| The carbonates in martian meteorite ALH84001 preserve a record of aqueous processes on Mars at 3.9 Ga, and have been suggested to contain signatures of ancient martian life. The conditions of the carbonate formation environment are critical for understanding possible evidence for life on Mars, the history of water on Mars, and the evolution of the martian atmosphere. However, the formation environment of the ALH84001 carbonates continues to be controversial.; New isotopic analyses of the ALH84001 carbonates, laboratory experiments, and geochemical modeling performed in this study provide quantitative constraints on the formation environment of the ALH84001 carbonates. Microscale carbon isotope analyses of ALH84001 carbonates reveal variable delta13C values ranging from +27‰ to +64‰ that are correlated with carbonate chemical compositions. Isotopic analyses of synthetic hydrothermal carbonates with chemical compositions similar to the ALH84001 carbonates do not show similar isotope compositions, correlations, or trends. Combined with earlier oxygen isotope analyses, these data are inconsistent with formation of the carbonates in previously proposed environments, and indicate that the carbonates formed in a short period of time (hours or days) from a low temperature, dynamic aqueous system. A combination of empirical and equilibrium thermodynamic modeling reveals that the precipitating fluids were Mg- and CO2-rich, and probably formed through low temperature (<100°C) leaching of rocks with similar compositions to ALH84001. Prior to precipitating the carbonates, the fluids must have had an Mg/Ca ratio greater than ∼4 and an Fe/Ca ratio greater than ∼1.; Three new hypotheses are proposed that involve low temperature (<100°C), dynamic aqueous processes: the carbonates formed (1) in a sublacustrine spring environment during the mixing of two fluids derived from separate chemical and isotopic reservoirs; (2) from high pH fluids that were exposed to a dense CO2-rich atmosphere and precipitate carbonate, similar to high pH springs on Earth; or (3) from a subsurface Mg- and CO 2-rich fluid that was exposed to a cold, dry, and thin atmosphere indicating that the early martian atmosphere may have been very similar to the atmosphere present today. |
