Noble gases, chemical composition, and cosmic-ray exposure age of the Yamato-74357 lodranite

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Cosmic-ray produced ^3He, ^<21>Ne, and ^<38>Ar concentrations and the chemical composition of the Yamato-74357 lodranite have been determined. Concordant concentrations of ^3He and ^<21>Ne have been obtained on both samples (Bern and Misasa). The concentration of cosmogenic ^<38>Ar is twice as high in the Bern as in the Misasa sample. The ^3He, ^<21>Ne and ^<38>Ar data for both samples yield an average cosmic-ray exposure age of 5.8±2.0Ma. This age indicates that Yamato-74357 belongs to the lodranite group, which probably originates from a common break-up event about 5Ma ago. Radiogenic ^4He and ^<40>Ar concentrations in the Bern sample are higher than those in the Misasa sample by more than a factor of two, suggesting that minerals enriched in U, Th and K are enriched in the finer grained fraction. The isotopic compositions of Kr and Xe are identical to those of the terrestrial atmosphere, except for the ^<129>Xe abundance. The small ^<129>Xe excess (^<129>Xe/^<132>Xe=1.15±0.04) is presumably due to the early formation of this meteorite.
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