ABSTRACT Title of Document: A STUDY OF THE SULFUR ISOTOPIC COMPOSITION OF MARTIAN METEORITES Heather B. Franz, Ph.D., 2012 Directed By: Professor James Farquhar, Department of Geology and ESSIC Sulfur is an important tracer for geochemical processes because it possesses four stable isotopes and forms natural compounds in a range of oxidation states. This element has been shown to undergo mass-independent isotopic fractionation (S-MIF) during laboratory photochemical experiments, which may provide clues to processes that have occurred both in the solar nebula and in planetary atmospheres. The surface of Mars has been found to contain ubiquitous sulfate minerals, marking this planet as an ideal candidate for sulfur isotope study. The shergottites comprise the youngest group of martian meteorites and the most representative of mantle-derived igneous rocks. Extraction and isotopic measurement of sulfur from 30 shergottites yield the first estimate of the juvenile martian sulfur composition, which matches within uncertainties that of Cañon Diablo Troilite. Analysis of martian meteorites spanning a range of ages from the shergottites, as young as ~150 Ma, to the nakhlites, ~1.3 Ga, reveals the presence of sulfur characterized by S-MIF compositions. These findings are interpreted as evidence for cycling of sulfur between an atmospheric reservoir where photochemical processing of sulfur-bearing gases occurred and a surface reservoir in which photochemical products were ultimately deposited. Anomalous sulfur has been detected in both sulfate and sulfide minerals, implying assimilation of sulfur from the martian surface into magmas. Differences in the S-MIF compositions of the nakhlites and shergottites may preserve a record of complementary sulfur formed by a single process or may indicate the operation of multiple photochemical processes at different times or geographical locations. Identification of the photochemical mechanism responsible for producing the anomalous sulfur observed in martian meteorites is important for constraining the atmospheric composition at the time the S-MIF signals were generated. Results of laboratory experiments with pure SO2 gas suggest that self-shielding is insufficient to explain the anomalous sulfur isotopic composition. This implies that an optically thick SO2 column in the martian atmosphere may not have been required for production of the observed signals. A STUDY OF THE SULFUR ISOTOPIC COMPOSITION OF MARTIAN METEORITES By Heather B. Franz Dissertation 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 Doctor of Philosophy 2012 Advisory Committee: Professor James Farquhar, Chair Professor Philip A. Candela Assistant Professor Sarah Penniston-Dorland Professor Richard J. Walker Dean’s Representative Professor Matthew C. Hansen © Copyright by Heather B. Franz 2012 Preface This document contains original material that has not been published elsewhere except selected data presented in the following abstracts: Franz, H. B., Farquhar, J., and Kim, S.-T. (2008) Sulfur isotopic composition of multiple mineral phases in shergottites. Lunar and Planetary Institute, #2433. Franz, H. B., Farquhar, J., and Irving, A. J. (2010) Acid-volatile sulfur isotopic composition of seven shergottites from northwest Africa. Lunar and Planetary Institute, #2341. Franz, H. B., Farquhar, J., and Irving, A. J. (2011) Acid-volatile sulfur isotopic composition of six shergottites. Lunar and Planetary Institute, #2338. ii Dedication This work is dedicated to Phoebe, my only light through the darkest days. I love you for all time, angel. I miss you. "All the darkness in the world cannot extinguish the light of a single candle." - Saint Francis of Assisi iii Acknowledgements I have so many people to acknowledge that I fear I will inadvertently miss someone. I must first thank my advisor, James Farquhar, for having the fortitude to travel this long and arduous path with me. I am grateful to many people at UM: Thanks to Rich Walker and Mike Brown for giving me the chance to pursue this crazy idea of becoming a geochemist (although Rich, you should have tried harder to talk me out of it); to my other committee members, Rich Walker, Phil Candela, Sarah Penniston-Dorland, and Matt Hansen, for their support; to Andy Campbell, who served on my proposal committee; to the many folks of the Farquhar group, past and present, who have provided lab assistance, especially Dave Johnston, Andy Masterson, Boz Wing, Sang-Tae Kim, Alexey Kamyshny, Harry Oduro, Nanping Wu, and Jon Banker; to Phil Piccoli for crucial and timely assistance with EPMA analyses; and to others in the Geology department who have been so helpful over the years – Sandy Romeo, Dorothy Brown, Todd Karwoski, Suzanne