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

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Science  14 Mar 1997:
Vol. 275, Issue 5306, pp. 1633-1638
DOI: 10.1126/science.275.5306.1633

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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 δ18O = +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 (δ18O = 4.6 ± 1.2‰). Secondary SiO2 has δ18O = 20.4‰. Carbon isotope ratios measured from the core of one concretion average δ13C = 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).

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