Noble gases and 81Kr-Kr exposure ages of non-Antarctic ordinarychondrites: An attempt to measure terrestrial ages of Antarctic meteorites

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Cosmogenic ^<81>Kr as well as other noble gases of three ordinary chondrites Long Island (L6), Densmore (1950) (H6), and Gladstone (stone) (H6) have been measured with the mass spectrometer newly installed in 1989 to examine the analytical accuracy using the production rates of cosmogenic noble gases proposed by EUGSTER (Geochim. Cosmochim. Acta., 52,1649,1988). For Long Island and Densmore, the ^<81>Kr-Kr ages are 16±2Ma and 7.7±0.5Ma, respectively, and are in agreement with the ages by ^<21>Ne and ^<83>Kr, which indicates the validity of both ^<81>Kr analysis and the production rates P_<21> and P_<83> used in this work. Cosmogenic ^3He contents in these meteorites are lower than those expected from the production rate P_3,which might be caused by partial loss of He. The ages T_<38> based on ^<38>Ar and P_<38> were shorter than the ages T_<21> and T_<81>. This may be due to the unreasonably high ^<38>Ar production rates calculated for these meteorites, whose low cosmogenic ^<22>Ne/^<21>Ne and ^<78>Kr/^<38>Kr ratios indicate heavy shielding against cosmicray irradiation. The constant production rate of cosmogenic ^<38>Ar indicates the better internal concordance than that as a function of shielding depth proposed by EUGSTER. Gladstone is a gas-rich meteorite, for which the abundant solar noble gases made it difficult to estimate the cosmogenic noble gas concentrations and the resulted ages have large uncertainties.
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