APXS OVERVIEW: WIDESPREAD, OPEN HYDRAULIC SYSTEM AND MULTIPLE FLUID CONDITIONS INDICATED BY SI-ENRICHMENT TRENDS IN GALE CRATER SEDIMENTS, MARS

Alpha Particle X-ray Spectrometer (APXS) rock analyses represent bulk geochemical compositions and integrated historical records of igneous protolith(s), physical processing, overprinting alteration, and dust coverage. The APXS measures abundances of major, minor, and some trace elements (Cr, Ni, Zn, Br, Ge) of ~1.7 cm spots on rocks and soils. Since Curiosity arrived at bedrock exposures of the lowermost units of Mount Sharp on sol 755, the APXS has in total examined 111 separate targets (104 rocks, 7 soils), including 44 rocks brushed by the Dust Removal Tool (DRT) and 11 drill fines. Rocks examined include basaltic to Si-rich mudstones and basaltic sandstone lenses of the Murray Fm. and unconformable, overlying basaltic sandstones of the Stimson Fm. In general, these units exhibit a greater chemical signal of open-system aqueous alteration than seen previously in the volcaniclastic Bradbury Group.

Both formations exhibit trends of Si-enrichment and Fe-depletion, but with different geologic occurrences. Within the pervasively altered Murray mudstones, two Si-enrichment trends are identified in layered deposits: (1) a high-Al₂O₃ trend (up to 13.2 wt%), typified by the jarosite-bearing drill target Mojave with moderate SiO₂ (53.0 wt%) and relatively elevated trace metal (Mn, Zn, Ni, Cr) concentrations that likely formed under acid-sulfate conditions; and (2) a low-Al₂O₃ trend (ranging to 3.5 wt%), typified by the crystalline silica-bearing drill target Buckskin with high SiO₂ (74.4 wt%) and up to ~90% depletion in trace metals formed by leaching by acidic fluids. The Stimson basaltic sandstone is less pervasively altered with a composition similar to local soils, suggesting isochemical alteration during diagenesis. Stimson is more variable than soil, however ranging to higher Al₂O₃ and lower K₂O (to 0.29 wt%), possibly reflecting varied provenance. Altered, SiO₂-rich Stimson targets (up to 66.7 wt%) occur as halos surrounding through-going fractures and are characterized by low Al, Fe, and trace metals, but with elevated SO₃ (up to 15.1 wt%) and abundant amorphous silica. The altering fluids were likely acidic, S-bearing, and possibly reached high temperatures. The various Si-enrichment trends and losses of more soluble elements in basaltic sediments point toward an open and widespread hydrologic system.