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

Abstract

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 δ 18 O = +9.5 to +20.5‰. Most of the core of one concretion is homogeneous (16.7 ± 1.2‰) and over 5‰ higher in δ 18 O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (δ 18 O = 4.6 ± 1.2‰). Secondary SiO 2 has δ 18 O = 20.4‰. Carbon isotope ratios measured from the core of one concretion average δ 13 C = 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).