40Ar-39Ar analyses of lunar anorthositic breccia, Yamato-82192 from Antarctica
Lunar anorthositic breccia, Yamato-82192 (Y-82192), collected at the Minami-Yamato Nunataks, Antarctica, was analysed by the ^<40>Ar-^<39>Ar method in order to reveal its chronology. Sample Y-82192,63D shows rather peculiar degassing patterns in that most Ar was degassed only in the highest temperature fraction (1600℃). Such patterns of Ar are observed in strongly shocked lunar samples, as observed in the glassy matrix of Y-791197 and some other lunar samples. Hence, the possibility of inherited Ar cannot be excluded. Thus, the apparent ^<40>Ar-^<39>Ar age of 4237±165 Ma (1σ) observed in the 1600℃ fraction indicates the upper limit for the degassing event in the lunar highland area. On the basis of the amounts of trapped and cosmogenic Ar observed in the present sample, it is inferred that Y-82192 was not exposed on the lunar surface directly but was shielded from both heavy dose of solar wind and galactic cosmic-ray irradiations after the degassing event such as a shock impact.