Geochemistry of a hydrothermal sediment core from the OBS vent-field, 21°N East Pacific Rise

We report a geochemical investigation of near-vent hydrothermal sediment from the East Pacific Rise, collected in a short (11 cm) core by submersible from the OBS vent-site at 21°N. The sediment has high concentrations of Fe (23–25%), Cu (10–22%), Zn (4–13%) and S (22–30%), but low Mn concentrations (108–279 ppm) indicating a much fresher sulfidic input than in previously reported near-vent sediments from the Mid-Atlantic Ridge. Further studies are required to fully characterise near-field hydrothermal sediments, but the data in this study allow the first comparisons with similar near-vent material collected previously from the Mid-Atlantic Ridge as well as ridge-flank data from the EPR and Juan de Fuca Ridge. Compared to ridge-flank sediments the near field sediments exhibit high ratios of Fe/Mn, Cu/Fe and Zn/Fe. Unlike Cu and Zn, Pb is not so significantly enriched. Pb isotope compositions are indistinguishable from local MORB indicating a dominant hydrothermal source for this element in these sediments. This may not necessarily be the case for ridge-flank sediments, however, where scavenging from seawater may also be important. REE distributions throughout the core exhibit low concentrations and characteristics typical of vent-fluids, with LREE enrichments and pronounced positive Eu-anomalies. These patterns are similar to those of relatively fresh sulfides, but there is also evidence for progressive oxidation downward into the core by ingressing seawater.