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AN ABSTRACT of the THESIS of Nicole D. Rocco for the Degree Of AN ABSTRACT OF THE THESIS OF Nicole D. Rocco for the degree of Master of Science in Geology presented on June 15, 2020. Title: Isotopic and Geochemical Signatures of Mantle and Crustal Contributions in Rhyolites from Okataina and Taupo Volcanoes, New Zealand Abstract approved: ______________________________________________________ Adam J.R. Kent Silicic caldera-forming eruptions are some of the largest and most destructive volcanic eruptions known, and present significant local and global hazards. The underlying processes within crustal magma plumbing systems that lead to the accumulation and eruption of large volumes of evolved magma remain enigmatic, yet there is broad consensus that interaction between mantle-derived magmas and surrounding crust is crucial to the generation of many silicic magmas. Constraining these processes are key to understanding the evolution of caldera-forming systems. Radiogenic isotopes are well-suited for deciphering mantle versus crustal contributions given they are not affected by fractionation and most sources have unique isotopic signatures. Here we present a suite of high precision whole pumice Pb and Sr isotope measurements from two caldera-producing volcanic centers, Okataina (OVC) and Taupo (TVC), in the world’s most active and voluminous rhyolitic volcanic system, Taupo Volcanic Zone (TVZ), New Zealand. Samples were collected from the most recent caldera-forming eruptions in the two volcanic centers along with smaller yet significant eruptions over the last c. 50 ka, with the aim of investigating spatial and temporal changes throughout a caldera cycle – the caldera- forming event and the smaller yet significant eruptions that occur between them. Glass major and trace elements complement our isotopic data, and help to elucidate contributions from mantle-derived and crustal sources and bring light to temporal changes surrounding a caldera cycle. Strontium and lead isotope compositions from both the OVC and TVC show significant offset from local basalts toward local upper crustal terranes (Waipapa and Torlesse metasediments), representative lower crust, and an enriched mantle component. Potential mechanisms for this include melting of the lower crust by intruding basalts and assimilation of the upper crust by more evolved magmas during storage and transport. Mixing trends show these possibilities along with the complexities presented by variable compositions of source endmembers. Two hypotheses are proposed for explaining the observed OVC and TVC Sr and Pb isotopic compositions: 1. Local TVZ basaltic magmas are interacting with upper crustal rocks – specifically, the Torlesse metasedimentary composite superterrane – that comprise the eastern North Island, integrating radiogenic Pb and Sr from the upper crust. 2. Local TVZ basalts are influenced by an enriched mantle component sourced in the deep mantle and are melting the lower crust (represented by granulitic xenoliths found in TVZ intermediate lavas) on their ascent, thus bringing more radiogenic Pb from the source as well as Pb and Sr from the lower crust. The data also reveal distinct signatures for eruptions sourced from each volcanic center. TVC eruptions are more radiogenic in both Sr and Pb isotope compositions, suggesting either greater extent of crustal input or distinctly different crustal Pb and Sr sources. Glass compositions support these notions, but on a more temporal scale. For each volcanic center, there is a marked change in major and trace element compositions after a large caldera-forming eruption, indicating a deepening of the magmatic system to less evolved compositions, higher pressures, and potentially more ability to melt and assimilate the crust. This is also reflected in Sr isotopic compositions; higher pressures correlate with more radiogenic Sr ratios, suggesting that deeper magmas may either have greater interaction with the crust or are interacting with more radiogenic crustal components. ©Copyright by Nicole D. Rocco June 15, 2020 All Rights Reserved Isotopic and Geochemical Signatures of Mantle and Crustal Contributions in Rhyolites from Okataina and Taupo volcanoes, New Zealand by Nicole D. Rocco A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented June 15, 2020 Commencement June 2021 Master of Science thesis of Nicole D. Rocco presented on June 15, 2020 APPROVED: _____________________________________________________________________ Major Professor, representing Geology _____________________________________________________________________ Dean of the College of Earth, Ocean, and Atmospheric Sciences _____________________________________________________________________ Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. _____________________________________________________________________ Nicole D. Rocco, Author ACKNOWLEDGEMENTS I would like to express my deepest gratitude, first and foremost, to my advisor, Adam Kent. Through the many challenges that graduate school presents, you have continually been a support and staunch advocate. I have never once worried about the many path changes that this degree has presented, as I have always known you have had my back. You have provided insight when needed and allowed me to grow as a critical thinker and scientist, pushed me when it was required, and reassured me in nebulous times. You have been the perfect advisor for me. I am honored to have Dave Graham and Kari Cooper as my committee members. Dave, your inbox and office door was continually open to all my isotope-related questions, and you have instilled in me deep wonder and love of isotope geochemistry. I am in constant amazement of your incredible depth of knowledge and am honored to have learned from you, in and away from the classroom. I am grateful to have Kari on my team as well. Spending two weeks in the field with you in New Zealand was an opportunity of a lifetime – you are inspirational as a woman and a scientist. The entire GeoPRISMS Tephra Team (Tyler Schlieder, Elizabeth Grant, Olivia Barbee, and Lydia Harmon) have been a continual support and resource. Our group chats always left me feeling stoked and inspired and I cannot wait to see the compilation of work our group produces. I was fortunate enough to meet Sarah Smithies three- quarters of the way through this project – thank you so very much for all our chats, sharing of data, and brainstorming. In addition to my GeoPRISMS collaborators, the entire VIPER (and honorary VIPERS – Danielle Woodring and Kali Melby) group has been a constant source of learning and community. I could not be prouder to call myself a VIPER. I could not have completed this project without the guidance of Frank Ramos. I cannot express enough my deep appreciation for you opening your lab and home to me to complete my data collection after the Burt 2 fire. I have learned more from you about isotopic analysis than I ever imagined I would during this project and am even more inspired to spend my career in a lab because of you. Thank you for always answering my questions, no matter how small, and for pouring a mean Long Island after a long day in the lab. I would also like to acknowledge the GAIA team and Kelly Rose from the Department of Energy National Energy Technology Laboratory in Albany, OR. My fellowship in the GAIA lab has bolstered my growth as a scientist and as a collaborator. I am grateful to have been a part of this team throughout my graduate work, and am honored for the opportunities afforded to me at NETL. I would have never made it through these last three years if not for my partner JJ. Your unwavering support has held me afloat in even the most challenging times. You have never questioned my drive to return to school later in life and have been my biggest cheerleader every step of the way. Nikki Moore and Kit Kleinschmidt fill the other slots on the cheerleading squad. Nikki – you are my biggest academic mentor (and colleague). You have been available and with me at every turn, from calming me down over a beer or guiding me through the world of geochemistry, volcanoes, and writing. Kit – you are always a pragmatic voice, a committed believer in facts and science, and the sister I never had. Our adventures in the wilderness have brought sanity, serenity, and purpose into my life. Thank you for both for reminding me to quiet the imposter voice in my head and to balance my work life with the other things that I love. The biggest inspiration in my life is my mom. You have encouraged me to follow my dreams throughout my entire time on this planet, by your words and by your actions. Watching you return to school while I was in high school showed me that anything was possible if I was willing to work hard. I have witnessed you reinvent yourself again over the last two decades, all the while urging me to continue to do what I love, no matter what that entailed. You are my rock in every sense of the word and I would absolutely not have made it here without you. Lastly, my research was made possible by funding from the National Science Foundation (GeoPRISMS) and various grants and scholarships from Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences. TABLE OF CONTENTS Page 1 Introduction ...........................................................................................................
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