Noble gases in ten, small Yamato chondrites

Concentrations of noble gases and isotopic compositions of He, Ne and Ar were determined for small Yamato chondrites. Five of the ten specimens studied showed high (^<22>(Ne)/^<21>(Ne))_c ratios, suggesting hard cosmic-ray irradiation in small bodies in space or near surfaces of meteoroids. Among them, Y-74447 (H) shows a cosmogenic ^<22>(Ne)/^<21>(Ne) ratio as high as 1.306±0.040,which suggests a very hard cosmic-ray irradiation in a small body. The preatmospheric size of this chondrite may be as small as inferred from its recovered mass (14.3g). No significant difference was found in cosmic-ray irradiation history between antarctic and non-antarctic chondrites. Cosmic-ray irradiation ages and U/Th-He and K-Ar ages are given. While Y-74138(H) was found to contain appreciable amounts of trapped, solar-type gases, Y-74060(H) shows complete loss of radiogenic ^4(He) and radiogenic ^<40>(Ar) as low as found in shocked L chondrites. The Y-74060 chondrite is one of those rare cases in which H chondrites lost radiogenic gases by shock or other metamorphic events. Trapped planetary gases are discussed in relation with petrologic types.