Magma generation conditions at the Akita-Komagatake volcano, Northeast Japan arc: Implications of across-arc variations in mantle melting parameters

Abstract

Akita-Komagatake is an active volcano in the Northeast Japan arc with a slab depth (H) of ∼110 km. In this study, magma generation conditions were estimated for basaltic products from the volcano to examine the thermal structure of the sub-arc mantle and the across-arc variations in the parameters related to magma genesis. The studied basalts have the highest MgO content (∼6 wt%) among the products from the volcano and contain ∼15% plagioclase phenocrysts, ∼5% olivine phenocrysts, and <1% pyroxene phenocrysts. The water content of the melt was estimated to be ∼4 wt% by melt inclusions in the phenocrysts using plagioclase–melt hygrometry. Multi-component thermodynamic analyses suggest that the primary magma was generated by ∼14% melting of the source mantle with ∼0.5 wt% H2O at ∼1275 °C and ∼1.3 GPa. The estimated pressure condition of ∼1.3 GPa coincides well with the low-velocity anomaly observed in the mantle at ∼40 km depth beneath the volcano. By comparing the results from Akita-Komagatake to the published results of the frontal-arc Iwate volcano (H = ∼90 km) and the rear-arc Sannome-gata volcano (H = ∼150 km), which are arranged along a cross-arc line in the Northeast Japan arc, we found that the pressure conditions of the magma generation are ∼1.3 GPa at both Iwate and Akita-Komagatake, which are significantly lower than the ∼1.8 GPa at the rear-arc Sannome-gata volcano. The source mantle temperature of ∼1275 °C at ∼1.3 GPa beneath Akita-Komagatake is higher than the ∼1250 °C at ∼1.3 GPa beneath Iwate, which suggests that the ascending flow of the mantle wedge corner flow may reach the shallowest levels beneath Akita-Komagatake. It was also demonstrated that the depth at which fluid was released from the subducting slab tends to increase for the volcanoes with increasing H.