CORRELATED CHEMICAL AND ISOTOPIC ZONING IN CARBONATES IN THE MARTIAN METEORITE ALH84001

The meteorite ALH84001, a sample of the ancient martian crust, contain s small quantities (similar to 1%) of strongly chemically zoned carbon ate. High spatial resolution (10 mu m) ion microprobe analyses show th at the chemical zoning is strongly correlated with variations in oxyge n isotope ratios. Early formed Ca,Fe-rich cores have delta(18)O simila r 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 ank erite appear to be isotopically lighter with delta(18)O similar to 1 p arts per thousand. The large range in delta(18)O requires a significan t range in either fluid isotopic composition, or temperature, or both, in the course of the deposition sequence. Our data are inconsistent w ith formation of the zoned carbonates by closed system Rayleigh fracti onation. There is no unique interpretation of the oxygen data, but the recent observation of existence of Delta(17)O excesses in the carbona te 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 < sim ilar to 400 degrees C, and which, at least in the early stages of form ation, 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-exis tence of thermophilic bacteria; temperatures during earlier stages of deposition are less likely to have been hospitable to bacteria. (C) 19 98 Elsevier Science B.V. All rights reserved.