Abstract
Abstract The unequilibrated hypersthene chondrite Yamato-74191 was studied mass spectrometrically for rare gases released at various temperatures. Cosmogenic gases dominate in He and Ne. The meteorite contains large amounts of trapped Ar, Kr and Xe, and radiogenic 40Ar and 129Xe. Cosmic-ray irradiation and K-Ar ages were determined. In addition to spallogenic components of Kr and Xe, isotopic excesses of 80Kr, 82Kr, 128Xe and 126Xe relative to AVCC-Kr and -Xe were found. The ratio of 80Kr-excess to 82Kr-excess is 2.66 after correction for spallogenic Kr. A correlation between 128Xe/132Xe and 129Xe/132Xe was found The 129Xe/132Xe ratio for trapped Xe in Yamato-74191 was determined as 1.12 ± 0.29 with the correlation plot. The excesses found in Yamato-74191 are best explained by epithermal neutron capture on Br and I, and by the 127I (n, 2nβ) 126 Xe reaction. Using neutron-produced 80Kr, the neutron slowing -down density was estimated to be 0.14 ± 0.03 cm-3sec-1. A minimum mass and a preatmospheric radius was estimated to be 470 kg and 32 cm, respectively.
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