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[SVC48-17] Petrological characteristics of Aso-ABCD tephra which erupted before Aso-4 pyroclastic eruption
Keywords:Aso, caldera-forming eruption, melt inclusion, Aso-ABCD tephra, Aso-4 pyroclastic eruption
Pumice and volcanic ash was sampled 20 km east of Aso caldera, where the thickness of Aso-ABCD tephra in total is 3 m. The samples were analyzed for XRF bulk-rock chemical analysis, EPMA mineral analyses, FT-IR analyses of melt inclusions. Phenocryst assemblage is plagioclase, clinopyroxene, orthopyroxene, and magnetite, with no hornblende which is common in Aso-4 products. Bulk composition of pumice ranges 63-66 wt. % silica, and mostly plot on Aso-3 trend, and not on Aso-4 trend of Kaneko et al. (2007, 2015). Melt inclusions in plagioclase and pyroxenes also show compositions similar to glass of Aso-3 with silica range mostly in 70-72 wt. %. Compositional range of phenocryst cores are An40-64 for plagioclase, Mg# =70-74 for orthopyroxene, and Mg#=74-81 for clinopyroxene. Water content was estimated to be 1.0-4.8 wt.% for melt inclusions in host minerals of An40-64 plagioclase, Mg#=70-74 orthopyroxene, and Mg#=74-81 clinopyroxene.
Equilibrium relationship between melt inclusion and host clinopyroxene provides temperature estimate of 860-950 ℃ and pressure estimate of 1.1-2.7 kbar (Putirka, 2008). The pressure corresponds to the depth of 3-9 km, comparable to the estimated depth (6 km) of present Kusasenri-ga-hama magma reservoir. Furukawa et al. (2006) showed a gradual change of tephra composition, estimated temperature, estimated water content, and estimated oxygen fugacity from Aso-3 to Aso-4. However, our study showed similarity of magma composition between Aso-ABCD tephra and Aso-3 products. Aso-4 magma reservoir was not yet prepared 9000 years before Aso-4 eruption. Or Aso-4 reservoir at that time was independent from, and had no interaction with, other magma supply system then.