Noble gases in the mantle wedge and lower crust: An inference from the isotopic analyses of xenoliths from Oki-Dogo and Ichinomegata, Japan

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 ratios (307–1870) of these samples are generally lower than that assumed for MORB-type. 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 ratios and a negligible change in ratios. A high He concentration (1.2 x 10-6 cm3STP/g) and 3He/4He ratio (10.5 x 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 vo lcan ism do not ascend directly from the mantle wedge but from the lower crust where they accumulate. © 1993, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.