Petrographic controls on baddeleyite occurrence in a suite of eight basaltic shergottites
basaltic shergottites, baddeleyite distribution, Mars
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.
Sheen A., Herd C., Hamilton J and Walton E. (2021) Petrographic controls on baddeleyite occurrence in a suite of eight basaltic shergottites. Meteoritics & Planetary Science 56: 1502-1530. doi: 10.1111/maps.13726.
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