The cosmic ray record in the San Juan Capistrano meteorite

Abstract

An integrated study of the cosmic ray exposure history of the San Juan Capistrano meteorite was carried out using measurements of rare gas isotopic abundances, particle track densities and radioisotope concentrations. Spallation systematics determined for Kr isotopes in lunar samples are shown to be valid also for the San Juan Capistrano and St. Severin meteorites, thus allowing us to determine a reliable 81Kr/ 83Kr production ratio as needed for applying the 81Kr-Kr dating method. The 81Kr-Kr age of SJC is 28.7 ± 2.0 Myr, about 35% longer than ages determined by spallation He or Ne. The minimum observed track production rate (2.6 × 10 5 tracks/cm 2 · Myr) sets a minimum of 8 cm for the preatmospheric radius of an assumed spherical body. Track density gradients and the low 60Co activity (<2.9 dpm/g Co) both set an upper limit of 10 cm to the radius. Track results show that ablation losses have averaged 6cm. The relative spallation yields of 78Kr and 83Kr, and the ratios 3He/ 21Ne and 22Ne/ 21Ne are all compatible with a hard irradiation as would be experienced by a sample depth of about 6 cm in a body of 8–10 cm. The low activities of 54Mn, 22Na and 26Al are also consistent with these irradiation conditions.