[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-P06] Gas geochemistry of caldera and stratovolcano in Japan

*佐野 有司¹、鹿児島 渉悟²、張 茂亮³、高畑 直人⁴ (1.高知大学海洋コア総合研究センター、2.富山大学大学院理工学研究科、3.天津大学地球システム科学科、4.東京大学大気海洋研究所)

キーワード：ヘリウム同位体、カルデラ火山、成層火山

Few studies have investigated relationships between morphological difference of caldera volcano and stratovolcano, and discharged fluid geochemistry. We have reported helium isotope ratios of geothermal fluids and groundwater inside and outside the caldera of Mt. Aso, Japan, and compare these to helium ratios in groundwater at Mt. Unzen, a stratovolcano in the same tectonic field of Southwest Japan [1]. Helium isotope ratios decrease in proportion to distance from the central part of the volcano in both cases as reported by general features [2]. However, the helium isotope ratios of inferred magmas are probably lower in Mt. Aso relative to Mt. Unzen, while Mt. Aso has more developed geothermal systems than Mt. Unzen. The former is fairly supported by seismic tomography data. The latter was suggested by case studies of Yellowstone and Taupo Zone [3]. We discuss here gas geochemistry of geothermal fluids and groundwater samples combined with helium isotopes. Nitrogen and carbon dioxide are the main gaseous components, and more than 5% methane is found in four samples. ⁴⁰Ar/³⁶Ar ratios are close to the atmospheric value of 295.5 in most samples, and the highest isotope ratios of 334.7 shows 12.5% excess ⁴⁰Ar. Nitrogen isotope ratios are mostly 0‰ with positive and negative anomalies in some samples. There is a weak positive correlation between d¹⁵N values and ³He/⁴He ratios [4]. Carbon isotope ratios varied from -26.3‰ to -3.9‰ with the average of -11.2‰. These values could be explained by a mixture of oxidized organic matter in sedimentary layers, marine carbonates, and mantle-derived carbon together with depletion by calcite precipitation [5].

[1] Sano,Y., Kagoshima, T., Zhang, M., Takahata, N., Onoue, T., Shibata, T., Nishio, Y., Chen, A.-T., Lee, H., Fischer, T.P. & Zhao, D. Older magma at Aso caldera than Unzen stratovolcano in south west Japan as recorded through helium isotopes. Communications Earth and Environment 4, 2 (2023).
[2] Sano, Y. & Fischer, T.P. The analysis and Interpretation of Noble Gases in Modern Hydrothermal Systems. In P. Burnard (ed.), The Noble Gases as Geochemical Tracers, Advances in Isotope Geochemistry. pp. 249-317, Springer-Verlag, Berlin (2013).
[3] Stimac, J., Goff, F. & Goff, C.J. Intrusion-Related Geothermal Systems. In J. Stimac (Ed), The Encyclopedia of Volcano, pp 799-822, Elsevier (2015).
[4] Kagoshima, T., Sano, Y., Takahata, N., Lee, H., Lan, T. & Ohba, T. Secular variations of helium and nitrogen isotopes related to the 2015 volcanic unrest of Mt. Hakone, central Japan. G-cubed 20, GC008544 (2019).
[5] Sano, Y. & Marty, B. Origin of carbon in fumarolic gas from island arcs. Chemical Geology 119, 265-274 (1995).