Magma accumulation process of new silicic caldera volcano: A case study on the Hijiori volcano, Northeastern Japan arc

In order to know how silicic caldera volcanos commence the activity, magma accumulation process of the Hijiori volcano was studied. The Hijiori volcano is one of the 108 active volcanoes in Japan, which erupted at about 12,000 years ago (in Calendar age) on the location where no volcanic body existed before the activity. Total eruptive volume of the Hijiori caldera is estimated to be about 2 cubic km. Though span of the activity is as short as the resolving power of radiocarbon dating, there are three major quiescences found by the field investigation. All the pumices from the three major eruptions are similar. They are phenocryst rich (50- vol.%) and have similar phenocryst assemblages (Pl, Qz, OPx, Hb, and Mt), bulk chemistry (64 ±2 wt. % H2O), and isotopic (Sr,Nd) compositions. Yet zoning profiles of Pl, Qz, OPx and Hb phenocrysts tend to be less complicated in the earlier stages, suggesting that the magma chamber of Hijiori volcano had been disturbed repeatedly by such as magma mixing that continued intermittently during the eruptive activities. Mt phenocrysts have no zoning profiles and their chemical compositions (Al2O3, Mg/Mn) are mostly unique through the eruptive sequences. The Mt phenocrysts suggest that the physicochemical conditions of the magma were the same just before the each eruption since Mt phenocrysts can easily be re-equilibrated with the environments within a few months. The observed discrepancy between Mt and the other phenocrysts implies that there was repeated input of crystal poor hot magma into a crystal rich cool magma chamber, and that the mass ratio of new magma input to the existing magma body may be small enough to retain the physical and chemical conditions of the crystal rich magma chamber almost the same. The depth where the magma mixing happened is estimated to be about 10 km from the observed water concentration in melt inclusions. Input of felsic magma into a long-lived felsic crystal mush in the upper crust may have played an important role on both the magma accumulation and the commencement of the first eruption.