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Magma plumbing system of Kirishima volcano revealed by helium isotopic ratio of olivine and pyroxene phenocrysts

Rai Yoneda 1, Hirochika Sumino2, Masataka Kawaguchi3, Toshiaki Hasenaka4 , Yasuhisa Tajima5

  • Affiliations: 1Department of Chemistry, School of Science, The University of Tokyo, Tokyo, Japan; 2Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo, Japan; 3Earthquake Research Institute, The University of Tokyo, Tokyo, Japan; 4Center for Water Cycle Marine Environment and Disaster Management, Kumamoto University, Kumamoto, Japan; 5Nippon Koei Co., Ltd. Tokyo, Japan

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 240

  • Programme No: 1.1.26

  • Theme 1 > Session 1

Abstract

Noble gases are useful geochemical tracers for the origin of materials on and in the Earth. For example, since helium isotopic ratio, 3He/4He, is 1.4×10-6 ≡ 1 RA in the atmosphere, while 3He/4He = 8 (±1) RA in the convecting mantle, helium in fumaroles or hot springs is attracting attention as a monitoring tool for volcanic activity (e.g.,1). However, 3He/4He of melt inclusions in mafic phenocrysts, expected to reflect magma itself, is necessary for understanding a magma plumbing system. In this study, the noble gas isotopic compositions of olivine and pyroxene phenocrysts from the Kirishima volcano group in Japan were measured. The 3He/4He of olivines in the pumice of the Shinmoe-dake 2011 eruption was 7.43 (±0.06) RA, while that of pyroxenes in the same pumice was 7.98 (±0.17) RA. Although the olivines and pyroxenes are considered to be crystallized in deep and shallow magma chambers, respectively2,3, 3He/4He indicate that melt inclusions in pyroxene have more mantle-like properties, which suggests the different crystallization timing. In addition, olivines in three lavas from Ohachi, erupted in 788, 1235, and 1350, respectively, had a high 3He/4He of ~8 RA to reinforce that Ohachi lavas have a deeper origin4. [1] Padrón, E. et al., Geology 41, 539-542 (2013). [2] Suzuki, Y. et al., JVGR 257, 184-204 (2013). [3] Aizawa, K. et al., JGR Solid Earth 118, 198-215 (2014). [4] Kagiyama, T. et al., Bull. Volcanol. Soc. Jpn. 42, S157-S165 (1997).