Chemical composition of magnetite in Martian meteorite ALH 84001: Revised appraisal from thermochemistry of phases in Fe-Mg-C-O |
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| Authors: | Allan H Treiman Eric J Essene |
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| Institution: | a Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, United States
b Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States |
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| Abstract: | Martian meteorite Allan Hills (ALH) 84001 contains sub-micron magnetite grains, suggested to be of biogenic origin, in its globules of Fe-Mg carbonate mineral. There is disagreement on whether the low Mg content of the magnetite could only arise from biological metabolism (
Treiman, 2003] and Thomas-Keprta et al., 2009]). However, constraints on the magnetite’s biogenicity are far less certain than had been inferred. The thermochemical bases for the equilibrium calculations are reviewed in detail; there are inconsistencies and gaps in fundamental data for siderite, macromolecular carbons, and magnesioferrite. The calculations of Treiman (2003), assuming formation of magnetite via “siderite = magnetite + CO2 + CO”, are incorrect because of a flaw in the computer code used. The corrected location of this equilibrium (Thomas-Keprta et al., 2009) is no longer crucial, because of recent finds that the magnetite grains are associated with macromolecular carbon; this implies that the dominant magnetite-forming reaction was “siderite = magnetite + CO2 + C”. From the location of this equilibrium, using the corrected computer code and best available thermochemical data, the Mg-poor magnetite grains (and macromolecular carbon) in carbonates in ALH 84001 could have formed by decomposition of the carbonates at geologically reasonable pressures and temperatures. The low-Mg compositions of the magnetite grains remain consistent with an abiotic origin within the known geological history of ALH 84001. |
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