Reconstruction of an Evolving Magma Chamber beneath Usu Volcano since the 1663 Eruption

Usu volcano, located in northern Japan, has erupted seven times since AD 1663. Before these seven eruptions, the volcano had a long repose period ( ≧ 5000 yr). The 1663 eruption was the firstand by far the largest among the seven, producing nearly aphyric rhyolitic pumice. Small mafic inclusions (‘micro-clots’J, consisting of glass, quenched crystals and abundant vesicles occur in the pumice. On the basis of petrological studies of the microclots, it is concluded that these are quenched melts of a mafic magma injected into the rhyolite. The products of the 1769 eruption (and those of the following five eruptions) were dacites with abundant (10–15 vol %) microphenocrysts. According to crystal size distribution (CSD) analysis, the new microphenocrysts appear to have crystallized at a considerably higher cooling rate (∼ 300 times) than the phenocrysts in the 1663 eruptive products. The contrasting petrologic features of the aphyric rhyolite and the following microphenocryst-rich dacites can be explained by mixing and rapid cooling of a mafic magma injected during the 1663 eruption. We estimate the size of the magma chamber beneath Usu volcano just after the 1663 eruption, using numerical calculations for a cooling magma chamber. If the magma chamber was sill-like, its thickness is estimated to have been several hundreds of meters.