10:00 AM - 10:15 AM
[HTT16-05] Origin of hot spring waters in Okayama and Tottori Prefectures, southwest Japan: lithium and strontium isotopes as indicators of deep-derived fluids
Keywords:Deep-derived fluids, Geofluids, Arima-type fluids, Hot spring, Groundwater, Lithium isotope
Slab dehydration occurs in the Philippine Sea Plate as it subducts beneath the forearc region of southwest Japan. This process is thought to generate Arima-type fluids, characterized by hydrogen and oxygen isotope compositions (δD and δ18O) that deviate from the meteoric water line. To date, Arima-type fluids have been reported in limited areas, such as the Arima and Takarazuka hot springs, which are located near active faults.
Although Arima-type fluids are believed to originate from slab dehydration, their reports remain limited. The conventional δD-δ18O indicator fails to detect Arima-type fluids when the Cl- concentration is below 3400 mg/L. However, lithium isotopes (δ7Li) enable their identification under low-salinity conditions. This suggests that δ7Li indicator can detect Arima-type fluids component in hot springs with lower salinity, which were previously undetectable using conventional methods.
In this study, we collected water samples, including hot spring and well water, from Okayama and Tottori Prefectures, southwest Japan, and analyzed their chemical compositions, δ7Li values, and 87Sr/86Sr ratios. The results revealed that the δ7Li and 87Sr/86Sr compositions of some water samples in Okayama Prefecture are similar to those of Arima-type fluids. Furthermore, these compositions cannot be easily explained by local rock interactions, residual hydrothermal or magmatic fluids, or seawater altered by rock interactions. Therefore, our findings suggest that Arima-type fluids may be more widely distributed in the forearc region of southwest Japan than previously recognized.
This presentation will highlight the findings of Akishiba and Nishio (2025; PEPS) and explore the applicability and potential of lithium isotope indicators in groundwater studies, including not only hot spring waters but also spring and well waters.
Although Arima-type fluids are believed to originate from slab dehydration, their reports remain limited. The conventional δD-δ18O indicator fails to detect Arima-type fluids when the Cl- concentration is below 3400 mg/L. However, lithium isotopes (δ7Li) enable their identification under low-salinity conditions. This suggests that δ7Li indicator can detect Arima-type fluids component in hot springs with lower salinity, which were previously undetectable using conventional methods.
In this study, we collected water samples, including hot spring and well water, from Okayama and Tottori Prefectures, southwest Japan, and analyzed their chemical compositions, δ7Li values, and 87Sr/86Sr ratios. The results revealed that the δ7Li and 87Sr/86Sr compositions of some water samples in Okayama Prefecture are similar to those of Arima-type fluids. Furthermore, these compositions cannot be easily explained by local rock interactions, residual hydrothermal or magmatic fluids, or seawater altered by rock interactions. Therefore, our findings suggest that Arima-type fluids may be more widely distributed in the forearc region of southwest Japan than previously recognized.
This presentation will highlight the findings of Akishiba and Nishio (2025; PEPS) and explore the applicability and potential of lithium isotope indicators in groundwater studies, including not only hot spring waters but also spring and well waters.