Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p23a1095y&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P23A-1095
Other
3617 Alteration And Weathering Processes (1039), 5400 Planetary Sciences: Solid Surface Planets, 5470 Surface Materials And Properties, 6225 Mars
Scientific paper
Out of the nearly 200 samples analyzed by the Alpha Particle X-ray Spectrometer (APXS) onboard the Mars Exploration Rover (MER) Spirit within Gusev Crater, 10 measurements exceed 60 wt% SiO2. The highest measured concentration of SiO2 is in excess of 90 wt%. All of these samples are found along the eastern margin of Home Plate, localized within a region less than 50 meters in diameter. These occurrences include rocks, subsurface soil excavated by the rover wheels, and centimeter-scale nodules. Higher concentrations of Si in these samples correspond to lower Fe, Mg, and Ni as well as generally higher Ti and Cr. In one example, after the basaltic soil contamination is removed, the renormalized composition consists of 95% SiO2, 1.2% TiO2, and 0.3% Cr2O3. Other remaining oxides include small amounts of MgO, Al2O3, and SO3. The elemental chemistry of these samples clearly indicates the presence of abundant silica. Excess silica (up to 35 wt%) is also found in the distinct, but related, light-toned Paso Robles class soils which are dominated by sulfates and likely fumarolic in origin. Silica is readily mobilized in aqueous solutions and its solubility is relatively independent of pH under acid to neutral conditions but strongly dependent on the temperature of the fluid (higher temperatures correspond to greater solubility). Geochemical indicators (Zn, Ni, and Cl trends) in other samples east of Home Plate are consistent with interaction with fluids at elevated temperatures. Thus one possible process for concentrating SiO2 in the measured samples involves the dissolution of silicate rocks in hydrothermal fluids and reprecipitation of silica as the solutions cooled and evaporated. Given the proximity to likely fumarolic deposits (the silica concentrations are found within 100 meters of a Paso Robles class soil exposure), an alternative process may involve interactions with acidic vapors and small amounts of water which leached many elements leaving behind a residual dominated by silica. This alternative is supported by the association with Ti, as acid-sulfate weathered analogs from Kilauea Volcano (Hawaii) are enriched in both amorphous silica and anatase. It is likely that a combination of these processes, both of which involve localized aqueous interactions, have been active along the eastern margin of Home Plate.
Athena Science Team
Clark Barry
Gellert Ralf
Hurowitz Joel
Ming Dengming
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