Chemical composition of magnetite in Martian meteorite ALH 84001: Revised appraisal from thermochemistry of phases in Fe–Mg–C–O

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; 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 + CO 2 + 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 + CO 2 + 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.