Martin, and Michelle Montero. I am also grateful to Mark Lewis, formerly of the Aerospace Engineering department, for being my stalwart supporter and friend. From NASA Goddard Space Flight Center, I would like to thank Pan Conrad, Stephanie Getty, Jim Golder, Amy McAdam, Ed Patrick (now at SwRI), Craig Roberts, Jen Stern, and Inge ten Kate (now at University of Utrecht) for moral support; Inge also for translating ancient sulfur-related papers from their original German ; Alex Pavlov, Melissa Trainer, and Reggie Hudson for discussions about photochemistry; Manuel Quijada and Tim Madison for measuring the D2 lamp spectrum; Mollie Powell for assistance with polishing the Y000593 meteorite section; iv Pan Conrad, Jason Dworkin, and Jen Eigenbrode for tossing around research topics; Jamie Elsila for consultation on small sample analysis with the MAT-253; and especially Paul Mahaffy for the resources to make it all possible. At UCLA, thanks to Kevin McKeegan, Rita Economos, and Axel Schmitt for ion microprobe analyses. Thanks also to Rhoda Tuit for her warm hospitality. Thanks to James Day of UCSD, John Jones of NASA Johnson Space Center, Sebastien Danielache of the Tokyo Institute of Technology, Chris Herd of the University of Alberta, and Sushil Atreya of the University of Michigan for insightful discussions. I’m also grateful to Andrew Steele of the Carnegie Institute of Washington for support with version #1 of the meteorite study. This work could not have been performed without access to martian meteorite samples, for which I gratefully acknowledge the following people: the Meteorite Working Group for RBT 04261, LAR 06319, EET 79001, ALH 77005, and QUE 94201; L. Welzenbach and T. McCoy of the Smithsonian National Museum of Natural History for Shergotty; S. Ralew of Chladni’s Heirs for NWA 4925; M. N. Rao and L. Nyquist of NASA Johnson Space Center for Zagami; J. Zipfel of Forschungsinstitut und Naturmuseum for DaG 476 and SaU 005; C. Smith of the British Natural History Museum for Los Angeles; H. Kojima of the National Institute of Polar Research for Y980459; A. Treiman of the Lunar and Planetary Institute for NWA 5789; T. Bunch for NWA 998; and B. Zanda of the Museum National d’Histoire Naturelle for Chassigny. The remaining meteorites were provided by A. J. Irving of the University of Washington. v Last but certainly not least, I must thank my wonderful family, who have always been there for me my parents, who raised me with an appreciation for education and the wonders of both art and science, and taught me that I could be whatever I wanted to be; my late grandparents on both sides, who provided unconditional love; my sisters, who have supported me throughout the journey with sympathetic ears; my furry four-legged kids, who remind me daily of what’s truly important; and my husband Bryan, who is still my best friend after all these years. vi Table of Contents Preface........................................................................................................................... ii Dedication .................................................................................................................... iii Acknowledgements ...................................................................................................... iv Table of Contents ........................................................................................................ vii List of Tables ................................................................................................................ x List of Figures .............................................................................................................. xi 1 Chapter 1: Introduction ................................................................................................ 1 1.1 Introduction to Mars ..................................................................................... 1 1.1.1 A Brief History of Our Neighboring Planet .............................................. 1 1.1.2 Sulfur on Mars .......................................................................................... 6 1.1.3 Martian Meteorites .................................................................................. 12 1.2 Sulfur Isotopes ........................................................................................... 18 1.2.1 Sulfur Isotope Systematics .................................................................. 18 1.2.2 Applications of Sulfur Isotope Ratios ................................................. 22 1.3.3 Previous Studies of Sulfur in Martian Meteorites............................... 25 1.3 Research Goals...........................................................................................