally, during aging of palagonites, there is an uptake of Si, Al, Mg, and K from sediment or low-temperature solution and gradual loss of Ti and Fe (Stroncik and Schmincke, 2001).

The high values of TiO2 and FeO (Fig. 7.6A) in the present samples vis-a-vis with the global samples suggest that the palagonites from the present study are more matured. This is evident by the occurrence of Ca-poor phillipsite that indicates grading of palagonites to a mature stage.

The chemical composition largely supports the finding of extensive alteration of sediments and palagonite formation. The major oxide data for the sediments show an enrichment of Al2O3, TiO2, MnO, MgO, Fe2O3 and depletion of SiO2 along with Na2O and

K2O in zone B with respect to zone A. Such enrichment in zone B could be due to addition of oxides through low-temperature fluids thereby affecting the chemistry of the sediments. The low values of SiO2, Na2O, and K2O indicate scavenging of oxides from zone B. This is further confirmed by the absence of radiolarians in zone B.

Different magnetic susceptibility parameters have been used in the present study to understand if there is any evidence that could be related to low-temperature alteration of the sediments and to examine the effect of in-situ alteration on the grain size in both the studied cores.

The core AAS-22/5 dominated with radiolarians and micronodules in the top 250 cm (zone A) shows an increasing trend in the concentration of magnetic minerals and grain size up to a depth of 125 cm and a decreasing trend from 125 cm to 250 cm while the lower portion (i.e. below 250 cm) of the core shows a decreasing trend. Ideally, the susceptibility values should show an inverse trend in both these parameters, but that is not the case in the top 125 cm of core AAS-22/5. The coarsening of the grains in the top half of zone A (0 to 125 cm) was identified to be mainly because of the occurrence of glass shards. These were noted to be extremely abundant at depth 10-15 cm and 110-115 cm (discussed in Chapter 8).

The concentration of magnetic minerals was observed to decrease with fining of the sediments in the lower portion of the core (zone B) (Fig. 7.11A). The decrease in magnetic concentration is also seen by the decrease in concentration of Fe2O3 below 250 cm of the core.

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The fine grained sediments in the lower portion could be a result of low-temperature alteration. Similarly, the concentration of magnetic minerals in the core AAS-22/3 showed higher values in the finer sediments occurring in zone A (top 60 cm). There is a decrease in the magnetic concentration with coarsening of the grain size upto a depth of 205 cm. The magnetic concentration is seen to reduce further but the SIRM/χlf shows a positive trend.

The high concentration of magnetic minerals (Fig. 7.11A, B) is not reflected in the chemical data of these two cores (Table 7.5A, B). In both the cores, the Fe2O3 values are significantly low in zone A (i.e., radiolarian-rich) and high in zone B (i.e. palagonite-rich) along with MgO, Al2O3 and Na2O and K2O and a contrast was observed in the magnetic data. The plausible reason for this observation could be that the micronodules observed in the CF may be contributing to the high concentration of χlf, ARM, and SIRM in both the cores. The unaffected area within the core (zone A) shows a well preserved magnetic signal (Fig. 7.11A, B). The decreasing trend in the magnetic mineral in zone B could be due to their dissolution by low-temperature hydrothermal fluids.

The effect of in-situ alteration and compositional changes within the two cores are quite evident from the grain size parameter. In the core AAS-22/5, SIRM/χlf ratio shows more fluctuations (Fig. 7.11A) in the grain size signal but only in the upper 125 cm, while below this depth the grain size has remained constant suggesting a steady depositional condition. On the other hand in the core AAS-22/3, the SIRM/χlf shows finer grain size in zone A associated with radiolarians and coarse grain in zone B (Fig. 7.11B). This could be due to the alteration of the sediments and formation of palagonite grains.

7.6 Conclusions A detailed study carried out of two 5 m long sediment cores, one from the transitional area of siliceous and pelagic clay (AAS-22/5) and the other from the red clay ooze (AAS- 22/3), reveals a hiatus in the occurrence of radiolarians. Despite the fact that many workers have studied several sediment cores of the CIOB but their data pertained to the disappearance of radiolarian in short cores of 40-50 cm length. In contrast, results obtained from this study from two long cores present a better picture of the depositional pattern and in-situ alteration of the sediments. This is for the first time that information concerning the processes that control large scale disappearance and the likely reasons for the same, are presented.

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Based on the above findings I conclude that: 1) The dissolution or non-preservation of radiolarians in the core AAS 22-5 (from 250 to 490 cm) and in the core AAS-22/3 (from 60-290 and 325-560 cm) could be due to intra-basinal volcanism and low-temperature alteration of sediments. 2) The in-situ alteration of sediments is supported by the large scale occurrence of palagonite and zeolites (phillipsite) in the radiolarian devoid zone. 3) The magnetic susceptibility data support the chemical signal of the sediment, thereby further confirming the observation of in-situ alteration of sediments. 4) The occurrence of Ca-poor phillipsite and with K more than Na indicates an early to final stages of palagonitisation of the core sediment.

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Table 7.1 A compilation of studies on disappearance of radiolarians in several sediment cores from the Central Indian Ocean Basin. Radiolarians present in core NR-1 is Collosphaera invaginata; NR-2 is Collosphaera tuberosa; and NR-3 is Stylatractus universus. Among these three species, NR-2 and NR-3 are present in all the studied cores, while NR-1 is missing in some of the cores. na = not availab

Core name Latitude Longitude Water Depth Core Recovery Disappearance /Appearance of References (S) (E) (m) (cm) radiolarians AAS-22/5 15o 06.619 78o 11.742 5030 490 i) 80-290 cm Present work ii) 315- 560 cm (core bottom)

AAS-22/3 16o 59.590 77 o 59.665 4760 560 250-500 cm Present work (core bottom) Core 9o 77o 5400 750 i) 430-460 cm Khadge (1998) ii) 470-486 cm iii) 580-605 cm AAS 38/4 13° 59’ 76’’ 74° 59’ 74’’ 4935 540 100 – 540 cm Sensarma et al. (2015) (core bottom) NR-1 species is missing AAS-61, BC-8 16° 75° 30’ 5010 20 6-20 cm Mascarenhas- (core bottom) Pereira et al. (2006) NR-1 species is present F-88B 12°43’ 77° 03’ 5427 88 NR-1 species is present Gupta (1988) F 200 B 12° 00’ 76° 45’ 5460 86 NR-1 species is present Gupta (1988) NR-1 (Core) 9.99° 77.92° 5250 28 NR-1 species is missing Banakar et al. (1991) NR-35 11.97° 78.49° 5450 23.5 NR-1 species is missing Banakar et al. (1991) NR-21 11° 78.49° 5325 32 Banakar et al. (1991) V 19-169 10° 13’ 81° 37’ 5110 na Core top contains radiolarians Kolla (1973) V 19-170 7° 54’ 81° 25’ 5218 na Radiolarians in bottom sediment are Kolla (1973) absent SS 2/94 19° 76° 4280 na NR-1 species is missing Iyer et al. (1999)

SS 2/89 14° 01 75° 59 4440 na NR-1 species present Iyer et al. (1997) SS 10/657 13° 59.86 76° 30 4440 37 NR-1 species present Iyer et al. (1997)

SS-667 12.5o 76o 5250 32 - Borole (1993) Core 10o 76o 5400 na - Khadge (2008) BC 37 16° 06 310 75o 26.04” 4251 25 NR-1 species present Nath et al. (2012) SVBC 16o 06 943 75o 25. 083’ 3990 40 NR-1 species present Nath et al. (2012) AAS 2/3 7o 48 80o 5463 512 NR-1 species present Gupta (2009)

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Table 7.2 Down-depth variations in the abundance (%) of the components in the sediment core AAS-22/5. Depth Palagonite Phillipsite Micro- Glass Biota (cm) grains grains nodules shards 250-255 73.25 1.15 7.79 7.12 10.11 255-260 73.59 1.75 7.45 5.95 10.75 260-265 73.68 2.32 8.84 3.26 11.89 265-270 72.09 1.54 14.01 2.47 9.89 270-275 76.42 1.97 12.45 1.42 7.75 275-280 74.71 1.6 13.13 1.17 9.39 280-285 76.92 1.87 15.27 1.54 4.4 285-290 78.74 1.12 14.74 1.8 3.6 290-295 80.65 1.96 11.64 1.38 4.38 295-300 80.46 1.61 12.3 1.61 4.02 300-305 85.05 2.07 10.69 0.85 1.46 305-310 83.33 1.55 11.9 1.07 2.14 310-315 79.64 1.02 16.38 0.8 2.16 315-320 77.86 0.78 16.13 0.33 4.89 320-325 79.55 1.25 14.66 0.57 3.98 325-330 82.16 0.94 12.09 0 4.81 330-335 80.18 0.57 15.23 0 4.01 335-340 78.21 1.12 16.87 0 6.17 340-345 79.64 0.68 16.95 0 2.73 345-350 78.48 2.4 14.08 0 3.7 350-355 76.42 2.4 14.08 0 7.1 355-360 77.01 1.43 15.73 0 5.83 360-365 80.55 1.04 13.58 0 4.83 365-370 79.28 2.6 14.16 0 3.96 370-375 77.35 2.1 15.36 0 5.19 375-380 74.39 2.44 17.64 0 5.53 380-385 74.87 1.71 15.61 0 7.81 385-390 75.68 1.08 14.59 0 8.65 390-395 73.45 1.99 16.68 0 7.87 395-400 76.00 1.52 14.77 0 7.71 400-405 76.00 1.95 17.48 0 4.46 405-410 74.71 1.17 19.32 0 4.8 410-415 76.00 1.95 16.5 0 5.54 415-420 78.56 1.68 14.48 0 5.27 420-425 74.71 1.92 17.82 0 5.55 425-430 77.61 1.66 15.52 0 5.21 430-435 76.84 1.32 17.56 0 4.28 435-440 78.04 1.67 16.83 0 3.46 440-445 74.79 2.56 17.41 0 5.24 445-450 72.69 2.7 19.31 0 5.3 450-455 70.99 2.33 21.81 0 4.87 455-460 77.52 1.11 18.27 0 3.1 460-465 77.35 0.99 18.78 0 2.87 465-470 76.59 1.86 18.27 0 3.28 470-475 78.04 1.34 16.95 0 3.68 475-480 71.14 2.64 23.68 0 2.54 480-485 73.92 3.7 19.96 0 2.43 485-490 75.35 2.8 19.27 0 2.58 490-500 67.5 1.93 27.97 0 2.6

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Table 7.3 Chemical composition of palagonite grains from the CIOB and from the world ocean. 1 to 10 = study area; 11 to 17 = palagonite from CIOB (Iyer 1999); 18 = DSDP Leg 37 (Scarfe and Smith, 1977); 19 = DSDP Leg 37 (Andrews, 1977); 20 = Melson (1973); 21 = Baragar et al. (1977); 22 = Furnes (1978). N=number of points analyzed per grain.

oxides N SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O H2O (wt%) 1 14 51.78 0.17 15.87 5.2 0.03 3.32 2.25 0.61 2.41 18.36 2 11 48.24 0.19 15.71 10.46 0.02 2.76 2.72 0.37 2.02 17.51 3 9 51.67 0.2 13.8 8.77 0.02 3.81 2.93 0.43 1.98 16.39 4 8 51.65 0.43 14.5 6.92 0.02 4.36 1.95 0.4 2.01 17.76 5 4 47.86 0.24 18.65 6.86 0.58 3.59 2.21 0.17 1.51 18.33 6 13 50.11 0.24 16.88 6.14 0.04 3.24 2.26 0.54 2.25 18.3 7 11 51.89 0.2 15.36 9.84 0.15 3.4 2.49 0.7 1.63 14.34 8 12 49.89 0.26 12.84 11.22 0.05 4.52 1.85 0.29 2.61 16.47 9 10 49.64 0.23 14.56 7.56 0.13 3.59 1.64 0.76 2.41 19.48 10 9 52.84 0.16 14.5 7.96 0.02 4.54 1.52 0.64 2.29 15.53 Avg. 10 50.56 0.23 15.27 8.09 0.11 3.71 2.18 0.49 2.11 17.25 11 43.64 0.68 14.8 6.82 3.71 0.97 1.56 5.42 3.74 15.79 12 39.59 2.02 14.29 13.85 1.71 3.41 3.66 2.58 1.97 12.39 13 41.72 1.09 15.34 12.53 0.3 4.81 3.08 2.77 1.91 12.14 14 41.18 1.63 17.84 12.01 0.25 1.84 1.34 2.59 2.82 1.41 15 38.28 1.79 13.16 13.21 3.89 1.49 1.53 2.92 3.18 17.34 16 37.28 1.31 13.86 10.54 6.75 1.52 1.19 3.18 3.01 17.98 17 37.1 1.41 14.08 11.19 7.29 1.58 1.22 3.6 2.94 16.97 18 4 42.05 2.13 11.43 19.31 na 3.95 1.48 0.78 2.73 na 19 6 53.42 2.38 15.6 20.58 0.06 5.15 1.11 0.14 3.49 na 20 3 44.19 2.81 15.42 15.72 na 2.82 0.6 2.14 3.23 na 21 6 41.8 1.9 12.15 19.96 0.02 4.06 0.72 0.99 3.72 na 22 4 37.88 1.21 15.38 11.97 0.21 7.56 5.44 2.12 0.5 10.99

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Table 7.4 Chemical composition of phillipsite crystals from the CIOB. 1 to 7 = my study, 8 = Avg of 7; 9 = Iyer et al. (2018). N = number of points analyzed per grain.

Oxides N SiO2 Al2O3 FeO MgO CaO Na2O K2O H2O (wt%) 1 6 59.35 19.96 1.15 0.39 2.59 1.49 5.07 10 2 5 57.13 21.39 2.73 0.82 2.06 1.26 3.86 10.76 3 4 63.46 20.32 0.97 0.31 1.8 1.43 6.32 5.39 4 5 58.44 19.55 1.24 0.4 1.6 1.32 5.49 11.96 5 8 56.41 20.73 2.47 0.38 1.78 1.38 4.68 12.17 6 4 57.36 20.39 1.25 0.52 1.67 1.88 5.58 11.35 7 7 58.84 20.65 1.65 0.43 2.39 1.47 4.99 9.58 8 7 58.71 20.43 1.64 0.46 1.98 1.46 5.14 10.17 9 58 57.48 19.36 1.64 0.34 0.24 2.48 10.25 8.02

Table 7.5A Down-depth major oxides (wt%) of sediment core AAS-22/5.

