Stable isotope systematics of the martian regolith
Creators: name: Loiselle, Liane
Publisher: name: The Australian National University
Year: 2021

description: The geologic history of a planetary body is recorded within its rock record. As such, meteorites provide an opportunity to answer questions about the geologic history of other planetary bodies. NWA 7034 and its paired stones represents a new class of martian meteorites that are the first examples of the martian surface available for analysis in terrestrial laboratories. Regolith breccia samples are composed of rock fragments from a diverse array of lithologic sources. As such, they consist of several lithologies that are not represented by other martian meteorites, providing an opportunity to further expand our understanding of Mars. The stable isotopic systematics of these martian regolith breccia (MRB) samples represents an opportunity to expand our understanding of geological processes and geochemical cycling on Mars, particularly those invoked of incorporating crustal or surficial deposits. A powerful tool when studying the isotope systematics of complex geological materials are SIMS techniques, as they offer unparalleled capability for the in situ isotopic analysis of samples at the microscale. The aims of this thesis were twofold: first to develop a high-resolution SIMS technique to be able to analyse the 16O, 17O, 18O isotopic compositions of achondrite materials, more specifically martian meteorites, and secondly, to assess the stable isotope (O and S) systematics of MRB samples with high-resolution, high-precision in situ microscale techniques (i.e., /\17O, /\33S and /\36S of MRB lithologies). First, I develop an analytical protocol whereby SIMS three O isotope measurements can be used to robustly distinguish achondrite (parent body) population values, including main-group pallasites (ca.-0.2permil) and martian meteorites (ca.+0.3permil), from the TFL (zero) to levels of analytical precision better than 0.10 permil. Then, it is applied to (1)samples of the SNC clan, (2)lithologies within ALH84001 and (3) MRB lithologies. The results of these analyses show (1)all SIMS isotopic analysis of SNC silicates lie within error of the MFL, and replicate whole-rock bulk analyses of the MFL; (2)The ALH84001 lithologies show more diversity than the SNCs, and evidence that the apparent variations within Fe-rich carbonates and silicate host rock within the specimen can be resolved during isotopic analysis with the in situ SIMS technique (3)MRB samples show non-uniform /\17O, with the lithologies assessed exhibiting a variation of more than 2permil. Also, several distinct /\17O compositions are seen in the rocks, with some feldspars (those with cryptoperthite textures), the matrix, and gabbroic clasts being significantly enriched, while pyroxene, oxide, and phosphate lithologies lie on the MFL. This may suggest interactions with distinct O sources, and maybe different geological processing. Lastly, I measure the 33S-MIF and 36S-MIF compositions of MRB sulphide minerals using SIMS isotopic analysis to address questions about the S source of these minerals to assess the distribution of S-MIF signatures in martian samples and inform our understanding of S cycling on Mars. Results suggest that MRB sulpides contain a heterogenous distribution of S-MIF signatures. The isotopic analysis of MRB sulphide grains do not show a uniform 33S composition, with heterogeneities spanning a 0.73permil range and show positive, near-0 and negative /\33S values within the same grain. However, the mostly +sign of 33-SMIF signatures suggests that at some point in its history the early martian atmospheric environment was favourable for MIF photochemical processing. This study shows that MRBs are isotopically heterogenous with variable O-MIF and S-MIF signatures that may record evidence of secondary planetary geological processes not documented in other martian rocks. Understanding these signatures has implications for atmosphere-regolith-crustal exchange interactions and increasing our understanding of the geologic history and evolution of Mars.