Cosmic Ray Produced Radionuclides in Antarctic Meteorites

概要

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Cosmic ray produced ^<53>(Mn) (t_<1/2>=3.7×(10)^6y) has been determined in seventeen antarctic meteorites, among which, two have extremely low contents of ^<53>(Mn) : 101±6dpm/kg Fe in Yamato-7301 and 22±3 in Allan Hills No. 8. Even after corrections are made for undersaturation based on published values of rare gas ages, saturation specific activity of ^<53>(Mn) in these two meteorites is abnormally low compared with the average value of 470±100dpm/kg Fe in other antarctic meteorites. ^<26>(Al) (t_<1/2>=7.2×(10)^5y) and ^<10>(Be) (t_<1/2>=1.6×(10)^6y) were also measured in these two meteorites and in several others. These results as well as those of rare gas isotopes are discussed in terms of terrestrial ages of the meteorites, shielding effects in the parent bodies, undersaturation of the radioactivities and multi-stage irradiation histories. These two meteorites apparently experienced a two-stage exposure history with a long preirradiation (>(10)^8y) under heavy shielding in a large body before the recent brief exposure (≲2×(10)^6y) in a much smaller body. These facts seem to illustrate the complex nature of the exposure histories of most meteorites. The consequence of these multi-stage exposures is also discussed. Terrestrial ages, estimated for a few antarctic meteorites from the ^<26>(Al)/^<53>(Mn) ratio, seem to be younger than 3×(10)^5 years.