Core Details, cm SiO2 Al2O3 TiO2 Fe2O3 MnO MgO CaO Na2O K2O P2O5 AAS 22-5, 0-5 50.60 9.37 0.34 4.28 0.99 2.32 1.48 7.57 2.06 0.31 AAS 22-5, 95-100 48.52 11.16 0.36 5.40 1.28 2.57 1.05 7.18 2.12 0.33 AAS 22-5, 195-200 47.11 12.41 0.46 6.09 1.39 2.91 1.10 7.18 2.13 0.37 AAS 22-5, 245-250 46.60 14.18 0.55 7.02 1.53 3.48 1.17 6.18 2.15 0.43 AAS 22-5, 295-300 43.03 13.82 0.55 6.90 1.55 3.90 1.10 6.88 1.99 0.38 AAS 22-5, 395-400 42.06 13.99 0.50 6.79 1.56 3.71 1.13 5.05 1.83 0.45 AAS 22-5, 495-500 44.57 14.53 0.49 6.46 1.56 4.16 1.20 5.74 2.00 0.54

Table 7.5B Down-depth major oxides (wt%) of sediment core AAS-22/3.

Core Details, cm SiO2 Al2O3 TiO2 Fe2O3 MnO MgO CaO Na2O K2O P2O5 AAS 22-3, 0-5 47.96 10.52 0.36 4.93 2.38 2.51 1.78 8.56 2.48 0.35 AAS 22-3, 45-50 44.09 12.38 0.50 6.59 2.07 2.67 1.41 5.85 2.18 0.63 AAS 22-3, 95-100 44.16 13.21 0.53 7.05 2.14 3.17 1.51 6.83 2.24 0.66 AAS 22-3, 145-150 43.05 14.06 0.57 7.46 2.28 3.35 1.55 5.87 2.32 0.69 AAS 22-3, 195-200 43.77 14.22 0.61 7.49 2.31 3.31 1.84 5.45 2.42 0.71 AAS 22-3, 245-250 42.84 13.86 0.57 7.30 2.33 3.44 1.53 6.10 2.33 0.68 AAS 22-3, 295-300 45.79 13.25 0.53 6.88 2.11 2.99 1.44 6.15 2.40 0.61 AAS 22-3, 345-350 44.24 13.19 0.51 7.03 2.11 3.05 1.52 5.95 2.24 0.66 AAS 22-3, 395-400 44.56 14.62 0.61 7.71 2.35 3.36 1.58 5.21 2.40 0.73 AAS 22-3, 445-450 44.20 14.48 0.62 7.65 2.28 3.26 1.58 5.04 2.47 0.73 AAS 22-3, 495-500 44.42 14.30 0.61 7.55 2.24 3.43 1.43 5.15 2.45 0.63 AAS 22-3, 545-550 45.46 14.80 0.52 7.17 2.28 2.56 1.88 4.73 3.41 1.05

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Table 7.6 Magnetic susceptibility data of core A=AAS-22/5 and B=AAS-22/3.

Core 22/5 (A) LF HF S-ratio ARM SIRM ARM / SIRM SIRM/ klf Magnetite % depth cm (1T) 0-5 8.3310 7.8199 0.9958 0.1663 2.0804 0.0799 0.2497 1.2623 5-10 6.2724 5.3150 0.4865 0.1672 2.5092 0.0666 0.4000 0.9504 10-15 6.8351 6.4357 1.6870 0.1721 0.9790 0.1758 0.1432 1.0356 15-20 6.0057 5.4679 0.9051 0.1832 1.4422 0.1270 0.2401 0.9100 20-25 6.1891 5.6472 1.0259 0.1886 1.3238 0.1424 0.2139 0.9377 25-30 7.3002 6.3797 0.1835 0.2029 7.8210 0.0259 1.0713 1.1061 30-35 28.306 27.869 4.8678 0.2585 1.6662 0.1551 0.0589 4.2888 35-40 7.5258 7.0103 0.9661 0.2379 1.7460 0.1362 0.2320 1.1403 40-45 6.8855 6.1511 1.0140 0.2217 1.5033 0.1475 0.2183 1.0433 45-50 7.6826 7.0814 1.0402 0.2348 1.5546 0.1510 0.2024 1.1640 50-55 7.5636 6.3974 0.8187 0.2141 1.9267 0.1111 0.2547 1.1460 55-60 7.1741 7.0311 1.0504 0.2302 1.5613 0.1475 0.2176 1.0870 60-65 7.0960 7.0655 1.0913 0.2306 1.5118 0.1526 0.2130 1.0751 65-70 8.2581 7.2175 0.9470 0.2303 1.7388 0.1324 0.2106 1.2512 70-75 8.3970 7.2666 0.9132 0.2290 1.8087 0.1266 0.2154 1.2723 75-80 8.3242 7.5924 1.0872 0.2384 1.5579 0.1530 0.1872 1.2612 80-85 8.1964 7.4634 0.8900 0.2352 1.9020 0.1237 0.2321 1.2419 85-90 8.4583 7.1503 1.0209 0.2681 1.6238 0.1651 0.1920 1.2816 90-95 8.0674 7.1710 1.4583 0.3075 1.1349 0.2709 0.1407 1.2223 95-100 9.0133 6.8422 0.9916 0.3318 1.7657 0.1879 0.1959 1.3656 100-105 8.7093 7.7994 0.9923 0.2644 1.7253 0.1532 0.1981 1.3196 105-110 8.6221 7.5247 0.9977 0.2996 1.7047 0.1757 0.1977 1.3064 110-115 8.2861 7.1994 1.0013 0.2899 1.6120 0.1798 0.1945 1.2555 115-120 8.2049 6.6685 0.9970 0.2925 1.5552 0.1881 0.1895 1.2432 120-125 13.138 11.643 1.0044 0.2964 2.4279 0.1221 0.1848 1.9906 125-130 7.4565 6.5309 0.9815 0.2878 1.4948 0.1925 0.2005 1.1298 130-135 8.1294 7.4301 0.9853 0.3144 1.6461 0.1910 0.2025 1.2317 135-140 8.3660 7.2204 1.0091 0.3002 1.6093 0.1866 0.1924 1.2676 140-145 6.9223 6.8309 0.9883 0.2790 1.5612 0.1787 0.2255 1.0488 145-150 7.4189 6.7502 1.0117 0.2459 1.5525 0.1584 0.2093 1.1241 150-155 7.2617 6.2739 0.9950 0.2288 1.4474 0.1581 0.1993 1.1003 155-160 7.3964 6.7064 0.9942 0.2491 1.5631 0.1594 0.2113 1.1207 160-165 6.4472 5.7572 0.9920 0.2129 1.3295 0.1601 0.2062 0.9769 165-170 6.5827 5.7435 0.9986 0.2140 1.3603 0.1573 0.2066 0.9974 170-175 6.8513 6.1467 0.9958 0.2182 1.3838 0.1577 0.2020 1.0381 175-180 8.0157 7.0792 0.9961 0.2511 1.5822 0.1587 0.1974 1.2145 180-185 6.4292 5.9891 0.9901 0.2156 1.3653 0.1579 0.2124 0.9741 185-190 8.2765 7.4040 0.9922 0.2658 1.6818 0.1580 0.2032 1.2540 190-195 6.5626 6.4705 0.9818 0.2243 1.4972 0.1498 0.2281 0.9943 195-200 7.6308 6.7631 0.9896 0.2406 1.5899 0.1513 0.2084 1.1562 200-205 7.4825 6.6036 0.9819 0.2392 1.5841 0.1510 0.2117 1.1337 205-210 7.4397 6.7826 0.9928 0.2457 1.5940 0.1542 0.2142 1.1272 210-215 8.3195 6.8977 0.9753 0.2193 1.5698 0.1397 0.1887 1.2605 215-220 6.8378 6.0980 0.9811 0.1862 1.3767 0.1352 0.2013 1.0360 220-225 7.2993 6.6249 0.9999 0.2212 1.4967 0.1478 0.2050 1.1060 225-230 7.0943 6.6069 0.9848 0.2375 1.5033 0.1580 0.2119 1.0749 230-235 7.5729 6.7005 1.0058 0.2090 1.4946 0.1398 0.1974 1.1474

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235-240 7.8570 6.8155 0.9992 0.2167 1.5192 0.1426 0.1934 1.1905 240-245 7.7569 7.2165 1.0013 0.2833 1.5825 0.1790 0.2040 1.1753 245-250 7.6825 6.7967 0.9543 0.2166 1.5787 0.1372 0.2055 1.1640 250-255 7.4269 6.7595 0.9813 0.2128 1.5376 0.1384 0.2070 1.1253 255-260 6.9027 6.6722 0.9788 0.2402 1.5279 0.1572 0.2214 1.0459 260-265 6.9921 6.3565 0.9909 0.2250 1.4894 0.1510 0.2130 1.0594 265-270 7.0158 6.3551 0.9912 0.2270 1.4691 0.1545 0.2094 1.0630 270-275 6.8518 6.1853 1.0065 0.2208 1.4179 0.1557 0.2069 1.0382 275-280 7.1380 6.2108 0.9859 0.2260 1.4625 0.1545 0.2049 1.0815 280-285 7.0045 6.4277 0.9902 0.2233 1.4498 0.1540 0.2070 1.0613 285-290 7.4276 6.6896 0.9802 0.2261 1.4829 0.1525 0.1996 1.1254 290-295 7.0447 6.4289 1.0130 0.2143 1.4375 0.1491 0.2041 1.0674 295-300 6.9815 6.1628 0.9881 0.2109 1.4070 0.1499 0.2015 1.0578 300-305 6.9908 6.3000 0.9935 0.2080 1.3875 0.1499 0.1985 1.0592 305-310 6.8638 6.2204 0.9874 0.2184 1.4289 0.1528 0.2082 1.0400 310-315 6.8056 6.0946 0.9758 0.2133 1.3814 0.1544 0.2030 1.0312 315-320 6.9183 6.0264 0.9982 0.2272 1.4092 0.1612 0.2037 1.0482 320-325 6.3833 5.7969 1.0021 0.2094 1.3178 0.1589 0.2065 0.9672 325-330 6.5887 6.0432 0.9954 0.2140 1.3429 0.1594 0.2038 0.9983 330-335 6.5637 6.1231 0.9812 0.2102 1.3567 0.1549 0.2067 0.9945 335-340 6.7522 6.1464 1.0103 0.2132 1.3619 0.1566 0.2017 1.0231 340-345 6.9772 6.0264 0.9780 0.2157 1.3675 0.1577 0.1960 1.0571 345-350 7.0092 6.2754 0.9872 0.2212 1.3830 0.1599 0.1973 1.0620 350-355 6.4391 5.9197 1.0009 0.2286 1.3935 0.1640 0.2164 0.9756 355-360 6.9293 6.2384 1.0027 0.2375 1.4518 0.1636 0.2095 1.0499 360-365 6.8034 6.3011 1.0024 0.2275 1.4089 0.1614 0.2071 1.0308 365-370 6.6716 6.1975 1.0068 0.2271 1.4350 0.1583 0.2151 1.0108 370-375 6.6600 6.1568 0.9928 0.2324 1.4095 0.1649 0.2116 1.0091 375-380 6.5666 6.0708 0.9930 0.2238 1.4090 0.1589 0.2146 0.9949 380-385 6.5485 5.9617 0.9972 0.2350 1.4239 0.1651 0.2174 0.9922 385-390 6.7031 5.9359 0.9924 0.2292 1.3881 0.1651 0.2071 1.0156 390-395 6.3323 5.7011 1.0065 0.2290 1.3738 0.1667 0.2169 0.9594 395-400 6.3459 5.8639 1.0080 0.2295 1.4220 0.1614 0.2241 0.9615 400-405 6.2831 5.7210 0.9822 0.2224 1.3894 0.1601 0.2211 0.9520 405-410 6.5167 5.7680 0.9856 0.2224 1.4096 0.1578 0.2163 0.9874 410-415 5.7310 5.1912 1.0038 0.1995 1.2546 0.1590 0.2189 0.8683 415-420 6.3377 5.9693 0.9891 0.2221 1.3731 0.1617 0.2167 0.9603 420-425 6.7935 6.0209 0.9919 0.2268 1.3880 0.1634 0.2043 1.0293 425-430 6.4496 5.8165 1.1198 0.2246 1.2268 0.1831 0.1902 0.9772 430-435 6.3524 5.8390 1.0078 0.2226 1.3412 0.1660 0.2111 0.9625 435-440 6.3789 5.9461 1.0007 0.2279 1.3417 0.1698 0.2103 0.9665 440-445 6.2080 5.6052 1.0018 0.2183 1.3053 0.1672 0.2103 0.9406 445-450 6.2538 5.7487 0.9893 0.2182 1.3364 0.1633 0.2137 0.9475 450-455 6.3268 5.7679 0.9851 0.2125 1.3336 0.1594 0.2108 0.9586 455-460 6.0289 5.6050 1.0290 0.2094 1.2756 0.1642 0.2116 0.9135 460-465 6.2013 5.6845 1.0099 0.2121 1.3004 0.1631 0.2097 0.9396 465-470 6.2074 5.5819 1.0004 0.2138 1.3085 0.1634 0.2108 0.9405 470-475 6.4159 5.6791 1.0130 0.2139 1.3298 0.1608 0.2073 0.9721 475-480 5.9671 5.6736 1.0136 0.2102 1.3148 0.1599 0.2203 0.9041 480-485 6.2156 5.7569 1.0059 0.2098 1.2964 0.1618 0.2086 0.9418 485-490 5.9486 5.5622 1.0121 0.2020 1.2562 0.1608 0.2112 0.9013 490-500 6.0957 5.6857 1.0003 0.2120 1.3309 0.1593 0.2183 0.9236

