Chemical models for martian weathering profiles: Insights into formation of layered phyllosilicate and sulfate deposits

1Mikhail Yu. Zolotov, 2Mikhail V. Mironenko
1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
2Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str., Moscow 119991, Russia

Numerical chemical models for water-basalt interaction have been used to constrain the formation of stratified mineralogical sequences of Noachian clay-bearing rocks exposed in the Mawrth Vallis region and in other places on cratered martian highlands. The numerical approaches are based on calculations of water-rock type chemical equilibria and models which include rates of mineral dissolution. Results show that the observed clay-bearing sequences could have formed through downward percolation and neutralization of acidic H2SO4-HCl solutions. A formation of weathering profiles by slightly acidic fluids equilibrated with current atmospheric CO2 requires large volumes of water and is inconsistent with observations. Weathering by solutions equilibrated with putative dense CO2 atmospheres leads to consumption of CO2 to abundant carbonates which are not observed in clay stratigraphies. Weathering by H2SO4-HCl solutions leads to formation of amorphous silica, Al-rich clays, ferric oxides/oxyhydroxides, and minor titanium oxide and alunite at the top of weathering profiles. Mg-Fe phyllosilicates, Ca sulfates, zeolites, and minor carbonates precipitate from neutral and alkaline solutions at depth. Acidic weathering causes leaching of Na, Mg, and Ca, from upper layers and accumulation of Mg-Na-Ca sulfate-chloride solutions at depth. Neutral MgSO4 type solutions dominate in middle parts of weathering profiles and could occur in deeper layers owing to incomplete alteration of Ca minerals and a limited trapping of Ca to sulfates. Although salts are not abundant in the Noachian geological formations, the results suggest the formation of Noachian salty solutions and their accumulation at depth. A partial freezing and migration of alteration solutions could have separated sulfate-rich compositions from low-temperature chloride brines and contributed to the observed diversity of salt deposits. A Hesperian remobilization and release of subsurface MgSO4 type solutions into newly-formed depressions could account for formation of massive layered sulfate deposits through freezing or evaporation. This scenario explains the observed deficiency of salts in Noachian formations, a paucity of Hesperian phyllosilicates, and the occurrence of sulfate deposits in Valles Marineris troughs, chaotic terrains, and some craters of the Hesperian age.

Reference
Zolotov MY, Mironenko MV (2016) Chemical models for martian weathering profiles: Insights into formation of layered phyllosilicate and sulfate deposits. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.04.011]
Copyright Elsevier

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