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Browsing by Author "Hamilton, Jarret S."

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    Geologic mapping of candidate source craters for martian meteorites
    (2019) Hamilton, Jarret S.; Herd, Christopher D. K.; Walton, Erin L.; Tornabene, Livio L.
    Martian meteorites are the only rock samples from Mars that are currently accessible for research in Earth-based laboratories. The meteorites are derived from the near-surface units adjacent to their source craters. These source craters eject material beyond the martian escape velocity during formation from random, hypervelocity impact on the planet’s surface. Specific source craters for any of the martian meteorites are unknown. This study uses results from a queried database to constrain potential source craters based on parameters such as ejection age, petrology, preservation, and crater diameter. Preliminary results indicate a number of candidate source craters that require detailed mapping to better understand their morphology, relative age, and volcanic context.
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    Petrographic controls on baddeleyite occurrence in a suite of eight basaltic shergottites
    (2021) Sheen, Alex I.; Herd, Christopher D. K.; Hamilton, Jarret S.; Walton, Erin L.
    Baddeleyite (ZrO2) is a common late-stage accessory mineral in basaltic shergottites and is a robust geochronometer for obtaining igneous crystallization ages via high-precision in situ SIMS U-Pb analysis. Amenability to SIMS U-Pb dating depends in large part on the size and abundance of baddeleyite grains, which are generally surveyed using microbeam methods. We examine the petrography, mineralogy, geochemistry, and baddeleyite distribution in eight basaltic shergottites to identify factors that may be used to predict baddeleyite distribution in unknown samples of Mars. Results suggest that fractional crystallization controls baddeleyite occurrence in shergottites to the first order; samples with pyroxene major element compositions extending beyond the 1-bar stability boundary generally have higher baddeleyite abundance compared with samples with pyroxene compositions terminating at or before the stability boundary. In samples which display two pyroxene composition trends (high-Ca and low-Ca), the largest baddeleyite grains tend to be associated with Fe-Ti oxides; in samples where pyroxene composition forms a continuous trend extending beyond the 1-bar stability boundary, the largest baddeleyite grains are typically associated with polymineralic late-stage pockets. Bulk HFSE content and fO2 do not appear to directly influence baddeleyite distribution. Based on our findings, we propose that pyroxene composition is a useful proxy for assessing baddeleyite abundance and distribution in shergottites and may aid in determining a sample’s feasibility for U-Pb geochronology prior to conducting detailed surveys for baddeleyite characterization.