Pyrite δ³⁴S and Δ³³S constraints on sulfur cycling at sublacustrine hydrothermal vents in Yellowstone Lake, Wyoming, USA

Sulfur isotope values (δ³⁴S and Δ³³S) of pyrite in sediment from steam-heated hydrothermal vents on the floor of Yellowstone Lake (WY) were measured using secondary ionization mass spectrometry (SIMS). The high resolution of the SIMS data place important constraints on sulfur cycling processes at/near the vent fluid-lake water interface. Pyrite with a distinct mantle-basalt (δ³⁴S = 0‰) isotope composition (δ³⁴S = +0.5 to +3.1‰) replaces pyrrhotite during incipient stages of alteration at moderately high temperature. Disseminated cubic pyrite (δ³⁴S = +2.0 to +5.3‰) occurs in zones where more extensive oxidation is likely. Framboidal pyrite with δ³⁴S values ranging from –5.2 to +4.1‰ and Δ³³S up to +0.30‰ suggest formation from low-temperature microbial sulfate reduction in sediments near but not directly in the vent fluid up-flow zone. The co-occurrence of pyrite with S isotope values characteristic of distinct formation processes, coupled with notable intra-crystal S isotope variations, suggests the venting locus is dynamic in time and space.