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

Sulfur isotope values (delta S-34 and Delta S-33) of pyrite in sediment from steam-heated hydrothermal vents on the floor of Yellow-stone 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 (delta S-34 = 0 parts per thousand) isotope composition (delta S-34 = +0.5 to +3.1 parts per thousand) replaces pyrrhotite during incipient stages of alteration at moderately high temperature. Disseminated cubic pyrite (delta S-34 = +2.0 to +5.3 parts per thousand) occurs in zones where more extensive oxidation is likely. Framboidal pyrite with delta S-34 values ranging from -5.2 to +4.1 parts per thousand and Delta S-33 up to +0.30 parts per thousand 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 intracrystal S isotope variations, suggests the venting locus is dynamic in time and space. (C) 2019 Elsevier Ltd. All rights reserved.