Amphibole–melt disequilibrium in silicic melt of the Aso-4 caldera-forming eruption at Aso Volcano, SW Japan

The most recent and largest caldera-forming eruption occurred at 90 ka at Aso Volcano, SW Japan, and is known as the “Aso-4 eruption.” We performed chemical analyses of amphibole phenocrysts from Aso-4 pyroclasts collected from the initial and largest pyroclastic unit (4I-1) of the eruption to infer the composition–temperature–pressure conditions of the melt that crystallized amphibole phenocrysts. Each amphibole phenocryst is largely chemically homogeneous, but inter-grain chemical variation is observed. Geothermometry, geobarometry, and melt–SiO 2 relationships based on amphibole single-phase compositions reveal that most amphibole phenocrysts were in equilibrium with hydrous melt comprising 63–69 wt