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Isotopic Systematics of Ultramafic and Mafic Rocks of the Taitao Ophiolite, Southern Chile

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Isotopic Systematics of Ultramafic and Mafic Rocks of the Taitao Ophiolite, Southern Chile ABSTRACT Title of Document: ISOTOPIC SYSTEMATICS OF ULTRAMAFIC AND MAFIC ROCKS OF THE TAITAO OPHIOLITE, SOUTHERN CHILE Ruth Frost Schulte, Master of Science, 2007 Directed By: Professor Richard J. Walker Department of Geology A variety of ultramafic and mafic rocks from a large, well-mapped sampling of the ca. 6 Ma Taitao Ophiolite, Chile, have been examined. Calculated initial 187Os/188Os ratios of the peridotitic rocks range from 0.1168 to 0.1282. Similar ranges of Os isotopic compositions have been reported for abyssal peridotites and peridotites and chromites from some other ophiolites. A correlation between the Mg # of primary olivine grains and Os isotopic compositions of whole rock peridotites suggests that the isotopic variability is due to variable extents of partial melting at approximately 1.5 Ga. This conclusion is also supported by a linear correlation between 187Re/188Os and 187Os/188Os, although how this relation was retained through a second, recent melting event is unclear. The ancient melting event requires that this km-scale block of the mantle remained isolated and unmixed within the convecting upper mantle for a period of ~1.5 Ga. Calculated initial 187Os/188Os ratios of the mafic rocks are substantially enriched relative to the peridotites, suggesting that these rocks were derived via partial melting of portions of the mantle with higher, long/term Re-Os ratios than the ambient upper mantle. The results are comparable to global observations of Os isotopic compositions in mid-ocean ridge basalts (MORB) versus abyssal peridotites. At Taitao, however, the geologic relations are generally well defined. ISOTOPIC SYSTEMATICS OF ULTRAMAFIC AND MAFIC ROCKS FROM THE TAITAO OPHIOLITE, SOUTHERN CHILE by Ruth Frost Schulte Thesis submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Master of Science 2007 Advisory Committee: Professor Richard J. Walker, Chair Professor Roberta Rudnick Assistant Research Scientist Igor Puchtel ACKNOWLEDGMENTS First, I want to thank Richard Walker for all his patience and assistance in guiding me through the graduate school process. I’m sure there were times when he scratched his head at my questions and comments. Secondly, I would thank Professor Ryo Anma at Tsukuba University and Professor Maruyama and Dr. Komiya at the Tokyo Institute of Technology for allowing me to select samples from their 2000-2001 and 2002-2003 expeditions to the Taitao Peninsula. You and your graduate students made me feel at ease and I treasure my memories of my time in Tokyo. I am particularly indebted to Kon-san and Shibuya-san for their hospitality and friendship. I would also like to thank Dr. Phil Piccoli for all the hours he spent on the microprobe with me and the hours afterwards helping me understand the results. In addition, thanks to Roberta Rudnick and Igor Puchtel for agreeing to be on my committee despite busy schedules of their own. Thank you to James Farquhar for agreeing to be an unofficial committee member and answering many questions about oxygen isotopes without getting irritated. I also owe my deep gratitude to Mary Horan at the Carnegie Institute of Washington for all her efforts in obtaining Sr and Nd isotopic data for the Taitao peridotites. Many thanks also to Jane Hammarstrom and John Slack at USGS in Reston, VA, for allowing me to use their wonderful Nikon microscope on days that I worked from home. I would like to thank my parents for always believing in me, loving me and supporting me even when there were times when they thought I might have lost my way and my mind. Mom, you are the steady voice inside my heart that keeps me walking even in the dark. Dad, you are the positive ray of light that brightens my mind and pushes me forward. ii To my husband, thank you for the endless encouragement, for the shoulder to lean on, the ear to bend and the hand to hold. You are my soul and teammate. I am so blessed to have found you. I cherish you; love you, and thank you for bringing out the best in me. To Celia, my beautiful daughter, thank you for allowing me the time away from you to finish my degree and for growing into such a wonderful little girl in spite of my many mistakes. To my second child who will be born several months after I have finished my thesis, I hope you will learn to love Earth and its environment with as much wonder as much as I do. I also want to thank my fellow graduate students for all their support, in particular Tracey Centorbi, Tom Ireland, Nick Geboy, and Margaret Anne Baker. There were countless times when each of you helped me in the lab or answered questions I’m sure I had already asked. I would not have been able to finish without you. Thanks also to Andy Masterson for all the hours he spent loading and running my olivine grains for oxygen analysis as well as the time he took to explain the analytical methods to me. Last but certainly not least, I would like to extend my deep appreciation to Manuel Schilling for the long hours he spent in the lab confirming my Os isotope analyses and for sharing his thoughts and resources on the Taitao Ophiolite. Muchas gracias para tu ayuda. I hope one day I am able to visit Chile and see the Taitao Peninsula myself. iii TABLE OF CONTENTS LIST OF TABLES.............................................................................................................. v LIST OF FIGURES ........................................................................................................... vi CHAPTER 1: INTRODUCTION....................................................................................... 1 1-1 Review of chemical heterogeneity in the mantle.................................................. 2 1-2 The Re-Os isotope system in the mantle............................................................... 7 1-3 Purpose of This Work .......................................................................................... 11 2-1 Regional Geology .................................................................................................. 13 2-2 Sample Descriptions ............................................................................................. 20 CHAPTER 3: ANALYTICAL METHODS ..................................................................... 23 3-1 Re-Os Isotopic Analysis ....................................................................................... 23 3-2 Electron Microprobe Analysis............................................................................. 27 3-3 XRF Analysis......................................................................................................... 29 3-4 Olivine Oxygen Isotope Analysis......................................................................... 32 3-5 Whole Rock Oxygen Isotope Analysis ................................................................ 33 3-6 Sm-Nd and Rb-Sr Isotope Analyses.................................................................... 34 CHAPTER 4: RESULTS.................................................................................................. 36 4-1 Major elements ..................................................................................................... 36 4-2 Trace elements ...................................................................................................... 43 4-3 Olivine and chromite............................................................................................ 50 4-4 Isotopic results ...................................................................................................... 56 CHAPTER 5: DISCUSSION............................................................................................ 64 5-1 Evidence for variable extents of partial melting................................................ 64 5-2 Alteration History................................................................................................. 66 5-3 Re-Os isotope systematics .................................................................................... 70 5-4 Chromite Separates .............................................................................................. 75 5-5 Mafic Rocks........................................................................................................... 76 5-6 Sm-Nd and Rb-Sr isotope systematics................................................................ 77 5-7 Implications of isotopic heterogeneities.............................................................. 83 CHAPTER 6: CONCLUSIONS ....................................................................................... 87 APPENDIX 1: TABLE OF MODAL MINERALOGY ................................................... 88 APPENDIX 2: THIN SECTION DESCRIPTIONS......................................................... 90 REFERENCES ............................................................................................................... 108 iv LIST OF TABLES Table 3-1. Total Analytical Blank (TAB) Data. .............................................................. 26 Table 3-2. Osmium standard analyses for VG Sector 54 TIMS...................................... 26 Table 3-3. Osmium standard analyses for NBS Bobcat TIMS........................................ 27 Table 3-4. Forsterite Fo90 primary olivine standard ....................................................... 28 Table 3-5. Bushveld chromite standard. .......................................................................... 29 Table
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