ABSTRACT

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.