Dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of H2O in the production of magmas at convergent plat boundaries. H2O released from the subducted slab forms hydrous peridotites at the base of forearc mantle wedge. Dehydration of the down-dragged hydrous peridotites results in the formation of three hydrous columns or potential magma sources in the mantle wedge above the temperature of 600°C and the pressures of 3.5 and 5.5 GPa of the surface of the slab. In normal subduction zones, partial melting takes place in two backarc-side hydrous columns; under anomalously high geothermal conditions, the trenchward hydrous colum n can produce primary andesite magmas.
High pressure experimental results have suggested that incompatible elements with larger ionic radii can be readily transported with H2O fluid phases through both dehydration and vesiculation processes: these processes may govern the geochemical characteristics of subduaction zone magmas.
Sr and Nd isotope compositions of Tertiary volcanic rocks in the NE Japan arc indicates that the subduction components do not increasingly pollute the magma source region during the continuous subduction of the oceanic lithosphere; the degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites.
The material movement in the mantle wedge during the episode of the backarc spreading has been discussed with reference to the migration of volcanic front in the NE Japan arc. The backarc extension of the Japan Sea was triggered by the injection of high-terperature asthenosphere into the mantle wedge. The asthenospheric mantle materials are identical to those of midoceanic ridge basalt source in Sr-Nd isotopic compositions.
