Stratigraphic variations in zoning patterns and abundance of phenocrysts in pre-caldera pyroclastic deposits of Akagi volcano were examined in order to reconstruct thermal and compositional structures of a magma chamber. The eruption of the Yunokuchi pumice fall deposit (UP), followed by the Garan pyroclastic flow, took place at ca. 45 ka just prior to the formation of the summit caldera with a dimension of 2 by 4 km.

Four zoning patterns are identified in hypersthene (hyp); Type a: uniform cores with low Mg#s of 61-65 in opx; Type b: low-Mg# core fringed with partial dissolution zone with high Mg#s; Type c: resorbed core showing partial dissolution; Type d: core similar to Type c enclosed by low-Mg# mantle. Plagioclase (pl) phenocrysts are divided into Type A (An75-85) and B (An92-94) based on An content of the cores. All pl phenocrysts have normally zoned rims with a plateau of An82-85 in the innermost rims.

We propose a magma chamber filled with mush consisting of Type a hyp, Type A pl and a small amount of melt. Supply of more mafic, hot magma into the chamber formed a melt-enriched pocket in the mush. Mushy magma adjacent to the injected magma was mixed with it, resulted in partial dissolution of Type a into Type b. As partial dissolution proceeded, Type b cores were completely replaced by Type c cores, which accumulated on the chamber floor. The accumulated Type c cores were reacted with infiltrated interstitial melt equilibrated with the lower mush, forming the low-Mg# mantle as seen in Type d hyp. Meanwhile, mushy magma on the ceiling of the melt pocket was heated and reacted with the melt to form Type b hyp. Thus, the magma chamber became stratified in respect to distribution of hyp types: Type a, d, c, b+c+melt, b and a in ascending order.

The last supply of new magma with Type B pl caused extensive mixing of the supplied magma with the mush, resulted in the reverse zoning in the outermost rim of the pre-existing Type A pl and hyp. Tapping the chamber first expelled a less viscous, crystal-poor mixed magma with a higher proportion of Type B pl and Type b and c hyp. As eruption proceeded, increasing amount of the mushy magma with Type A pl and Type a hyp were incorporated into the effusing magma.