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Core 22/3 ARM / SIRM/ LF HF cfd S-ratio ARM SIRM Magnetite (B) SIRM klf depth cm % (1T) % 0-5 5.7310 5.5290 3.5256 0.9776 0.1385 1.1640 0.1190 0.2031 0.8683 5-10 7.6945 7.0723 8.0863 0.9643 0.2052 1.6398 0.1251 0.2131 1.1658 10-15 8.1050 7.3866 8.8643 0.9716 0.2127 1.7193 0.1237 0.2121 1.2280 15-20 9.3452 8.5073 8.9655 0.9877 0.2445 1.9086 0.1281 0.2042 1.4159 20-25 10.2309 9.0617 11.4286 0.9806 0.2617 2.0762 0.1260 0.2029 1.5501 25-30 10.7873 9.6505 10.5376 0.9825 0.2884 2.2534 0.1280 0.2089 1.6344 30-35 10.4240 9.7681 6.2921 0.9710 0.2895 2.2145 0.1307 0.2124 1.5794 35-40 11.0110 9.7717 11.2554 0.9736 0.2977 2.2649 0.1315 0.2057 1.6683 40-45 10.7775 9.9170 7.9840 0.9671 0.2853 2.2146 0.1288 0.2055 1.6330 45-50 10.5583 9.5814 9.2527 0.9635 0.2780 2.1582 0.1288 0.2044 1.5997 50-55 11.0855 9.7528 12.0219 0.9644 0.2813 2.2169 0.1269 0.2000 1.6796 55-60 8.9217 8.0513 9.7561 0.9680 0.2239 1.7679 0.1267 0.1982 1.3518 75-80 11.4073 10.0493 11.9048 0.9710 0.2890 2.2128 0.1306 0.1940 1.7284 80-85 10.4184 9.5072 8.7459 0.9679 0.2759 2.0886 0.1321 0.2005 1.5785 85-90 13.1492 11.9670 8.9905 0.9589 0.3291 2.5680 0.1282 0.1953 1.9923 90-95 10.2147 9.4642 7.3469 0.9826 0.2704 2.0790 0.1301 0.2035 1.5477 95-100 10.6920 9.8419 7.9505 0.9569 0.2860 2.1230 0.1347 0.1986 1.6200 100-105 10.4587 9.4165 9.9650 0.9993 0.2747 2.0979 0.1309 0.2006 1.5847 105-110 10.4586 9.4917 9.2450 0.9700 0.2755 2.1012 0.1311 0.2009 1.5846 110-115 10.2043 9.3428 8.4425 0.9793 0.2693 2.0272 0.1328 0.1987 1.5461 115-120 10.3114 9.1249 11.5068 0.9773 0.2649 2.0039 0.1322 0.1943 1.5623 120-125 10.9599 9.5422 12.9353 0.9690 0.2759 2.0808 0.1326 0.1899 1.6606 125-130 10.7762 9.8680 8.4277 0.9938 0.2774 2.0770 0.1336 0.1927 1.6328 130-135 10.4494 9.5853 8.2697 0.9871 0.2704 2.0294 0.1333 0.1942 1.5832 135-140 10.4515 9.7587 6.6291 0.9726 0.2748 2.0737 0.1325 0.1984 1.5836 140-145 10.4214 9.3729 10.0606 0.9916 0.2678 1.9793 0.1353 0.1899 1.5790 145-150 9.4662 8.6580 8.5380 0.9804 0.2534 1.8722 0.1353 0.1978 1.4343 150-155 10.3561 9.3503 9.7122 0.9908 0.2624 2.0040 0.1309 0.1935 1.5691 155-160 10.2791 9.3468 9.0695 0.9619 0.2632 2.0200 0.1303 0.1965 1.5574 160-165 10.5487 9.3490 11.3725 0.9792 0.2673 2.0192 0.1324 0.1914 1.5983 165-170 9.4620 8.5012 10.1535 0.9846 0.2445 1.8248 0.1340 0.1929 1.4336 170-175 10.1631 8.9059 12.3698 0.9583 0.2512 1.9137 0.1313 0.1883 1.5399 175-180 10.7619 9.5302 11.4451 0.9880 0.2606 2.0841 0.1251 0.1937 1.6306 180-185 9.6781 8.7295 9.8016 0.9912 0.2427 1.8446 0.1316 0.1906 1.4664 185-190 10.4745 9.6428 7.9402 0.9844 0.2627 2.0156 0.1304 0.1924 1.5870 190-195 9.9724 8.9701 10.0503 0.9814 0.2456 1.8951 0.1296 0.1900 1.5110 195-200 9.5088 8.7612 7.8627 0.9826 0.2466 1.8740 0.1316 0.1971 1.4407 200-205 10.4668 9.2911 11.2332 0.9862 0.2447 1.9613 0.1247 0.1874 1.5859 205-210 10.3262 9.2141 10.7692 0.9734 0.2456 1.9168 0.1281 0.1856 1.5646 210-215 10.3027 9.0930 11.7409 0.9799 0.2418 1.9102 0.1266 0.1854 1.5610

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215-220 9.9923 9.1019 8.9109 0.9877 0.2431 1.9078 0.1274 0.1909 1.5140 220-225 9.5741 8.6688 9.4563 0.9870 0.2390 1.8728 0.1276 0.1956 1.4506 225-230 8.4854 8.6051 11.3229 0.9804 0.2442 1.8614 0.1312 0.2194 1.2857 230-235 9.2540 8.4534 8.6514 1.0081 0.2410 1.8226 0.1322 0.1970 1.4021 235-240 9.2384 8.0876 12.4567 0.9793 0.2438 1.8042 0.1351 0.1953 1.3998 240-245 9.2599 8.2193 11.2374 0.9815 0.2455 1.8358 0.1337 0.1983 1.4030 245-250 9.2883 8.2655 11.0115 0.9798 0.2438 1.8297 0.1332 0.1970 1.4073 250-255 9.0460 8.4335 6.7717 0.9826 0.2346 1.8511 0.1268 0.2046 1.3706 255-260 9.0084 8.1232 9.8266 0.9988 0.2425 1.8438 0.1315 0.2047 1.3649 260-265 8.7042 8.0763 7.2136 0.9756 0.2360 1.7876 0.1320 0.2054 1.3188 265-270 8.9169 42.4921 5.4570 0.9672 0.2388 1.8139 0.1317 0.2034 1.3510 270-275 8.9621 8.1532 9.0253 0.9690 0.2441 1.8275 0.1336 0.2039 1.3579 280-285 9.2320 8.1892 11.2957 0.9725 0.2499 1.8593 0.1344 0.2014 1.3988 285-290 9.1428 8.4049 8.0706 0.9874 0.2436 1.8868 0.1291 0.2064 1.3853 290-295 8.0282 7.4010 7.8125 0.9678 0.2264 1.6948 0.1336 0.2111 1.2164 295-300 8.8611 8.1070 8.5106 0.9690 0.2481 1.8466 0.1343 0.2084 1.3426 305-310 10.4056 9.2563 11.0454 0.9723 0.2838 2.1348 0.1329 0.2052 1.5766 310-315 10.8125 9.6458 10.7900 0.9604 0.2893 2.2258 0.1300 0.2059 1.6383 315-320 9.5528 8.7049 8.8757 0.9684 0.2546 1.9784 0.1287 0.2071 1.4474 320-325 9.2239 8.5919 6.8513 0.9772 0.2616 2.0144 0.1299 0.2184 1.3976 325-330 10.7433 9.6021 10.6227 0.9367 0.2782 2.1511 0.1293 0.2002 1.6278 330-335 9.7493 8.6927 10.8374 0.9481 0.2561 1.9408 0.1320 0.1991 1.4772 335-340 10.9866 9.8741 10.1266 0.9687 0.2853 2.1441 0.1331 0.1952 1.6646 340-345 11.5688 10.3938 10.1563 0.9779 0.2913 2.2060 0.1321 0.1907 1.7528 345-350 11.3348 10.2580 9.5000 0.9486 0.2861 2.2106 0.1294 0.1950 1.7174 350-355 10.9097 9.6547 11.5031 0.9556 0.2855 2.1027 0.1358 0.1927 1.6530 355-360 10.6640 9.6314 9.6831 0.9781 0.2820 2.1199 0.1330 0.1988 1.6158 360-365 10.7328 9.7613 9.0517 0.9199 0.2891 2.3172 0.1248 0.2159 1.6262 365-370 11.1733 9.8455 11.8837 1.0580 0.2809 1.9773 0.1421 0.1770 1.6929 370-375 10.7824 9.7247 9.8089 0.9498 0.2776 2.1059 0.1318 0.1953 1.6337 375-380 10.2517 9.3101 9.1850 0.9774 0.2757 2.0619 0.1337 0.2011 1.5533 380-385 10.4467 9.3403 10.5911 0.9766 0.2749 2.0242 0.1358 0.1938 1.5828 385-390 10.0750 9.3478 7.2180 0.9771 0.2629 1.9570 0.1343 0.1942 1.5265 390-395 10.0388 9.3384 6.9767 0.9727 0.2753 2.0399 0.1350 0.2032 1.5210 395-400 10.0049 9.1712 8.3333 0.9631 0.2670 1.9761 0.1351 0.1975 1.5159 400-405 10.3583 9.1446 11.7166 0.9572 0.2651 1.9634 0.1350 0.1896 1.5694 405-410 9.9832 9.0272 9.5759 0.9627 0.2565 1.9476 0.1317 0.1951 1.5126 410-415 10.0967 9.0777 10.0917 0.9691 0.2589 1.9635 0.1318 0.1945 1.5298 415-420 10.3783 9.1869 11.4793 0.9715 0.2645 1.9967 0.1325 0.1924 1.5725 420-425 10.4180 9.1635 12.0419 0.9737 0.2599 1.9753 0.1316 0.1896 1.5785 425-430 10.2810 9.2916 9.6234 0.9708 0.2664 2.0153 0.1322 0.1960 1.5577 430-435 10.3924 9.3766 9.7744 0.9783 0.2667 2.0505 0.1301 0.1973 1.5746 435-440 10.0360 9.1365 8.9629 0.9761 0.2552 1.9720 0.1294 0.1965 1.5206 440-445 10.1168 9.3438 7.6408 0.9551 0.2578 1.9728 0.1307 0.1950 1.5328 445-450 9.1852 8.5266 7.1703 0.9624 0.2400 1.8284 0.1312 0.1991 1.3917

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450-455 10.1319 9.0288 10.8876 0.9736 0.2529 1.9295 0.1311 0.1904 1.5351 455-460 9.5215 8.1738 14.1542 0.9593 0.2425 1.8828 0.1288 0.1977 1.4427 460-465 9.3513 8.1839 12.4845 0.9822 0.2382 1.8118 0.1315 0.1938 1.4169 465-470 9.3600 8.3400 10.8984 0.9817 0.2422 1.8431 0.1314 0.1969 1.4182 470-475 8.7507 8.0352 8.1761 0.9851 0.2324 1.7461 0.1331 0.1995 1.3259 475-480 8.9593 7.9497 11.2693 0.9712 0.2435 1.8066 0.1348 0.2016 1.3575 480-485 9.3373 8.3793 10.2599 0.9658 0.2429 1.8386 0.1321 0.1969 1.4147 485-490 8.9643 8.0657 10.0244 0.9684 0.2461 1.8248 0.1348 0.2036 1.3582 490-495 9.2029 8.2679 10.1604 0.9864 0.2456 1.8550 0.1324 0.2016 1.3944 495-500 9.2383 8.3339 9.7893 0.9719 0.2457 1.8537 0.1325 0.2006 1.3997 500-505 9.1096 8.4021 7.7670 0.9802 0.2438 1.8619 0.1309 0.2044 1.3802 505-510 8.8817 8.1258 8.5106 0.9801 0.2446 1.8711 0.1307 0.2107 1.3457 510-515 8.3648 7.5548 9.6828 0.9678 0.2322 1.7425 0.1332 0.2083 1.2674 515-520 8.2674 7.5512 8.6626 0.9692 0.2439 1.7493 0.1394 0.2116 1.2526 520-525 8.1826 7.2906 10.9012 0.9719 0.2274 1.6993 0.1338 0.2077 1.2398 525-530 8.5627 7.8054 8.8435 0.9612 0.2495 1.8214 0.1370 0.2127 1.2974 530-535 8.3844 7.5567 9.8720 0.9736 0.2487 1.7413 0.1428 0.2077 1.2704 535-540 8.2112 7.4188 9.6506 0.9798 0.2554 1.7529 0.1457 0.2135 1.2441 540-545 7.6325 6.9243 9.2784 0.9751 0.2490 1.6834 0.1479 0.2206 1.1564 545-550 7.7199 7.0955 8.0882 0.9737 0.2491 1.6682 0.1493 0.2161 1.1697 550-555 7.9987 7.3447 8.1761 0.9723 0.2563 1.7200 0.1490 0.2150 1.2119 555-560 7.7765 7.0846 8.8975 0.9890 0.2473 1.6626 0.1488 0.2138 1.1783

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CHAPTER 8

VOLCANIC GLASS SHARDS AND TEPHROCHRONOLOGY

Chapter 8

8.1 Introduction Volcanic glass shards (henceforth referred as ‘glass shards’ or ‘shards’) are pyroclastic material derived during a violent aerial or submarine magmatic eruption. Occurrence of these in the marine sediments are used to understand the cyclicity of volcanism, volcanic production rates, geochemical character of the source and temporal relations between paleoclimate and volcanism (Ninkovich et al., 1978; Chesner et al., 199; Lowe, 2011; Austine et al., 2014) and as stratigraphic event marker to derive the tephrochronology (Lane et al., 2017). On a larger perspective, the shards effectively help in understanding the global sedimentary system (Scudder et al., 2009, 2016). With the advances in the identification of successive marine tephra layers, it is also equally important to date and correlate these to discern their proximal source. This correlation is mainly achieved through the direct dating of sediments. However, in the marine environment dating gets highly complicated as the tephra layers are not well defined i.e. they are often blended with the sediments (Lane et al., 2012). In such cases indirect methods such as biostratigraphy (Nigrini et al., 2005, Gupta 1988, 1996, 2009), radiogenic U-Th isotope dating (Banakar et al., 1991; Nath et al., 2013) or microtektites (Lee and Wei, 2000; Glass and Koberl, 2006; Prasad et al., 2007; Pattan et al., 2010) have been used to derive the age of the associated sediment.

In the Indian Ocean, the volcanic glass shards that co-exist with the sediments are reported to be derived from the Indonesian Arc volcanism. The Toba volcano located in the Indonesian Volcanic Arc (IVA; Fig.1) has erupted several times in the geological past and contributed to four major and many minor ash-flows.

