Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p12a0357e&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P12A-0357
Mathematics
Logic
1045 Low-Temperature Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5405 Atmospheres: Composition And Chemistry, 5415 Erosion And Weathering, 6225 Mars
Scientific paper
The aqueous and solid products of chemical weathering of Mars-analog materials have been predicted using a thermodynamic equilibrium reaction-path model. Aqueous and solid compositions are monitored as rock (CIPW normative compositions of terrestrial basalt or Shergotty SNC meteorite material) is titrated into 1 liter of aqueous solution, buffered by CO2 and O2 at fixed fugacity. Constraints on Mars weathering processes are inferred based on the predicted effects of varying fO2, fCO2, water/rock ratio, percent primary material weathered, and initial solution chemistry on the final equilibrium assemblage. The equilibrium products predicted from the interaction of meteoric water or Na-Ca-Cl brine with basalt and Shergotty composition rocks suggest that hydrologic systems on Mars were (are?) dominated by Na-Ca-Mg-K-Cl solutions that evolve to produce low-temperature (221-218K) eutectic brines through cooling/evaporation processes. Most importantly, such brines are stable at current Mars surface P, T conditions because of the lowered triple-point temperature and vapor pressure.
Secondary minerals predicted from the model include calcite, dolomite, kaolinite, hematite, dawsonite, clinoptilolite, and SiO2. The equilibrium assemblage is unaffected by changes in water/rock ratio (100 to 0.1). The only change in weathering products predicted with varying fO2 occurred at the hematite/magnetite buffer - hematite is stable above fO2=10-71, siderite is stable at lower fO2. However, much smaller variations in fCO2 result in significant changes in the equilibrium assemblage. At fCO2 > 10-4, dolomite, calcite, and dawsonite are stable; at lower fCO2, calcite is the only stable carbonate. Calcite abundance decreases when fCO2 falls below 10-5, and calcite is unstable at fCO2 < 10-7. Kaolinite, paragonite, saponite, prehnite, and analcime are only stable over limited ranges of fCO2. The strong dependence of the weathering assemblage on fCO2 offers the possibility to constrain past or present fCO2 conditions on Mars based on in-situ mineralogical observations. The current fCO2 of the Martian atmosphere is 10-2.2 and the fO2 is 10-5. In the model, fCO2 < 10-5 results in little or no carbonate precipitation. Because CO2 solubility in aqueous solutions decreases with increasing salt content, highly saline aqueous fluids near the surface of Mars will not precipitate carbonates. The lack of carbonates observed on the surface of Mars suggests that high salinity brines are involved in chemical weathering processes on the planet.
Bodnar Robert J.
Elwood Madden Megan E.
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