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Li, Hf and Os ISOTOPE SYSTEMATICS of AZORES BASALTS and a NEW MICROWAVE DIGESTION METHOD for Os ISOTOPIC ANALYSIS

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Li, Hf and Os ISOTOPE SYSTEMATICS of AZORES BASALTS and a NEW MICROWAVE DIGESTION METHOD for Os ISOTOPIC ANALYSIS MIAMI UNIVERSITY The Graduate School Certificate for Approving the Dissertation We hereby approve the Dissertation of Huimin Yu Candidate for the Degree: Doctor of Philosophy Elisabeth Widom, Director William K. Hart, Reader Michael R. Brudzinski, Reader Paul B. Tomascak, Reader Michael W. Crowder, Graduate School Representative ABSTRACT Li, Hf AND Os ISOTOPE SYSTEMATICS OF AZORES BASALTS AND A NEW MICROWAVE DIGESTION METHOD FOR Os ISOTOPIC ANALYSIS by Huimin Yu This dissertation includes three projects related to the isotope geochemistry of ocean island basalts (OIB) in the Azores archipelago. Detailed studies of Hf, Os and Li isotope systematics are combined with Sr-Nd-Pb isotopes and trace elements to investigate the nature and origin of mantle heterogeneity beneath the Azores. In addition, new microwave digestion methods have been developed and tested for dissolution of samples for Os isotope analysis. The first project focuses on Hf-Os isotope systematics of basalts from the Azores Central Group islands (Faial, Pico, São Jorge and Terceira) with HIMU and EM-type 187 188 signatures. Sub-chondritic Os/ Os and ΔεHf signatures on or slightly below the terrestrial εHf - εNd array indicate that the mantle sources of these basalts do not contain significant recycled crustal material. Rather, the sources of the basalts are interpreted to include variable and geographically controlled mixtures of a deeply derived enriched mantle plume, relatively depleted mantle similar to that beneath the Mid-Atlantic ridge, and recycled metasomatized mantle wedge. The second project focuses on assessing the utility of Li isotopes as a tracer of heterogeneous mantle sources. The δ7Li data of Central Group island (Faial, Pico and Terceira) and São Miguel basalts vary only slightly (+3.1 to +4.7‰), and are all within the range of normal MORB, despite large variations in radiogenic isotopes. Nevertheless, the Central Group island basalts have, on average, slightly higher δ7Li than São Miguel, and exhibit positive correlations with Sr and Os isotopes, and negative correlations with Pb, Nd and Hf isotopes, and are consistent with the interpretations of the Hf-Os isotope study. New diffusion modeling furthermore suggests that mantle heterogeneities induced by subduction processes may be maintained in the mantle for timescales of >2.5Ga. The third project assesses the utility of microwave digestion for Os isotopic analysis. Compared to conventional Carius tube digestions, microwave digestion is faster and safer, and allows for the use of HF to achieve complete dissolution of silicate samples. This study demonstrated that microwave digestions successfully achieve spike-sample equilibration, have acceptably low processing blanks, and produce yields >90%. Li, Hf AND Os ISOTOPE SYSTEMATICS OF AZORES BASALTS AND A NEW MICROWAVE DIGESTION METHOD FOR Os ISOTOPIC ANALYSIS A DISSERTATION Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Geology and Environmental Earth Science by Huimin Yu Miami University Oxford, Ohio 2011 Dissertation Director: Elisabeth Widom, Ph.D. TABLE OF CONTENTS Chapter 1: Introduction Introduction 2 References 6 Chapter 2: Hafnium and osmium isotopic systematics of ocean island basalts in the Central Group islands of the Azores Archipelago Abstract 10 Body Text 11 References 39 Chapter 3: Lithium isotope systematics of basalts from the Azores Archipelago: constraints on the origin of the mantle sources Abstract 86 Body Text 88 References 114 Chapter 4: Microwave Digestion Method for Os analysis Abstract 155 Body Text 156 References 169 Chapter 5: Conclusions Summary 181 References 183 ii LIST OF TABLES Chapter 2: Hafnium and osmium isotopic systematics of ocean island basalts in the Central Group islands of the Azores Archipelago 1. Major element, trace element and isotopic ratios of the Central Group island basalts 48 2. The compositions of end-members in Os-Sr-Pb mixing models 59 3. The compositions of end-members in Nd-Hf mixing models 60 Chapter 3: Lithium isotope systematics of basalts from the Azores Archipelago: constraints on the origin of the mantle sources 1. Compositions of Azores basalts samples including lithium concentrations and isotopic compositions 125 2. δ7Li values of four olivine-whole rock pairs 127 Chapter 4: Microwave Digestion Method for Os analysis 1. Comparison of microwave digestion and Carius tube digestion 170 2. Osmium yields of microwave digestion 171 iii LIST OF FIGURES Chapter 2: Hafnium and osmium isotopic systematics of ocean island basalts in the Central Group islands of the Azores Archipelago 1. Map of Azores Archipelago 61 2. 