Microscale sulfur isotope imaging analysis with NanoSIMS: A new methodology for arbitrary area selection and its application to Archean sedimentary pyrite

We present a method for in situ sulfur (S) isotope analysis in arbitrarily small areas (<1 μm²) within pyrite using ion imaging mode of nanoscale secondary ion mass spectrometry (NanoSIMS). The precision and accuracy of δ³⁴S values obtained using this method were evaluated with reference pyrite with homogeneous S isotope ratios. The internal precision of the δ³⁴S values in any region of interest (ROI) as small as 0.94 μm² was approximately 1.0 ‰ at the 1σ weighted standard deviation (termed wSD) level for a single ROI. The in situ ion imaging method developed here enables the selection of any ROI smaller than 1 μm² within a raster area and is accurate and precise enough to detect δ³⁴S variations in small pyrite (<10 μm) commonly found in ancient sedimentary rocks. This technique was applied to measure δ³⁴S values of sedimentary pyrite from early Archean cherts, yielding results consistent with those obtained through conventional spot analysis. The δ³⁴S values of small pyrite exhibited a fractionation of over 20 ‰ from those of Archean seawater sulfate. One plausible explanation for this fractionation is that these pyrites formed from sulfur that underwent isotopic fractionation via biological metabolism, such as microbial sulfate reduction or/and microbial sulfur disproportionation. Although further data are needed to strengthen interpretations of origins of the analyzed pyrite, our results demonstrate that the imaging mode of NanoSIMS is a powerful tool for high spatial resolution isotope analysis. This approach has the potential to provide insights into sulfur metabolizing activity on the early Earth.