Abnormally High δ34S Values of Edifices at the Mid-Atlantic Ridge: The Closed System Effect at the Sulfide Generation Zone

Abstract The S isotope composition of single sulfide grains was studied taking into account the sequence of their formation in samples from the sulfide structures of hydrothermal fields confined to the Mid-Atlantic Ridge. The ranges of δ34S values obtained for the structures of the Logachev, Krasnov, and Rainbow fields are the following (‰): from +3.9 to +6, from +7.1 to +9.8, and from +2.1 to +8.4, respectively. Sulfides of later generations within each of these fields are regularly enriched in the 34S isotope relative to sulfides of earlier generations. Here, the S isotope composition does not depend on the composition and structure of the mineral. The data obtained show that in the sulfide generation zone, where seawater sulfate reduction takes place, the system is closed relatively to fluid. This leads to an increase in δ34S values of reduced sulfur compounds in the hydrothermal fluid and to sulfides being crystallized.