[S-GL21] Geochronology and Isotope Geology

[SGL21-P09] Stable Strontium isotope composition (δ88/86Sr) of Arima-type saline water originated from slab-fluids generated at forearc depth

*Tomomi Kani¹, Keiji Misawa², Noritoshi Morikawa³, Kohei Kazahaya³, Fumitake Kusuhara⁴, Shigekazu Yoneda⁵, Yasutaka Terakado⁶ (1.Department of Earth and Environmental Sciences, Kumamoto University, 2.National Institute of Polar Research , 3.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST) , 4.Central Research Institute of Electric Power Industry , 5.National Museum of Nature and Science, Tsukuba, Japan., 6.Graduate School of Human Development and Environment, Kobe University)

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

Strontium (Sr), its dual isotope systems i.e., radiogenic and mass-dependent isotope systems are useful to understand mass transfer and water budget in the subduction zone. In the southwest Japan forearc, slab-fluid generated from subducted materials migrate up to crustal level and appear as nonvolcanic Arima-type spring water with high salinity. We analyzed stable Sr isotope values (δ88Sr) of Arima-type saline water using by TRITON TIMS with correction of isotope fractionation during mass spectrometry with 87Sr-84Sr double spike. The obtained δ88Sr values of the Arima saline water which range from +0.122‰ to +0.153‰ are lighter than those of local rocks (Rokko granites and Arima rhyolitic welded tuff). Whereas the δ88Sr value of Tansansen spring water as the near-surface water almost identical that of the Rokko granite. These results suggest that the Sr isotope compositions of the Arima saline water should be regarded as those of slab-fluids without significant isotopic disturbances at crustal level. The Sr isotope characteristics of the saline water can be explained by isotope fractionation during fluid extraction from subducted altered oceanic crust and overlying sediments at forearc depth.