Sources of chlorine in deep fluids beneath the Japanese island, inferred from the application of the long-lived radionuclide 36Cl

Several types of volcanic and non-volcanic fluids are widely distributed beneath the Japanese island. The knowledge of the sources of chlorine in these fluids and their origins are of considerable significance, since they could be associated with its possible impact of chemical corrosion on potential nuclear waste repositories as well as the condition of groundwater flow in its vicinity. Besides major and trace elements, various types of isotopes in water can provide valuable insights into the origins of water and/or solutes. This study focuses on the application of the long-lived radionuclide 36Cl (half-life 301,000 years) to better elucidate the sources of chlorine in deep fluids in Japan. Thousands of hot springs existing in Japan offers an ideal opportunity for this purpose. Deep fluid samples were obtained from over 100 hot spring wells all across the Japanese island, and analyzed for 36Cl as well as other chemical and isotopic substances. The obtained 36Cl/Cl ratios in the fluid samples were largely in the range less than 1 × 10-14. The observed variation in 36Cl/Cl ratios would depend on the source of chlorine, fluid residence time in the crust, and the mixing of shallow groundwater. Among the fluid samples along the two major tectonic lines in the Kinki area, southwestern Japan, the samples obtained nearby the Arima-Takatsuki Tectonic Line (Arima-type brine; Matsubaya et al., 1973) exhibited particularly low 36Cl/Cl ratios almost equal to the seawater value (1-2 × 10-15). The Arima-type brine is characterized by high chloride concentration (up to twice that of seawater), high δ18O value approximately close to that of magmatic water (Giggenbach, 1992), and high 3He/4He ratios similar to that of the upper mantle (Morikawa et al., 2008). An inverse correlation was found between 3He/4He and 36Cl/Cl ratios for the two tectonic lines, suggesting a possible source of these fluids. Specifically, high 3He/4He ratios accompanied by low 36Cl/Cl ratios indicate a deeper source of the fluids possibly linked with the dehydration of the subducting slab, and also a relatively short residence time in the crust without significant production of nucleogenic 36Cl. Once in the crust, the fluids accumulate radiogenic 4He and nucleogenic 36Cl, which leads to a lower 3He/4He ratio and a higher 36Cl/Cl ratio approaching to a secular equilibrium value of surrounding rocks. Conversely, the 36Cl/Cl ratios for the fluid samples unassociated with major fault systems or volcanic activity such as those obtained in the interior of sedimentary basins were relatively high, which might be derived from a long-time residence of fossil seawater or the deep-seated fluids in the crust. The observed 36Cl/Cl ratios in deep fluids in Japan demonstrate that 36Cl can be an effective tool for elucidating the sources of chlorine and/or fluids, especially when coupled with helium isotopes. References: Giggenbach, W.F. 1992. Earth Planet. Sci. Lett., 113, 495-510. Matsubaya, O. et al. 1973. Geochem. J., 7, 123-151. Morikawa, N. et al. 2008. Geochem. J., 42, 61-74.