NOBLE-GASES IN ORTHOPYROXENITE ALH84001 - A DIFFERENT KIND OF MARTIANMETEORITE WITH AN ATMOSPHERIC SIGNATURE

Meteorite ALH84001, a recently identified martian orthopyroxenite, has xenon with higher ratios of Xe-129/Xe-132 (up to 2.15) than any previ ously identified martian sample except the glass in EETA79001, suggest ing that it has incorporated a relatively large amount of martian atmo sphere. As such, it should be a fruitful sample to study in order to t ry to determine incorporation mechanisms, but it is petrogenetically v ery different from other martian meteorites with similar noble gas sig natures. We determine a cosmic-ray exposure age of 15.8 +/- 1.6 Ma, lo nger than any other martian meteorites, requiring either another impac t event on Mars, or another breakup of a martian fragment en route to Earth. Argon systematics suggest an age much older than the other mart ian meteorites and may indicate a substantial amount of martian atmosp heric argon as well. Excesses of Kr-80 and Kr-82 in one sample are con sistent with neutron capture on bromine in situ, but not with an atmos phere rich in Br-derived isotopes.