Lithium Isotope Systematics of Arima Hot Spring Waters and Groundwaters in Kii Peninsula

Slab-dehydrated fluid is considered to be involved in island arc magmatism. In this study, Li isotope ratios were determined for deep groundwater samples from the non-volcanic forearc region of SW Japan. The contribution of the slab-dehydrated fluid from the Philippine Sea Plate (PHS) was investigated in the Arima area and the eastern Kii Peninsula area, corresponding to the slab depths of similar to 60 km and 20-30 km, respectively. In the Arima area, the high-temperature thermal waters with high salinity called the Arima-type fluid were clarified to have low delta Li-7 values of +1-+3%. The low d7Li values with high Li concentrations are thought to be the result of fluid-rock interactions at high temperatures, which is consistent with the characteristics of the slab-dehydrated fluid. On the other hand, the delta Li-7 values of deep groundwaters in the eastern Kii Peninsula showed a wide range of +2 to +29%. Several groundwater samples had a similar chemical property to the Arima-type fluid: low delta Li-7 values, low Cl/Li ratios, and Sr-87/Sr-86 ratios around 0.708-0.710. They are distributed along the Median Tectonic Line (MTL) and within about 20 km south of the MTL. These results indicate a common supply of slab-dehydrated fluids to the non-volcanic forearc region in SW Japan from PHS along the large faults, and Li isotope ratios will be a useful indicator to detect their contribution.