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Carbonate‐Phyllosilicate Parageneses and Environments of Aqueous Alteration in Nili Fossae and Mars

The equilibrium thermodynamics of Fe/Mg‐phyllosilicates and carbonates have been investigated using numerical modeling, with emphasis on the effects of evaporation and temperature (hydrothermalism) under conditions related to present‐day and primitive Mars, that is, low or high CO2 partial pressure, under oxidizing or reducing environments. Findings are discussed in relation to alteration and weathering of minerals found in the Nili Fossae region. Results show that nontronite precipitates at very high water‐to‐precipitate ratio and low temperature, in association with ferrihydrite. During evaporation, this assemblage leads to carbonates: calcite and magnesite, and to low temperature iron‐rich serpentine phyllosilicates such as cronstedtite or berthierine under reducing conditions. At high temperature, the initial paragenesis dominated by nontronite leads to hematite and berthierine at low CO2 partial pressure, and to talc, magnesite and antigorite at high CO2 partial pressure. Thus, the Fe/Mg‐smectites, Mg‐carbonate, and Mg‐serpentine found near Nili Fossae may result from several alteration events. Weathering of primitive bedrock formed nontronite, with subsequent or concurrent hydrothermalism leading to Mg‐serpentine. Further carbonation due to CO2‐rich fluids led to talc, magnesite and to hydrogen/carbon monoxide/methane in the presence of iron, but at relatively low partial pressures (up to 10−4 bar). Unless the serpentinization process acted for extended periods of time and over a large portion of the martian surface, it is unlikely it significantly contributed to the atmospheric composition, except maybe for H2. However, such water‐rich active environments with the presence of redox reactions could have been suitable for the potential emergence of biological activity.