Sulfur isotopic systematics of hydrothermal precipitates in the Okinawa Trough: Implication for the effect of sediments and magmatic volatiles

The Okinawa Trough is a sediment-covered intra-continental back-arc basin with active hydrothermal activities. The sulfur isotopic systematics of the hydrothermal systems in the Okinawa Trough remain unclear owing to the complex sulfur reservoirs. To address these uncertainties, we conducted in situ S isotopic combined with whole-rock geochemical, Pb isotopic, and barite Sr isotopic analyses of typical hydrothermal precipitates from three hydrothermal fields with varying sediments thickness (Iheya North Knoll (INK) < Yonaguni Knoll IV (YK) < Noho) in the Okinawa Trough. The delta S-34 values of barite from INK (+16.4 parts per thousand to +19.6 parts per thousand, median + 18.5 parts per thousand) and YK (+16.2 parts per thousand to +22.2 parts per thousand, median + 19.5 parts per thousand) mostly indicate lower values than those obtained for seawater (- +21 parts per thousand), combined with the observation of high S and O fugacity (fO(2)-fS(2)) mineral assemblages (e.g., low-Fe sphalerite +/- enargite +/- tennantite +/- chalcopyrite), and high Cu and Au contents of hydrothermal precipitates, consistent with the addition of magmatic volatile to these two fields. In contrast, the in situ delta S-34 values of sulfide minerals from INK (+5.6 parts per thousand to +10.4 parts per thousand, median + 9.0 parts per thousand) and YK (+3.5 parts per thousand to +5.5 parts per thousand, median + 4.7 parts per thousand) fields were all positive, inconsistent with the S isotopic characteristics of H2S formed from SO2 disproportionation. We conclude that a large proportion of thermochemical SO42- reduction-derived heavy sulfur likely obscure the characteristics of H2S sourced from disproportionation. This was likely resulting from (1) sediment controlled long-scale lateral hydrothermal circulation, (2) the existence of early precipitated anhydrite, and (3) high concentrations of H-2 in the high-temperature reaction zone. The Noho field have low delta S-34 values (-0.2 parts per thousand to +5.3 parts per thousand, median + 0.9 parts per thousand) of sulfide minerals, combined with high sediment contributions reflected by high Pb isotopic ratios and low-fS(2)-fO(2) mineral assemblages (e.g., high-Fe sphalerite +/- pyrrhotite +/- isocubanite), which indicates that the low delta S-34 value sulfur was derived from leaching of biogenic pyrite in thick sedimentary strata by hydrothermal fluid. Near surface, the high-temperature discharging hydrothermal fluids interacted with different thicknesses of sedimentary strata and leached different proportions of biogenetic pyrite, resulting in a negative correlation of sedimentary strata thickness and sulfide delta S-34 values of the Okinawa Trough hydrothermal fields.