Haranggoal Dacitic Tuff (HDT), the first large-scale eruption with a total surge of 35 km3 (Chesner and Rose, 1991), was dated to be of 1.2 Ma by fission-track method (Nishimura et al., 1977). This was followed by the Oldest Toba Tuff (OTT) which erupted from the southern part of the Toba caldera and it expelled tephra and dense rock equivalent that spread over 1,000 km3 to 2,300 km3 (Lee et al., 2004; Pearce et al., 2014). Using 40Ar/39Ar dating, Chesner et al. (1991) reported an age of 0.80 Ma for the OTT. These tephra were welded and deposited on marine sediments of Palaeozoic and Mesozoic ages (Knight et al., 1986). The glass shards of the OTT eruption is well preserved in the marine sediments of the Central Indian Ocean Basin (CIOB) (Glass and Koeberl, 2006; Pattan et al., 2010; Iyer et al., 2012; Pearce et al., 2014).

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The Middle Toba Tuff (MTT) with an erupted volume of 60 km3 (Chesner et al., 1991), is limited in aerial extent and has been identified in Leg 121 ODP Site 758 core (henceforth referred to as ODP Site 758, Fig. 8.1). The sanidine phenocrysts that were dated using the 40Ar/39Ar method defined an age of 0.501 ± 0.005 Ma (Diehl et al., 1987; Chesner and Rose, 1991; Mark et al., 2017). Dehn et al. (1991) suggested Layer C (see below for details of layers identified) as MTT while based on δ18O stages Imbrie et al. (1984) suggested an age of 0.513 to 0.538 Ma.

Of all the four eruptions, the Youngest Toba Tuff (YTT) was a super-eruption that resulted in a large-scale dispersion of the ash that covered an area of at least 4x106 km2 (Rose and Chesner, 1987). 40Ar/39Ar method indicated an age of 0.074 Ma i.e. 74 ka for the YTT shards (Ninkovich et al., 1978; Chesner et al.,1991; Storey et al., 2012; Mark et al., 2017). Using age dispersal model, Costa et al. (2014) estimated the YTT fallout to be 8,600 km3 of rhyolitic glass shards that covered an area of ~40 million km2 and the volume of eruptive materials to be 5,800 km3. The trace of this ash fallout in the north-eastern Indian Ocean has been well studied (Ninkovich et al., 1979; Kennett, 1981; Dehn et al., 1991). Later work has revealed dispersal of YTT as far as Antarctica (Song et al., 2000), in the Indian subcontinent (Acharyya and Basu, 1993; Shane et al., 1995; Westgate et al., 1998), CIOB (Iyer et al., 1997; Pattan et al., 1999, 2010; Pearce et al., 2014), Bay of Bengal (Gasparotto et al., 2000); Arabian Sea (Nambiar and Sukumaran,2002),and South China Sea (Wiesner et al., 1995; Song et al., 2000; Bühring et al., 2000; Liu et al., 2007).

The YTT eruption occurred near the marine isotope stage 5-4 boundary (Schulz et al., 2002) and led to global cooling (Oppenheimer, 2002; Williams et al., 2009). Rampino and Self (1993) suggested that the super-eruption of Toba resulted in a volcanic winter and accelerated glaciations but this was contested by Schulz et al. (2002). On the contrary, those authors opined that the Toba eruption may have had a minor impact on the evolution of the low-latitude monsoonal climate. The Toba eruption perhaps affected the tropical ecosystem, increased deforestation and caused a near extinction of humans (Rampino and Self, 1993; Gathorne-Hardy and Harcourt-Smith, 2003; Williams et al., 2009).

Apart from these major eruptions, the IVA has also resulted in minor events of lesser intensity. The ODP Site 758 (5°23.049' N, 90°21.673' E, water depth 2923.6 m, Fig. 8.1) is considered as a repository for glass shards derived from the IVA. To-date, 15 layers i.e., Layers A to M, and d and h (Dehn et al. 1991) and another layer of 8 Ma (Padmakumari and

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Ahmad, 2004) give an insight into the cyclic volcanism of Toba that occurred over millions of years. The sequential deposition of Toba tephra at this site draws attention for a comparative study. Besides this ODP Site 758, there is no study concerning the lateral spread of these minor eruptions in the Indian Ocean.

The objective of this chapter is to understand the distribution of glass shards in the CIOB. The long unsolved understanding of whether the silicic glass shards occurring in the CIOB are solely from the IVA, especially Toba eruptions or is there a possibility of their in- situ formation has been dealt in this chapter. The other objective is to obtain a first-hand tephrochronology for the glass shards found in the basin.

8.2 Earlier studies of the CIOB Glass Shards Literature review was carried out to understand the work existing concerning glass shards in the CIOB and these reports suggest that glass shards in the basin are either of ex-situ or in-situ origin. The source for ex-situ glass shards has been ascribed to the Toba volcano (IVA) (Martin-Barajas and Lallier-Verges, 1993; Mascarenhas-Pereira et al., 2006; Glass and Koeberl, 2006; Pattan et al., 2010). So far, only the OTT and YTT are reported from CIOB (Glass and Koeberl, 2006; Pattan et al., 2010; Iyer et al., 2012; Pearce et al., 2014). In contrast, there are limited reports of the in-situ or intra-basin formation of glass shards in the CIOB (Iyer et al., 1997). Mascarenhas-Pereira et al. (2016) have reported in-situ strombolian- type glass shards that correspond to 85, 107-109, and 142-146 ka and a cryptotephra layer of 34 ka. The details of the literature review with respect to glass shards are given in Table 8.1 and locations are plotted in Fig. 8.1.

8.3 Study area Selection of the study area was made based on the literature review and the knowledge gap so as to understand the distribution of glass shards in the CIOB. To achieve this, four gravity sediment cores were examined (Table 8.1). These cores were sampled along an N-S transect line (79°E FZ) to yield better coverage and understand the stratigraphic continuity of glass shards (Fig. 8.1).

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Fig. 8.1 (Left panel) Map showing the major physiographic features of the CIOB. The referred site ODP 758 has been marked in red solid star symbol. The Toba caldera is marked with a red solid dot. (Right panel) The detailed map of the study area with locations of studied sediment cores are marked as solid deep red stars while the referenced sites are marked with a black solid dot (details are provided in Table8.1).The siliceous sediments occur between 5°S and 15°S while the pelagic sediments occur below 15ºS and upto 25ºS in the CIOB

8.4 Results and Interpretations A total of 434 coarse fractions (CF) obtained (see Chapter 2 for details) from four gravity cores were observed under a binocular microscope. The morphology and down depth variations in the CF of each core are discussed below.

8.4.1 Core descriptions: The northernmost core AAS-22/8, located in the siliceous sediment domain (Fig. 8.1), throughout its length is moderate yellowish-brown (10YR 5/4). Lenses of dark yellowish-brown (10YR 4/2) occur at a core depth of 38-40 cm (below seafloor). FeMn nodules (smooth, nearly rounded and up to 2 cm diameter) were present at a depth of 245-250 cm. The core AAS-22/7 (Fig. 8.1) is located 330 km south of core AAS- 22/8 and has a similar textural appearance and does not show much variations in terms of physical properties. The other two cores were retrieved from the transition of siliceous-red

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Chapter 8 clay (AAS-22/5; Fig. 8.1) and from red clay (AAS-22/3; Fig. 8.1). Core AAS-22/5 has a moderate brown colour (5YR 4/4) throughout with FeMn micronodules (~1 cm) occurring at a depth of 20-30 cm. The southernmost core AAS-22/3 (Fig. 8.1) has a deep brown colour (5YR 3/4) with nodules (size 1-2 cm) occurring at depths of 0-10 cm and 40-45 cm.

Binocular microscopy of >63 µm CF revealed an assemblage of components mainly radiolarians, FeMn micronodules (<1 cm size), basaltic fragments, palagonite grains, volcanic glass shards, microtektites, pumice pieces, bread crust (oxidised basaltic glass), magnetite spherules, biotite flakes and phillipsite crystals (Amonkar and Iyer, 2017). The plot of CF against core depth (Fig. 8.2) portrays the down-core variations. Fluctuations in the major components such as radiolarians, glass shards, micronodules, and palagonite grains correlate well with microscopy observations. Radiolarians are over-whelming in the northern cores (AAS-22/7 and AAS-22/8) but are substantially less in the southern cores (AAS-22/3 and AAS-22/5). Glass shards were observed in high abundance in a few fractions, and these are referred to as SRH (shard-rich horizons). Core details and the identified depths at which the SRH occur are listed below together with the associated components (Table 8.2).

Fig. 8.2 Down-core variation of CF% in the four studied cores of the CIOB. Legends: N= FeMn micronodules; G=Glass shards; V=Volcanic material; P=Palagonite grains.

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Core AAS-22/8: A linear trend is observed in CF% up to 100 cm with values ranging from 2 to 3% while below 100 cm, the CF shows increasing trends (Fig. 8.2). Distinct peaks (with CF ~6-7%) occur at core depths of 210-230, 430-435 and 520-530 cm and relatively smaller peaks at 135-140, 345-350 and 365-370 cm. Most of these peaks (with CF ~3.5-5%) coincide with SRH, while high CF% at depths 345-350 and 365-370 is mainly due to FeMn micronodules. The microtektites occurring at depth 205-210 cm are confirmed to be from the Australasian tektite strewn field (0.77 Ma) (Prasad et al., 2007).

Six SRH occur at depths 135-140, 190-195, 205-210, 430-435, 485-490 and 520-525 cm (Table 8.2; Fig. 8.3). Glass shards in most of these SRH are colourless and have morphology of cuspate and Y-shaped intersections while those at depths 485-490 and 520-525 cm are stubby/blocky with a smooth surface.

Core AAS-22/7: Drastic fluctuations are observed in CF% down-core with values ranging from 10 to 2%. The entire core is dominated by radiolarians, while abundant Fe-rich spherules and fine basaltic material occur in the top 5 cm and at depths 280-355 and 470-490 cm (discussed in Chapter 6). Two distinct SRH occur at depths of 0-5 and 220-225 cm (Table 8.2). The latter SRH is associated with abundant microtektites that are light green to pale yellow in colour, rounded to sub-rounded in shape and was easily distinguished by their pitted/cratered surface. These microtektites at a depth of 220-225 cm correspond to the Australasian tektite strewn field (Prasad et al., 2007).

Core AAS-22/5: The down-core plot of CF% shows a higher CF in the top 5 cm of the core (10%) but reduces between 5 and 100 cm (~3%) while below 100 cm till 290 cm the CF reduces drastically (5 to <1%) (Fig. 8.2). The broad peak between depths 290-370 cm is due to plentiful palagonite grains and FeMn micronodules. Three SRH occur at depths 10-15, 110 -115, and 245-250 cm (Table 8.2), of which the SRH occurring at depth 110-115 cm is associated with microtektites (Prasad et al., 2007).

Core AAS-22/3: Compared to all the other cores mentioned above, CF% in this core is very low (mostly 2-3%). The CF is slightly higher in the surface with few peaks occurring at depth 280-300 (4%), 430-435 (3.8%), and 480-485 cm (4.1%) and these are ascribed to the presence of FeMn micronodules (Fig. 8.2). Eight SRH occur at depths of 10-15, 60-65, 170-175, 295- 300, 325-330, 335-340, 350-355 and 365-370 cm.

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A significantly higher number of glass shards are observed in the top section of all the three cores occurring at 0-5 cm in AAS-22/7 (4593 number), 10-15 cm in AAS-22/5 (3044) and 10-15 cm in AAS-2/3 (4973) (Table 8.2). A similar abundance is observed in AAS-22/8 at depth 205-210 cm (4607), AAS-22/7 220-225 cm (4170), AAS-22/5 110-115 cm (2915), and AAS-22/3 60-65 cm (3322) suggesting that the shards could be products of major volcanic eruptions.

8.4.2 Morphology of shards (Shape, Size, Texture) Shape, size and texture are important parameters to understand the distribution, formation, and origin of the shards. In the present study, based on the morphological observations (Table 8.3) the glass shards are classified into two types. Shards with cuspate and Y-shaped intersections and containing vesicles (Fig. 8.3A) are classified as type (I) while shards with smooth-surfaced and blocky morphology and devoid of vesicles are classified as type (II) (Fig. 8.3B). The glass shards range in size from 30 to 720 µm.

Fig. 8.3 Electron micrographs of the volcanic shards. A:(Type I) cuspate and with Y- shape intersections. B: (Type II) platy and blocky.

8.4.3 Chemical Composition of glass shards SEM-EDS analysis of about 200 glass shards from 20 SRH shows a high content of

SiO2 (77 to 79 wt. %) with high total alkali (Na2O+K2O ~8-9 wt %). Average values of multiple shards occurring within each SRH and the standard deviation are given along with chemical composition (Table 8.3). The identified SRH consist of shards of rhyolitic composition, irrespective of the core depth at which they occur. Similar observations were

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Chapter 8 reported for shards occurring in other core sites in the CIOB (Iyer et al., 1997; Pattan et al., 1999, 2010; Mascarenhas-Pereira et al., 2016).

The compositional data of the shards from the four cores were compared with their relative equivalent layers (Table 8.4) as reported by other workers and plotted on the TAS diagram (Fig. 8.4).

Fig.8.4 The chemical composition of SRH plotted on the TAS diagram show shards from the four cores from the CIOB to be rhyolitic

The nomenclature fields are after LeBas et al. (1986). Abbreviations: Ph=Phonolite; TPh=Tephriphonolite; PhT=Phonotephrite, T=Tephrite; Ba=Basanite; PBas=picrobasalt; TrBas=Trachybasalt; BaTrAnd=Basaltic trachyandesite; TrAnd=Trachyandesite; BasAnd=Basaltic andesite. The compositions of the shards from the study area (red solid symbols) are compared with their equivalent layers (black and white symbols dots). See Tables 8.3 and 8.5 for sources of data.

8.4.4 Sedimentation rate and age of the cores The rate of sedimentation and chronological dates for the CIOB sediments have been determined using sequential biostratigraphy and radiolarians proxies (Gupta 1988, 1996, 2009) and U-Th isotopic ratio (Banakar et al., 1991; Nath et al., 2013 and references therein). Besides these methods, microtektites occurring within the sediment cores have also been used to obtain the sediment age and sedimentation rate. Microtektites occurring in the present study, are from the youngest Australasian tektite strewn field (0.77 Ma), which is one of the largest (covers an area of over more than 50 million km2) of the four major fallouts during the Cenozoic times and well-identified in the CIOB (Kunz et al., 1995; Prasad et al., 2010, 2013; Mark et al., 2017). This microtektite layer have been used as a chronological marker horizon in the CIOB, to calculate the sedimentation rates (e.g., Glass and Koberl, 2006; Prasad et al.,

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2007; Pattan et al., 2010). A similar approach of using the Australasian microtektites was made by Lee et al. (2003) for three deep-sea cores along an east-west transaction across the South China Sea. These cores were located 1800-2500 km away from the Toba caldera.

