Keywords:silicic magma, magma diversity, rhyolite, caldera-forming eruption, crustal melting
It has been widely accepted that large scaled silicic magma eruptions were caused by the mafic injections into large silicic magma storage system. In some cases, the mafic injection is considered as a trigger of eruption to produce mingled magma between the mafic and silicic magmas. On the other hand, it has been also discussed that a zoned magma chamber could be formed before the eruption by the mafic injection into the silicic magma chamber. In both cases, many previous studies have considered that the silicic magma was nearly homogeneous. However, we have recognized the possible diversity in the silicic magma from many large silicic eruptions. In case of caldera-forming eruption, such as 42 ka Shikotsu, 120 ka Kutcharo and 7.3 ka Kikai-Akahoya ones, voluminous silicic magma erupted with small amount of mafic magma. Thus, it has been concluded that mafic injection occurred just before eruption. However, there exists possible diversity of the silicic magma. The silicic magma shows compositional variations, ranging from rhyolite to dacite. In addition, many major and trace elements and isotope ratios exhibit single linear trend in SiO2 variation diagrams. It should be noted that these linear trends do not continue to coexisted mafic magma, ranging from dacite to basaltic andesite. Thus, it can be concluded that mafic magma(s) injected into the silicic magma, which showed distinct, compositional variations. Phenocrystic minerals in the silicic magma can be compositionally distinguished from those derived from the mafic magma. These minerals, such as plagioclase and pyroxenes, show relatively wide variations. In addition, normally and reversely zoned phenocrysts coexist in a single silicic sample. These and linear trends in SiO2 variation diagrams indicate that the silicic magma is mixing products between two silicic end-member magmas such as rhyolitic and dacitic ones. Considering isotope ratios of the silicic magma, these two end-member magmas were derived from distinct source materials. In addition, these silicic magmas could not be produced by simple differentiation processes from the mafic magmas. Thus, it can be assumed that the silicic magmas could be formed by crustal melting. Although crustal materials are usually heterogeneous, there should exist considerable difference in the region of crustal melting to produce contrasted two silicic end-member magmas. Analysis of compositional zoning profiles of phenocrystic minerals suggests that mixing between silicic magmas had occurred several hundred years before eruption. The mixing could form a zoned, large, silicic magma chamber, in which mafic magma injected just before eruption. On the other hand, eruptions of VEI=5 class, such as AD 1640 Hokkaido-Komagatake and AD 1667 Tarumai ones, also show small but possible compositional variations of the silicic magma, dacitic one. However, the variations are smaller than those in caldera-forming eruptions (VEI=7). This might correspond to the difference in volume of the region of the crustal melting to form silicic magma.