What question did this study set out to answer?

To reconstruct the alteration history of submarine pumice from hydrothermal fields in the Okinawa Trough.

March 26, 2026GEOCHEMICAL JOURNAL0 citationsOpen Access

Deconvolving low-temperature alteration and post-sampling oxidation in submarine pumice from active hydrothermal fields of the middle Okinawa Trough

ZSZijun SongTokyo University of Marine Science and Technology SYShinsuke Yagyu NKNaoya KannaSphere Institute

Key Points

To reconstruct the alteration history of submarine pumice from hydrothermal fields in the Okinawa Trough.
Analyzed major-element geochemistry and sulfur isotopes of pore fluids.
Conducted isocon analysis to assess element mass-transfer properties.
Examined mineralogical observations to identify alteration characteristics.
Some pumice samples show Fe and Mg enrichment and K depletion, indicating significant alteration.
Most samples display evidence of seawater–rock interaction before recovery.
Alteration occurred under low-temperature conditions (<50–60 °C), not high-temperature hydrothermal conditions.

Abstract

This study deconvolves natural subseafloor alteration processes and post-sampling modification recorded in submarine pumices recovered from active hydrothermal fields at Iheya North Knoll and Izena Hole in the middle Okinawa Trough. Major-element geochemistry, isocon analysis, mineralogical observations, and sulfur isotopes of associated pore fluids were integrated to reconstruct the alteration history of the pumices. Although some pumice samples exhibit intense alteration characterized by Fe and Mg enrichment, K depletion, and high Chemical Index of Alteration (CIA) values, all samples show evidence of seawater–rock interaction associated with subseafloor seawater recharge prior to recovery. Isocon slope values and element mass-transfer coefficients indicate systematic loss of Si, Na, and Ca and net gain of Mg and/or Fe, constraining alteration to low-temperature conditions (<50–60 °C) rather than high-temperature hydrothermal alteration. The pore fluids display extremely high sulfate concentrations (~722 mmol/L) and negative δ34S values generally close to those of authigenic pyrite within the pumices, indicating that most sulfate was generated by oxidation of the pyrite during long-term freezer storage after sampling. δ34S values nevertheless preserve information on sulfur sources, showing that the pyrite originally formed predominantly via bacterial sulfate reduction within permeable pumice layers acting as seawater recharge conduits. REE patterns of the fluids resemble those of the host pumices, reflecting low-temperature fluid–rock interaction prior to recovery, whereas weak Ce anomalies are attributed to minor seawater contamination during sampling. Overall, the pumice-hosted geochemical record represents a multi-stage palimpsest involving low-temperature subseafloor alteration, pyritization under active seawater recharge, and superimposed post-sampling oxidative overprinting.

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