Off‐Mount Calibration and One New Potential Pyrrhotite Reference Material for Sulfur Isotope Measurement by Secondary Ion Mass Spectrometry

Sulfur isotope measurements in three sulfide (two pyrite and one pyrrhotite) samples on two epoxy mounts showed that the mount‐to‐mount variation of raw δ34S values was negligible when secondary ion mass spectrometry (SIMS) analytical settings remained stable. In consequence, an off‐mount calibration procedure for SIMS sulfur isotope analysis was applied in this study. YP136 is a pyrrhotite sample collected from northern Finland. Examination of thin sections with a polarising microscope, backscattered electron image analyses and wavelength dispersive spectrometry mapping showed that the sample grains display no internal growth or other zoning. A total of 318 sulfur isotope (spot) measurements conducted on more than 100 randomly selected grains yielded highly consistent sulfur isotope ratios. The repeatability of all the analytical results of 34S/32S was 0.3‰ (2s, n = 318), which is the same as that of the well‐characterised pyrite reference materials PPP‐1 and UWPy‐1. Its δ34S value determined by gas mass spectrometry was 1.5 ± 0.1‰ (2s, n = 11), which agrees with the SIMS data (1.5 ± 0.3‰, 2s) calibrated by pyrrhotite reference material Po‐10. Therefore, YP136 pyrrhotite is considered a candidate reference material for in situ sulfur isotope determination. Key Points Off‐mount calibration is applicable to SIMS pyrite and pyrrhotite sulfur isotope analyses. YP136 is a new potential pyrrhotite reference material suitable for calibrating sulfur isotope determination by SIMS. SIMS δ34S value = 1.5 ± 0.3‰ (2s)