Intensive Hydration of the Wedge Mantle at the Kuril Arc–NE Japan Arc Junction: Implications from Mafic Lavas from Usu Volcano, Northern Japan

The generation and evolution of basaltic magmas at Usu volcano, located at the junction between the NE Japan arc and the Kuril arc, have been investigated. The mafic products, which form the somma edifice of the volcano, consist of basalt (49·6–51·3 wt % SiO2) and basaltic andesite (52·0–54·9 wt % SiO2) lavas. The basaltic lavas show relatively tight compositional trends, and ⁸⁷Sr/⁸⁶Sr ratios tend to decrease with increasing whole-rock SiO2content. The water content of the basaltic magmas was determined to be ∼4·8 wt % based on plagioclase–melt thermodynamic equilibrium. Using this information and an olivine maximum fractionation model, the water content of the primary Usu magma was estimated to be 3·9 wt %. Multi-component thermodynamic calculations suggest that the primary magma was generated by ∼23% melting of the source mantle with ∼0·94 wt % H2O at ∼1300°C and ∼1·4 GPa. The 0·94 wt % water content of the source mantle is significantly higher than that beneath volcanoes in the main NE Japan arc (generally <0·7 wt % H2O); this implies that the wedge mantle at the arc–arc junction is intensively hydrated. The temperature of the wedge mantle of ∼1300°C at ∼1·4 GPa is also significantly higher than that of the mantle in the main NE Japan arc. Unlike the basaltic lavas, the whole-rock compositions of the basaltic andesite lavas are scattered in Harker variation diagrams. This observation suggests that the compositional diversity was produced by at least two independent processes. To elucidate the processes responsible for this compositional diversity, principal component analysis was applied to the major element compositions of the samples. This suggests that 47% of the diversity of the whole-rock compositions can be explained by mixing with partial melts of lower crustal materials, 25% is explained by redistribution of plagioclase phenocrysts, and 16% is explained by fractionation of accessory minerals.