Experimental evidence for the hydrothermal formation of native sulfur by synproportionation

Elemental sulfur (S-0) is known to form in submarine acid-sulfate vents by disproportionation of magmatic SO2. S-0 formed upon disproportionation shows delta S-34(S) values considerably lower than the influxing magmatic SO2, which results in delta S-34(S) values typically <0 parts per thousand. The peculiar occurrence of isotopically heavy sulfur in the Kemp Caldera hydrothermal system (delta S-34(S) > 5 parts per thousand) and Niua North (delta S-34(S) = 3.1 parts per thousand) led to the suggestion that disproportionation is not the only sulfur forming process in submarine hydrothermal systems. We conducted hydrothermal experiments to investigate if synproportionation of SO2 and H2S can explain the occurrence and isotopic composition of S-0 observed in some vent fields. Provided that SO2 and H2S are both abundant, this formation mechanism is thermodynamically conceivable, but it has not yet been demonstrated experimentally that this process actually takes place in submarine hydrothermal systems. We conducted the experiments in collapsible Ti-cells under pT-conditions (20-30 MPa, 220 degrees C) that are relevant to S-0 formation in submarine hydrothermal systems. We used starting concentrations of 10 mM sulfite and 20 mM sulfide of known isotopic composition. Under acidic conditions (pH(25 degrees C) = 1.2), S-0 was the most abundant reaction product, but small amounts of sulfate were also produced. A Rayleigh fractionation model was applied to determine the isotopic composition of SO42-, SO2, H2S and S-0 expected to form by SO2 disproportionation, H2S oxidation, and SO2-H2S synproportionation. The sulfur isotopic signatures of the sulfur produced in the experiments can only be explained by synproportionation of sulfite and sulfide. These results provide strong evidence that synproportionation is likely responsible for exceptionally high delta S-34(S) values observed in S-0 from some arc/back-arc hydrothermal environments, like the Kemp Caldera in the South Sandwich arc. Coeval degassing of H2S and SO2 is likely required to have this particular reaction dominate in the H-S-O reaction network and produce noticeable accumulations of isotopically heavy native sulfur at the seafloor.