Microtektites observed in my cores at depths 205-210 cm (AAS-22/8), 220-225 cm (AAS- 22/7), 110-115 cm (AAS-22/5) and 60-65 cm (AAS-22/3) are compared with the earlier reported depths for the same reported cores (cf. Prasad et al., 2007). The identified depths match well indicating that these depths represent the datum plane of 0.77 Ma age. The occurrence of microtektites at a shallower depth along the transect line from north to south could be due to the variation in the sedimentation rate from north to south in the basin (Udintsev, 1975; Banakar et al., 1991; Borole, 1993; Gupta et al., 1988; Nath et al., 1989, 2001).

Pattan et al. (2010) reported the occurrence of OTT shards with the microtektites layer at a depth of 60-65 cm and YTT at 10-15 cm, as also observed in the presently studied core (AAS- 22/3). Those authors opined that in the Indian Ocean, the occurrence of these two layers i.e. OTT and the microtektites are merged and hence could be used as a marker. Based on this, they estimated a sedimentation rate of 1.7 mm/ka (above YTT) and 0.7 mm/ka (YTT to OTT). Based on the above evidence, microtektites have been used to calculate the sedimentation rate for the four cores that I have studied.

The calculated sedimentation rate for the core AAS-22/8 is 2.69 mm/ka microtektites co- occurs with glass shards (OTT) at a depth of 205-210 cm. Sedimentation rate for the core AAS-22/7 is calculated to be 0.34 mm/ka up to YTT and 3.1 mm/ka from YTT to OTT (Table 8.5). The high sedimentation rate is supported by a high content of CF, and abundance of radiolarian and diatoms in the northernmost cores indicates this area to be influenced by equatorial upwelling. The sedimentation rates for the southern core AAS-22/5 is 1.68 mm/ka up to YTT and 1.43 mm/ka from YTT to OTT and for the core AAS-22/3 the rates are 1.68 mm/ka from the surface to a depth of 15 cm (YTT) and 0.718 mm/ka between YTT and OTT. The calculated values for the core AAS-22/3 closely match with those of 1.7 mm/ka (above YTT) and 0.7 mm/ka (YTT to OTT) (cf. Pattan et al., 2010).

Sedimentation rates in the CIOB have also been calculated by other methods. Goldberg and Koide (1963) recorded a sedimentation rate of 2.75 mm/ka using Ionium/Thorium methods for the Indian Ocean sediments while Banakar et al. (1991) and Borole (1993) determined a rate of 1-5 mm/ka using the 230Th method. Based on radiolarian biostratigraphy,

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Gupta (1991) reported a sedimentation rate of 2.5mm/ka for the CIOB sediments, while 230Th decay studies indicated the rate of sedimentation to be 2.3 mm/ka along 10°S to 14°S latitudes and 75°E longitude (Mascarenhas-Pereira et al., 2006). Therefore, it is observed that the sedimentation rates in the CIOB vary between 1 and 5 mm/ka and results of the present study are well within these rates (Table 8.1). Hence, the derived ages indicates that the presence of mictotektites is an excellent proxy for deep-sea sediments.

8.4.5 Tephrochronology of the CIOB Shards

Based on the global distribution of the YTT (Lane et al., 2017), its occurrences in the CIOB (Pearce et al., 2014), distinct glass morphology (cuspate and Y-shape intersections) and rhyolitic chemical composition, perhaps the SRH in cores AAS-22/7 (0-5 cm) and AAS-22/5 (10-15 cm) could be equivalent to the YTT (as would be discussed later). This fact is also attested by a synthesis of the previous studies (Table 8.1). Based on the literature review it is clear that the YTT occurs at a depth <35 cm in the CIOB sediments irrespective of the sedimentation rates. Also, there has been no major volcanic event in either in the vicinity of the CIOB or in-situ that may have contributed the glass shards to the surface sediments. This fact further confirms that the SRH present above 35 cm are YTT.

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Fig. 8.5 Schematic section of the core showing occurrence of SRH at different depths. Cores AAS-22/8 and AAS-22/7 are sited in the siliceous domain and are pale yellow while cores AAS-22/5 and AAS-22/3 are from the transition of red clay and red clay and are deep- red. SRH (Shard-Rich Horizons) identified in the cores are depicted here. The Layer E (OTT) along with microtektites is used as a datum plane to calculate the sedimentation rate. Linear extrapolations of the rate of sedimentation have helped to compare the identified SRH with tephrochronology of ODP Site 758 (cf. Dehn et al., 1991). The two SRH (blue) occurring in core AAS-22/8 with type II shards could be of an intra basinal origin while the other SRH are derived from Toba eruptions.

Core AAS-22/8: In this core six SRH were identified at depths of 135-140, 190-195, 205-210, 430-435, 485-490 and 520-520 cm (Fig. 8.5). The calculated sedimentation rate is 2.69 mm/ka. Based on the sedimentation rates, SRH occurring at depth of 135-140 cm corresponds to an age of 0.502-0.520 Ma (Table 8.5). This age is compared with that given by Dehn et al., (1991) and the closest best fit is with the MTT (0.49 to 0.538 Ma). There are a few studies that pertain to the MTT (also known as Layer C) at ODP Site 758 in the Indian Ocean (Imbrie et al., 1984; Chesner, 1988; Dehn et al., 1991). This Layer C (MTT) that occurs between the δ18O stage 13.2 and 14.2 was assigned an age of 0.49 to 0.538 Ma (Chesner, 1988; Dehn et al., 1991). So far there are no reports of MTT layer in the CIOB and this finding could probably be the first report of MTT in the CIOB. The absence of information related to MTT in the CIOB could be because of restricted sampling for sediments, scarcity of long cores (≥ 5 m long) and low sedimentation rates. The occurrence of Layer C (MTT) only in AAS-22/8 further supports the fact that during the MTT eruption the volume of the ash surge must have been less and this has been reflected in the shard counts (Table 8.2). The SRH occurring between 190 and 195 cm correspond to an age of 0.706-0.725 Ma and could be similar to Layer D (0.731-0.750 Ma) of ODP Site 758. Since this Layer D was deposited subsequent to OTT (0.840 ± 0.30) within a short time span, it was believed that some of these layers may have been merged (Pattan et al., 2010) and hence not traceable in the CIOB sediment cores.

In ODP Site 758, the Layer E occurs in the middle of δ18O stage 21 and with an age of 0.775 Ma is closest to that of OTT (0.840 ± 0.30 Ma; Diehl et al., 1987). Further, Pattan et al. (2010) used the age of microtektites and suggested the co-occurring glass shards to be 0.77 Ma. The Layer F occurs between δ18O stages 45 and 46 and corresponds to 1.273 and 1.294 Ma (Dehn et al., 1991). Similarly, an age of 1.2 ± 0.16 Ma was determined using fission-track method (Nishimura et al., 1977). These ages match well with the HDT of the IVA. But in the present study, such SRH that coincide with this age is not identified. This suggests that the

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HDT layer may not have been preserved or the ash fallout of the HDT perhaps did not reach the CIOB seafloor due to the very less volume of ash surge (35 km3) that was generated during the eruption (Chesner and Rose, 1991). Interestingly, some of the FeMn oxides-coated pumices in the CIOB are the HDT products of 1.2 Ma (Kalangutkar et al., 2011; 2015). The next successive layer in the core AAS-22/8 was observed at depth 430-435 cm which corresponds to an age of 1.598-1.617 Ma and can be co-relatable with Layer G of ODP Site 758 that is 1.596-1.653 Ma (Dehn et al., 1991) (Fig. 8.5).

In addition to the above-identified horizons, the two identified SRH at depths 485-490 and 520-525 cm, correspond to an age of 1.802-1.821 and 1.933-1.951 Ma, respectively. Only these two layers have a distinct type II morphology (as mentioned above). Further, it was noted that the age calculated for these two SRH does not correlate with those reported for ODP Site 758 (Dehn et al., 1991) indicating that these could be of in-situ origin (Table 8.2).

Core AAS-22/7: Two SRH were identified in this core, the first one in the top 5 cm and the second one at 220-225 cm (Fig. 8.5). The former SRH was identified to be YTT while the latter, associated with microtektites (0.77 Ma) is 0.696-0.712 Ma indicate these to be OTT.

Core AAS-22/5: In this core, three SRH (Fig. 8.5) occur at 10-15, 110-115 and 245-250 cm. The first SRH is the newly recognized YTT and the second SRH is the reported OTT (Prasad et al., 2007). The third SRH between 245 and 250 cm (Fig. 8.4) of 1.713-1.748 Ma is equivalent to Layer G (1.596-1.636 Ma) of ODP758 (cf. Dehn et al., 1991).

Core AAS-22/3: In this core, eight SRH are present at 10-15 (YTT), 60-65 (OTT), 170-175, 295-300, 325-330, 335-340, 350-355 and 360-370 cm (Fig. 8.5). Based on the sedimentation rates the SRH at 170-175 cm (2.367 - 2.436 Ma) could be equivalent to Layer H of ODP Site 758 (cf. Dehn et al., 1991). The SRH at 295-300 (4.108-4.178 Ma) and 325-330 cm (4.526- 4.596 Ma) could be similar to Layers I (4.124-4.131 Ma) and J (4.526-4.596), respectively.

The SRH occurring at depth 335-340 (4.665-4.735 Ma) and 350-355 cm(4.874-4.944 Ma) could be similar to Layers K (4.666-4.667 Ma) and L (4.759-4.765 Ma), respectively of ODP Site 758 (cf. Dehn et al., 1991). While the bottom-most SRH occurring at depth 365-370 cm with an age of 5.083 to 5.153 Ma is equivalent to Layer M of ODP Site 758 (5.062-5.063 Ma; Dehn et al., 1991)

It was noted that the time span covered by the other three studied cores (AAS-22/3, AAS- 22/5, and AAS-22/8) does not exceed 4 Ma and this fact constrains the identification of

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Chapter 8 comparable layers in the CIOB as found in ODP Site 758. To date, the existence of layers H, I, J, K, L, and M was not identified in the CIOB sediments and this could be due to limitations in the recovery of long sediment cores and their locations in the basin.

8.5 Discussion

Morphology, size, and distribution of volcanic pyroclasts such as glass shards and their deposition in a submarine environment can be used as tools to understand the volcanic cyclicity, type of eruption and intensity. The distribution of the glass shards is dependent on the volume of the surge and the extent of lateral spread. Morphology is another characteristic of glass shards, with the shape been an indicator of the type of eruption (aerial or submarine) and the size could also help to understand the distance of travel of the tephra fallout. In the present study, morphology, size and major oxide chemical composition of the glass shard have been used to understand their distribution in the undisturbed sediments of the CIOB. Further, the results were compared with the ODP Site 758.

It is significant that irrespective of the depth of occurrence the glass shards are rhyolitic and have a distinct morphology ranging from cuspate to Y-shaped intersections (Fig. 4A). So far, only two layers (i.e., YTT and OTT) of the total of 16 layers have been reported to occur in the CIOB (Iyer et al., 1997; Pattan et al., 1999, 2010; Pearce et al., 2014). Based on my present study it is evident that the YTT shards are confined up to a core depth of <35 cm irrespective of the rate of sedimentation in the basin. This observation is consistent with the earlier studies (Table 8.1).

It is recognised that the CIOB seafloor is carpeted largely by the YTT but this layer was not observed in the surface sediments of the core AAS-22/8 (Fig. 8.5). The absence of YTT could be due to the erosion of sediments by the Antarctic Bottom Watermass (AABW) that pass through the basin (Lavergne et al., 2017). This is corroborated by the absence of the radiolarian species Collosphaera invaginate from the CIOB surface sediments (Johnson and Nigrini, 1982; Banakar et al., 1991).

Apart from the YTT and OTT, there were no other reports of Toba tephra in the CIOB. The present study has helped to reveal for the first time the existence of other Toba eruptives. The identified SRH have been compared based on chemistry and morphology with the 16 reported layers (Table 8.4) identified in ODP Site 758. The chemical composition of the

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shards shows high SiO2 (77 to 79 wt. %) and high total alkali (Na2O+K2O ~8-9 wt. %) which is typical of Toba eruptives. The TAS diagram (Fig. 8.4) also indicates a rhyolitic source for the glass shards. Standard deviation and the average of each layer do not show much variation in the mean values. The morphology of glass shards suggests a bimodal morphology. Similarly, size analysis carried for glass shards from all the SRH shows a mixed grains size with a range from 30 to 720 µm.

The SRH in the northernmost core AAS-22/8, i.e. Layer C (MTT) and Layer D had a significantly lower count of glass shards compared to the OTT layer. This could be due to the proximity of the cores to the IVA source and the surge output. The layers occurring at depths of 430-435 (AAS-22/8) and 245-250 (AAS-22/5) is related to Layer G of the ODP Site 758. The likely reasons for the absence of Layer G in AAS-22/7 is perhaps because of the high sedimentation rate of 3.1 mm/ka and the limiting age of 1.74 Ma for this 5.5 m long core. This layer was also not observed in the southernmost core AAS-22/3 (5.6 m). The possible reason for the absence of Layer G could be the due to the location of this core is far from the source and the shards may have not have travelled so far (17⁰ S).

Though all the four cores are at least 5 m long, the northern two cores have maximum ages of 2.08 Ma (AAS-22/8) and 1.74 Ma (AAS-22/7) compared to the southern cores that are of 3.49 Ma (AAS-22/5) and 7.79 Ma (AAS-22/3). This suggests that the northern part of the study area witnesses higher sedimentation input, thereby making it difficult to identify SRH beyond the age of the core i.e. layers H to M. On the other hand, the southern cores with low sedimentation rates have a better probability of occurrence and preservation of the other layers as found in ODP Site 758 (Dehn et al., 1991). The SRH occurring at depth 170-175 cm (2.367-2.436 Ma) is equivalent to layer H (2.238-2.265 Ma). Similarly, SRH occurring at depth 295-300 (4.108-4.178 Ma) is correlated with Layer I (4.124-4.131 Ma), and the SRH at depth 325-330 (4.526-4.596 Ma) is equivalent to Layer J (4.552-4.560 Ma). The next well preserved SRH identified at depth 335-340 (4.665-4.735 Ma) is correlated with Layer K (4.666-4.667 Ma).The oldest two layers occurring at depth 3350-355 (4.87-4.944 Ma) and 365-370 cm (5.083-5.153 Ma) is correlated with Layer L (4.759-4.765) and M (5.062-5.063 Ma), respectively.

