ASSESSING THE HETEROGENEITY OF THE TISSINT SHERGOTTITE STREWN FIELD USING RB-SR, SM-ND AND LU-HF ISOTOPE SYSTEMATICS --------------------------------------------------- A Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston -------------------------------------------------------- In Partial Fulfillment of the Requirements for the Degree Master of Science ------------------------------------------------------- By Stephanie Elaine Suarez August 2019 ASSESSING THE HETEROGENEITY OF THE TISSINT SHERGOTTITE STREWN FIELD USING RB-SR, SM-ND AND LU-HF ISOTOPE SYSTEMATICS ___________________________________________ Stephanie Elaine Suarez APPROVED: ___________________________________________ Dr. Thomas Lapen, Advisor ___________________________________________ Dr. Alexander Robinson ___________________________________________ Dr. Brian Beard ___________________________________________ Dr. Dan E. Wells, Dean, College of Natural Sciences and Mathematics ii ACKNOWLEDGEMENTS I would first like to acknowledge my advisor Dr. Tom Lapen for the opportunity to work on this amazing project. Thank you for your patience and guidance throughout this entire process. Next, I would like to thank my committee members Dr. Brian Beard for his assistance operating the TIMS and Dr. Alexander Robinson for his input and assistance. Thank you to Dr. Minako Righter, for her assistance operating the MC-ICP-MS, training me in the clean lab, and instilling the importance of having work-life balance. Our collaborator Anthony ‘Tony’ Irving for his wisdom on Martian geology. I would also like to thank our other collaborators for insightful conversations. My mother Cecelia 'Cece' Suarez, my reason for striving for a career that will one day give us a better life. Our numerous dogs that we have rescued and cared for during the course of this thesis. Your tiny paws will always have a special place in my heart. Thank you to the supportive faculty, staff and students I have come across at the Earth and Atmospheric Science department. I treasure the daily conversations we have had on our best and worst days. Thank you to my former undergraduate research advisor Dr. Elizabeth Catlos for giving me the opportunity to become a researcher as well as our collaborator Dr. Michael Brookfield for his continued guidance. And lastly, anyone who I may have overlooked that has mentored me over the years or inspired me to keep going. iii ASSESSING THE HETEROGENEITY OF THE TISSINT SHERGOTTITE STREWN FIELD USING RB-SR, SM-ND AND LU-HF ISOTOPE SYSTEMATICS --------------------------------------------------- An Abstract of a Thesis Presented to the Faculty of the Department of Earth and Atmospheric Sciences University of Houston -------------------------------------------------------- In Partial Fulfillment of the Requirements for the Degree Master of Science ------------------------------------------------------- By Stephanie Elaine Suarez August 2019 iv Abstract Tissint, the 5th witnessed Martian meteorite fall, occurred on July 18, 2011 east of Tata, Morocco. While most studies of Tissint classify it as a shergottite relatively depleted in lithophile incompatible trace elements (e.g., light REE), crystallization age determinations have been variable. Currently, there is a discrepancy in crystallization age determination amongst three separate labs (UH: University of Houston; LLNL: Lawrence Livermore National Labs; and NASA-JSC: NASA Johnson Space Center) using combinations of Lu-Hf, Sm-Nd, and Rb-Sr analyses. Each lab tested one single fragment from the entire Tissint meteorite strewn field, which included samples (UWB1), (ASU#1744), (UNM#645). Sm-Nd analyses were performed at all three labs and produced two different dates. Analyses from two of the three labs, UH and LLNL, are in agreement and give a combined Sm-Nd age of 593±25 Ma for samples UWB1 and ASU#1744. Analysis at NASA-JSC provided a date of 472±36 Ma (sample UNM#645). This is approximately a 120 Ma difference in crystallization ages for separate samples analyzed from the strewn field. To confirm whether materials of different isotopic compositions and ages were co-mingled resulting in a heterogeneous strewn field, eight separately-collected fragments from the Tissint strewn field, including the sample analyzed at JSC (UNM#645) were examined using Rb-Sr, Sm-Nd and Lu-Hf isotopic systems. Mineral and leached bulk rocks analyzed at UH and LLNL define isochrons of 571±84 Ma, 559±39 Ma and 590±49 Ma, respectively. In all three isotopic systems, the analyses performed at UH and LLNL plot along the same isochron with slopes consistent with a v ~590 Ma igneous crystallization age, including UNM#645 dated at 495±35 Ma by NASA-JSC. While the Lu-Hf isotopic system indicates no evidence of contamination or element mobility, the Rb-Sr and Sm-Nd isotopic systems show evidence for element mobility and potential mixing with an isotopic component not in equilibrium with the igneous phases. Despite these issues, there is no evidence for multiple igneous sources. Fragments identified so far are cogenetic and the Tissint strewn field appears to be homogeneous. vi Table of Contents Introduction ........................................................................................................................ 1 Sample Descriptions ......................................................................................................... 10 Methods ............................................................................................................................ 12 3.1 Sample Preparation ........................................................................................................... 12 3.2 Sample Digestion ................................................................................................................ 12 3.3 Ion-exchange chromatography ......................................................................................... 13 3.3.1 Rb-Sr ...........................................................................................................................................13 3.3.2 Lu-Hf and Sm-Nd ......................................................................................................................14 3.4 Mass Spectrometry ............................................................................................................ 15 3.4.1 Multi-collector Inductively Coupled Mass Spectrometry (MC-ICPMS) .............................15 3.4.1.1 Lu .........................................................................................................................................15 3.4.1.2 Hf .........................................................................................................................................16 3.4.1.3 Sm ........................................................................................................................................17 3.4.1.4 Nd .........................................................................................................................................17 3.4.2 Thermal Ionization Mass Spectrometry (TIMS) ....................................................................18 Results .............................................................................................................................. 19 4.1 Lu-Hf ................................................................................................................................... 19 4.2 Rb-Sr ................................................................................................................................... 22 4.3 Sm-Nd ................................................................................................................................. 24 Discussion ........................................................................................................................ 26 5.1 Crystallization Age Comparison ...................................................................................... 26 5.2 Isotope systematics of leachates and residues ................................................................. 28 Conclusions ...................................................................................................................... 32 References ........................................................................................................................ 34 vii List of Figures, Tables and Equations Figures 1. Mixing curves for calculated source compositions for Martian meteorites 4 2. Hand specimens of Tissint collected in Morroco in 2011 5 3. Proposed mechanism for a heterogenous strewn field 9 4. Hand specimens of Tissint used in this study 11 5. Lu-Hf isochron 21 6. Rb-Sr isochron 23 7. Sm-Nd isochron 25 8. All internal isochron ages 27 9. Rb-Sr Leachate of Tissint and other 1.1Ma ejected Martian meteorites 31 Tables 1. Compilation of Tissint ages from LLNL, JSC, and UH 7 2. Lu-Hf Data for Tissint 20 3. Rb-Sr Data for Tissint 22 4. Sm-Nd Data for Tissint 24 5. Ages determined for Tissint from previously published data and this study 27 Equations 1. Isochron Age 19 viii Chapter 1 Introduction Martian meteorites are rocks that were once a part of Mars and have since landed on Earth. Meteoroids that impact Mars’ surface have the potential to eject materials into space; some of which crosses Earth's orbit and fall as meteorites. (Melosh, 1984; Head and Melosh, 2000). Analysis of these Martian meteorites, our only available physical material from Mars,