Insights into the formation of Fe- and Mg-rich aqueous solutions on early Mars provided by the ALH 84001 carbonates

The chemical and isotopic pattern of the zoned carbonate globules in the ALH 84001 meteorite reveals a unique aqueous environment on early Mars. If the evolution of the fluid composition was dictated primarily by carbonate precipitation, the zoning pattern of the carbonates can constrain the fluid to have had an Mg/Ca mole ratio > ~5.3 and a Fe/Ca mole ratio > ~1 prior to the formation of the carbonates. Chemical equilibrium modeling of water–rock interactions indicates that low temperatures and low pH favor the formation of an aqueous solution with elevated Mg and Fe concentrations. The modeling shows that a sufficiently Fe- and Mg-rich fluid could have formed through low-temperature (< 100 °C) subsurface aqueous alteration of an ALH 84001-type rock at pH 5–7. This range of pH corresponds to an elevated CO2 fugacity (~ 0.1–1 bar). Formation of ALH 84001 carbonates could have been driven by degassing of CO2 and corresponding pH increase in near-surface environments during an upwelling of subsurface CO2-rich solutions. This scenario is consistent with the unaltered nature of the ALH 84001 rock and with chemical and isotopic composition of its carbonates.