Sample preparation and sulfur isotope analysis

Pyrite concretions were first split by a rock saw and then slabs were polished before sampling. Pyrite samples were collected by a hand-held micromill from the polished slabs. To capture the isotopic variations within a single pyrite concretion, multiple samples were sequentially collected from rim to core. Sulfur isotopic compositions of pyrite were measured at the Oxy-Anion Stable Isotope Consortium in Louisiana State University, at the EPS Stable Isotope Laboratory in McGill University, and at the Geochemistry laboratory in University of Maryland.

For δ³⁴S analysis, about 0.05 mg pyrite-rich powder was mixed with 1–2 mg V₂O₅, and was analyzed for S isotopic compositions on an Isoprime 100 gas source mass spectrometer coupled with a Vario Microcube Elemental Analyzer. Sulfur isotopic compositions are expressed in standard δ-notation as permil (‰) deviations from the Vienna-Canyon Diablo Troilite standard. The analytical error is <0.2‰ based on replicate analyses of samples and laboratory standards. Samples were calibrated on two internal standards: LSU-Ag₂S-1: −4.3‰; LSU-Ag₂S-2: +20.2‰.

Multiple sulfur isotope analyses were conducted by converting pyrite samples to H₂S_(g) via the chromium reduction method. H₂S gas was then carried through a N₂ gas stream to a Zn acetate solution where it precipitated as ZnS. ZnS was then converted to Ag₂S through addition of AgNO₃. Samples were then filtered and dried at 80 °C. Ag₂S samples were converted to SF₆(g) through reaction with F₂(g) in heated Ni bombs. Generated gas was then purified through a series of cryo-focusing steps followed by gas chromatography. The purified samples were then analyzed on a Thermo MAT-253 on dual-inlet mode. The total error on the entire analytical procedure is less than (1σ) 0.1‰ for δ³⁴S and 0.01‰ for ∆³³S.