Correlated chemical and isotopic zoning in carbonates in the Martian meteorite ALH84001

The meteorite ALH84001, a sample of the ancient martian crust, contains small quantities (similar to 1%) of strongly chemically zoned carbonate. High spatial resolution (10 mu m) ion microprobe analyses show that the chemical zoning is strongly correlated with variations in oxygen isotope ratios. Early formed Ca,Fe-rich cores have delta(18)O similar to 7 parts per thousand increasing to 22 parts per thousand SMOW in the more Mg-rich outer cores and magnesite rims. Isolated areas of ankerite appear to be isotopically lighter with delta(18)O similar to 1 parts per thousand. The large range in delta(18)O requires a significant range in either fluid isotopic composition, or temperature, or both, in the course of the deposition sequence. Our data are inconsistent with formation of the zoned carbonates by closed system Rayleigh fractionation. There is no unique interpretation of the oxygen data, but the recent observation of existence of Delta(17)O excesses in the carbonate appears to rule out models which involve high temperature isotopic exchange with silicate. Comparison with terrestrial analogues suggests that ALH84001 carbonates formed in a hydrothermal system with T < similar to 400 degrees C, and which, at least in the early stages of formation, may have involved water with delta(18)O < 0 parts per thousand SMOW. The later stages of deposition probably occurred at temperatures below 150 degrees C, a conclusion which does not preclude the co-existence of thermophilic bacteria; temperatures during earlier stages of deposition are less likely to have been hospitable to bacteria. (C) 1998 Elsevier Science B.V. All rights reserved.