Noble Gases in Japanese Chondrites: Their Gas Retention and Cosmic-Ray Exposure Histories.

Noble gas concentrations and isotopic ratios were measured for 12 Japanese chondrites. Cosmic-ray exposure ages and gas retention ages obtained for these chondrites are discussed with those for other Japanese chondrites already reported. Cosmic-ray exposure ages of L chondrites range from 5 to 60 Ma without clear cluster, while H chondrites belong to two clusters around 7 and 30~40 Ma. More than half of L chondrites have U, Th-He, and K-Ar ages lower than 3.0 Ga, indicating gas loss by shock event. The concentrations of trapped Ar, Kr, and Xe agree with those inferred from correlation reported between the trapped gas content and the petrographic type. Shiraiwa (H4), find, is the only exception, whose Xe concentration is in the range of type 6. Noble gas data confirm that Kariyasu is a fragment of the Mino (Gifu) metrorite shower, and Hashima and Kasamatsu are different falls. The enstatite chondrite Kishima has a low concentration of subsolar noble gas component. Because the measured concentrations of the subsolar type noble gases are lower than reported ones in Fukutomi (L4-5), the carrier which provide such noble gases should be heterogeneously distributed in Fukutomi.