[SCG62-P05] Chemical characteristics of Hot spring gas and water and geological structure around Aira caldera
Keywords:Aira caldera, Hot spring geochemistry, Fault, Kagoshima rift, Sakurajima
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.