Keywords:hot spring, slab-derived fluid, geochemistry, CO2, Na-Cl, hypersaline
Active empirical researches on the relation between hot spring and deep-generated aqueous fluid accompanied by plate subduction (slab-derived fluid) have been carried out in recent years (e.g., Kazahaya et al., 2014; Kusuda et al., 2014). We began the pursuit of their relevance by focusing on saline hot springs along the Median Tectonic Line (Amita et al., 2004; Ohsawa, 2004). In consequence, we made an educated guess at CO2-rich Na-Cl type saline spring waters as closely related to a slab-derived fluid and it may be expected regularity between occurrence depth and Li/B or CH4/CO2 ratios of slab-derived fluids (Ohsawa et al., 2010; Amita et al., 2014). On the other hand, researches to explore the actual situation of slab-derived fluid using aqueous fluid inclusions in geological samples produced in subduction zones have been promoted (e.g., Nishimura et al., 2008; Yoshida et al., 2011; Yoshida et al., 2015). From studies of aqueous fluid inclusions in mantle xenoliths, slab-derived fluid is estimated to be CO2-rich Na-Cl type saline waters (Kawamoto et al., 2013; Kumagai et al., 2014), and from the similarity of the chemical composition of the fluid inclusions and the Arima hot spring water, commentary also began to appear that the Arima-type thermal water is originated from the slab-derived fluid.
After that, when we examined saline hot spring waters and some associated gas in Arima and surrounding areas in southwestern Hyogo Prefecture, we found the discharge of hypersaline hot spring water with similar hydrochemical facies (CO2-rich Na-Cl type) but different hydrogen and oxygen isotope composition from Arima-type thermal water and also found that He and CO2 with the hypersaline hot spring water are of crustal origin (Ohsawa et al., 2015). This findings show that hypersaline hot spring water of Na-Cl type rich in CO2 is not always related to slab-derived fluid, and it will be a continued issue to find a definite geochemical indicator in hot spring water which shows clearly that the origin is the slab.