K-Ar Ages of Yamato-74 Meteorites

Measurements have been carried out on the isotopic compositions and contents of helium, neon and argon for three kinds of Yamato meteorites, which were petrologically classified into (a) L-chondrite (Yamato-74190-01), (b) H-chondrite (Yamato-74640-02) and (c) eucrite (Yamato-74159-01). K-Ar age for each sample was calculated to be (a) ~2.1 b.y., (b) ~4.5 b.y. and (c) ~4.3 b.y. From the isotopic correlation diagram for neon, it has been suggested that the neon of each sample consisted of a mixture of spallogenic neon and trapped one. ^3(He) exposure ages for the samples (a), (b) and (c) were calculated to be 25,12 and 27 m.y. and ^<21>(Ne) exposure ages to be 25,11 and 32 m.y., respectively.