87Sr/86Sr versus 206Pb/204Pb ratios for basalts of the Central Group islands, Azores archipelago 63 3. Total alkalis versus SiO2 and plot of Ni versus MgO for basalts from the Central Group islands 65 4. Variations of major element versus MgO for the basalts from the Central Group islands 67 5. Chondrite-normalized REE patterns for the Central Group island basalts 69 6. Primitive-mantle normalized trace element patterns for the Central Group island basalts 71 7. Trace element compositions of the Central Group island basalts 73 8. Whole-rock isotope signatures of the Central Group island basalts 75 9. Osmium isotope signatures of the Central Group island basalts 77 10. Calculated mixing trends between enriched Azores mantle plume (AMP) and potential recycle crustal components 79 11. Neodymium and Hf isotope signatures of the Central Group island basalts and calculated mixing trends between enriched Azores mantle plume (AMP) and potential recycle crustal components 81 12. Model of proposed mantle sources beneath the Azores archipelago 83 Chapter 3: Lithium isotope systematics of basalts from the Azores Archipelago: constraints on the origin of the mantle sources 1. Lithium isotope signatures of Earth reservoirs 128 2. Map of Azores Archipelago 130 87 86 206 204 3. Sr/ Sr versus Pb/ Pb for basalts of the Azores archipelago 132 iv 4. Deviation of individual analyses from the mean of replicate analyses of all standards (L-SVEC 50ppb) 134 7 5. Comparison of δ Li values between whole rocks and olivine separates 136 6. Histogram of Li isotopic composition of the Azores basalts and MORB 138 7. δ7Li values of whole rock powder from the Central Group islands versus radiogenic isotopes 140 7 87 86 8. δ Li versus Sr/ Sr of global OIB 142 9. Osmium concentrations and isotopic compositions of the Central Group island basalts 144 7 10. Major and trace element compositions versus δ Li for Azores basalts 146 11. Isotopic ratios and major and trace element compositions of the Azores basalts 148 12. Calculated mixing trends between basalt and pelagic sediment 150 7 13. Diffusion models for Li concentrations and δ Li of recycled material 152 Chapter 4: Microwave Digestion Method for Os analysis 1. The temperature and pressure change during heating of aqua regia in the microwave 172 2. The temperature and pressure change during heating of HF+EtOH+HCl in the microwave 174 3. Distillation system for Os separation 176 4. Osmium concentrations and 187Os/188Os ratios of sample aliquots digested by Carius tube and microwave 178 v ACKNOWLEDGMENTS I would like to express my sincerely gratitude to the many people who have helped me during my study at Miami University. The first and the foremost is my advisor Dr. Elisabeth Widom. Everything she taught me over last six years is extremely valuable in my research career and in my whole life. She encouraged me to develop independent thinking skills. I am really grateful to her patience of my scientific writing and guidance when I lost direction during my research. I respect her as an extremely knowledgeable geologist as well as a successful mentor. Thanks to all the people who have served on my committees: Dr. William K. Hart, Dr. Michael R. Brudzinski and Dr. Michael W. Crowder. I owe a special thanks to Dr. Paul B. Tomascak for making the time to come to Oxford for my defense. I would also like to thank Dave Kuentz and John Morton for their assistance in the labs in Miami University. Thank you to Lin Qiu and Roberta Rudnick at University, Richard Carlson at DTM for their assistance in their labs during my visit. I thank all faculty and staff in the Department of Geology at Miami University, especially Cathy Edwards and Jeanne Johnston for their assistance with day to day questions. I am thankful to Amy Gelinas and Qing Meng for their assistance with sample analysis and all of my lab mates, Elise Conte, George Daly, Rebecca Tortorello and Fara Rasoazanamparany to share ideas, discuss my research and help my English. I also would like to thank Shanshan Ji, Yun Luo, Qing Meng, Jing Zhang, Qiuyuan Huang and Shizuko Watanabe for their friendships. Last but not least, I would like to thank my family. My husband, Yongjian Han, always understands me, supports me and encourages me. My daughter Grace Han and my son Steven Han bring a lot of fun and are my source of happiness. I owe a special thanks to my parents Guangshan Yu and Wenqing Shi. Without their support and understanding, I could not finish my doctoral dissertation. These projects were supported by the National Science Foundation (NSF EAR # 0510598 and NSF EAR # 0549552 to Elisabeth Widom) and the Geological Society of America (8696-07 to Huimin Yu). vi CHAPTER 1 Introduction 1 Ocean island basalts (OIB) exhibit large variations in their chemical and isotopic compositions, indicating that they are produced by melting
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