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.