Importance of pumiceous sediment in influencing chemistry of hydrothermal fluids in Okinawa Trough: insights from boron isotopic ratios

Abstract The involvement of sediments in the Okinawa Trough hydrothermal system is well-established. However, data on boron in sediments are scarce, with minimal quantitative analysis. In this study, we collected hydrothermal fluids and sediments from different hydrothermal areas, such as the southern and middle Okinawa Troughs, and measured their boron concentrations and isotopic ratios. Regarding boron in hydrothermal fluids, after correcting for the influence of seawater mixing, it exhibited the world’s highest boron concentration and the world’s lowest boron isotope ratio, yet showed significant diversity between hydrothermal systems. Regarding boron in sediments, values other than pumice fell within the range reported for silicious sediments worldwide. On the other hand, pumices showed values equivalent to those reported for volcanic rocks in the Okinawa Trough. The large diversity observed among hydrothermal systems indicate that the reacted solid phases are diverse in several hydrothermal systems of the Okinawa Trough. In the Iheya North Knoll, the boron data in the hydrothermal fluids could not be explained without the influence of pumice. On the other hand, in the Hatoma Knoll, the data can be explained by the reaction between volcanic rocks and seawater. In the Daiyon-Yonaguni Knoll, it is difficult to explain by the reaction of either volcanic rocks or sediments with seawater, suggesting that the involvement of both solid phases is also necessary. Furthermore, at the JADE site of the Izena Hole, different hydrothermal fluids collected from the site reacted with different solid phases, suggesting the possibility of heterogeneity in the area around the discharge zone.