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 center dot 6-51 center dot 3 wt % SiO2) and basaltic andesite (52 center dot 0-54 center dot 9 wt % SiO2) lavas. The basaltic lavas show relatively tight compositional trends, and Sr-87/Sr-86 ratios tend to decrease with increasing whole-rock SiO2 content. The water content of the basaltic magmas was determined to be similar to 4 center dot 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 center dot 9 wt %. Multi-component thermodynamic calculations suggest that the primary magma was generated by similar to 23% melting of the source mantle with similar to 0 center dot 94 wt % H2O at similar to 1300A degrees C and similar to 1 center dot 4 GPa. The 0 center dot 94 wt % water content of the source mantle is significantly higher than that beneath volcanoes in the main NE Japan arc (generally < 0 center dot 7 wt % H2O); this implies that the wedge mantle at the arc-arc junction is intensively hydrated. The temperature of the wedge mantle of similar to 1300A degrees C at similar to 1 center dot 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