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Published March 14, 1997 | public
Journal Article

Low-Temperature Carbonate Concretions in the Martian Meteorite ALH84001: Evidence from Stable Isotopes and Mineralogy


The martian meteorite ALH84001 contains small, disk-shaped concretions of carbonate with concentric chemical and mineralogical zonation. Oxygen isotope compositions of these concretions, measured by ion microprobe, range from δ^(18)O = +9.5 to +20.5‰. Most of the core of one concretion is homogeneous (16.7 ± 1.2‰) and over 5‰ higher in δ18O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (δ^(18)O = 4.6 ± 1.2‰). Secondary SiO_2 has δ^(18)O = 20.4‰. Carbon isotope ratios measured from the core of one concretion average δ^(13)C = 46 ± 8‰, consistent with formation on Mars. The isotopic variations and mineral compositions offer no evidence for high temperature (>650°C) carbonate precipitation and suggest non-equilibrium processes at low temperatures (<∼300°C).

Additional Information

© 1997 American Association for the Advancement of Science. Received 10 February 1997; accepted 21 February 1997. J. Craven aided in all aspects of the ion microprobe analysis in Edinburgh and M. Spicuzza analyzed carbonate standards in Madison. J. Muhl measured ion probe pit depth and volume by optical interferometer. B. Hess mounted and carefully polished the sample. J. Fournelle, T. Patterson, S. Burgess, and P. Hill assisted in electron microprobe analysis. M. Diman drafted the figures and B. Barker aided in photography. G. Rossman and R. Reeder donated carbonates as potential standards. We thank three anonymous reviewers for prompt, helpful comments. This research was supported by NSF, NASA, DOE and NERC.

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