Magmatic volatiles at rifts and arcs: Sources and fractionation effects PDF
Preview Magmatic volatiles at rifts and arcs: Sources and fractionation effects
University of New Mexico UNM Digital Repository Earth and Planetary Sciences ETDs Electronic Theses and Dissertations 7-10-2013 Magmatic volatiles at rifts and arcs: Sources and fractionation effects Joost Maarten de Moor Follow this and additional works at:https://digitalrepository.unm.edu/eps_etds Recommended Citation de Moor, Joost Maarten. "Magmatic volatiles at rifts and arcs: Sources and fractionation effects." (2013). https://digitalrepository.unm.edu/eps_etds/16 This Dissertation is brought to you for free and open access by the Electronic Theses and Dissertations at UNM Digital Repository. It has been accepted for inclusion in Earth and Planetary Sciences ETDs by an authorized administrator of UNM Digital Repository. For more information, please [email protected]. i J. Maarten de Moor Candidate Earth and Planetary Sciences Department This dissertation is approved, and it is acceptable in quality and form for publication: Approved by the Dissertation Committee: Tobias Fischer , Chairperson Zachary Sharp , Co-chairperson Penelope King , Co-chairperson Charles Mandeville Charles Shearer ii MAGMATIC VOLATILES AT RIFTS AND ARCS: SOURCES AND FRACTIONATION EFFECTS by J. MAARTEN DE MOOR B.A., Earth and Environmental Science, Wesleyan University, 2003 M.S., Earth and Planetary Sciences, University of New Mexico, 2005 DISSERTATION Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Earth and Planetary Sciences The University of New Mexico Albuquerque, New Mexico May, 2013 iii © 2013, J. Maarten de Moor iv DEDICATION This work is dedicated to my grandparents, Joost and Roelfina de Moor and Herman and Truus van’t Hof, who have played such important roles in my education v ACKNOWLEDGEMENTS I would like to express my deep gratitude to all of those who have supported me and my research during the course of Ph.D. studies. Firstly, none of this work would have been possible without my advisors. Dr. Tobias Fischer has been a true mentor during both my M.S. and Ph.D. and provided me with strong support and encouragement in every aspect of my development as a volcanologist and geochemist. We have shared many incredible experiences in pursuit of gas samples, some of which were incomparably awe inspiring and some of which were simply terrifying. I will always be grateful for Tobias’ generous support, guidance, and trust. Similarly, Dr. Zach Sharp has had a huge influence on my development as a stable isotopist and geochemist and has been incredibly supportive during both my M.S. and Ph.D. work. Zach has taught me to question at a fundamental level and to explore ideas creatively. Working with him has always been a pleasure and an adventure in itself. Likewise, Dr. Penny King has been a great mentor and extremely supportive during my Ph.D. work. Her breadth of knowledge has been inspiring and she has taught me to interrogate data rigorously and critically. Penny’s support and inspiration has allowed me to accomplish analyses that I would not have otherwise even considered. I have enormous respect for my advisors and am extremely grateful for their support. I would like to thank all of those who have contributed to this work in the field and laboratory. Drs. Rick Hervig, Roman Botcharnikov, Bernard Marty, Dave Hilton, Erik Hauri, and Mike Zelenski have been inspiring and supportive in the laboratory and field and have significantly contributed to my scientific thought processes. Drs. Viorel Atudorei, Vitchko Tsanev, Mike Spilde, Lynda Williams, Adrian Brearley, and Mehdi Ali are all sincerely thanked for assistance with analytical methods. I would especially like to thank Drs. Charlie Mandeville, Liz Cottrell, and John Stix for sharing their analytical standards, influential discussions, and their work that has been inspirational to me. Carlos Ramirez, Frederick Mangasini, Jessica Rowland, Dr. Dereje Atalay, Dr. Peter Barry, and Andrés Ulloa all provided assistance and good company in the field. I would vi like to gratefully acknowledge the support and encouragement from the faculty in the Department of Earth and Planetary Sciences and the Institute of Meteoritics at UNM. The people of Engare Sero village, especially Olomolik (“Tall”) and Medimuni Juma, are thanked for their hospitality and logistical support during fieldwork at Oldoinyo Lengai. The Afar people of Kusa Wat and Durubu villages are thanked for armed protection, shelter, and resisting the temptation to shoot us themselves. This work would not have been possible without financial support. In particular, the NASA Earth and Space Science Fellowship provided funding in support of my research from 2009-2012. The MARGINS-NSF program and a GSA student research grant supported research on Anatahan. Work on Oldoinyo Lengai was funded by NSF Grant EAR0827352, the National Geographic Society, a GSA student research grant, and NSF funding in support of the Arizona State University National SIMS Facility. Research on Erta Ale and Masaya was supported by NSF Grant EAR1049891, the NASA Earth and Space Science Fellowship, and the Caswell-Silver Foundation. Finally, I would like to thank my family and friends from the bottom of my heart for bringing light to my life. Mom, Dad, Zand, Em, Oma Jobs, Oma, Jansie, Diederik, Andra, Odette, Monique, Liesbelle, Meghan, Zach, Scotty, Matt, Leah, Paul, Big Zach, Oli, Ant, Tiappa, Carol, Kana, Dylan, Spruce, Ryan, Kym, Amber. Love you all. vii MAGMATIC VOLATILES AT RIFTS AND ARCS: SOURCES AND FRACTIONATION EFFECTS by J. MAARTEN DE MOOR B.A., Earth and Environmental Science, Wesleyan University, 2003 M.S., Earth and Planetary Sciences, University of New Mexico, 2005 ABSTRACT Mantle degassing is a fundamental process that modifies the chemical composition of the atmosphere, crust, and deep Earth. Elements are returned to the mantle from