Record of fluid–rock interactions on Mars from the meteorite ALH84001

Abstract

ALLAN HILLS (ALH) 84001 is the most recently recognized¹ member of a suite of meteorites—the SNCs—that almost certainly originated on Mars². Several factors distinguish ALH84001 from the other SNC meteorites. Preliminary studies^3,4 suggest that it may be older than other martian meteorites. Moreover, it contains abundant, zoned domains of calcium–iron–magnesium carbonate that are indigenous to the sample¹ and thus may hold important clues regarding near-surface processes on Mars and the evolution of the martian atmosphere. We report here analyses of the carbon and oxygen stable-isotope compositions of the carbonates that place constraints on their formation conditions. Our results imply the presence of at least two chemically distinct carbonates—one Ca,Fe-rich, the other Mg-rich—that are enriched in ¹³C relative to terrestrial carbonates (δ¹³C≈ +41‰), consistent with martian atmospheric CO₂ as the carbon source. The oxygen isotope compositions of the carbonates indicate that they precipitated from a low-temperature fluid in the martian crust. Combined with textural and bulk geochemical considerations, the isotope data suggest that carbonate deposition took place in an open-system environment in which the ambient temperature fluctuated.