Formation of Martian Silica-rich Deposits through Rock Alteration: a Theoretical Assessment

Introduction: The secondary mineralogy of martian surface materials is consistent with aqueous alteration of mafic and ultramafic rocks [e.g., 1-3]. Alteration of mafic silicates results in the release of SiO2 into aqueous solution, followed by the precipitation of silica [4] and/or silica-rich minerals. The presence of these phases is consistent with data obtained at the Mars Pathfinder [4] and Opportunity rover [5] landing sites and with thermal infrared spectra of large surface regions [e.g., 6]. Recently, silica-rich outcrops and soil (up to 95 wt% SiO2) have been found in Gusev crater [7, 8]. These samples generally also have high Ti concentrations (up to 1.2 wt% TiO2 [8]). Thermal infrared spectra of these materials are consistent with opaline silica [9]. Here we use theoretical models to investigate conditions under which abundant silica can form by alteration of olivine basalt. Approach: We are exploring the effects of pH, solution/rock mass ratio (W/R) and temperature (T) on secondary mineralogy, solution chemistry, and timing of alteration. Equilibrium mineral assemblages are calculated with the GEOCHEQ code [10]. The bulk chemical composition of Adirondack-type olivine basalt from Gusev crater [11] is used to represent the protolith. For 0oC models, aqueous solutions of H2SO4-HCl with pH<5 are used, because of preferential deposition of silica at low-pH [12]. High T work is exploring a broader range of pHs. Some models are constant-pH open systems while others are closed, allowing pH to increase with weathering. Temperatures ≤200oC are used because of the inability of metastable amorphous silica to form in higher T solutionrock systems [13]. To explore the timescale of silica precipitation we are using a coupled kinetic-thermodynamic model developed for acid weathering [14]. This model considers mineral dissolution rates and solubility-controlled precipitation. We are exploring the timing of acid weathering of 0.1 mm mineral grains with the normative mineralogy of the basalt [11] exposed to different amounts of H2SO4-HCl solutions with initial pHs<5. Results at 0oC: Amorphous silica forms over a wide range of solution pH at W/Rs<~10 (Fig. 1). This high W/R boundary implies that acid ground water, spring waters and surface streams would be saturated with silica. Evaporation, freezing, or other consumption (e.g., rock hydration) of these solutions would result in silica deposition. Less silica precipitates at higher pH, corresponding to the increase in silica solubility [12] and decrease in solubility of secondary silicates. At pH<~1-1.5, silica is the only precipitate. Deposits with 60-100 wt.% silica form at pHs<~3 (Fig. 1).