Development of Arsenopyrite Reference Materials (Sacra and Niko) for In situ Multiple Sulfur Isotope Measurement

Advances in high‐resolution in situ techniques, such as SIMS and LA‐ICP‐MS, now enable precise multiple sulfur isotope measurements at spatial resolutions of tens of micrometres, providing new insights into redox evolution, metal behaviour and fluid pathways in ore‐forming systems. The accuracy of the measurement results depends critically on the availability of matrix‐matched reference materials, which have not yet been developed for arsenopyrite. To address this gap, we introduce two new arsenopyrite reference materials, Niko and Sacra. Bulk isotope ratio mass spectrometry analyses yield mean compositions of δ 33 S = +2.28 ± 0.01‰, δ 34 S = +4.44 ± 0.04‰, Δ 33 S = ‐0.002 ± 0.009‰, Δ 36 S = ‐0.142 ± 0.342‰ for Niko, and δ 33 S = +0.38 ± 0.07‰, δ 34 S = +0.76 ± 0.14‰, Δ 33 S = ‐0.007 ± 0.003‰, Δ 36 S = ‐0.275 ± 0.185‰ for Sacra. All uncertainties are reported as 2 s of replicate bulk analyses (Niko n = 3; Sacra n = 3). SIMS analyses showed lower measurement repeatability for Niko (δ 34 S = 0.39‰, and Δ 33 S = 0.151‰; 2 s , n = 35) as compared with Sacra (0.98‰ and Δ 33 S = 0.330‰, 2 s , n = 44). These results support the use of Niko as a primary matrix‐matched reference material for in situ multiple sulfur isotope measurement, with Sacra serving as a secondary reference and quality control material. The applicability of Niko and Sacra is currently validated for arsenopyrite compositions within or close to the compositional range represented by these two materials; the effects of more extreme variations in S/As ratio and Fe abundance on matrix‐dependent instrumental mass fractionation remain to be evaluated. Niko and Sacra therefore provide new matrix‐matched materials for improving the reproducibility of in situ multiple sulfur isotope analyses of near‐stoichiometric arsenopyrite, thereby strengthening SIMS‐based investigations of sulfur cycling and ore‐forming processes.