ABSTRACT

The concentrations and isotope ratios of molybdenum (Mo) and tungsten (W) are expected to serve as proxies for paleoceanography. Although submarine hydrothermal activities are potentially major sources and sinks of Mo and W, data on these elements in hydrothermal fluids are scarce. In this study, we present the concentrations and isotope ratios of dissolved Mo and W in hydrothermal fluid samples collected from nine active sites—Higashi Ensei, Iheya North, JADE, Hakurei, Higashi Izena, Daisan Kume, Yokosuka, Futagoyama, and Hatoma Knoll—within the Okinawa Trough, which is a back-arc basin. The Mo concentrations ranged from 4.2–411 nmol/kg, δ^98/95Mo values ranged from –0.17 to 3.92‰, W concentrations ranged from 1.7–238 nmol/kg, and δ^186/184W values ranged from –0.03 to 1.08‰. We found that both δ^98/95 Mo and δ^186/184W were close to 0‰ when Mo and W were dissolved from rocks and sediments during hydrothermal circulation. The phase separation of the hydrothermal fluids concentrates Mo and W in the liquid phase. Sediments can be a significant source of W at high temperatures and Mo even at low temperatures. The scavenging of Mo by Fe sulfides reduces its concentrations and elevates δ ^98/95Mo values during the emission of high-temperature hydrothermal fluids, while having an insignificant effect on W. The scavenging of W by Fe hydroxides reduces its concentrations and elevates δ ^186/184W values during mixing of hydrothermal fluids and seawater, while having an insignificant effect on Mo. Therefore, the data on Mo and W are useful for investigating the processes that occur during hydrothermal circulation.