title: 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 delta18O = +9.5 to +20.5 per thousand. Most of the core of one concretion is homogeneous (16.7 +/- 1.2 per thousand) and over 5 per thousand higher in delta18O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (delta18O = 4.6 +/- 1.2 per thousand). Secondary SiO2 has delta18O = 20.4 per thousand. Carbon isotope ratios measured from the core of one concretion average delta13C = 46 +/- 8 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 (< approximately 300 degrees C).
authors.affiliation: Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Valley
authors.forename: J M
authors.initials: JM
authors.lastname: Eiler
authors.forename: C M
authors.initials: CM
authors.lastname: Graham
authors.forename: E K
authors.initials: EK
authors.lastname: Gibson
authors.forename: C S
authors.initials: CS
authors.lastname: Romanek
authors.forename: E M
authors.initials: EM
authors.lastname: Stolper
mesh_terms: D002247:Carbon Isotopes; D002254:Carbonates / Q000737:chemistry*; D011232:Chemical Precipitation; D018535:Mars; D019110:Meteoroids; D008903:Minerals / Q000737:chemistry; D010103:Oxygen Isotopes; D012822:Silicon Dioxide / Q000737:chemistry; D013696:Temperature
chemical_list: D002247:Carbon Isotopes; D002254:Carbonates; D008903:Minerals; D010103:Oxygen Isotopes; C092478:pyroxene; D012822:Silicon Dioxide