Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Evening Poster

S (Solid Earth Sciences) » S-CG Complex & General

[S-CG62] Crustal fluids and deformation

Wed. May 23, 2018 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Naoji Koizumi(School of Environmental Science,the University of Shiga Prefecture), Koji Umeda(Hirosaki University), Norio Matsumoto(産業技術総合研究所地質調査総合センター地震地下水研究グループ, 共同), Hidemi Tanaka(School of Science, The University of Tokyo)

[SCG62-P05] Chemical characteristics of Hot spring gas and water and geological structure around Aira caldera

*Kuniyo Kawabata1,2, Noritoshi Morikawa3, Kohei Kazahaya3, Hiroshi A Takahashi3, Masaaki Takahashi3, Hitoshi Kodamatani1, Kazuhiko Kano3, Yujin Kitamura1, Takashi Tomiyasu1 (1.Graduate School of Science and Engineering, KAGOSHIMA UNIVERSITY, 2.Kagoshima University Museum, 3.AIST, Geological Survey of Japan,Research Institute Earthquake and Volcano Geology)

Keywords:Aira caldera, Hot spring geochemistry, Fault, Kagoshima rift, Sakurajima

Aira caldera located in the Kagoshima Bay, in southern part of Kyushu, Japan is likely to have a large magma chamber, supplying magma to active volcanoes Sakurajima and Wakamiko in the caldera (Roullear et al, 2013). At Sakurajima volcano, rise of magma was observed on August 2015 that brought a raise in the volcanic alert level from 3 to 4 for the following month. Increase in CO2 release from Wakamiko was also reported on 2015. These observations imply that the activity of Aira caldera is increasing.

Around Aira caldera, there are many hot springs, which are presumed to be affected by the volcanic activity. We tried to clarify relationship between hot springs and the magma under Aira caldera via examining hot springs components in this study. We also investigated the geological structure in relation to the magma and hot springs because it is directly linked with the conduits and aquifer of the hot springs. We sampled 24 hot springs water and analyzed stable hydrogen and oxygen isotopes ratio (δD and δ18O), helium isotopes ratio (3He/4He), dissolved ions and radon isotope (222Rn) concentration.

The result of δD and δ18O show that most of all hot springs around Aira caldera is meteoric water origin. High 3He/4He was recognized at hot springs around northern region from Kagoshima Bay and hot springs in Sakurajima. 3He/4He and 20Ne results present that gas in hot springs having higher 3He/4He can be explained by mixing between mantle helium and air. The high 3He/4He hot springs are also related to active fault distribution in northern region from Kagoshima Bay. Hot springs closer to an active fault show higher 3He/4He in the region. In Sakurajima, only hot springs at the north and south parts have higher 3He/4He, which is hardly related to the directions of magma intrusions. Several faults showing similar orientation with Kagoshima rift have been recognized within Kagoshima Bay (Hayasaka, 1984). Presence of a large fault on the south of Sakurajima was also suggested by Hayasaka (1984). A line connecting hot springs having 3He/4He in north of Kagoshima Bay and those in Sakurajima, displays a consistency with the Kagoshima rift and the southern extension of the line includes the large fault reported by Hayasaka (1984). These results suggest that the hot springs with higher 3He/4He is possibly linked with the large structures of the Kagoshima rift.