Abstract

Research articles : Atmospheric and hydrospheric sciences

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

Tomohiro Toki, Naoya Ebina, Yuki Mitsutome, Ke-Han Song, Ryuichi Shinjo, Jun-ichiro Ishibashi, Shinsuke Kawagucci
Keywords:
Earth Sciences, Geochemistry, Geology, Historical Geology, Palaeoceanography, Sedimentology

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.

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Left) Boron concentrations and isotope ratios in hydrothermal fluids collected from two hydrothermal systems in the southern Okinawa Trough (Hatoma Knoll and Daiyon-Yonaguni Knoll) are shown as a red triangle and a red x mark, respectively. In contrast, the combinations of boron concentration and isotope ratio in hydrothermal fluids formed by mass exchange between volcanic rocks and seawater at 350°C for various water/rock ratios are shown by the black curve. The combinations formed by mass exchange between sediments and seawater at 350°C are shown by the brown curve. The boron concentration and isotope ratio combinations in low-temperature hydrothermal fluids formed by sediment-seawater reactions at 150°C are shown by a green curve. The boron data in the hydrothermal fluids from the Hatoma Knoll lie on the reaction curve between volcanic rock and seawater. The boron data in the hydrothermal fluids at Daiyon-Yonaguni Knoll lie between the curves for reactions with volcanic rock and sediment. This indicates that boron in the hydrothermal fluids at Hatoma Knoll is formed by reactions between volcanic rock and seawater, whereas boron in the hydrothermal fluids at Daiyon-Yonaguni Knoll would be formed by reactions between seawater and two solid phases: both volcanic rock and sediment.
Right) Boron concentrations and isotope ratios in hydrothermal fluids collected from four hydrothermal sites (Iheya North Knoll, JADE site and HAKUREI site in Izena Hole, and Minami-Ensei Knoll) within three hydrothermal systems in the middle Okinawa Trough are shown by red symbols. The black curve shows calculated boron concentrations and isotope ratios in hydrothermal fluids formed from volcanic rocks collected from the middle Okinawa Trough reacting with seawater at 350°C, calculated for various water/rock ratios. Similarly, the gray curve shows the theoretical plot for pumice collected from the middle Okinawa Trough reacting with seawater at 350°C. The brown curve shows sediment reacting with seawater at 350°C, and the green curve shows sediment reacting with seawater at 150°C. These results indicate that the boron in the Iheya North Knoll hydrothermal fluids can only be explained by a reaction between pumice and seawater at 350°C. For HAKUREI site and Minami-Ensei Knoll, it was suggested that the hydrothermal fluids formed from the reaction of both pumice and sediment with seawater at 350°C. Furthermore, multiple endmembers have been reported for hydrothermal fluids collected from the JADE site, suggesting varying degrees of sediment involvement.
These findings suggest that the effects of hydrothermal-sediment interactions, often treated as a single subject, can vary significantly due to spatial or qualitative differences such as sediment composition heterogeneity and hydrothermal flow pathways.

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