Nitrogen, noble gases, and nuclear tracks in lunar meteorites MAC88104/105

Abstract

The nuclear tracks and the noble gases in different samples of the lunar meteorites MAC88104 and MAC88105 are studied in order to understand the exposure histories of the samples. The trapped solar wind component is very small and no solar flare irradiation is found in the track record. The cosmogenic noble gases give an exposure age of 220 +/− 45 Ma for both meteorites, confirming their pairing. The nuclear track data, along with cosmogenic radionuclide data, constrain the transit time from moon to earth to within 0.2-0.6 Ma. Almost all the noble gases in these meteorites were thus produced during exposure on the moon at a shielding of 60 g/sq cm or greater. Solar and spallation N components are determined and an excess N component of nonsolar wind origin is found in four samples. The value for the spallation components strongly suggests that the excess N is probably of indigenous lunar origin. (C.D.)