Noble gas isotopic composition, cosmic ray exposure history, and terrestrial age of the meteorite Allan Hills A81005 from the Moon

We present a comprehensive study of the elemental and isotopic abundances of the noble gases He, Ne, Ar, Kr, and Xe in the meteorite Allan Hills A81005 from the Moon. In addition to a bulk sample five grain size fractions were analyzed. Chemical abundances relevant for the interpretation of the cosmic-ray-produced noble gases were determined and indicate that the grain size fractions are chemically uniform. Except for the fact that the trapped noble gas concentrations appear to be grain size correlated, the isotopic and elemental pattern of the trapped solar wind noble gases in A81005 are very similar to those observed in lunar soils and breccias. The A81005 material resided during (580±180) m.y. in the nuclear active zone of the lunar regolith at an average shielding depth of about 40 g/cm2. From literature data we conclude that the Moon-Earth transit time lasted less than a few million years. Finally, A81005 was captured by the Earth more than 140,000 years ago as obtained from the abundance of cosmic-ray-produced 81Kr. (orig.)