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 δ¹⁸O = +9.5 to +20.5⁰/₀₀. Most of the core of one concretion is homogeneous (16.7±1.2⁰/₀₀) and over 5⁰/₀₀ higher in δ¹⁸O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (δ¹⁸O = 4.6±1.2⁰/₀₀). Secondary SiO₂ has δ¹⁸O = 20.4⁰/₀₀. Carbon isotope ratios measured from the core of one concretion average δ¹³C = 46±8⁰/₀₀, 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).