High spatial resolution and precision NanoSIMS for sulfur isotope analysis

Nano-scale secondary ion mass spectroscopy (NanoSIMS) is a powerful tool for determining the sulfur isotope composition of micrometer-sized minerals. However, the high spatial resolution of delta S-34 analysis comes at the expense of accuracy, which limits the applicability of NanoSIMS to sulfur isotope analysis. The method proposed here couples high lateral resolution (>1 x 1 mu m(2)) with high precision. A Faraday cup (FC, for S-32)-electron multiplier (EM, for S-34) detector combination is selected to improve static counting and mitigate the quasi-simultaneous arrival (QSA) effect. The instrument is set and tuned to achieve high transmission (similar to 70%) and mass resolution power (MRP similar to 5600 for S-32). A primary beam of 3.5 pA was rastered on a scan area of 1 x 1 mu m(2) to acquire the delta S-34 isotope ratio. The delta S-34 ratios of the four analyzed standard samples are consistent with previously reported values, with reproducibility (1 SD) better than 0.5 parts per thousand. Because EM aging affects the accuracy of the sulfur analysis results, periodically adjusting the maximum value of the peak-height distribution (PHDmax) of the EM during the analysis is necessary. Careful optimization of the height (Z position) at different locations (samples) ensures precise and accurate delta S-34 values. The S isotopic characteristics of framboidal pyrite from the Shuiyingdong Carlin-type Au deposit are determined using this method. The results indicate that the pyrite crystallite comprising framboidal pyrite is rimmed by ore pyrite, supporting the magmatic hydrothermal origin of the deposit. The developed method can be used for analyzing the delta S-34 of pyrite samples with a limited analyzable region (>1 x 1 mu m(2)) with high precision.

Automated summary

Nano-scale secondary ion mass spectroscopy (NanoSIMS) is a powerful tool for determining the sulfur isotope composition of micrometer-sized minerals. The proposed method in this study combines high lateral resolution with high precision, providing accurate results for delta S-34 analysis. The method has been successfully used to analyze framboidal pyrite samples, showing its applicability in determining the S isotopic characteristics of mineral deposits.