Asthenospheric injection and back-arc opening: Isotopic evidence from northeast Japan

Abstract

Nd and Sr isotopic compositions were determined for the Tertiary volcanics from the back-arc side of the NE Japan arc. Sr isotopes show a linear trend through time from an enriched signature ( 87 Sr 86 Sr = 0.705437 ) to a depleted signature ( 87 Sr 86 Sr = 0.70270 ). In a complementary fashion, Nd isotopes start at low value ( ϵ Nd = −0.80) and show a gradual increase ( ϵ Nd=8.3) with decreasing age. Isotopic change of Nd and Sr from the enriched signature to the depleted one is synchronous with the opening of the Japan Sea at ∼ 15 Ma. This synchronism indicates that the opening of the Japan Sea was initiated by the injection of the asthenosphere. During the pre-opening stage, the mantle wedge was composed of a two-layered structure: the sub-continental lithosphere and the underlying asthenosphere. The volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub-continental lithosphere. The sub-continental lithosphere of the back-arc side was thinned by the injection of the depleted asthenosphere, which accelerated the growth of the MORB source within the mantle wedge of the back-arc side and resulted in magma generation with the depleted isotopic signature of Nd and Sr at the post-opening stage.