Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p34c..03z&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P34C-03
Mathematics
Logic
[1009] Geochemistry / Geochemical Modeling, [1039] Geochemistry / Alteration And Weathering Processes, [3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The composition of martian phyllosilicates could be used to constrain ancient aqueous environments. This requires understanding of effects of solution chemistry, pH, Eh, temperature, pressure, and duration of processes on composition of precipitated minerals. In addition to laboratory data and terrestrial analogs, the formation and fate of phyllosilicates could be evaluated with numerical physical-chemical models of water-rock interaction. Advanced models handle multicomponent and multiphase systems with non-ideal aqueous, solid, and gas solutions. These models consider solubilities of solids and gases in aqueous solution and constrain conditions of mineral saturation. Additional procedures quantify pH-dependent rates of mineral dissolution coupled with chemical equilibria in solution and percolation of fluids. With these models, formation and compositional evolution of phyllosilicates could be linked to a specific stage and/or setting of water-rock interaction. The models tie the composition and pH of solution with the mineralogical assemblage, which may contain phyllosilicates and other secondary phases together with unaltered minerals. If several minerals are observed in a natural locality, the models help to narrow conditions of phyllosilicate formation. Analogous models could be used to evaluate pathways and rates of phyllosilicate alteration through chemical weathering, diagenesis, and fluid-assisted metamorphism. The applicability of these models to natural environments is restricted by uncertain thermodynamic data for phyllosilicates (especially for smectites), limited data on dissolution rates, large compositional variations and uncertain surface area of theses minerals, common formation in poorly crystallized and mixed structures, and oversaturation of solutions. Another problem is that composition of depositional phyllosilicates and other phases (e.g. in deltas, eolian deposits, and soils) could reflect diverse environments of their initial formation rather than sedimentation. Nevertheless, our models demonstrate sequential formation of phyllosilicates (kaolinite then compositionally variable smectites) through neutralization of acidic fluids interacting with martian basalt. Modeling of percolation of acidic fluids through fragmented basaltic materials predicts formation of the vertical sequence of dominated minerals (from the top): amorphous silica - kaolinite and/or montmorillonite - ferrous chlorite - Fe-Mg smectites. This series reflects leaching of elements and neutralization of fluids with depth, and is similar to succession observed in the Mawrth Vallis region. Other results show relatively high stability of many clay minerals with respect to acid weathering, which strongly affects basaltic glasses, primary Fe-Mg minerals, and carbonates. Silica and kaolinite form through acid weathering of many clay minerals. Phyllosilicates could form together with sulfates, and this assemblage (except kaolinite-rich cases) does not indicate acidic environments.
Mironenko Mikhail V.
Zolotov Mikhail Yu.
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