Oxygen isotopic composition of carbonate in the Nakhla meteorite: Implications for the hydrosphere and atmosphere of Mars

We have located small arc:as of siderite within the mesostasis of the Nakhl a meteorite. High concentrations of Mn (up to similar to 50% rhodochrosite) and elevated D/H ratios indicate that the siderite is not a terrestrial al teration product. The isotopic composition of oxygen in the siderite has be en determined with the Manchester ISOLAB 54 ion microprobe. The siderite ha s delta(18)O = +34 +/- 1 parts per thousand, which is higher than any other martian carbonate yet reported. If the delta(18)O value is the result of e quilibration with water at <60 degrees C, then the carbonate could have for med from CO2/H2O produced during the degassing of Mars, and not modified su bsequently isotopically. Formation from a water rich fluid at >60 degrees C requires that the fluid was heavy isotope enriched relative to fluids prod uced during planetary degassing. An enrichment of 8 -15 parts per thousand is consistent with theoretical outgassing models that are able to account f or enhancements of Delta(17)O in martian alteration products. Estimated dep osition temperatures would be raised to 80-170 degrees C. The effect of a g lobal scale fixation of martian CO2 as carbonate operates in the opposite d irection and could lead to a reduction in delta(18)O of the martian hydrosp here of a few permil. Copyright (C) 2000 Elsevier Science Ltd.