Decreasing CO2 partial pressure triggered Mg–Fe–Ca carbonate formation in ancient Martian crust preserved in the ALH84001 Meteorite
We retrace hydrogeochemical processes leading to the formation of Mg–Fe–Ca carbonate concretions (first distinct carbonate population, FDCP) in Martian meteorite ALH84001 by generic hydrogeochemical equilibrium and mass transfer modeling. Our simple conceptual models assume isochemical equilibration of orthopyroxenite minerals with pure water at varying water-to-rock ratios, temperatures and CO2 partial pressures. Modeled scenarios include CO2 partial pressures ranging from 10.1325 to 0.0001 MPa at water-to-rock ratios between 4380 and 43.8 mol mol−1 and different temperatures (278, 303 and 348 K) and enable the precipitation of Mg–Fe–Ca solid solution carbonate. Modeled range and trend of carbonate compositional variation from magnesio-siderite (core) to magnesite (rim), and the precipitation of amorphous SiO2 and magnetite coupled to magnesite-rich carbonate are similar to measured compositional variation. The results of this study suggest that the early Martian subsurface had been exposed to a dynamic gas pressure regime with decreasing CO2 partial pressure at low temperatures (approximately 1.0133 to 0.0001 MPa at 278 K or 6 to 0.0001 MPa at 303 K). Moderate water-to-rock ratios of ca. 438 mol mol−1 and isochemical weathering of orthopyroxenite are additional key prerequisites for the formation of secondary phase assemblages similar to ALH84001’s ‘FDCP’. Outbursts of water and CO2(g) from confined ground water in fractured orthopyroxenite rocks below an unstable CO2 hydrate-containing cryosphere provide adequate environments on the early Martian surface.