[S-VC35] Hydrothermal systems of volcanoes

[SVC35-P05] Compositional variability of rare earth elements in groundwaters around Kuju Volcano

*Hitomi Nakamura¹, Noritoshi Morikawa¹, Yoko Togo¹, Hikaru Iwamori², Shuhei Sakata² (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Earthquake Research Institute, The University of Tokyo)

Keywords:Kuji, volcano, hot spring, rare earth

The Kuju Mountain Range consists of approximately 20 volcanoes spread across an area of about 13 km from east to west and 10 km from north to south. The volcanic edifice consists of small stratovolcanoes and lava domes composed of basaltic andesite to dacite. The Kuju Mountain Range can be topographically divided into western, central, and eastern parts, with volcanic activity generally becoming more recent from west to east. (Kawabe et al., 2015). High heat flows of 400–700°C at a depth of approximately 5 km have been observed beneath Kuju Volcano, despite the magma chamber not being in a fully molten state. (Ehara, 1992). Volcanic earthquakes occur in the shallow areas around Mount Hossho in the central region, as well as at depths of several kilometers or more around Sujiyu and Otake, located west of Mount Kuroiwa in the western part of the range (Japan Meteorological Agency, 2013). In addition, the New Energy and Industrial Technology Development Organization (NEDO) conducted an assessment of Supercritical Geothermal Resources (Kuju region), and a high-density magnetotelluric survey detected a low resistivity zone extending from a depth of 5 km in the direction of Otake to a depth of 1 km in the direction of Ioyama (Aizawa et al., 2022). Combined with a reflection survey, the existence of a supercritical reservoir has been pointed out in this region (Nishijima et al., 2024). Alternatively, a comparison of the theoretical values of hydrothermal fluids released in each state of the magma chamber with actual data from groundwater samples revealed that the type and state of magma differs from the west to the east, which is interpreted as the solidification and generation of felsic magma in the west and central parts, while in the east, activity of mafic magma associated with deep low-frequency earthquakes occurring at depths of 10 to 31 km has also been suggested (Kazahaya et al., 2024). In this study, we aimed to understand the fluid circulation beneath Kuju Mountain Range, focusing on the previously noted difference between the northwest and southeast directions. We collected groundwater samples (hot spring water and spring water) and conducted geochemical analysis. As a result, we obtained data on the rare earth element composition from approximately 50 samples, and we will report on the diversity of their compositions and their spatial distribution.
This study was supported by the Secretariat of the Nuclear Regulation Authority, Japan.