抄録
The noble gas isotopic compositions of xenoliths sampled from Oki-Dogo island and Ichinomegata crater, Japan, have been measured to understand better the origin of noble gases in the mantle wedge and lower crust beneath the Japanese Island Arc which represents a typical convergent plate boundary. The 3He/4He ratios of lherzolites and a websterite derived from the upper mantle are the same as MORB-type He isotopic ratios, i.e., 8 ± 1 times the atmospheric value. The 40Ar/36Ar ratios (307–1870) of these samples are generally lower than that assumed for MORB-type Ar (>20000). These results indicate that the He and Ar initially present in the mantle wedge were similar to the MORB-type gases but were later contaminated by atmospheric noble gases in the subducting slab. This resulted in the decreased 40Ar/36Ar ratios and a negligible change in 3He/4He ratios. A high He concentration (1.2 × 10–6 cm3 STP/g) and 3He/4He ratio (10.5 × 10–6), similar to arc volcanic gases, was observed in an amphibolite sample which represents lower crust beneath Ichinomegata. This He isotopic ratio implies that magmatic noble gases associated with arc volcanism do not ascend directly from the mantle wedge but from the lower crust where they accumulate.
