Simultaneous fractionation of sulfur dioxide explains mass independent fractionation of sulfur isotopes in Archean sedimentary pyrites

The relationship between Delta S-36 and Delta S-33 in Archean sedimentary pyrites has been used to evaluate early geologic processes, including photochemical reactions in the anoxic atmosphere, biological activity and thermochemical alteration during sediment deposition. We have applied statistical methods to quadruple S isotope analyses of Archean sedimentary pyrites, using data compiled from the literature. Most of the best-fit lines, on plots of Delta S-36 against Delta S-33, have Archean reference array-like Delta S-36/Delta S-33 slopes that vary between -1.5 and - 0.9. Rigorous statistical tests were conducted to calculate the probability of the best-fit lines passing through the origin. Seventeen of 23 Delta S-36-Delta S-33 regression lines, which pass our reliability filter of R-2 >= 75% and Delta S-33 range >= 2 parts per thousand, have positive intercepts on the Delta S-36 axis, and 13 of these have a probability of < 5% of a zero intercept on the Delta S-36 axis. The observed Delta S-36/Delta S-33 slopes and the non-negative intercepts, which requires at least two mass-independent fractionation source reactions to operate simultaneously, can be produced by UV radiation in the atmosphere at low SO2 partial pressures by combining collision-induced intersystem crossing in the SO2 photoexcitation band (240-340 nm), with the self-shielding effect in the SO2 photolysis band (190-220 nm). The two SO2 photochemical processes must occur simultaneously in a single atmospheric reservoir in order that the fraction contributed by the end-member process remains constant across the full range of Delta S-33 values. We call this process simultaneous fractionation. We applied a two-end-member model to calculate the fraction of S contributed by the SO2 photoexcitation end-member (f) needed to produce the observed Delta S-36/Delta S-33 gradients and variable intercepts on the Delta S-36 axis in the Archean sedimentary pyrites, when the other end-member is SO2 photolysis with the self-shielding. The simplest explanation for variations in f, and therefore variations in Delta S-36/ Delta S-36 gradients, is that it is controlled by changes in the partial pressure of SO2 in the atmosphere.