Noble gases and 81Kr-terrestrial age of Asuka-881757 lunar meteorite
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Noble gas compositions of the Asuka-881757 lunar meteorite were determined using 10.2 and 395.3mg specimens. This meteorite has virtually no solar type noble gases. Small amounts of trapped Ar, Kr and Xe have relative abundances similar to those of atmospheric and planetary noble gases. K-Ar age is 3.75±0.35Ga, which agrees with the ages by other methods such as U-Th-Pb, Rb-Sr and Ar-Ar. Cosmogenic isotope concentrations are the lowest among the lunar meteorites reported until now. These isotopes were produced in space by 4π geometry irradiation. The history of A-881757 based on the cosmic-ray produced stable isotopes and ^<81>Kr is as follows : It has been shielded from the cosmic ray irradiation until it was ejected from the moon 1.35±0.17Ma ago and fell on the earth 0.06±0.04Ma ago after a moon-earth transit time of 1.29±0.13Ma. These results require an ejection event for A-881757 in addition to those responsible for the other lunar meteorites.