The identified rhyolitic glass shards from the present study with type I morphology is compared compositionally with its equivalent layers occurring in the Indian Ocean. The SRH occurring in the top section of the cores were evaluated with an average of 92 YTT shards

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Chapter 8 reported by Pattan et al. (1992). Though the reports about the occurrence of the MTT are very few, the SRH layer identified as MTT was further compared with an average of 18 shards (cf. Westgate et al., 1988), the OTT shards with an average of 19 glass shards (cf. Pattan et al., 2010) and shards in Layers D, G to M with ODP Site 758 (cf. Dehn et al., 1991). The chemical composition of the referenced tephra layers (Table 8.4) distinctly corroborates with those identified using sedimentation rates and microtektites. The reference data when plotted in the TAS diagram (Fig. 8.4) also signified a similar composition.

The two SRH observed at depths of 485-490 and 520-525 cm in the core AAS-22/8 have glass shards of type II morphology. Though these glass shards are abundant (2,972 shards in

430-435 cm and 2,586 shards in 520-525 cm), rhyolitic in composition with high SiO2 (77.96 to 78.12 wt %) and correspond to ages of 1.802-1.821 and 1.933-1.951 Ma, respectively but these do not correlate with the ages derived for ODP Site 758 (Dehn et al., 1991).

The shape of the glass shards is directly related to the mode of formation (Wohletz, 1983). The vesicular glass shards with cuspate and Y-shape intersections morphology are indicative of sub-aerial eruptions; while the blocky glass shards with scarce vesicles are known to form due to hydroclastic processes (Fisher and Schmincke 1984; Helo, 2013).

The blocky glass with a smooth surface are poorly vesiculated and were perhaps formed from intraplate volcanism under high hydrostatic pressure from a silicic melt during a phase of a localized strombolian type volcanism. Detailed studies of sediments from other sites of the CIOB could reveal the occurrence of such volcanic events.

8.5.1 Source and Formation of shards Since more than a couple of decades, the source of glass shards present in the CIOB has been controversial. There are two schools of thoughts: (i) the cuspate glass shards belong to the YTT (Pattan et al., 1999), except in some cases wherein the glass shards are of OTT (Glass and Koeberl, 2006; Pattan et al., 2010), while (ii) blocky shards resulted in intraplate volcanism in the CIOB (Iyer et al., 1997, Mascarenhas-Pereira et al., 2016). Mascarenhas- Pereira et al. (2006) examined the likely sources for the CIOB glass shards by comparing these with those that originated from the neighboring terrestrial volcanoes. Based on the chemical composition of the CIOB glass shards (Table 8.3), the volcanic sources such as the Reunion hotspot, Kerguelen plume, Broken Ridge, Afro-Arabian flood volcanic and Taupo volcanic center were ruled out because these sources have contributed basaltic materials also the ages of the shards do not correlate with those from the terrestrial sources. Considering

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Chapter 8 these reports and present observations, it can be suggested that the shards occurring in shallow depth within the sediments are mainly the YTT, while those occurring at deeper depths are either from earlier eruptions of Toba and/or were formed in-situ.

In recent years, there has been growing evidence of pyroclastic eruptions in the deep-sea environment and various models have been developed to understand their mechanism (Clague, 2009; Clague et al., 2003; Helo et al., 2008, 2011). These submarine eruptions of strombolian style are known to transfer gases directly to the cold surrounding deep waters resulting in pyroclastic deposits (Head and Wilson, 2003). The fragmentation of pyroclasts in the deep-sea could be driven by the release of CO2 (Helo, 2013) and this fact indicates that pyroclastic fragments deposited at abyssal depths are independent of their source and types of origin. The bubble wall glass shards are known to have a hydrovolcanic origin (Schipper and White, 2010) and the process is well identified in the sub-aqueous environment. In contrast, blocky glass shards devoid of vesicles (Fig. 8.3B) are the products of a rapid interaction of lava and seawater during phreatomagmatic eruptions (Maicher and White, 2001).

It is noteworthy that an analogous observation was made of glass morphology and chemistry in four short cores (max. 40 cm) recovered from the CIOB. The SRH in these cores were reportedly formed by hydrovolcanic activities that occurred during 85, 107-109, and 142-146 ka ago and also a crypto-tephra layer of 34 ka (Mascarenhas-Pereira et al., 2016). Considering the above facts, it is evident that tephra form Toba (especially YTT) are common in the CIOB sediments, but the possibility of an intra-basinal origin of glass shards cannot be ruled out especially for those SRH that cannot be correlated with the well-established layers present in ODP Site 758.

8.6 Conclusions The investigation of four long sediment cores (average length 5 m) that were retrieved along the transect line located in the north (7º E) to the south (17º E) reveals the occurrence of multiple shard-rich horizons (SRH). These SRH provide the first detailed tephrochronology for the volcanic glass shards identified in the CIOB sediments. Sedimentation rate derived using the abundance peak of Australasian microtektites (age 0.77 Ma), suggest variations across the basin with the highest rate (3.1mm/ka) been observed in the northern part of the study area while the rates were extremely low (0.72mm/ka) in the southern part of the basin. The estimated sedimentation rates are well

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correlated with ages derived by other workers and fit remarkably with those reported for Leg 21 ODP Site 758 which was dated using paleomagnetic and δ18O methods.

The major conclusions drawn from this study are:  Identification of new tephra layers MTT (= Layer C) and D that occur between the ubiquitous YTT and OTT (Layer E). Additionally, nine layers (G, H, I, J, K, L, M and two unknown) were recognised below the OTT.  Based on a comparative study of surface sediments floored it is noted that irrespective of the rate of sedimentation, YTT in the basin is restricted to a depth of <35 cm.  The two unknown SRH, between layers G and H, have compositional similarities but are morphologically dissimilar to the Toba tephra. These differences attest to SRH formation during short and localized in-situ phreatomagmatic volcanic episodes in the CIOB.  An important revelation is that the glass shards in the CIOB are not solely from the Indonesian Volcanic Arc (especially Toba eruptions) but are also contributed by in- situ intraplate eruptions.

Table 8.1 A compilation of reported glass shards along with their depth of occurrence in the Central Indian Ocean Basin. The details of the four gravity cores used in the present study are presented in serial numbers 1 to 4. Legends: * = shards occurring in these horizons were reported to be of intra basinal origin derived by submarine hydrovolcanic activity, ** = Cryptotephra deposit of 34 ka, YTT = Youngest Toba Tuff (74 ka), OTT = Oldest Toba Tuff (0.84 Ma). na = not available.

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Sr Sediment core Latitude Longitude Water Core Occurrence depth of References No. (°S) (°E) Depth (m) recovery shards (in cm below length (cm) sea floor)

1 AAS-22/8 7° 05.289 78° 07.712 5180 560 205-210 (OTT) Present study, Prasad et al. (2007), Pearce et al. (2014) AAS-22/7 10° 00.988 78° 08.971 5240 550 0-5 (YTT) Present work 2 220-225 (OTT) AAS-22/5 15° 06.619 78° 11.742 5030 490 10-15 (YTT) Present study, Prasad et al. (2007), Pearce et 3 115-120 (OTT) al. (2014) 4 AAS-22/3 16° 59.590 77° 59.665 4760 560 10-15 (YTT) Present study, Prasad et al. (2007), Pattan et 60-65 (OTT) al. (2010), Pearce et al. (2014) 5 NR-1 9° 99.00 77° 92.00 5250 28 0-10 (YTT) Banakar et al. (1991), Pattan et al. (2002) 6 NR-35 11.56° 78.49° 5450 23.5 14-26 (YTT) Banakar et al. (1991), Pattan et al. (2002) 7 NR-21 11.30° 78.49° 5325 32 20-30 (YTT) Banakar et al. (1991), Pattan et al. (2002) 8 NR-54 7° 78.15° 5200 70 30-35 (YTT) Pattan et al. (2002) 9 SK-226 13.08° 75.00° 5270 36 18-26 (YTT) Pattan et al. (2002) 10 SS-657 14.00° 76.00° 5050 36 6-14 (YTT) Pattan et al. (2002) 11 AAS-38/2 8° 29.868 77° 59.761 5438 na 185-190 (OTT) Prasad et al. (2007), Pearce et al. (2014) 12 AAS-V/GC-02 13° 01.48 75° 27.04 5099 na 165-170 (OTT) Pattan et al. (2010), Pearce et al. (2014) 13 BC-14 14° 00.231 75° 30.064 5145 26 18-20 (YTT), Mascarenhas-Pereira et al. (2016) 22-24 (85 ka)*, 8-10 (34ka)** 14 BC-20 12° 00.066 75° 29.893 5200 34 16-18 (YTT), Mascarenhas-Pereira et al. (2016) 24-26 (107-109 ka)*, 32-34 (141-146 ka)* 15 BC-25 10° 59.964 75° 29.927 5300 34 16-18 (YTT), Mascarenhas-Pereira et al. (2016) 24-26 (107-109 ka)*, 32-34 (141-146 ka)*

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Chapter 8 Table 8.2 The coarse fraction components are listed in decreasing order of abundance. Also shown are counts of glass shards made by visual observation in 0.1 mg of the CF.

Number of Core Depth in shards per Microscopy Description Number core (cm) 0.1mg AAS-22/8 135-140 2133 Radiolarians, glass shards, micronodules (few). 190-195 3482 Glass shards, radiolarians. 205-210 4607 Glass shards, radiolarians, tektites. 430-435 2972 Radiolarians, glass shards. 485-490 2586 Radiolarians, glass shards. 520-525 3507 Glass shards, radiolarians. AAS-22/7 0-5 4593 Radiolarians, glass shards, micronodules, bread crust, magnetite spherules. 220-225 4170 Radiolarians, glass shards, micronodules, basaltic pieces, tektites. AAS-22/5 10-15 3044 Radiolarians, glass shards, micronodules (few).

110-115 2915 Radiolarians, glass shards, micronodules, tektites. 245-250 1690 Glass shards, palagonite, and micronodules (few). AAS-22/3 10-15 4973 Glass shards, radiolarians, micronodules (few). 60-65 3322 Glass shards, radiolarians, tektites, Fe-coated rock pieces (few). 170-175 2025 Glass shards, palagonite, micronodules, biota, shark teeth (few) 285-290 1600 Glass shards, micronodules, palagonite, biota 295-300 3214 Glass shards, radiolarians (milky white), micronodules, palagonite grains. 325-330 2800 Glass shards, palagonite. 335-340 1882 Glass shards, micronodules, palagonite, sharks’ teeth. 350-355 2153 Glass shards, palagonite, micronodules, sharks’ teeth. 365-370 2100 Glass shards, palagonite, micronodules (few).

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Chapter 8 Table 8.3 EDS composition of the glass shards recovered from different shard-rich horizons (SRH) in the four studied cores. Averages and standard deviation are listed below each SRH. nd = not detected; n = number of shards analysed. The compositional data are plotted in the TAS diagram (Fig. 8.4).

Size in Sample Id Spectrum Na2O Al2O3 SiO2 K2O CaO FeO total µm 1 3.18 11.80 78.51 5.11 0.61 0.75 99.94 100*100

2 2.50 11.52 78.90 5.48 0.67 0.92 99.98 130*100

3 2.42 10.93 75.61 5.79 0.80 0.91 96.46 200*50

4 2.65 12.08 78.26 5.32 0.70 0.92 99.93 110*95

5 2.63 11.94 78.02 5.70 0.76 0.95 99.99 95*80

6 2.53 11.71 78.30 5.63 0.79 0.92 99.88 110*130

7 2.76 11.89 78.49 5.34 0.64 0.73 99.84 120*150

8 2.67 11.88 78.50 5.38 0.73 0.82 99.97 450*200

9 2.64 11.43 78.50 5.41 0.69 0.88 99.55 110*140

10 2.96 12.22 78.06 5.25 0.76 0.75 99.98 130*80

AAS-22/8 Stdv 0.23 0.37 0.92 0.21 0.06 0.09 1.09 (135-140) Avg 10 2.69 11.74 78.11 5.44 0.71 0.85 99.55

1 2.71 11.21 78.23 5.68 0.74 0.88 99.45 220*50

2 2.48 11.49 78.27 5.85 0.83 0.85 99.77 280*100

3 2.76 11.84 77.86 5.52 0.73 0.85 99.55 300*80

4 2.37 11.51 78.16 5.89 0.81 0.99 99.71 150*100

5 2.21 11.66 76.53 5.58 0.83 0.86 97.67 100*190

6 2.37 11.39 76.47 5.95 0.86 1.12 98.14 120*100

7 2.78 11.75 76.74 5.82 0.77 1.57 99.43 20*130

8 3.38 12.25 77.64 4.98 0.68 0.72 99.64 200*130

9 2.35 11.57 77.81 5.65 0.79 0.89 99.04 220*120

10 2.77 11.49 77.76 6.03 0.81 1.08 99.93 170*50

AAS-22/8 Stdv 0.34 0.28 0.70 0.30 0.06 0.24 0.75 (190-195) Avg 10 2.62 11.61 77.55 5.69 0.78 0.98 99.23

1 2.51 11.20 78.52 5.99 0.78 0.97 99.97 200*50

2 2.20 11.54 76.38 5.95 0.82 1.08 97.97 180*70

3 2.41 11.39 78.80 5.82 0.73 0.83 99.98 145*60

4 2.85 12.07 77.90 5.72 0.68 0.97 100.19 100*70

5 2.81 11.52 78.17 5.25 0.69 98.44 110*120

6 3.36 11.93 77.04 4.80 0.60 0.82 98.55 50*70

7 2.29 11.11 78.98 5.14 0.76 1.13 99.40 100*150

8 2.97 11.02 78.65 5.39 0.59 0.82 99.43 200*230

9 2.31 11.38 78.43 5.01 0.84 1.06 99.02 200*210

AAS-22/8 Stdv 0.39 0.35 0.86 0.43 0.09 0.13 0.78 (205-210) Avg 9 2.63 11.46 78.10 5.45 0.72 0.96 99.22

