Multistage Magma Mixing Revealed in Phenocryst Zoning of the Yunokuchi Pumice Akagi Volcano, Japan

Abstract

Detailed study of the texture and chemical zoning of phenocrysts in the compositional and thermal histories at the sites a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, where they grew. Only recently have such studies been Japan, permits reconstruction of the subvolcanic magma chamber. undertaken focusing on magma chambers as the sites Hypersthene phenocrysts are complexly zoned; some have spongy of phenocryst formation (Tait, 1988; de Silva, 1989; cellular textures whereas others exhibit simple reverse zoning. Augite Umino & Iwano, 1992; Fodor et al., 1993; Nakada et phenocrysts have texturally similar cores but Al and Ti vs mgal., 1994). The magma chambers depicted by these number zoning patterns are correlatable with hypersthene cores. studies are mainly composed of phenocryst-poor magma Most plagioclase phenocrysts have a uniform calcic core enclosed surrounded by a comparatively thin crystallizing boundby a normally zoned rim; others have variable An contents and ary layer. Phenocrysts are interpreted to have crysmottled textures. These characteristics and melt (glass) compositions tallized either within the main body of magma, or in suggest a complex, multistage mixing of low-T (940–960°C) situ within the boundary layer. highly silicic and high-T (980–1060°C) less silicic magmas. Combination of geophysical and petrological data, Stratigraphic variations of the phenocryst types show that the earlier together with fluid dynamical simulations of magmatic erupted magma inherited phenocrysts from more high-T magma processes beneath mid-oceanic ridges has shown that a components. We propose the presence of an inhomogeneous mushy mush of crystals and interstitial melt (25–55 vol. % chamber filled with a highly silicic magma which was recharged crystals) probably makes up the majority of the magma periodically by less silicic magmas. The large viscosity contrast and chamber and only a small volume exists as a meltdensity balance of the new magma relative to the mush controlled enriched lens (e.g. Sinton & Detrick, 1992; Nicolas et al., the efficiency of mixing and the position of emplacement of the high1993; Barth et al., 1994). Likewise, it is highly probable T magma in the mush. Tapping the chamber first expelled the that magma chambers beneath island-arc volcanoes fluidal mixed magma, followed by progressive erosion of the mush largely consist of a mush of crystals and interstitial melt. as the eruption continued. Recent studies on arc volcanic products have presented evidence for the existence of such ‘mushy’ magma chambers. These include an estimation of very high crystallinity