17:15 〜 18:45
[SVC26-P17] Isotopic Compositions of Noble Gases in Olivine in Pumice of Shinmoe-dake 2011 Eruption
キーワード：新燃岳、希ガス、かんらん石、メルトインクルージョン、火道
Noble gases are chemically inert and have different isotopic ratios in different reservoirs, making them useful geochemical tracers for the origin of materials on and in the Earth. For example, isotope ratio of helium, 3He/4He, is lower in the crust because of the decay of uranium and thorium, which are enriched in the crust, while it is higher in the mantle because uranium and thorium are depleted and a significant portion of primordial helium having high 3He/4He is still preserved in the mantle. Of some samples containing noble gases, olivine is one of the minerals relatively often found in volcanic rocks, and diffusion of noble gases in olivine is slow. Therefore, the isotopic composition that had been preserved in the olivine phenocryst since the mineral crystallization in a magma chamber can be measured.
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 basaltic and silicic magma chambers 10 km and 5 km below the surface, respectively1,2. In addition, data on noble gas isotopes in the fumaroles have been accumulated including nearby volcanoes3. 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. After crushing the pumice and picking olivines, whose diameters are 0.5-1 mm, noble gases were extracted by crushing them in a vacuum. The isotopic ratio of helium resulted in 3He/4He = 7.46 ± 0.09 RA (1 RA = 1.4×10-6). This is lower than that of the recent fumaroles from Iwo-yama (~7.9 RA), which is adjacent to Shinmoe-dake. Both the Iwo-yama and Shinmoe-dake magmas are thought to be derived from the silicic magma chamber fed by the deeper basaltic magma chamber2. Therefore, it seems to be inconsistent with the proposed subsurface structure that the phenocryst, whose origin should be deeper than the fumaroles, has more contribution of the crustal helium with low 3He/4He. 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.
1. Suzuki, Y. et al. Volcanol. Geotherm. Res. 257, 184-204 (2013).
2. Aizawa, K. et al. J. Geophys. Res. Solid Earth, 119, 198–215 (2014).
3. Sumino, H. et al. The Volcanological Society of Japan 2020 Fall Meeting, O1-24 (2020).
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 basaltic and silicic magma chambers 10 km and 5 km below the surface, respectively1,2. In addition, data on noble gas isotopes in the fumaroles have been accumulated including nearby volcanoes3. 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. After crushing the pumice and picking olivines, whose diameters are 0.5-1 mm, noble gases were extracted by crushing them in a vacuum. The isotopic ratio of helium resulted in 3He/4He = 7.46 ± 0.09 RA (1 RA = 1.4×10-6). This is lower than that of the recent fumaroles from Iwo-yama (~7.9 RA), which is adjacent to Shinmoe-dake. Both the Iwo-yama and Shinmoe-dake magmas are thought to be derived from the silicic magma chamber fed by the deeper basaltic magma chamber2. Therefore, it seems to be inconsistent with the proposed subsurface structure that the phenocryst, whose origin should be deeper than the fumaroles, has more contribution of the crustal helium with low 3He/4He. 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.
1. Suzuki, Y. et al. Volcanol. Geotherm. Res. 257, 184-204 (2013).
2. Aizawa, K. et al. J. Geophys. Res. Solid Earth, 119, 198–215 (2014).
3. Sumino, H. et al. The Volcanological Society of Japan 2020 Fall Meeting, O1-24 (2020).