Evolution of the magma chamber beneath Usu volcano since 1663: A natural laboratory for observing changing phenocryst compositions and textures

We have investigated the evolution of an active silicic magma-feeding system beneath Usu volcano, Japan, where eight eruptions have been recorded since ad 1663. All magmatic products contain similar types of plagioclase and orthopyroxene phenocrysts that consist of homogeneous cores with uniform compositions, and a zoned mantle that increases in size with time. The compositions of plagioclase and orthopyroxene phenocrysts vary gradually and regularly with time, as do the bulk-rock compositions. The texture of these phenocrysts also changes systematically, caused by progressive crystal growth, dissolution and diffusion. On the basis of these observations, we conclude that the same magma-feeding system has persisted at Usu volcano since ad 1663. Compositional variation of magnetite phenocrysts differs from that of plagioclase and orthopyroxene, because magnetite has large diffusion coefficients and should represent magmatic conditions immediately before the eruption. Most pumices from Usu volcano contain two types of magnetite phenocryst, each with a different composition and crystallization temperature, indicating that two magmas mixed before each eruption (approximately several days before). The end-members changed with time: rhyolite + basaltic andesite (1663); dacite +/- rhyolite (1769, 1822, 1853); dacite +/- dacite (1977, 2000). The temperature of the magma apparently increases with time, and the increase can be explained by sequential tapping from a magma chamber with a thermal and chemical gradient in addition to injection of high-temperature magma.