surface reservoirs through subduction at convergent margins. Melts generated in the mantle carry dissolved volatiles from depth and upon decompression exsolve a fluid or vapour phase. These volatiles drive explosive eruptions, change climate, and form economic metal deposits. Thus, the sources and processes involved in degassing of the mantle and melts are a crucial topic in geochemistry. This dissertation is composed of four chapters, each contributing original and new insights into sources and processes involved in mantle degassing. Chapters 1 and 2 focus on volatiles and the origin of carbon-rich magmatic emissions in the East African Rift, which is perhaps the least understood tectonic system on Earth in terms of mantle degassing. East Africa hosts a diverse array of enigmatic magma compositions, best exemplified by Oldoinyo Lengai volcano in Tanzania, which is the only active carbonatite volcano on Earth and an important alkaline endmember magmatic system. In Chapter 1 (published in Earth and Planetary Sciences in January, 2013) we show that nephelinite melts at Oldoinyo Lengai are the most carbon-rich natural silicate melts known to science. However, we argue that the mantle source for these melts is not unusually rich in carbon. Rather, extreme degrees of fractional crystallization and the viii high carbon solubility in these alkali-rich melts are responsible for the high observed carbon contents. We also show that the Oldoinyo Lengai magmatic system is rich in water, contrary to prior assumption, and that water degassing plays a fundamental role in the eruptive behaviour and magmatic evolution at Oldoinyo Lengai. The appendix to Chapter 1 contains additional data (including carbon, oxygen ,and sulfur isotope compositions and assessment of sulfur behaviour in the magma system) from Oldoinyo Lengai that have not been published. Chapter 2 presents the first bulk gas and nitrogen isotope compositions from the Rungwe Volcanic Province, which is the southernmost volcanic manifestation of rifting in East Africa. In this paper (currently in press in Chemical Geology), we show that the gases emitted at Rungwe are CO -rich, presenting a hazard to the inhabitants of this 2 fertile and thus heavily populated area. We find that the gas compositions record high temperatures in the deeper hydrothermal system, which could be a valuable geothermal resource for local economic development. The nitrogen isotope compositions and gas ratio tracers are consistent with an upper mantle source for the gases, and this signature is strongest in the central part of the province where the intersection of deep crustal structures provide direct conduits for mantle degassing. Interestingly, the mantle signatures are associated with the lowest temperature emissions in the region. The structures provide conduits for melts as well as gases, leading to the coincidence of mantle gas emission and volcanic edifice building in the central Rungwe Volcanic Province. These high elevation mountains are the recharge zones for the shallow aquifer and the mantle gases thus equilibrate with cold meteoric water close to the surface. Chapters 3 and 4 are studies of S degassing at rift and arc volcanoes through the use of S isotopes. Sulfur is extremely important for eruption prediction because SO is 2 the only gas routinely measured remotely at active volcanoes. Explosive eruptions can inject large quantities of S into the stratosphere where it oxidizes to sulfate, which can cause decadal climate cooling. Chapter 3 examines S degassing during an explosive eruption at Anatahan ( Mariana Arc) that emitted about 250 ktons of SO in the first ten 2 ix days of the eruption. The results of this study show that the source of erupted sulfur was from ultimately from the mantle, with little addition from subducted seawater sulfate, or the pre-eruptive hydrothermal system. Sulfur isotopes fractionate during the degassing process, and the change in sulfur isotope compositions through the eruption are consistent with closed system degassing of a magma body. Chapter 4 presents a detailed assessment of the S cycle at persistently degassing basaltic volcanoes. In this study, we constrain the conditions of degassing (oxygen fugacity, temperature) as rigorously as possible to utilize S isotope compositions of gases and melts to address equilibrium versus non-equilibrium degassing and Earth’s sulfur cycle. We find that S degassing is not an equilibrium process and that S partitioning into the gas phase is associated with a kinetic fractionation effect reflected in the S isotope compositions. Erta Ale, a reduced magmatic system, convincingly demonstrates a kinetic effect because S2- (rather than SO 2-) is the dominant S species in the melt. Equilibrium 4 degassing should result in preferential partitioning of the heavy S isotope into the gas, however we observe that the gas is isotopically lighter than the melt. This is consistent with faster diffusion of the light isotope. At Masaya, an oxidized magmatic system, the equilibrium and kinetic isotope fractionation effects both favor isotopically light gas, which makes the contributions from equilibrium versus kinetic effects more ambiguous. However, under steady state conditions the gas phase is representative of the sulfur isotope composition of the source and the isotope composition of gas from Masaya indicates recycling of oxidized S through the subduction zone. Mass balance calculations show that only a small fraction of the S subducted at the Central American arc is returned to surface reservoirs. However, the flux of oxidized S from the subducted slab is high enough to rapidly oxidize iron in the mantle wedge, providing an explanation why arc magmas are more oxidized than those at rifts. Finally, the S retained in the subducted slab is isotopically light, potentially carrying the signature of microbial life into the deep mantle.
Description:The list of books you might like