1 2.64 11.63 76.93 5.50 0.89 0.90 98.48 220*100

2 3.69 12.19 77.53 4.49 0.55 0.51 98.96 210*90

3 3.17 11.91 77.23 4.99 0.72 0.90 98.92 160*140

4 2.20 11.87 78.02 5.88 0.67 1.28 99.92 130*80

5 2.39 11.35 79.18 5.34 0.58 1.15 99.99 180*170

6 2.20 11.87 77.72 5.88 0.67 1.28 99.62 70*140

7 2.90 11.71 77.62 4.91 0.72 0.64 98.48 200*110

8 2.21 11.53 78.40 5.85 0.95 1.02 99.96 110*200

9 2.33 11.71 77.98 6.02 0.90 1.02 99.94 150*130

10 3.00 11.90 77.51 4.90 0.85 0.85 99.01 300*90

11 2.23 11.39 78.64 5.02 0.68 1.22 99.18 180*100

12 3.10 12.14 77.96 5.07 0.78 0.82 99.87 130*150

AAS-22/8 Stdv 0.49 0.27 0.62 0.50 0.13 0.24 0.59 (430-435) Avg 12 2.67 11.77 77.89 5.32 0.75 0.97 99.36 1 2.11 11.06 77.70 6.01 0.88 1.02 98.78 110*100

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Chapter 8 2 2.04 11.13 76.52 5.85 0.94 0.79 97.27 70*150

3 2.68 11.85 77.55 5.18 0.63 0.86 98.75 90*60

4 2.23 11.61 79.79 6.20 0.97 1.10 101.90 170*150

5 3.19 11.83 78.18 4.98 0.65 0.75 99.57 140*200

6 2.33 11.41 78.54 5.83 0.73 1.10 99.94 110*120

7 2.97 11.67 77.70 4.92 0.69 0.77 98.72 200*110

8 2.92 11.18 77.61 5.30 1.29 1.30 99.58 100*210

9 2.97 11.33 79.45 5.15 0.95 1.08 100.93 110*120

AAS-22/8 Stdv 0.43 0.30 1.01 0.48 0.21 0.19 1.35 (485-490) Avg 9 2.60 11.45 78.12 5.49 0.86 0.97 99.49 1 2.43 11.64 77.28 5.65 1.07 1.87 99.94 150*200

2 2.43 11.64 77.28 5.65 1.07 1.87 99.94 170*200

3 2.38 11.66 79.05 5.03 0.83 0.91 99.86 210*160

4 2.41 11.15 78.17 5.29 0.95 1.39 99.35 100*150

5 3.10 11.33 77.19 5.63 0.77 1.61 99.63 50*60

6 3.45 12.19 78.13 4.77 0.67 0.66 99.87 130*90

7 2.51 11.46 78.59 5.72 0.73 0.96 99.97 200*160

8 3.10 12.05 77.69 5.50 0.72 0.91 99.97 100*170

9 2.72 11.89 77.99 5.22 0.75 0.93 99.50 120*130

10 2.91 12.27 78.08 5.12 0.61 0.77 99.76 140*90

AAS-22/8 Stdv 0.38 0.37 0.60 0.32 0.16 0.46 0.22 (520-525) Avg 10 2.74 11.73 77.94 5.36 0.82 1.19 99.78

Sample Id Spectrum Na2O Al2O3 SiO2 K2O CaO FeO total size in um 1 3.59 12.51 77.59 4.53 0.67 0.91 99.80 140*190 2 3.04 12.22 77.96 4.39 1.08 1.24 99.93 240*150 3 2.92 11.86 78.31 5.23 0.78 0.89 99.98 180*250 4 3.08 11.67 76.26 5.04 0.73 0.91 97.69 160*70 5 2.66 11.44 77.53 6.03 0.87 1.21 99.74 190*80 6 2.72 11.27 78.80 5.16 0.80 1.11 99.85 170*160 7 1.59 10.47 76.03 7.34 0.97 1.93 98.33 100*150 8 2.24 10.51 77.34 7.19 1.10 1.27 99.65 70*140 9 2.48 11.77 78.26 5.61 0.83 0.96 99.90 130*120 10 2.99 11.50 78.62 5.13 0.67 0.83 99.74 150*80 11 2.83 11.18 78.40 5.67 0.78 1.06 99.92 180*90 AAS -22/7 Stdv 0.52 0.63 0.91 0.96 0.15 0.31 0.76 (0-5) Avg 11 2.74 11.49 77.74 5.57 0.84 1.12 99.50

1 4.99 11.73 76.36 5.16 0.56 0.63 99.43 220*70 2 3.83 11.93 76.79 4.95 0.72 98.22 200*160 3 2.90 11.60 77.15 6.08 0.83 0.73 99.29 200*190 4 2.75 11.59 77.02 5.33 0.74 1.11 98.54 260*110 5 2.88 11.17 76.34 6.57 0.94 0.75 98.65 80*100 6 2.48 11.81 77.75 5.53 0.79 0.96 99.32 120*150 7 2.87 12.11 77.21 5.86 0.95 0.95 99.95 100*170 AAS -22/7 Stdv 0.88 0.30 0.50 0.57 0.14 0.17 0.60 (220-225) Avg 7 3.24 11.71 76.95 5.64 0.80 0.84 99.06

Sample Id Spectrum Na2O Al2O3 SiO2 K2O CaO FeO total size in um 1 2.57 11.44 78.38 5.73 0.83 0.99 99.94 280*100 2 2.92 12.30 77.81 5.15 0.73 1.08 99.99 130*200 3 2.31 11.07 77.92 6.44 0.79 1.08 99.61 170*165 4 2.66 11.73 77.07 5.11 0.81 0.87 98.25 180*90 5 2.23 11.29 78.26 6.18 0.98 0.97 99.91 120*80 6 2.30 11.56 77.41 6.00 0.92 1.26 99.45 160*90 7 1.80 10.89 77.23 7.01 1.14 1.59 99.66 70*270

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Chapter 8 8 2.72 11.78 78.12 5.55 0.84 0.85 99.86 160*70 9 2.23 11.04 77.35 5.89 0.79 1.20 98.50 210*100 10 2.41 11.92 78.13 5.20 0.95 1.08 99.69 190*200 AAS -22/5 Stdv 0.32 0.44 0.47 0.61 0.12 0.22 0.61 (10-15) Avg 10 2.42 11.50 77.77 5.83 0.88 1.10 99.49

1 2.88 11.82 77.99 5.28 0.69 0.88 99.54 90*150 2 3.05 11.68 78.05 5.25 0.72 0.82 99.57 270*80 3 2.27 11.56 78.96 5.58 0.54 1.06 99.97 170*150 4 2.95 11.83 77.06 5.55 0.82 0.94 99.15 130*40 5 2.46 11.28 78.00 5.52 0.70 0.85 98.81 170*150 6 2.47 11.76 78.91 5.42 0.63 0.80 99.99 230*140 AAS -22/5 Stdv 0.32 0.21 0.70 0.14 0.09 0.10 0.46 (110-115) Avg 6 2.68 11.66 78.16 5.43 0.68 0.89 99.51

1 2.39 10.82 75.52 6.18 1.39 1.37 97.67 250*160 2 2.10 11.07 78.40 6.34 0.88 1.20 99.99 150*80 3 2.76 11.65 77.71 5.88 0.91 0.94 99.85 120*90 4 3.05 11.91 77.66 5.25 0.74 0.80 99.41 220*160 5 3.10 12.39 77.54 5.27 0.57 0.74 99.61 220*260 AAS -22/5 Stdv 0.43 0.63 1.09 0.51 0.31 0.27 0.94 (245-250) Avg 5 2.68 11.57 77.37 5.78 0.90 1.01 99.31

Sample Id Spectrum Na2O Al2O3 SiO2 K2O CaO FeO total size in um 1 3.90 12.05 77.80 4.66 0.75 0.74 99.90 180*80 2 2.48 11.84 77.59 5.85 0.75 1.24 99.75 100*200 3 2.05 10.77 76.55 7.20 1.09 1.60 99.26 90*140 4 2.75 11.70 77.02 5.43 0.61 0.93 98.44 60*130 5 2.70 11.50 77.85 5.87 0.98 1.09 99.99 90*130 6 3.28 11.52 78.11 4.83 0.58 0.67 98.99 100*90 7 2.41 11.42 78.09 5.82 0.98 1.12 99.83 140*160 8 2.47 11.64 78.12 5.92 0.87 0.88 99.90 170*150 9 2.65 11.49 77.42 5.18 0.75 1.07 98.56 100*390 10 3.86 12.08 76.61 4.76 0.73 0.81 98.85 70*120 AAS -22/3 Stdv 0.62 0.37 0.60 0.76 0.17 0.27 (10-15) Avg 10 2.85 11.60 77.52 5.55 0.81 1.01 99.35 1 2.62 11.05 77.92 5.69 0.81 0.96 99.05 110*150 2 2.88 11.75 78.36 5.52 0.72 0.75 99.98 200*80 3 2.77 11.37 78.13 5.78 0.84 0.94 99.83 220*210 4 2.62 11.96 78.58 5.19 0.75 0.88 99.98 200*110 5 3.37 11.50 76.71 6.20 0.92 0.97 99.66 100*80 6 2.36 11.44 78.31 6.00 0.74 0.97 99.82 150*140 7 3.12 11.89 78.13 5.13 0.77 0.70 99.74 140*100 8 2.19 11.11 77.83 6.14 0.88 1.04 99.19 70*10 AAS -22/3 Stdv 0.38 0.34 0.57 0.41 0.07 0.12 0.35 (60-65) Avg 8 2.74 11.51 78.00 5.71 0.80 0.90 99.66 1 3.23 11.63 76.80 4.66 2.21 0.71 99.24 190*150 2 1.20 9.64 79.23 7.19 1.04 1.30 99.60 140*80 3 3.61 11.83 76.45 4.61 0.65 0.73 97.88 70*120 4 2.52 11.72 78.73 5.40 0.79 0.82 99.98 100*160 5 2.70 11.52 77.00 5.30 0.92 1.47 98.91 250*130 6 2.55 11.33 78.12 5.48 0.75 0.95 99.18 30*90 7 2.82 11.75 78.15 5.14 0.98 1.12 99.96 160*130 8 3.07 11.97 77.68 5.34 0.71 0.92 99.69 180*140 9 2.52 11.51 78.90 5.17 0.87 0.94 99.91 120*90

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Chapter 8

AAS-22/3 Stdv 0.67 0.70 0.98 0.75 0.47 0.26 0.68 (170-175) Avg 9 2.69 11.43 77.90 5.37 0.99 1.00 99.37 1 2.7 12.91 74.9 5.37 0.81 0.86 99.85 150*80 2 3.23 12.63 74.85 5.34 0.83 0.88 99.84 120*90 3 2.75 12.17 74.78 5.44 0.78 1.07 99.7 170*1650 4 3.08 12.46 75.03 5.41 0.79 0.95 99.83 180*90 5 4.04 12.26 74.01 4.81 0.75 0.84 99.54 120*80 6 2.7 12.29 74.9 5.76 0.83 1.04 99.74 160*90 7 3.16 12.47 75.05 5.2 0.85 1.02 99.83 70*270 8 3.43 13.18 74.81 4.95 0.7 0.83 99.84 400*270 9 3.09 12.52 74.72 5.28 0.74 1.07 99.71 170*80 10 3.1 12.95 74.81 5.15 0.72 0.82 99.73 110*70 AAS- 22/3 Stdv 0.40 0.33 0.29 0.27 0.05 0.10 0.10 (285-290) Avg 10 3.128 12.584 74.786 5.271 0.78 0.938 99.761 70*270 1 3.43 11.25 78.18 4.87 0.77 0.96 99.45 120*300 2 2.60 11.83 77.31 5.42 0.89 0.94 98.99 250*200 3 3.26 11.66 77.77 4.87 0.53 0.59 98.68 140*180 4 2.24 11.50 77.61 6.45 0.96 1.18 99.94 420*130 5 2.69 11.51 78.23 5.61 0.90 0.86 99.80 80*100 6 2.69 12.14 77.55 5.18 0.79 0.84 99.19 100*40 7 2.93 11.90 77.48 5.22 0.97 0.90 99.40 80*30 8 2.91 11.82 77.20 5.03 0.80 0.86 98.62 300*110 9 2.45 11.42 77.98 5.41 0.85 0.85 98.96 110*220 10 3.21 12.00 77.59 5.02 0.91 0.95 99.68 200*80 AAS -22/3 Stdv 0.38 0.28 0.35 0.47 0.13 0.15 0.46 (295-300) Avg 10 2.84 11.70 77.69 5.31 0.84 0.89 99.27 1 3.16 11.61 78.21 5.24 0.71 0.76 99.69 100*270 2 2.98 11.73 76.35 5.47 1.89 1.55 99.97 170*80 3 3.01 11.93 77.23 5.03 0.65 0.83 98.68 110*220 4 2.97 11.75 77.54 5.38 0.77 0.88 99.27 240*130 5 3.19 11.58 78.32 5.28 0.77 0.85 99.99 170*120 6 2.26 11.37 78.67 5.51 0.81 1.01 99.63 100*190 7 2.26 11.37 78.67 5.51 0.81 1.01 99.63 100*130 8 3.32 11.79 78.20 5.24 0.70 0.73 99.98 100*180 9 2.90 11.74 78.11 5.32 0.95 0.91 99.93 130*210 AAS -22/3 Stdv 0.38 0.19 0.76 0.16 0.38 0.25 0.43 (325-330) Avg 9 2.89 11.65 77.92 5.33 0.90 0.95 99.64 1 3.1 12.95 74.81 5.15 0.72 0.82 97.55 130*110 2 2.59 12.89 74.89 5.67 0.75 1.01 97.8 120*115 3 4.04 12.26 74.01 4.81 0.75 0.84 96.71 120*140 4 2.7 12.29 74.9 5.76 0.83 1.04 97.52 160*105 5 3.09 12.52 74.72 5.28 0.74 1.07 97.42 100*120 6 3.37 12.92 74.28 5.21 0.7 0.9 97.38 110*95 7 3.29 12.94 73.97 5.24 0.73 1.05 97.22 110*130 8 3.33 12.95 74.19 5.18 0.7 0.97 97.32 145*110 9 2.86 12.73 74.06 5.48 0.79 1.1 97.02 120*100 AAS -22/3 Stdv 0.43 0.29 0.40 0.29 0.04 0.10 0.32 (335-340) Avg 9 3.15 12.72 74.43 5.31 0.75 0.98 97.33 1 3.34 12.8 74.47 5 0.85 1.14 97.6 90*120 2 3.08 12.46 75.03 5.41 0.79 0.95 97.72 135*100 3 3.2 74.94 4.79 0.85 1.19 84.97 130*110 4 2.75 12.17 74.78 5.44 0.78 1.07 96.99 85*100 5 2.94 12.35 74.56 5.38 0.89 0.97 97.09 100*110 6 3.16 12.47 75.05 5.2 0.85 1.02 97.75 130*90

