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 a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, permits reconstruction of the subvolcanic magma chamber. Hypersthene phenocrysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have texturally similar cores but Al and Ti vs mg-number zoning patterns are correlatable with hypersthene cores. Most plagioclase phenocrysts have a uniform calcic core enclosed by a normally zoned rim; others have variable An contents and mottled textures. These characteristics and melt (glass) compositions suggest a complex, multistage mixing of lot-T (940–960°C) highly silicic and high-T (980–1060°C) less silicic magmas. Stratigraphic variations of the phenocryst types show that the earlier erupted magma inherited phenocrysts from more high-T magma components. We propose the presence of an inhomogeneous mushy chamber filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density balance of the new magma relative to the mush controlled the efficiency of mixing and the position of emplacement of the high-T magma in the mush. Tapping the chamber first expelled the fluidal mixed magma. Followed by progressive erosion of the mush as the eruption continued.