Primary multiple sulfur isotopic compositions of pyrite in 2.7Â Ga shales from the Joy Lake sequence (Superior Province) show felsic volcanic array-like signature

Multiple sulfur isotopes provide a powerful tool to study photochemical and biological processes controlling the Archean sulfur cycle and infer related atmospheric and marine environments. However, our understanding of early Earth's environment remains limited by the availability of well-preserved geological samples, as most Archean sedimentary rocks have experienced some degree of metamorphic alteration. To evaluate sulfur isotopic behavior during post-depositional processes and elucidate the sulfur cycle at 2.7 Ga, we use high-resolution in situ analytical techniques (EPMA and SIMS) to determine elemental compositions and multiple sulfur isotopic compositions of large diagenetic pyrite nodules and fine-grained secondary pyrite disseminated in quartz veins (formed during a lower greenschist metamorphic event) in shales from the 2.7 Ga Joy Lake sequence in the southwest Superior Province. Results show that trace metals and sulfur in the secondary pyrite were derived from both metamorphic fluid and pre-existing diagenetic pyrite. Diagenetic pyrite nodules could have been partially dissolved by metamorphic fluid. But the surviving nodules show elemental and isotopic features different from those of the deduced metamorphic fluid endmember, suggesting the nodules were not geochemically altered by metamorphism, and thus preserve primary isotopic signatures acquired during diagenesis. The sulfur isotopic ratios of pyrite nodules show strong variations, with decreasing δ34S values and increasing Δ33S values from cores to rims. This negative Δ33S-δ34S relationship is different from the commonly observed 'Archean reference line' defined by most Archean pyrite data, but similar to the 'felsic volcanic array'. Our observation provides a first possible case from 2.7 Ga, the age of peak crustal growth in the Archean, to support the hypothesis that photochemical pathways could be different under conditions of intense volcanic emission. This study also shows that high-resolution SIMS and EPMA analyses are a robust method for distinguishing primary geochemical signatures from post-depositional alteration, and thus may be used to evaluate the sulfur cycle in Archean sedimentary rocks with low-grade metamorphic overprints.