[A-OS20] Chemical and Biological Oceanography

[AOS20-03] Anomaly of concentration and isotope ratio of Mo and W in hydrothermal fluids at a back-arc basin, Okinawa Trough

*Matsuoka Kohei¹, Yoshiki Sohrin¹, Shinsuke Kawagucci², Tomohiro Toki³ (1.Kyoto University, 2.Japan Agency for Marine-Earth Science and Technology, 3.Facaltyof the science, University of the Ryukyus)

Keywords:Molybdenum, Tungsten, Isotope ratio, Hydrothermal fluid, Okinawa Trough

Concentrations and isotope ratios of molybdenum (Mo) and tungsten (W) are expected to be proxies for paleoceanography. We found that the concentrations and isotope ratios of Mo and W are uniform in the modern ocean but that they are affected by anthropogenic sources (Matsuoka et al., 2023). However, the effects of other sources on the isotope ratios of Mo and W are still unknown. In this study, we observed the concentrations and isotope ratios of dissolved Mo and W in hydrothermal fluids at a back-arc basin, the Okinawa Trough. We sampled hydrothermal fluids from 9 sites (Higashi Ensei, Iheya North, JADE, Hakurei, Higashi Izena, Daisan Kume, Yokosuka, Futagoyama, and Hatoma Knoll). Excluding the Hatoma Knoll, Mo concentration was from 8.1 nmol /kg (Izena) to 154.6 nmol /kg (Izena), and δ ^98/95Mo was from -0.17‰ (Izena) to 3.92‰ (Yokosuka). W concentration was from 1.7 nmol/kg (Yokosuka) to 238.0 nmol/kg (Izena), and δ ^186/184W was from -0.03 ‰ (Izena) to 0.11 ‰ (Izena). At the Hatoma Knoll, Mo concentration was 4.2–410.9 nmol/kg, δ ^98/95Mo was 1.27–3.07, W concentration was 0.9–98.9 nmol/kg, and δ ^186/184W was 0.05–1.08‰. For the endmembers of high temperature hydrothermal fluids, W concentrations were 200–5200 times higher than those in seawater, and δ ^186/184W ranged from 0.03 to 0.14. Mo concentrations were less than 10% of those in seawater due to removal of thiomolybdate. In contrast, Mo concentrations in low temperature fluids were up to ~4 times higher than those in seawater. In addition, W was effectively removed from low temperature fluids with high concentrations of Fe. These contrasting behaviors of Mo and W will be useful for characterizing hydrothermal fluids, not only in modern but also in geological hydrothermal activities.