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

Abstract

14 Slab-dehydrated fluid is considered to be involved in island arc magmatism. In this study, Li 15 isotope ratios were determined for deep groundwater samples from the non-volcanic forearc 16 region of SW Japan. The contribution of the slab-dehydrated fluid from the Philippine Sea 17 Plate (PHS) was investigated in the Arima area and the eastern Kii Peninsula area, 18 corresponding to the slab depths of ~60 km and 20-30 km, respectively. In the Arima area, the 19 high-temperature thermal waters with high salinity called the Arima-type fluid were clarified 20 to have low δ 7 Li values of +1-+3‰. The low δ 7 Li values with high Li concentrations are 21 thought to be the result of fluid-rock interactions at high temperatures, which is consistent 22 with the characteristics of the slab-dehydrated fluid. On the other hand, the δ 7 Li values of 23 deep groundwaters in the eastern Kii Peninsula showed a wide range of +2 to +29‰. Several 24 groundwater samples had a similar chemical property to the Arima-type fluid: low δ 7 Li 25 values, low Cl/Li ratios, and 87 Sr/ 86 Sr ratios around 0.708-0.710. They are distributed along 26 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 28 Japan from PHS along the large faults, and Li isotope ratios will be a useful indicator to detect their contribution.