[S-GC40] Volatile Cycles in the Deep Earth - from Subduction Zone to Hot Spot

[SGC40-10] Helium isotope variations in subduction-type volcanic hydrothermal systems

*佐野 有司¹、鹿児島 渉悟¹、高畑 直人¹、尾上 哲治²、柴田 智郎³、Tobias Fischer⁴ (1.東京大学大気海洋研究所海洋地球システム研究系、2.熊本大学先端科学研究部基礎科学部門地球環境科学講座、3.京都大学地球熱学研究施設、4.ニューメキシコ大学地球惑星科学教室)

キーワード：volcano、subduction

Large variations of ³He/⁴He ratios of gas and fluid samples were observed in subduction-type modern hydrothermal systems (0.3Ra – 8Ra where Ra is the atmospheric ratio of 1.38x10^-6) compared with those of mid-ocean ridge basalt (MORB) glasses (7.5Ra – 9.5Ra). These variations are principally attributable to binary mixing of two components: mantle He (8Ra) and crustal helium (0.1Ra). Major source of the latter component could be due to radiogenic helium in the subsurface crustal material. Thus, shallow assimilation would result in helium isotope variations around an isolated volcano. Early case study on the Ontake volcanic hydrothermal system showed that high-Ra was located in hot spring close to the central cone and decreased with distance from the volcano. Subsequent studies around Mt Nevado del Ruiz and Mt Kusatsu-Shirane confirmed the negative correlation between the ³He/⁴He ratio and the distance. There may be three necessary conditions to produce such a tendency. First, the volcano is independent strato-type and there is no other volcano close to the subject. Second, the magma chamber is located just beneath the volcano so that the conduit is not inclined significantly. Third, the hydrothermal system is well developed and the emission of mantle helium is stable for a long time. In order to apply these hypothetical conditions to a caldera volcano, we have carried out helium isotope survey around Aso volcano in Southwest Japan. Mt Aso is the largest active volcano in Japan with a caldera 25 km north-south and 18 km east-west. Post caldera activity is currently observed in central cone at Mt Nakadake. We have collected hot spring gas and fluid samples within the Aso caldera together with the outside and measured ³He/⁴He and ⁴He/²⁰Ne ratios by a noble gas mass spectrometer. The helium isotopes, after correction of air contribution, decrease with the distance from the central cone in the region. The tendency is consistent with those of strato-volcanoes reported in literatures.