CAUSES OF GEOCHEMICAL DIVERSITY IN THREE GALE CRATER SEDIMENTARY ROCK FORMATIONS

Bulk compositions of clastic sedimentary rocks are records of detrital grains from a source region, sorted by transport, and cemented. We compiled and summarized Alpha-Particle X-ray Spectraometer (APXS) geochemical compositions, variability and trends for samples in the fluvio-deltaic Bradbury group, lacustrine Murray formation, and eolian Stimson formation in Gale crater. The fluvio-deltaic Bradbury group is characterized by the most geochemical diversity, which can be explained primarily as a result of significant mineral sorting during transport and the variety of grain sizes in measured samples; the geochemical trends are centered around a source with an alkali-rich igneous composition, and the geochemical diversity originated from transport and sorting rather than source rock variations. There is, however, one distinctive source input for the Bradbury, which has high sanidine and a distinctive K₂O-rich chemical signature. The lacustrine Murray formation has multiple geochemical facies but is consistently compositionally distinct from average Mars crust. Geochemical trends in the Murray are interpreted to record shifts between mafic and silicic volcanic source rocks, the balance between clastic input and authigenic cement production in the lake water, and inputs from later diagenetic fluids. The eolian Stimson formation has low geochemical diversity and a composition very similar to average Mars crust, reflecting the regional integration of compositions of a variety of source rocks by wind. Several diagenetic processes in each of these formations have altered the compositions; the Bradbury group has localized Mg-rich and MnO-rich fracture fills, the Murray has high SiO₂ regions, the Stimson shows high SiO₂ haloes around fractures, and all units include late-stage sulfate-filled veins. The geochemical characteristics of each of these Gale crater formations distinctly reflect their depositional and post-depositional environments.