Noble gases and 81Kr terrestrial ages of Antarctic eucrites

Noble gas isotopic compositions and concentrations were measured for two monomict eucrites Yamato-791186 and -792510 and for three polymict eucrites Yamato-791960, -791962 and -792769. Terrestrial ages calculated on the concentrations of cosmogenic radionuclide 81Kr are in the range of 0.23-0.35 Ma, which are older than those reported for Yamato meteorites including eucrites. The noble gas result suggests that the monomict eucrites, Y-791186 and Y-792510, and the polymict eucrites, Y-791960 and Y-791962, are paired. If shielding effect on the cosmogenic 78Kr/83Kr ratio was considered, the preatmospheric radius of Y-791186 and Y-792510 was estimated to be small (about 10 cm in diameter). It was also calculated that Y-791960 was near the surface and Y-791962 was at the center of a preatmospheric body of about 1m in diameter. At least five pairs are identified for eucrites in the Yamato ice field referred to the published data. (author)