Sulfur formation associated with coexisting sulfide minerals in the Kemp Caldera hydrothermal system, Scotia Sea

Abstract

The Kemp Caldera is a submarine caldera located in the southernmost part of the intra-oceanic South Sandwich arc, Scotia Sea. The caldera comprises a unique hydrothermal system in which elemental sulfur (S0) coexists with sulfide minerals at unusually high pH-values (pH25 °C > 5). During the 2019 R/V Polarstern PS119 expedition, samples from the white smoker vent fields in the center of the caldera were recovered by a remotely operated vehicle. The sampling of elemental sulfur took place at the vent sites “Great Wall” and “Toxic Castle”, which are located < 80 m apart. At Great Wall, sulfur is fine-crystalline, while at Toxic Castle S0 occurs in liquid form, forming amorphous and globular structures. Here, tabular plates of covellite are found as inclusions in the quenched sulfur. The fluids from both sites have pH-values of about 5.4 to 5.7 (at 25 °C) and show a relatively wide temperature range from 63 to > 200 °C. The isotope values of sulfide and elemental sulfur range from 4.6 to 5.8 ‰, while sulfate concentrations at Great Wall are lower than in seawater.Elemental sulfur in submarine arc/back-arc systems is commonly believed to form by disproportionation of SO2. It generally shows negative δ34S values and precipitates from acid-sulfate type hydrothermal fluids with pH-values as low as < 1. This formation mechanism, however, cannot explain the high pH and low sulfate concentration of the Great Wall fluid alongside high δ34S values of elemental sulfur and massive sulfides found at Kemp. Instead, we suggest that S0 formation at Kemp Caldera can be attributed to synproportionation of SO2 and H2S. This formation mechanism is thermodynamically feasible but has not been demonstrated to actually take place in hydrothermal systems. The association of elemental sulfur with covellite is also uncommon, but not unexpected in magmatic-hydrothermal systems. The uncommonly high pH-value is likely responsible for the precipitation of the metal sulfides, which are unusual for known acid-sulfate systems studied to date. Our study shows that the geochemical behavior of sulfur in arc/back-arc hydrothermal systems is more diverse than previously recognized.