Secular variation of magma source compositions beneath the Northeast Japan arc

Abstract

Sr and Nd isotopic compositions were determined for Tertiary volcanic rocks with ages of 22, 16 and 8 Ma along the volcanic front in the NE Japan arc in order to examine the secular variation of magma source compositions in the mantle wedge. These rocks show no through-time variation in both Sr and Nd isotope ratios (0.70412–0.704759 and 2.09–4.12 ∈-units, respectively). It follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere. The degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites, because H 2O which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab-derived fluid beneath the fore-arc region.