Variations in Nd and Sr isotope ratios of Quaternary volcanic rocks from the southwestern Kurile arc: an implication for migration of fluid phases in the subduction zone

Variations in 87Sr/86Sr and 143Nd/144Nd ratios of basalt-andesite lavas from the Quaternary volcanoes of the southwestern Kurile arc show seesaw lateral trends toward the back-arc side. Combining trace element data, the complex lateral variations in the isotopic compositions can be ascribed to addition of fluids released from amphibole in the mantle wedge beneath the volcanic front and from phlogopite in the wedge on the back-arc side. The basalts of Rishiri volcano, which is located farthest away from the trench, shift to MORB composition because addition of fluids from the subducted oceanic lithosphere is not expected. It seems that interaction between the subducted oceanic lithosphere and mantle wedge governs the geochemical characteristics of arc magmas.