GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 20-11
Presentation Time: 11:00 AM


GELLERT, R.1, SCHMIDT, Mariek2, BERGER, Jeff A.3, BOYD, Nicholas1, DESOUZA, Elstan1, O’CONNELL-COOPER, Catherine4, PERRETT, Glynis5, THOMPSON, Lucy M.6, VANBOMMEL, Scott1 and YEN, Albert S.7, (1)Dept. of Physics, University of Guelph, Guelph, ON N1G 2W1, Canada, (2)Department of Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON L2S 3A1, Canada, (3)Department of Earth Sciences, Western University, 1151 Richmond St N, BGS Rm 1026, London, ON N6A 5B7, Canada, (4)Department of Earth Sciences, University of New Brunswick, 2 Bailey Drive, Fredericton, NB NB E3B 5A3, Canada, (5)Department of Astronomy, Cornell University, 616A Space Science Building, Ithaca, NY 14853, (6)Planetary and Space Science Centre, Department of Earth Sciences, University of New Brunswick, 2 Bailey Drive, Fredericton, NB E3B 5A3, Canada, (7)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109,

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.