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 μm 2 ) within pyrite using ion imaging mode of nanoscale secondary ion mass spectrometry (NanoSIMS). The precision and accuracy of δ 34 S values obtained using this method were evaluated with reference pyrite with homogeneous S isotope ratios. The internal precision of the δ 34 S values in any region of interest (ROI) as small as 0.94 μm 2 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 μm 2 within a raster area and is accurate and precise enough to detect δ 34 S variations in small pyrite (<10 μm) commonly found in ancient sedimentary rocks. This technique was applied to measure δ 34 S values of sedimentary pyrite from early Archean cherts, yielding results consistent with those obtained through conventional spot analysis. The δ 34 S 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. • We evaluated the accuracy and precision of a new sulfur isotope analysis method using the ion imaging mode of NanoSIMS. • This method allowed sulfur isotope analysis on pyrite of arbitrarily selected areas smaller than 1μm 2 with approximately 1‰ in analytical precision. • This technique allowed to eliminate heterogeneity in ion counts caused by coexisting organic sulfur and fine microquartz quantitatively. • This method demonstrate that Archean small pyrite showed sulfur isotope fractionation suggestive of microbial sulfur metabolism.