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Petrological Investigations of Regolithic Howardites and Carbonaceous Chondrite Impact Melts

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Petrological Investigations of Regolithic Howardites and Carbonaceous Chondrite Impact Melts University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 8-2015 Insights into planetesimal evolution: Petrological investigations of regolithic howardites and carbonaceous chondrite impact melts Nicole Gabriel Lunning University of Tennessee - Knoxville, [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Part of the Cosmochemistry Commons, Geology Commons, and the The Sun and the Solar System Commons Recommended Citation Lunning, Nicole Gabriel, "Insights into planetesimal evolution: Petrological investigations of regolithic howardites and carbonaceous chondrite impact melts. " PhD diss., University of Tennessee, 2015. https://trace.tennessee.edu/utk_graddiss/3510 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Nicole Gabriel Lunning entitled "Insights into planetesimal evolution: Petrological investigations of regolithic howardites and carbonaceous chondrite impact melts." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Geology. Harry Y. McSween, Major Professor We have read this dissertation and recommend its acceptance: Joshua P. Emery, Theodore C. Labotka, Timothy J. McCoy, Michael J. Sepaniak Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) Insights into planetesimal evolution: Petrological investigations of regolithic howardites and carbonaceous chondrite impact melts A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Nicole Gabriel Lunning August 2015 DEDICATION To Zack and my family ii ACKNOWLEDGEMENTS I would like to thank Hap McSween for accepting me as a PhD student, and for his guidance, encouragement, and support. Immediately after I decided to pursue my PhD at University of Tennessee with him as my advisor, Hap looked me in the eye and said “You made the right choice.” And I absolutely did. I have left almost every meeting with Hap even more excited about my research than when I entered, and eager to get back to work. Under his supervision, I have become a better petrologist, teacher, and writer. I would like to thank Tim McCoy and Cari Corrigan for giving me the opportunity to work at the Smithsonian and for taking the time to teach me about meteorites. Thank you, Tim and Cari, for your support when I decided to go back to graduate school and for continuing to mentor me during my PhD. I am grateful to Josh Emery, Ted Labotka, and Michael Sepaniak for serving on my committee, giving me constructive feedback, and helping me grow professionally. I would like to thank Devon Burr and Larry Taylor for constructive comments and advice on professional presentations. I am grateful to Allan Patchen for sharing his electron microprobe expertise and dry wit. I would like to thank my friends and fellow “Hap students” Arya Udry and Andrew Beck for guidance concerning the road immediately ahead of me, and Chris Wetteland and Tim Hahn for camaraderie and meteorite discussions. Chloe Beddingfield, Ashley Dameron, Sam Peel, Brianne Smith, Mike Lucas, Eric Maclennan, Robert Jacobsen, Richard Cartwright, Latisha Brengman, Sarah Sheffield, Nate Wigton, Cam Hughes, Chanda Drennen, Driss Takir, and many more people—sometimes grad school was rough—having all of you as friends and colleagues has made it a lot more fun! I would also like to thank the rest of the EPS department— particularly Melody, Angie, and Teresa—for support, encouragement, and for generally making the department a warm and welcoming place. I would like to thank my family for sending love and encouragement from the north. Your support and confidence in me throughout my life has been instrumental to this achievement and to much of my personal happiness. I thank my mom for her unwavering confidence in my intelligence and potential. I thank my dad for encouraging my interest in science and space from an early age. Andy, Nise, Tim, and James: You are among the smartest people I know, and growing up with you definitely made me smarter. Last, but far from least, I would like to thank Zack for being a well of support that even though I draw from it heavily seems never in danger of running dry. Thank you. iii ABSTRACT Asteroidal meteorites are the only available geologic samples from the early part of our solar system’s history. These meteorites contain evidence regarding how the earliest protoplanetary bodies formed and evolved. I use petrological and geochemical techniques to investigate the evolution of these early planetesimals, focusing on two meteorite types: Howardites, which are brecciated samples of a differentiated parent body (thought to be the asteroid 4 Vesta), and CV chondrites, which are primitive chondrites that have not undergone differentiation on their parent body. Quantitative petrological analysis and characterization of paired regolithic (solar wind-rich) howardites indicate that this large sample of the surface regolith of Vesta is assembled from a range of diverse source materials ( > 18 basaltic and ultramafic lithologies). The abundance of plagioclase in these howardites is depleted relative to unbrecciated source lithologies, which provides evidence that plagioclase is preferentially comminuted by impact gardening on Vesta, as previous studies have suggested it is in lunar regolith. The extent of plagioclase depletion in solar wind-rich howardites may be an indicator of regolith maturity. The regolithic howardites I studied contain fragments of Mg-rich olivine and pyroxene in low modal abundance ( < 1%); we present geochemical evidence that these mineral fragments are the first recognized mantle samples from Vesta. In general, mantle samples from differentiated meteorite parent bodies are lacking in meteorite collections, which presents an obstacle for understanding the differentiation of these early planetesimals. The geochemical signatures of these Mg-rich olivine and pyroxene fragments suggest that Vesta’s mantle may have only partially melted during its differentiation, rather than forming a whole-mantle magma ocean. In a regolithic howardite and two CV chondrites, I found impact melt clasts derived from primitive chondrite precursors (CV and CM chondrites). Previously, it had been hypothesized that impact melts might not form from carbonaceous chondrites because their high volatile content might explosively disrupt the formation of a cohesive melt. These clasts provide evidence that impact melts of these precursors can form, that these melts experience some volatile loss, and that they retain some of the redox trends of their precursors. iv TABLE OF CONTENTS INTRODUCTION .............................................................................................................. 1 References ....................................................................................................................... 4 CHAPTER I Grosvenor Mountains 95 howardite pairing group: Insights into the surface regolith of asteroid 4 vesta .................................................................................................. 6 Abstract ........................................................................................................................... 7 Introduction ..................................................................................................................... 7 Materials and methods .................................................................................................. 10 Meteorite samples ..................................................................................................... 10 Mineral chemistry and modal assemblages .............................................................. 11 Cosmogenic nuclides ................................................................................................ 12 Results ........................................................................................................................... 12 Textures and shock effects ........................................................................................ 12 Minerals .................................................................................................................... 13 Polymineralic (lithic) clasts ...................................................................................... 14 Cosmogenic nuclides ................................................................................................ 17 Discussion ..................................................................................................................... 18 GRO 95 howardite pairing ........................................................................................ 18 Diversity of source materials .................................................................................... 20 Eucrite:diogenite mixing ratio .................................................................................. 21 Plagioclase depletion
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