Intensive hydration of the wedge mantle at the Kuril arc – NE Japan arc junction: implications from mafic lavas from Usu Volcano, northern Japan

Abstract:

The southwestern part of Hokkaido, northern Japan, is located at the junction of the NE Japan arc and the Kuril arc. The subducting Pacific plate under this region shows a hinge-like shape due to the dip change of the subducting plate along the trench. Because of the interest in this unique tectonic setting, this arc-arc junction has been the focus of extensive geophysical studies (e.g. Kita et al., 2010, Morishige and van Keken, 2014; Wada et al., 2015). This region is also known as an area in which magmatism has been intense; there are many active volcanoes such as Usu, Tarumae, and Komagatake, and large calderas including Toya, Shikotsu, and Kuttara. In this region, the temporal and spatial evolution of the volcanism and the chemical compositions of the volcanic rocks are well characterized (e.g. Nakagawa, 1992). However, the generation conditions of magmas have not been estimated for these volcanoes, probably because of the scarcity of basaltic products. Therefore, a possible link between the tectonic setting and the intense magmatism is still unclear.

In this study, we carried out a petrological and geochemical study on mafic lavas (49.6–51.3 wt.% SiO₂) from Usu Volcano, and estimated the conditions under which the magmas were generated. By application of a plagioclase-melt hygrometer to the plagioclase and the host magma, the water content of ~6.5 wt.% was obtained for the basaltic magma. Using this information, as well as the olivine maximum fractionation model (Tatsumi et al., 1983), the composition of the primary magma is estimated to be 47.9 wt.% SiO₂, 15.1 wt.% MgO, and 4.1 wt.% H₂O. Analyses using the multi-component thermodynamics suggest that the primary magma was generated in the source mantle with 0.9 wt.% H₂O at 1310ºC and at 1.6 GPa. The water content of 0.9 wt.% of the source mantle is significantly higher than the estimates for the source mantle in the main NE Japan arc (<0.7 wt.% H₂O); this implies that the flux of slab-derived fluids is higher at the arc-arc junction than the main NE Japan arc. Beneath the arc-arc junction, the subducting Pacific plate shows a hinge-like shape, and many fractures might have been developed in the slab; this may have resulted in the intensive hydration of the wedge mantle due to efficient release of the fluids from the slab.