[S-CG54] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG54-P11] Strontium isotope compositions of nonvolcanic spring water in Kyushu, Japan

*Tomomi Kani¹, Noritoshi Morikawa², Masaaki Takahashi² (1.Department of Earth and Environmental Sciences, Kumamoto University, 2.National Institute of Advanced Industrial Science and Technology (AIST) )

Keywords:Sr isotope, spring water , slab-fluid, subduction

The slab-fluid generated from subducted materials travel through the pores in mantle up to crustal level and appear as nonvolcanic Arima-type saline water along the faults in southwest Japan. It is previously reported that the strontium (Sr) isotope compositions of the saline water in Arima area and Kashio area are retained that of the slab fluid without an isotopic overprint at the shallow crust (Kani et al., 2023). We analyzed the Sr isotope compositions of nonvolcanic (Arima-type) spring water which have non-meteoric oxygen and hydrogen isotopic compositions along faults in the forearc region of Kyushu, Japan. The six water samples were collected within 25 km square in Oita prefecture, more than 20 km northeast from the area of the Kuju Volcano group. The basement rocks around the sampling sites in this study are various as Cretaceous granite, Cenozoic ryolite and Permian oceanic sediments, respectively. The obtained ⁸⁷Sr/⁸⁶Sr ratios ranged widely from 0.7068 to 0.7086. These results suggest the Sr isotope compositions of the nonvolcanic spring water in the studied area, at least some of them, were overprinted by the interaction with the basement rocks at the shallow crust. The dual Sr isotope (δ⁸⁸Sr and ⁸⁷Sr/⁸⁶Sr) analyses of Arima-type brines, which are a new indicator of slab fluids, are needed in future studies to clarify whether the Sr isotopic characteristics of the nonvolcanic spring water in this study area retain those of the slab-fluid or an overprint by isotopic disturbances during ascending in the crust.