Master of Science (MS)
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Martian poikilitic shergottite meteorites are cumulate rocks that can be used to understand melt evolution on Mars from the bottom of the crust (~10 kbar) all the way up to near-surface shallow depths (~1 kbar). There is a general lack of knowledge regarding shergottites including, parental magma compositions, crystallization ages, and the location of the enriched geochemical source of the shergottites in the martian mantle. Through a comprehensive petrographic and geochemical study, we have attempted to better understand poikilitic shergottite formation and the evolution of the martian interior during the Amazonian period (~3 Ga – present day). We studied a comprehensive suite of poikilitic shergottites including Northwest Africa (NWA) 7755, NWA 11043, NWA 11065, NWA 10618 and Alan Hills (ALHA) 77005 using major, minor, and trace element compositions acquired through analyses of olivine hosted melt inclusions found within both the early-evolutional stage textural domain and late-evolution stage textural domain of the poikilitic shergottites, as well as crystallization age of NWA 7755. We accounted for diffusion and calculated accurate parental trapped liquid compositions from these melt inclusions, representative of the parental melt, using Petrolog3 and MELTS thermodynamic modeling software. Major elements reflected in our MI indicate magmatic evolution between the early and late textural stages linking them petrogenetically. Calculated parental melt compositions from these melt inclusion analyses yielded results that also petrogenetically links the poikilitic shergottites with other shergottite subgroups. Trace element ratios for the poikilitic shergottites in our sample suite were parallel to that of their bulk rock counterparts, suggesting they are sampling from a single source and the lack of scatter in trace element data suggests minimal post-emplacement processes. These parental melt compositions also displayed K-enrichment in the late-stage domain MI found in NWA 7755, NWA 10618, and NWA 11065, suggesting an exogenous K-rich component addition likely from k-rich material being introduced during magma ascent. The176Lu/177Hf versus 176Hf/177Hf 5-point isochron crystallization age of 223 ± 46 Ma yielded for NWA 7755 fits into the expected range for enriched shergottites of ~165 Ma to 225 Ma. Northwest Africa 7755 crystallization age, 176Lu/177Hf and 147Sm/144Nd source composition, being so similar to that of the other enriched shergottites suggests NWA 7755 likely shares a long-lived geochemical source with these samples that has lasted for at least 75 Ma or is derived from another geochemically enriched source. In addition, although isotopic composition shows that NWA 11043 might be intermediate, we cannot say for sure as we were not able to obtain a specific crystallization age. This is likely due to the effects of desert alteration and/or potentially source mixing in NWA 11043 making the sample extremely difficult to study isotopically.
Crystallization age; Martian magmatism; Martian meteorites; Melt inclusions; Parental magma compositions; Shergottites
University of Nevada, Las Vegas
O'neal, Evan W., "Source and Parental Melts of Poikilitic Shergottites: Implications for Martian Magmatism" (2022). UNLV Theses, Dissertations, Professional Papers, and Capstones. 4527.
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