Detection of slab-derived fluid in coastal area of Seto Inland Sea, Japan, based on ¹²⁹I/¹²⁷I ratios and halogen composition

We collected 14 saline groundwater samples from coastal land areas of Hiroshima and Hyogo Prefectures, and Shodoshima of Kagawa Prefecture. Data on δD, δ¹⁸O, Li/Cl, Br/Cl, I/Cl, and ¹²⁹I/¹²⁷I for the samples reveal that the groundwater collected from Hiroshima Pref. are mainly derived from relatively young seawater, whereas samples from Hyogo Pref. and Shodoshima clearly contain slab-derived fluid characterized by>0.001 Li/Cl weight ratios, and show 1) <1.25×10^-3 Br/Cl mole ratios, 2) >50×10^-6 I/Cl mole ratios, and 3) low ¹²⁹I/¹²⁷I ratios (i.e., old iodine age). In addition, the latter sampling sites were characterized by shallow cutoff depth (D90: the depth above which 90% of earthquakes occur), suggesting that D90 becomes shallow where slab-derived fluids ascend to shallow depths. Halogen composition and ¹²⁹I/¹²⁷I ratio are concluded to be good indicators of slab-derived fluid contained in saline groundwater. (author)