Evaluating intragrain gradients in the δ³⁴S of pyrite using a 7f-GEO SIMS

Recent work has highlighted the need to distinguish the contribution(s) of different populations of petrographically and isotopically distinct pyrites to interpret bulk δ³⁴S_pyr sedimentary values. To address this, we developed a method for micron-scale isotopic characterization of petrographically diverse sedimentary pyrites within a multimineralic matrix using ion imaging with a 7f-GEO secondary ion mass spectrometer (SIMS), aiming to rapidly and precisely analyze the isotopic composition of pyrites over relatively wide fields of view. By using primary beam currents in the range 1–11 pA and raster widths of 15–80 μm, precise data can reproducibly be acquired over a relatively large field of view, with negligible penetration into underlying mineral phases. Instrumental artifacts are a factor in cases where phosphate or halide minerals are intergrown with pyrite or where surface topographic variation is significant. However, a 1 pA beam method with smaller rasters can eliminate these artifacts if those situations cannot be avoided. The precision (1σ) on individual regions of interest (ROI) is <1‰ at all beam currents tested, provided the total accumulated number of measured ³⁴S counts exceeds one million. Isotopic gradients within grains can be resolved across areas as small as 20 μm² (5 μm diameter) and features as small as 1.5 μm² (∼1.4 μm diameter) can be resolved with a precision of 1.5‰ (1σ). The achievable sub-micron spatial resolution with a 1pA primary current allowed for data to be extracted from specifically desired regions within pyrite (e.g., distinguishing primary/early diagenetic pyrite from late-stage components), with minimal contamination from neighboring minerals.