Noble gases in orthopyroxenite ALH84001: A different kind of martian meteorite with an atmospheric signature

Meteorite ALH84001, a recently identified martian orthopyroxenite, has xenon with higher ratios of ¹²⁹Xe ¹³²Xe (up to 2.15) than any previously identified martian sample except the glass in EETA79001, suggesting that it has incorporated a relatively large amount of martian atmosphere. As such, it should be a fruitful sample to study in order to try to determine incorporation mechanisms, but it is petrogenetically very different from other martian meteorites with similar noble gas signatures. We determine a cosmic-ray exposure age of 15.8±1.6 Ma, longer than any other martian meteorites, requiring either another impact event on Mars, or another breakup of a martian fragment en route to Earth. Argon systematics suggest an age much older than the other martian meteorites and may indicate a substantial amount of martian atmospheric argon as well. Excesses of ⁸⁰Kr and ⁸²Kr in one sample are consistent with neutron capture on bromine in situ, but not with an atmosphere rich in Br-derived isotopes.