Conditions of magma generation at the Me-akan volcano, northern Japan

Abstract

The Me-akan volcano is one of the most active volcanoes in Japan. In this study, the conditions of magma generation were estimated for basaltic products derived from Akan-fuji, the youngest mafic volcanic edifice at Me-akan, to elucidate the magmatic system. The volcanic products show significant variations in their whole-rock compositions (50.1 to 52.6 wt% SiO2), and can be divided into a low-SiO2 group (<50.4 wt% SiO2) and a main group (>50.7 wt% SiO2) based on Pb isotopic variations. The rocks exhibit the following two groups of phenocryst assemblages: one group contains high-Fo [100 × Mg/(Mg + Fe)] olivine and high-An [100 × Ca/(Ca + Na)] plagioclase phenocrysts (A group), and the other consists of low-Fo olivine, pyroxene, and low-An plagioclase phenocrysts (B group). Our study demonstrates that both the low-SiO2 group and the main group magmas were produced by assimilation and fractional crystallization of distinct basaltic magmas and via subsequent mixing with more primitive magmas in lower crustal magma chambers. It is suggested that the A-group phenocrysts were derived from the primitive magmas, whereas the B-group phenocrysts crystallized in situ before and/or after the occurrence of magma mixing. Multi-component thermodynamic analyses suggest that the primary magmas were segregated from the mantle at 1210–1230 °C and 0.9–1.0 GPa. The observations on estimated melt segregation depths of ~30 km are consistent with seismological observations that low-frequency micro-earthquakes (M 0.0–2.5) occur actively in the uppermost mantle. The generation of primary magmas involved slab-derived fluids of similar composition for both groups. The primary magma for the main group was generated by a greater contribution of the slab-derived fluid relative to the mantle peridotite than that of the low-SiO2 group.