Lithium isotope systematics of Arima hot spring waters and groundwaters in Kii Peninsula

Abstract

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 ~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 δ 7 Li values of +1-+3‰. The low δ 7 Li 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 δ 7 Li 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 δ 7 Li values, low Cl/Li ratios, and 87 Sr/ 86 Sr 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