[A-HW23] Isotope Hydrology 2021

[AHW23-P01] Detection of slab-derived fluid in saline ground water collected from coastal area of Seto Inland Sea, Japan

★Invited Papers

*Yoko Togo¹, Kohei Kazahaya¹, Yuki Tosaki¹, Masaaki Takahashi¹, Noritoshi Morikawa¹, Hiroshi A Takahashi¹, Yuri Nakamura¹, Akihiko Inamura¹, Hinako Shimizu¹, Hiroyuki Matsuzaki² (1.National Institute of Advanced Industrial Science and Technology, 2.University of Tokyo)

Keywords:Slab-derived fluid, Halogen, Groundwater

Southwest Japan is a place where slab-derived fluid supplies as hydrothermal fluids in the forearc region, as typified by the Arima hot springs (Kazahaya et al., 2014). In this study, oxygen and hydrogen isotope ratios, Li/Cl, Br/Cl, I/Cl ratios, and iodine isotope ratios of saline groundwater and spring water collected in Hiroshima, Hyogo, and Kagawa prefectures (Shodo-shima) were analyzed to investigate halogen characteristics of slab-derived fluid.

The chlorine concentrations in the groundwater samples ranged from 1760 to 16600 mg/L. Oxygen and hydrogen isotope ratios indicated that the samples from the western area with high chlorine concentrations were mainly seawater, while the samples from the eastern area with low chlorine concentrations were mainly meteoric water. According to Kazahaya et al. (2014), high Li/Cl ratios of groundwater are characteristic of slab-derived fluids. The Li/Cl ratio of groundwater was higher in the eastern area, suggesting the presence of slab-derived fluid. The samples with high Li/Cl ratios were characterized by low Br/Cl ratios and high I/Cl ratios compared to seawater. In addition, iodine isotope ratios (¹²⁹I/¹²⁷I) showed relatively lower in the eastern region (i.e., older iodine age), suggesting that low iodine isotope ratios are characteristic of slab-derived fluids in this region. The characteristics of halogen ratios and iodine isotope ratios of slab-derived fluids should be investigated and compared in various regions for further investigation.