Magma Plumbing System beneath Kirishima-Ohachi Volcano: Constraints from Olivine-hosted Melt Inclusions of the AD 1235 Takaharu Scoria Eruption

Major element composition analyses are conducted on melt inclusions (MIs) and their host olivine phenocrysts in Takaharu scoria, which erupted at AD 1235 from Ohachi volcano, Kirishima volcano group, SW Japan, to investigate the magma plumbing system beneath the volcano. Olivine-hosted MIs are found in two of the three fall scoria units of the eruption: the first (ThT-a) and the second (ThT-b) units. In ThT-a scoria, olivine-hosted MIs are categorized into two clusters: one with S-rich basaltic compositions and the other with S-poor basaltic andesite compositions. In contrast, olivine phenocrysts from the ThT-b unit contained MIs of basaltic andesite composition. The compositions of the ThT-a S-poor and the ThT-b MIs are consistent with those of groundmass glasses of each scoria, respectively, indicating they record information on pre-eruptive conditions. The pre-eruptive H2O content of the MIs is estimated based on olivine-melt CaO partitioning to be 6.6 ± 1.4 wt%. This corresponds to the depth of H2O saturation in the melt of ∼17 km, which is consistent with H2O-saturated plagioclase liquidus and is further supported by observed depths of deep low-frequency earthquakes. Based on the results, the magma plumbing system beneath the Ohachi volcano is discussed.