Sulfur Isotope Fractionation During Sulfide Generation in the Hydrothermal Submarine Systems: The Case of Logatchev, Krasnov, and Rainbow Hydrothermal Fields, Mid-Atlantic Ridge

The sulfur isotope composition of single sulfide grains has been studied taking into account their paragenetic sequence in the samples collected from three hydrothermal fields (Logatchev, Krasnov, and Rainbow), located in the northern part of the Mid-Atlantic Ridge. The δ34S values of sulfides are +3.9 to +6, +7.1 to +9.8, and +2.1 to +8.4 ‰ in the Logatchev, Krasnov, and Rainbow fields, respectively. Late-generation sulfides within the studied domains (1–10 mm) of the samples are enriched in 34S isotope relative to early generations, and the sulfur isotope composition shows no relationship with the composition of sulfide minerals. In single grains of hydrothermal barite, the δ34S values exceed the generally accepted range for modern seawater sulfates. A model of the sulfur isotope fractionation in the hydrothermal system is proposed, based on the assumption of thermogenic sulfate reduction during fluid–rock interaction in the system, which is closed with respect to fluid. The model also takes into account simultaneous sulfur leaching from host rocks, the proportion of released sulfur from which depends on the progress of thermogenic sulfate reduction. Application of this model explains some well-known contradictions found in studying the sulfur isotope composition of sulfides from other hydrothermal fields in the World Ocean.