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 delta(18)O = +9.5 to +20.5 parts per thousand, Most of the core of one concretion is homogeneous (16.7 +/- 1.2 parts per thousand) and over 5 parts per thousand higher in delta(18)O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (delta(18)O = 4.6 +/- 1.2 parts per thousand). Secondary SiO2 has delta(18)O = 20.4 parts per thousand. Carbon isotope ratios measured from the core of one concretion average delta(13)C = 46 +/- 8 parts per thousand, consistent with formation on Mars. The isotopic variations and mineral compositions offer no evidence for high temperature (>650 degrees C) carbonate precipitation and suggest non-equilibrium processes at low temperatures (<similar to 300 degrees C).