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Chapter 8 7 2.39 11.9 73.78 6.28 0.91 1.45 96.71 120*100 8 3.99 12.24 74.37 4.69 0.73 0.89 96.91 90*120

9 3.47 12.63 74.75 5.1 0.75 1.00 97.7 130*100 10 2.96 12.43 74.75 5.26 1.08 1.1 97.58 95*100 AAS -22/3 Stdv 0.43 0.26 0.38 0.44 0.10 0.16 3.93 (350-355) Avg 10 3.13 12.38 74.65 5.26 0.85 1.08 96.10 1 3.43 13.18 74.81 4.95 0.7 0.83 97.9 85*100 2 2.76 12.68 75.49 5.27 0.64 0.87 97.71 95*110 3 2.64 12.74 74.69 5.58 0.77 0.97 97.39 100*100 4 3.59 12.16 73.73 5.11 0.97 0.93 96.49 70*130 5 2.7 12.91 74.9 5.37 0.81 0.86 97.55 140*80 6 3.23 12.63 74.85 5.34 0.83 0.88 97.76 70*110 7 3.1 12.65 74.97 5.3 0.74 0.92 97.68 80*110 8 2.81 12.54 75.34 5.3 0.82 0.9 97.71 70*130 9 3.11 12.76 74.86 5.3 0.82 0.87 97.72 100*10 10 2.46 12.36 74.31 5.84 0.91 1.19 97.07 120*100 AAS -22/3 Stdv 0.37 0.28 0.50 0.24 0.10 0.10 0.42 (365-370) Avg 10 2.98 12.66 74.80 5.34 0.80 0.92 97.50

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Chapter 8

Table 8.4 Geochemical composition of the glass shards reported as per the different layers encountered in the CIOB sediment. The list of referenced data: Pattan et al. (1992) (YTT); Westgate et al. (1988) (Middle Toba Tuff, MTT); Pattan et al. (2010) (OTT); while Layer D, and Layers G to M are from Dehn et al. (1991). The layers represented by NI = are Not Identified in the study area hence no data are provided. na= data not available. N = Number of shards analyzed. The following data have been plotted in the TAS diagram (Fig. 8.4) and also compared with our data (Table 8.3). H2O is by difference (as reported). See text for explanation.

Layer N SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total H2O A (YTT) 92 76.81 0.07 12.77 na 0.92 0.06 0.05 0.79 3.41 5.08 0.16 100.12 B NI C (MTT) 18 77.57 0.06 12.24 1.02 0.04 0.04 0.63 3.55 4.73 0.12 na 100 D 13 74.00 0.07 11.76 0.83 na 0 0.02 0.14 2.79 2.79 0.01 95.1 4.92 d NI E (OTT) 19 77.33 0.05 12.7 na 0.86 0.07 0.07 0.77 2.84 5.18 0.2 99.37 F NI G 9 73.31 0.13 11.38 0.69 na na 0.04 0.59 3.07 4.91 0.02 94.14 5.86 H 13 72.76 0.04 11.64 0.75 na na 0.02 0.45 3.35 4.30 0.02 93.34 6.66 h 8 73.07 0.05 11.65 0.66 na na 0.01 0.64 3.22 4.72 0.02 94.10 5.91 I 5 73.63 0.04 11.89 0.62 na na 0.03 0.67 3.11 4.35 0 94.34 5.66 J 10 73.02 0.08 11.92 0.69 na na 0.08 0.82 3.16 4.40 0.03 94.20 5.81 K 10 71.87 0.08 11.62 1.02 na na 0.04 0.61 3.43 4.35 0.02 93.02 6.98 L 8 68.06 0.35 12.70 2.38 na na 0.26 1.40 3.77 4.42 0.06 93.38 6.63 M 10 71.33 0.11 11.85 0.97 na na 0.11 1.05 3.31 4.05 0.04 92.81 7.19

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Chapter 8

Table 8.5 The occurrence of SRH at variable depths within the four cores along with the calculated ages. These ages are compared with the paleomagnetic (Diehl et al., 1987) and δ18O dates from Dehn et al. (1991). The layers which do not correlate to the tephra layers have also been listed. YTT = Youngest Toba Tuff (74 ka), OTT = Oldest Toba Tuff (0.84 Ma).

Layer Reported Ages Calculated ages are based on the known layers YTT AND OTT (Pattan et al., 2010) and the co-occurring (Dehn et al., microtektite layer of 0.77Ma (Prasad et al., 2007). 1991) AAS-22/8 AAS-22/7 AAS-22/5 AAS-22/3 Depth Calculated Depth Calculated Depth Calculated Depth Calculated (cm) Age(Ma) (cm) Age (Ma) (cm) Age (Ma) (cm) Age (Ma) A 0.071-0.110 0-5 0.00-0.148 10-15 0.060-0.089 10-15 0.060-0.089 B 0.071-0.121 C 0.512-0.538 135-140 0.502-0.520 D 0.731-0.750 190-195 0.706-0.725 d 0.756-0.750 E 0.774-0.780 205-210 0.762-0.781 220-225 0.696-0.712 110-115 0.769-0.804 60-65 0.776-0.816 F 1.273-1.294 G 1.596-1.653 430-435 1.598-1.617 245-250 1.713-1.748 485-490 1.802-1.821 520-525 1.933-1.951 H 2.238-2.265 170-175 2.367-2.436 h 3.664-3.666 I 4.124-4.131 295-300 4.108-4.178 J 4.552-4.560 325-330 4.526-4.596 K 4.666-4.667 335-340 4.665-4.735 L 4.759-4.765 350-355 4.87-4.944 M 5.062-5.063 365-370 5.083-5.153

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CHAPTER 9

SUMMARY AND CONCLUSIONS

Chapter 9

9.1 Summary and Conclusions Deep-sea volcanism and hydrothermal activities result in various types of rocks, pyroclasts and metallogenesis. These play an important role in estimating crustal growth, volcanic episodes and understand the (paleo) tectonic framework. Volcanism and hydrothermal activity are seen mostly associated with the mid-oceanic ridges and at some sites of seamounts. But there is a lacunae in the knowledge concerning similar studies in the intraplate regions of the world ocean.

In the present thesis, an investigation was carried out to understand the role of morpho-tectonic features and the processes governing the formation of volcanogenic hydrothermal materials (vhm) in an intraplate region. The vhm are composed of volcanic glass shards, Fe-rich magnetic spherules, palagonite, basaltic glass (fresh and altered) and metal-rich grains.

The area of research was the Central Indian Ocean Basin (CIOB) which is an 60 Ma old basin and has an average water depth of 5,000 m. In this ancient basin there has been continuous volcanism and intermittent hydrothermal activity since 60 Ma to as recent as 100 years. I selected four gravity sediment cores each of at least 5 m in length, 95 surface sediment samples and several rock samples from four seamounts. The cores were collected along the 79° E fracture zone (FZ, Indrani FZ) and were from the terrigenous sediment in the north (7°S/78°E) to the red-clay domain in the south (16°S/78°E) to understand the role of the FZ. The rocks were recovered from the foothill, slope and near the crest of seamounts while the surface sediments were collected in an area clustered with seamounts (12-14°S/74-75°E).

The salient conclusions that were arrived at from my study are as follows.

1. During the third phase of break-up of Gondwanaland during 60 ± 5 Ma the spreading rate was around 95 mm/year (half rate) with India moving towards Eurasia and during this period the CIOB was formed. The basin during its developmental stages was subjected to deformations that resulted in fissures, crenulations, and FZ. Contemporaneously the basin also witnessed extensive volcanism on the seafloor and production of seamounts. The study carried on morphological, petrological and geochemical parameters on four seamounts helped in understanding the formation.

At the seamounts basalts are common but there are instances where vhm are also found. These indicate that at older seamounts there occurred episodes of later volcanism

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Chapter 9 which added magmatic mass to the existing seamounts and also formation of vhm by hydrothermal activity.

2. The Fe-rich magnetic spherules from two sediment types and tectonic settings offer a new understanding of their genesis at a water depth of 5,000 m. The Fe-rich magnetic spherules of different morpho-types and textures are unique and associated with FZ and seamounts. The spherules were formed by a process of MCFI (molten fuel-coolant interaction) and the fluid-driven hydrovolcanic activity was perhaps enhanced by the release of CO2 and formation of a vapour-phase. This allowed the melt to get dispersed and with a simultaneous lowering of the temperature. The dispersed melt either resulted in their quenching (non-crystalline spherules) or produced well-crystallized Fe-rich magnetic spherules and titanomagnetite grains.

3. The occurrence of metallic grains (both native and non-native forms) such as Al, Ag and Cu along with Ba-S, Ba-S-Pb, Cu-Zn complexes has helped to understand their origin. The discovery of metal particles from seamount clustered area and from the FZ provides evidence that the porous deep-sea sediments, fissures and FZ perhaps acted as channels for the upward migrating hydrothermal fluids and remobilization of elements from seafloor basalts and sediments. The significant presence of metal-rich grains supports the existence of black/white smoker-kind of condition in the CIOB.

4. The occurrence of two basaltic layers within the core AAS-22/7 (345-355 and 470- 490 cm) along with Fe-rich magnetic spherules suggests later seepage of the magmatic activity into the porous sediments from the 79°E FZ. The geochemical studies of the sediment revealed a baking effect below 250 cm while the top 250 cm was unaffected by the magmatic activity. 5. A study carried out on two sediment cores (AAS-22/5 and AAS-22/3), reveals a hiatus in the occurrence of radiolarians and provides a new understanding of in-situ low temperature alteration of the CIOB sediments. The crux of the finding is a large scale disappearance of radiolarians between depths 250 and 490 cm (AAS/22-5) and 60-290 and 325-560 cm (AAS-22/3) extensive occurrence of grains of palagonite and phillipsite. The susceptibility of radiolarian shell been very low to changes in the environment of deposition could be a reason for such a major scale disappearance of radiolarians. The presence of palagonite grains suggest low temperature alteration of sediments while the Ca-poor phillipsite grains indicate an early to final stages of palagonitisation of the sediment cores.

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6. The present study presents the first detailed tephrochronology for the shard-rich horizons (SRH) identified in the CIOB sediment cores. The four long sediment cores (average length 5 m) helped to better understand spatial and temporal variations in the distribution of the glass shards. The presence of Australasian microtektites (age 0.77 Ma), that co-occur with the shards, was used as a proxy to determine the sedimentation rates.

The study helped to identify new tephra layers i.e., MTT (= Layer C) and D that occur between the ubiquitous YTT and OTT (Layer E) along with nine other layers (G, H, I, J, K, L, M and two unknown) that are present below the OTT. Additionally, two unknown SRH were identified and these are attributed to short lived and localized in-situ phreatomagmatic volcanic episodes. An important revelation of this study was that the glass shards in the CIOB are not solely from the Indonesian Volcanic Arc (especially Toba eruptions) but are also contributed by in-situ intraplate eruptions.

In general, the study helped to track the various events that occurred post formation of the CIOB i.e., after 60 Ma. A model (Fig. 9.1) is proposed to account for the different volcanic and hydrothermal activities that the CIOB was subjected to since its inception 60 million years ago and the chronological events are tabulated (Table 9.1.). In the model are shown the possible ways of formation of seamount basalts, vhm, iron-rich magnetic spherules, metallic grains, glass shards, mass disappearance of radiolarians, extensive palagonitization and baked sediments in the CIOB.

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Fig. 9.1 A schematic sketch to portray different processes that either occurred and/or ongoing in the CIOB. The studied cores are marked in blue circle adjacent to the FZ and the details are shown as insets in the circles to the right. Colour codes: Metallic grains are in purple, vhm and magma body are in red colour, brown textured area shows palagonite zone and blue dotted line is fissures/FZ present on the ocean floor. (Figure is not to scale).

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Table 9.1 Gist of different volcanogenic and hydrothermal events from the CIOB. Some of the ages are compiled from different sources (cited in the thesis).

Volcanic / Hydrothermal Events Mode of Ages in Ma (unless Formation mentioned differently) Hydrothermal alteration In-situ 100 yr Cryptotephra In-situ ~34 ka In-situ glass shards In-situ 70-75 ka In-situ glass shards In-situ ~85 ka In-situ glass shards In-situ 107-109 ka In-situ glass shards In-situ 142-146 ka Vhm near seamounts In-situ 475-625 ka Entrapped basalts in core formed due to lateral In-situ Not known seepage of lava from 79oE FZ Fe-rich magnetic spherules In-situ <0.70 A (YTT glass shards) Ex-situ 0.060-0.089 C (MTT glass shards) Ex-situ 0.502-0.520 D (Toba glass shards) Ex-situ 0.706-0.725 E (OTT glass shards) + Australasian microtektites Ex-situ 0.762-0.781 G (Toba glass shards) Ex-situ 1.598-1.617 In-situ glass shards In-situ 1.802-1.821 In-situ glass shards In-situ 1.933-1.951 H (Toba glass shards) Ex-situ 2.367-2.436 I (Toba glass shards) Ex-situ 4.108-4.178 J (Toba glass shards) Ex-situ 4.526-4.596 K (Toba glass shards) Ex-situ 4.665-4.735 L (Toba glass shards) Ex-situ 4.87-4.944 M (Toba glass shards) Ex-situ 5.083-5.153 Seamount basalts In-situ <60 CIOB Seafloor/ Oceanic crust formation In-situ 60-55

9.2 Scope for future study

In the present study the possibilities of formation of volcanics and associated hydrothermal materials at abyssal depth (5,000 m water depth) were presented. Similar investigations need to be carried out in the world ocean to help understand such enigmatic processes and resultant products.

A close-grid sampling in the world ocean could assist to comprehend and map the occurrence of metal-rich particles, which are uncommon in deep-sea environments. Such grains could be potential proxies to locate hydrothermal vents (active or inactive) in an oceanic intraplate setting.

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