SEARCHING FOR THE GREAT OXIDATION EVENT IN NORTH AMERICA: A REAPPRAISAL OF THE "MIF–MDF TRANSITION" IN THE HURONIAN SUPERGROUP BY SIMS SULFUR FOUR-ISOTOPE ANALYSIS

Sedimentological observations from the Paleoproterozoic Huronian Supergroup are suggested to mark the rise in atmospheric oxygen at that time, which is commonly known as the Great Oxidation Event (GOE) and typically coupled with a transition from mass-independent fractionation (MIF) to mass-dependent fractionation (MDF) of sulfur isotopes. An early in situ study of S three-isotopes across the Huronian Supergroup by Papineau et al. (2007) identified a weak MIF–MDF transition. However, the interpretation and stratigraphic placement of this transition are ambiguous. In this study, all four S isotopes were analyzed for the first time in two Huronian drill cores by secondary ion mass spectrometer (SIMS) and both Δ³³S and Δ³⁶S were calculated. Based on improved precision and detailed petrography, we reinterpret the dominance of pyrrhotite in the studied sections, which was previously proposed as "early authigenic" in origin, as resulting from regional metamorphism. Small but analytically resolvable non-zero values of Δ³³S (from –0.07‰ to +0.38‰) and Δ³⁶S (from –4.1‰ to +1.0‰) persist throughout the lower Huronian Supergroup. Neither pronounced MIF-S signals nor a MIF–MDF transition are seen in this study. Four scenarios are proposed for the genesis of small non-zero Δ³³S and Δ³⁶S values in Huronian: homogenization by regional metamorphism; recycling from older pyrite; dilution by magmatic fluids; and the occurrence of MDF. We argue that the precise location of the MIF–MDF transition in Huronian remains unsolved. This putative transition may have been erased by post-depositional processes in the lower Huronian Supergroup, or may be located in the upper Huronian Supergroup. Our study highlights the importance of integrated SEM and SIMS techniques in deep-time studies, and suggests that different analytical methods (bulk vs. SIMS) and diagenetic history (primary vs. metamorphic) among different basins may have caused inconsistent interpretations of S isotope profiles of the GOE successions at a global scale.