MULTISTAGE MAGMA MIXING REVEALED IN PHENOCRYST ZONING OF THE YUNOKUCHI PUMICE, AKAGI VOLCANO, JAPAN

Detailed study of the texture and chemical zoning of phenocrysts in a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, per mits reconstruction of the subvolcanic magma chamber Hypersthene pheno crysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have textura lly similar cores but Al and Ti vs mg-number zoning patterns are corre latable with hypersthene cores. Most plagioclase phenocrysts have a un iform calcic core enclosed by a normally zoned rim; others have variab le An contents and mottled textures. These characteristics and melt (g lass) compositions suggest a complex, multistage mixing of low-T (940- 960 degrees C) highly silicic and high-T (980-1060 degrees C) less sil icic magmas. Stratigraphic variations of the phenocryst types show tha t the earlier erupted magma inherited phenocrysts from more high-T mag ma components. We propose the presence of an inhomogeneous mushy chamb er filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density bala nce 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 mus h. Tapping the chamber first expelled the fluidal mixed magma, followe d by progressive erosion of the mush as the eruption continued.