OXYGEN ISOTOPIC COMPOSITION OF CARBONATE IN THE NAKHLA METEORITE: IMPLICATIONS FOR THE HYDROSPHERE AND ATMOSPHERE OF MARS

Abstract

We have located small areas of siderite within the mesostasis of the Nakhla meteorite. High concentrations of Mn (up to ∼50% rhodochrosite) and elevated D/H ratios indicate that the siderite is not a terrestrial alteration product. The isotopic composition of oxygen in the siderite has been determined with the Manchester ISOLAB 54 ion microprobe. The siderite has δ18O = +34 ± 1‰, which is higher than any other martian carbonate yet reported. If the δ18O value is the result of equilibration with water at <60°C, then the carbonate could have formed from CO2/H2O produced during the degassing of Mars, and not modified subsequently isotopically. Formation from a water rich fluid at >60°C requires that the fluid was heavy isotope enriched relative to fluids produced during planetary degassing. An enrichment of 8–15‰ is consistent with theoretical outgassing models that are able to account for enhancements of Δ17O in martian alteration products. Estimated deposition temperatures would be raised to 80–170°C. The effect of a global scale fixation of martian CO2 as carbonate operates in the opposite direction and could lead to a reduction in δ18O of the martian hydrosphere of a few permil.