Timing of Acid Weathering and Oxidation on Mars

Introduction: Martian surface materials are partially oxidized and reveal signs of alteration at low pH. Acidic conditions are consistent with the detection of jarosite and ferric sulfates, depleted concentrations of Fe and Mg in surface layers of basalts, and correlations between P, S, and Cl in some rocks/soils [1-5]. The presence of Fe phases in soils/rocks (hematite, goethite, nano-phase oxides, sulfates, nontronite) demonstrates oxidation of primary ferrous iron [e.g., 1,2,6]. However, common occurrences of Fe-Mg silicates (olivine, pyroxenes) [e.g., 1,2,7,8] and the presence of magnetite in dust [9] show limited weathering and oxidation that is consistent with cold and dry climate throughout history [10]. Although experimental and theoretical attempts have been made to consider alteration pathways on Mars [11-17], the conditions and timescales of weathering remain uncertain. Here we develop physical-chemistry models for timing of aqueous weathering. Coupling kinetics and thermodynamics to model weathering: Weathering of olivine basalt by H2SO4-HCl aqueous solutions was investigated through numerical modeling in a system open with respect to CO2 and O2 only. The model includes dissolution rates of primary and secondary minerals, oxidation rate of aqueous Fe, as well as chemical equilibration among solutes, dissolved gases, and precipitates. The model is based on the following assumptions: 1) Secondary solids form through dissolution of primary minerals; 2) Oxidation of Fe by dissolved O2 is kinetically controlled; 3) All species in aqueous solution are in chemical equilibrium; 4) Precipitation of secondary phases is controlled by their solubility and occurs faster than dissolution. Weathering is modeled as a series of consecutive equilibribraions in aqueous solution calculated at each time step. For each computation, elemental mass balance is calculated from the composition of aqueous solution at the previous step and current rates of mineral dissolution (mainly from [18]), which depend on surface area exposed to solution. Kinetics of Fe oxidation is considered as consecutive dissolution and oxidation reactions, Fe(in solids) → Fe(aqueous) → Fe(aqueous). At each time step, the amount of consumed O2 is calculated based on current pH, concentrations of dissolved Fe and O2, and rate equations and constants from [19,20]. The thermodynamic block of the model is based on the GEOCHEQ code [21], which uses the Gibbs free energy minimization method. Calculations are performed for 0°C in the system O-H-Mg-Ca-Al-Si-Na-FeS-Cl-C, which was open with respect to CO2 and O2 in the martian atmosphere. The martian achondrite EETA 79001-A [22] was used as a proxy for phase composition of olivine basalt. In our model, all opaque minerals are represented by magnetite. H2SO4-HCl solutions correspond to a S/Cl mole ratio of 5.2, consistent with the composition of martian soils. Nominal calculations are performed at original pH of 1.2. We also vary original solution pH, grain size, water to rock ratio, degree of exposure of mineral surface to solution, and partial pressure (P) of atmospheric O2. Results: The results show fast dissolution of Fe-Mg minerals at lower pH, followed by preferential dissolution of plagioclase at higher pH. Correspondingly, solutions evolve from acidic, Mg-Fe-rich compositions toward Na-rich alkaline fluids. Amorphous silica precipitates first, dissolves at pH > ~3, but remains the most abundant secondary phase until pH ~6-7 (Figs. 1 and 2). Goethite forms second; first stages of its formation is attributed to dissolution of magnetite. Oxidation of Fe becomes a minor source of goethite only at pH > 4-5. Precipitation of silicates (kaolinite, Fesaponite, and Fe-chlorite) consumes Si, Al, and Fe from solution. Abundant zeolites (stellerite, then stilbite) form after a long period of time when the solution becomes alkaline. Dolomite and Mg-saponite reach saturation at approximate neutral conditions. Subsequent weathering in alkaline solutions is characterized by increasing amounts of zeolites, clay minerals, and carbonates. Dissolution of plagioclase supplies Na, which incorporates in smectites (Na-saponite and Na-montmorillonite) and stilbite. The period over which neutralization and mineral precipitation occur is shorter at higher initial pH, lower water to rock ratios, and larger mineral surface areas (e.g., at smaller grain sizes). Modeled weathering of rock fragments and coarse sands show that significant periods of time are needed to reach neutralization and abundant precipitation of carbonates, zeolites, and smectites (Fig. 2). However, through acid weathering of dust particles can occur within a year. At PO2 < ~10 bar, which characterizes present (PO2 = ~10) and past martian conditions, formation of goethite is accompanied by deposition of Fe phyllosilicates (Fesaponite, daphnite). Siderite can also form at intermediate stages of weathering. At PO2 < ~10 bar, formation of goethite is primarily caused by dissolution of magnetite, which supplies Fe. Without initial Fe-phases, Fe oxidation requires higher PO2 values and pH > 4-5. At the present PO2, even a minor formation of goethite through aqueous oxidation needs some time since the beginning of weathering.