2015 年 60 巻 3 号 p. 381-397
Volcanic deposits from Tsumaya pyroclastic flow and Osumi pumice fall, which composed the first half of Aira pyroclastic eruption occurred about 29,000 ago, were studied to clarify magma plumbing system of the eruption. Both volcanic deposits contain phenocrysts (15-20 vol.%) of quartz, plagioclase, orthopyroxene, magnetite and ilmenite. In addition, a very small amount of amphibole is observed in Osumi pumice fall deposits. The chemical composition of the phenocrysts and their melt inclusions were analyzed by EPMA. The core compositions of the phenocryst plagioclase and orthopyroxene cluster around An42 and Mg#=48, respectively. Although some cores of plagioclase and orthopyroxene phenocrysts show remnants of magma mixing, the rim compositions having essentially the same composition as the clustered core composition suggest that the magma erupted from an approximately homogeneous magma reservoir. Application of geothermo- and geobarometers (Fe-Ti oxides, orthopyroxene-liquid, amphibole) reveals that the condition of magma storage was 790-850℃, 110 MPa, and FMQ to 0.6 orders of magnitude above it. Estimated water contents of unleaked melt inclusions hosted in quartz, plagioclase and orthopyroxene determined by reflection FT-IR technique were 3.1-5.4 wt.% with an average of 4.5 wt.%. Coexistence of bubbles in some melt inclusions suggests that the magma was water-saturated prior to eruption. Phenocrysts content calculated by MELTS program agreed with the observed one when the given pressure was 80-110 MPa. In summary, all estimates indicate that the pressure at the top of the magma chamber is probably around 100 MPa. This pressure is equivalent to a depth of 4-5 km. The suggested depth is quite shallower than the previously considered depth of magma chamber, i.e. 8-10 km, which was responsible for the Aira pyroclastic eruption.