HR: 0800h
AN: P11A-0255    [Abstracts]
TI: Formation of Silica Deposits on Mars by Acid Weathering: Physical-Chemical Constraints
AU: * McAdam, A C
EM: amcadam@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, United States
AU: Zolotov, M Y
EM: zolotov@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, United States
AU: Mironenko, M V
EM: mironenko@geokhi.ru
AF: Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str., Moscow, 119991, Russian Federation
AU: Sharp, T G
EM: tom.sharp@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, United States
AB: Many chemical and mineralogical characteristics of martian surface materials are indicative of exposure to acidic solutions. Silica-rich outcrops and soils found in Gusev crater could also result from acid weathering. We used thermochemical equilibrium and coupled kinetic-thermodynamic models to investigate conditions under which abundant amorphous silica forms by low-temperature (273 K) H₂SO₄-HCl acid weathering of mafic/ultramafic rocks. We explored the effects of pH, solution/rock ratio (W/R) and rock composition on secondary mineralogy, solution chemistry, and timing of weathering. Modeling shows that silica-rich deposits form mainly under very acid conditions (pHs<~2-3) and at high W/R ratios (~10²-10³). High W/R ratios may represent solution discharge in spring environments, groundwater flow, or lake settings. If solution pH does not change much during high W/R alteration, silica-rich (>90 vol% silica) mineral assemblages form at pHs<~2. If acid solutions with original pH<~2 are neutralized as weathering proceeds, abundant silica precipitates at early stages of weathering (W/Rs >~10²) below pH ~3. Calculated timing of silica deposition is consistent with these inferences. Kinetic models also demonstrate that silica can dissolve when the solution neutralizes. Silica precipitation conditions are similar for several potential martian protoliths, however, the volume precipitated increases with increasing rock SiO₂ content (Shergotty basalt > Adirondack olivine basalt > Chassigny dunite). Low-pH deposition of amorphous silica is consistent with experimental Mars analog studies and theoretical models directed at understanding martian acid weathering. Observed Ti enrichment in some of the high silica deposits in Gusev crater is also consistent with a low-pH process. Rapid dissolution of mafic minerals and glass in cold, low-pH fluids could have been followed by the precipitation of silica and Ti-oxide owing to their low and comparable solubilities in these solutions. However, a high-temperature (e.g., hydrothermal) origin for the observed deposits remains a possibility.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1039 Alteration and weathering processes (3617)
DE: 5415 Erosion and weathering
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting