Upper mantle isotopic components beneath the Ryukyu arc system: Evidence for ‘back-arc’ entrapment of Pacific MORB mantle

New Sr, Nd, and Pb isotopic data for Cenozoic basalts from northwest Kyushu, Japan and the Okinawa Trough, suggest that mantle sources feeding the Ryukyu ‘back-arc’ side volcanoes, represent a mixture of Pacific N-MORB mantle with EM2-rich components, with little or no EM1. The compositions of these basalts differ fundamentally from those characterizing northern Kyushu and the Japan Sea, which tap DUPAL-like (i.e. EM1-rich) sources similar to those prevailing beneath east and southeast Asia and western Pacific marginal basins. The apparent decrease in mantle DUPAL-like character to the east of northern Kyushu and the Sea of Japan, and its relative insignificance in mantle beneath the Ryukyu ‘back-arc’ region, suggest that Pacific MORB mantle may have been trapped within the mantle wedge during Philippine Sea plate subduction, thereby confining sub-Eurasian DUPAL-type asthenosphere to the west and restricting its flow to the east.