Other
Scientific paper
Aug 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.160..811s&link_type=abstract
Earth and Planetary Science Letters, vol. 160, Issue 3-4, pp.811-822
Other
17
Meteorites, Carbonates, Oxygen, Isotopes, Temperature
Scientific paper
The meteorite ALH84001, a sample of the ancient martian crust, contains small quantities (~1%) of strongly chemically zoned carbonate. High spatial resolution (10 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 18 O ~ 7 increasing to 22 SMOW in the more Mg-rich outer cores and magnesite rims. Isolated areas of ankerite appear to be isotopically lighter with 18 O ~ 1 . The large range in 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 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 <~400°C, and which, at least in the early stages of formation, may have involved water with 18 O < 0 SMOW. The later stages of deposition probably occurred at temperatures below 150°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.
Lyon Ian C.
Saxton John M.
Turner Gary
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