Low pH weathering of Mars analog lithologies: Thermodynamic modeling

Abstract

Antarctic and martian soils are rich in sulfates that may imply involvement of sulfuric acid solutions in weathering processes (e.g., Banin et al., 1997). We calculated thermochemical equilibria among sulfuric acid solutions and basaltic rocks. Equilibrium compositions of solids and aqueous solution are being calculated as functions of solution to rock (W/R) mass ratio (0.1–10⁵), initial solution pH (1–6), opening or closing the system to atmospheric CO₂ and O₂, and rock composition. Bulk compositions of the Antarctic Ferrar dolerite and a martian olivine basalt from Gusev crater (McSween et al., 2006) are used to characterize rocks. Calculations are performed at 0 °C and 1 bar with the GEOCHEQ code (Mironenko et al., 2000). Results for a dolerite composition in a system open to Earth's atmosphere show that with low pH (1–3) initial solutions and high W/R (∼10⁴–10⁵), mainly amorphous silica forms. At W/Rs of ∼10³–10⁵ (calculated pH ∼3–7), kaolinite and silica can form, as well as illite and goethite. At higher calculated pHs and lower W/R, which may characterize advanced stages of weathering and neutralization, goethite and dolomite dominate. Gypsum forms at calculated pHs of ∼1–7 and W/Rs of ∼10–10³⁻, but only for initially very acidic solutions (pH ≈ 1) which provide enough sulfur. For solutions with higher initial pH (4–6), high W/R (∼10³⁻–10⁵) results in mainly silica, kaolinite, and zeolites. At lower W/R, goethite and dolomite dominate. Weathering of Gusev basalt in a system open to Mars' atmosphere results in less Ca-zeolites, more goethite, and in magnesite in addition to dolomite, consistent with a greater Mg + Fe to Ca ratio in Gusev basalt. Results agree with some characteristics of Antarctic and Mars materials. Ferrar-derived soil fines contain clays, zeolites, and Ca-sulfates. Lack of abundant carbonates in martian, and some Antarctic, materials may indicate an early stage of acid weathering. Incomplete weathering also plays a role. Amorphous silica is formed during the first stages of weathering (high W/R), at all initial pHs. Formation of silica is consistent with terrestrial observations, experiments, and may contribute to silica-rich martian surfaces (e.g., Kraft et al., 2003).

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