Cosmic-ray exposure histories of the lunar meteorites AaU 012 and Shişr 166

Abstract

We measured the concentrations and isotopic compositions of the stable isotopes of He, Ne, Ar, Kr, and Xe in the two lunar impact-melt breccias Abar al’ Uj (AaU) 012 and Shişr 166 to obtain information on their cosmic-ray exposure histories and possible launch pairing; the latter was suggested because of their similar chemical composition. AaU 012 has higher gas concentrations than Shişr 166 and clearly contains implanted solar wind gases, indicating a shallow to moderate shielding for this meteorite in the lunar regolith. The maximum shielding depth of AaU 012 was most likely ≤310 g cm−2 and its lunar regolith residence time was ≥420 ± 70 Ma. Our results indicate that in Shişr 166 the trapped component is a mixture of air and solar wind. The low concentration of cosmogenic and solar wind gases indicate substantial diffusive gas loss and a shielding depth of <700 g cm−2 on the Moon for Shişr 166. All differences seen in the concentrations and isotopic compositions of the noble gases suggest that AaU 012 and Shişr 166 are most likely not launch pairs, although a different exposure history on the Moon does not exclude the possibility that the two meteorites were ejected by a single, large impact event.