Terrestrial 81Kr-ages of four Yamato meteorites

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The terrestrial ages of four Yamato achondrites have been determined from ^<81>Kr-Kr-exposure ages and cosmogenic ^<38>Ar concentrations. A comparison of terrestrial ages and exposure ages shows that Y-74450 and Y-790007 are paired. The terrestrial age is less than 4×(10)^4y and the exposure age is about 70×(10)^6y. This group of polymict eucrites is different from Y-790122,Y-790260 and Y-790266. The diogenite Y-75032 has an exposure age of 17×(10)^6y and a terrestrial age of less than 7×(10)^4y. The howardite Y-790727 contains large amounts of trapped gases. The exposure age is about 20×(10)^6y. The terrestrial ages of Yamato meteorites are younger than those from the Allan Hills region. This is possibly due to a young age of the Yamato ice field. The relatively short terrestrial ages also exclude a longer time of transportation within the ice and a special concentration mechanism due to ice flow.
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