Brine samples from the wells in the Kashio mineral spring (an “Arima-type” hot spring at Ooshika-Mura, central Japan) were analyzed to determine the original chemical and isotopic compositions of the deep brine end-member before its dilution by meteoric water and to elucidate the origin of the end-member. The trends of variation between Cl, δD, and δ18O indicated the existence of a two-component mixing system and a systematic variation in the mixing ratio, which were mentioned in previous studies. By carefully tracking the variation in tritium (3H) and atmospheric noble gas in the brine, the Cl concentration in the end-member was determined to be 24,000 mg/L. This value is consistent with the result of previous studies. Based on the estimated composition and other related data, we inferred that the end-member originated from slab-derived fluid, which in turn may have undergone oxygen isotope exchange reactions with minerals. Although both the Arima and Kashio brines are considered to be derived from fluid dehydrated from the Philippine Sea slab, the chemical and isotopic compositions of the Kashio end-member are different from those of the Arima end-member. In particular, the Kashio end-member is characterized by low Cl concentration (~ 40% lower than that in the Arima end-member), low hydrogen isotope ratio, and low 3He/4He ratio (1.4 Ra). These results indicate that the chemical and isotopic compositions of the slab-derived fluid are different for each location. The significant difference in δD could reflect the difference in the dehydration depth. Finally, the low temperature and relatively low 3He/4He ratio of the brine end-member could be explained by its long residence time within the crust.

For selecting the HLW geological disposal site, it is necessary to evaluate the groundwater chemical and physical stability and inflowing of deep-seated fluids which is idiomatically classified as slab-derived fluid, fossil seawater, and oil-field brine. Among them, the slab-derived fluid is found in some areas in Japan, they often have high temperature, high CO2 gas, and low pH characters. The features may have adverse impact for geological disposal systems, hence, consolidation of the methodology for finding the fluid is required. However, according to previous studies, the terminology of deep-seated fluids is somewhat ambiguous, and chemical characteristics of each deep-seated fluids does not well organized. In this study, the authors classify the terminology of deep-seated fluids by the formation mechanisms and the reduction of common chemical features of deep-seated fluids are conducted based on the own field surveys over 30 springs and previous studies. The origin of the deep-seated fluids could be classified by dissolved chemicals such as concentration of Cl, He isotope ratio and stable isotope ratio combined with concentrations of other dissolved ions and gases. Dissolved chemicals could be also useful tracer to find out the area of influence of the deep-seated fluid.

In the southwest Japan forearc, slab‐fluids produced from subducted materials migrate to crustal levels and appear as deep‐seated brine. We have analyzed for the first‐time stable strontium isotopes in non‐volcanic spring water with high salinity, referred to as Arima‐type saline water that likely originate from slab‐fluid that upwelled along major faults. The stable strontium isotope compositions of the saline water are isotopically light (δ88/86Sr = 0.122–0.157‰) and different from those of local bedrock and near‐surface water. The light strontium‐enriched and radiogenic signature of the saline water reflects the primary characteristic of slab‐fluids without an isotopic overprint in the crust. The Arima‐type brines show signatures of slab‐fluids at forearc depth, which is different from the slab‐fluids at subarc depth estimated from arc lavas. The characteristic features of the Arima‐type brines are explained by a larger contribution of subducted sediments and strontium isotope fractionation during fluids generation at shallower depth.