Acid sulfate alteration of fluorapatite, basaltic glass and olivine by hydrothermal vapors and fluids: Implications for fumarolic activity and secondary phosphate phases in sulfate-rich Paso Robles soil at Gusev Crater, Mars

Phosphate‐rich rocks and a nearby phosphate‐rich soil, Paso Robles, were analyzed in Gusev Crater, Mars, by the Mars Exploration Rover Spirit and interpreted to be highly altered, possibly by hydrothermal or fumarolic alteration of primary, phosphate‐rich material. To test mineral phases resulting from such alteration, we performed hydrothermal acid‐vapor and acid‐fluid experiments on olivine (Ol), fluorapatite (Ap), and basaltic glass (Gl) as single phases and a mixture of phases. Minerals formed include Ca‐, Al‐, Fe‐ and Mg‐sulfates with different hydration states (anhydrite, bassanite, gypsum; alunogen; hexahydrite, and pentahydrite). Phosphate‐bearing minerals formed included monocalcium phosphate monohydrate (MCP) (acid‐vapor and acid‐fluid alteration of fluorapatite only) and ferrian giniite (acid‐fluid alteration of the Ol + Gl + Ap mixture). MCP is likely present in Paso Robles if primary Ca‐phosphate minerals reacted with sulfuric acid with little transport of phosphate. Under fluid:rock ratios allowing transport of phosphate, a ferric phosphate phase such as ferrian giniite might form instead. Mössbauer measurements of ferrian giniite‐bearing alteration products and synthetic ferrian giniite are consistent with Spirit's Mössbauer measurements of the ferric‐bearing phase in Paso Robes soil, but are also consistent with ferric sulfate phases in the low‐P soil Arad_Samra. Therefore, Mössbauer data alone do not constrain the fluid:rock ratio. However, the excess iron (hematite) in Paso Robles soil, which implies aqueous transport, combined with our laboratory experiments, suggest acid‐sulfate alteration in a hydrothermal (fumarolic) environment at fluid:rock ratios sufficient to allow dissolution, transport, and precipitation of secondary chemical components including a ferric phosphate such as ferrian giniite. Key pointsAcid sulfate alteration of phosphate minerals indicate water‐rock ratioMonocalcium phosphate indicates low and ferrian giniite high water‐rock ratiosResults suggest formation conditions at Paso Robles had a high water-rock ratio