Noble gases,KrKr ages, andBe of chrondrites from China

A comprehensive study of the cosmic-ray exposure history of five ordinary chondrites from China was carried out using measurements of the noble gas isotopic abundances and¹⁰Be concentrations. The following average cosmic-ray exposure ages, based on cosmogenic²¹Ne and on⁸¹KrKr dating were obtained: Zhaodong (L4) — 15.7 ± 3.0 m.y., Nan Yang Pao (L6) — 48 ± 10.0 m.y., Guangrao (L6) — 16.8 ± 3.5 m.y., and Lunan (H6) — 26.7 ± 5.0 m.y. The H5 chondrite Zaoyang was exposed for only 0.90 ± 0.12 m.y. to galactic cosmic rays as calculated from the¹⁰Be activity and from the low amounts of cosmic-ray-produced noble gases. The Zhaodong chondrite contains large amounts of⁸⁰Kr and⁸²Kr produced by neutron capture of bromine. From the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite.