The subseafloor thermal gradient at Iheya North Knoll, Okinawa Trough, based on oxygen and hydrogen isotope ratios of clay minerals

We measured the oxygen and hydrogen isotope ratios of smectite, chlorite, and illite in hydrothermally active sediments at the Iheya North Knoll, Okinawa Trough, obtained by seafloor drilling of active hydrothermal fields during the Integrated Ocean Drilling Program (IODP) Expedition 331. Formation temperatures calculated from oxygen isotope data were compared to the downhole temperatures that were measured during the same expedition. The δ18O values ranged from +9.9‰ to +13.3‰ for smectite, +1.3 to +3.0‰ for chlorite or chlorite/smectite mixed-layer mineral, and +2.1‰ to +5.1‰ for illite or illite/smectite mixed-layer mineral with minor chlorite. For samples of illite that contained minor chlorite, δ18O values for illite or illite/smectite mixed-layer mineral of +4.4‰ to +6.1‰ were calculated, based on the proportion of the two clay minerals calculated from chemical composition of the individual mineral and the mixture determined by TEM-EDS and EPMA. We estimated the formation temperatures of the minerals using oxygen isotope fractionation data and assumed values of 0 to +1.2‰ for the fluid that was equilibrated with the minerals, based on the reported values of the seawater and vent fluid measured in previous studies. The temperature range calculated was 111–175 °C for a shallow layer less than about 25 m below seafloor (mbsf) that contained smectite and a minor kaolinite component, 177–297 °C for a deeper layer between about 25 and 40 mbsf that was dominated by chlorite or chlorite/smectite mixed-layer mineral, and 220–314 °C for the deepest layer between about 40 and 120 mbsf that contained illite or illite/smectite mixed-layer mineral with less chlorite. The temperatures of mineral formation calculated from the oxygen stable isotope data were slightly higher at some depths, but both calculated and measured temperatures demonstrated a steep, step-wise thermal gradient below the active hydrothermal field, where temperatures of >200 °C were determined at as little as 50 mbsf. Hydrogen isotope values mostly ranged between −40 and −55‰ and did not vary significantly or systematically between the layers, except in the clay fractions containing smectite above 25 mbsf that had relatively low values between −65 and −72‰. Hydrogen isotope results of the clay minerals were problematic to interpret and not consistent with interpretation of the oxygen data. Calculated temperatures from illite and smectite were mostly negative, whereas temperatures calculated from chlorite were more positive (average 145 °C), but lower than those from the oxygen isotope data. Additional work is still required to determine the hydrogen isotope fractionation between clay minerals and water with respect to temperature.