ON THE BUR-GHELUAI H5 CHONDRITE AND OTHER METEORITES WITH COMPLEX EXPOSURE HISTORIES

We present the C-14, Al-26, Be-10, He-3, He-4, Ne-20, Ne-21, Ne-22, Ar-36, Ar-38, and Ar-40 concentrations and the track densities measured in up to 13 samples of the Bur Gheluai (H5) meteorite fall. Only a multi-stage exposure history can explain the data in a self-consistent way. Parameters for a model two-stage history obtained by simultaneous, least-squares fitting of the concentrations of C-14, Al-26, Be-10, and Ne-21 were: first stage duration approximately 10 Ma and radius >2 m; second stage duration approximately 0.6 Ma and radius 40-100 cm. Nominal one-stage Ne-21 production rates (P21) inferred from Al-26 in Bur Gheluai samples exceed those inferred from Be-10 as expected for a meteorite with a complex history. Nonetheless, data for other meteorites indicate that multi-stage irradiations alone do not account for all the high reported values of P21 based on Al-26: The equations describing the production of cosmogenic nuclides show that uncorrected shielding effects may also play a role. A compilation of ordinary, solar-gas-poor chondrites for which two-stage histories have been proposed includes many with short second stages but none with unambiguously long first stages (>0.2 Ga).