High spatial resolution and precision NanoSIMS for sulfur isotope analysis

Abstract

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 δ³⁴S 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 × 1 μm²) with high precision. A Faraday cup (FC, for ³²S)–electron multiplier (EM, for ³⁴S) 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 (∼70%) and mass resolution power (MRP ∼ 5600 for ³²S). A primary beam of 3.5 pA was rastered on a scan area of 1 × 1 μm² to acquire the δ³⁴S isotope ratio. The δ³⁴S ratios of the four analyzed standard samples are consistent with previously reported values, with reproducibility (1 SD) better than 0.5‰. Because EM aging affects the accuracy of the sulfur analysis results, periodically adjusting the maximum value of the peak-height distribution (PHD_max) of the EM during the analysis is necessary. Careful optimization of the height (Z position) at different locations (samples) ensures precise and accurate δ³⁴S 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 δ³⁴S of pyrite samples with a limited analyzable region (>1 × 1 μm²) with high precision.