Venus: A phase equilibria approach to model surface alteration as a function of rock composition, oxygen- and sulfur fugacities

Rock alteration processes on Venus are still poorly understood due to the limited geochemical data on surface rocks and uncertainties in atmospheric compositions. We use phase equilibria calculations to constrain mineral stabilities at Venus surface conditions for different rock and gas compositions resulting in the chemical system SiO2-TiO2-Al2O3-FeO-MgO-CaO-Na2O-K2O with C-O-H-S gas at varying O-2 and S-2 fugacities. While the low concentrations of H2O in the present-day atmosphere result in conditions, under which anhydrous mineral assemblages dominate, higher amounts of water, possibly during an earlier stage in Venus' history, could have resulted in the formation of amphibole and biotite. Even in a sulfur-free atmosphere, carbonates would be stable only in alkali-rich basalts. The presence of SO2 in the atmosphere, however, causes the formation of anhydrite. The stabilities of iron oxides and sulfides are highly sensitive to gas fugacities (i.e., the composition of the atmosphere), as well as temperature. While the modeled magnetite-hematite transition is located close to conditions relevant for planetary radius, the assemblage of anhydrite + hematite +/- pyrite may be stable at higher elevations if a similar range of fO(2) as at the lowlands is assumed. Therefore, our model agrees with pyrite as the proposed cause of the high radar backscatter observed at high elevations in the northern highlands.