演題概要

2P-22（1A-O1-1515）

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火山岩中かんらん石斑晶の希ガス同位体比からの霧島火山群マグマ供給系への示唆

(¹東大・ ²熊本大・ ³日本工営)
^o米田羅生¹・ 角野浩史¹・ 川口允孝¹・ 長谷中利昭²・ 田島靖久³

Here, we report isotopic compositions of noble gases in olivine in pumice of the Shinmoe-dake 2011 eruption. Geophysical observations and petrological investigations on the Kirishima volcano group, including Shinmoe-dake, have suggested that there are deep basaltic and shallow silicic magma chambers) (Suzuki et al., 2013; Aizawa et al., 2014). In addition, data on noble gas isotopes in the fumaroles have been accumulated including nearby volcanoes (Sumino, et al., 2020). This study aimed to discuss the consistency between the proposed subsurface structure and the isotopic compositions of noble gases through a comparison of the isotopic ratios of noble gases in the phenocrysts and fumaroles. The isotopic ratios of helium resulted in ³He/⁴He = 7.46 ± 0.09 R_A (1 R_A = 1.4×10^-6). This is lower than that of Iwo-yama (~7.9 R_A) (Sumino, et al., 2020). Both the Iwo-yama and Shinmoe-dake magmas are thought to be derived from the silicic magma chamber fed by the deeper basaltic magma chamber (Aizawa et al., 2014). This suggests that a gas flow pathway directly connects the deeper magma chamber and the Iwo-yama fumaroles or that the crustal assimilation process of the magma ascending during the eruption was recorded in the olivine phenocrysts.