Improved precision and spatial resolution of sulfur isotope analysis using NanoSIMS

Abstract

High precision analyses of all four sulfur isotopes in four pyrite and three sphalerite standards and in working reference samples were carried out using a CAMECA NanoSIMS 50L instrument. The measurements were made using three different settings of the Faraday cup (FC) and/or electron multiplier (EM) detectors, which meet different requirements for spatial resolution. The effects of EM aging and quasi-simultaneous arrival were corrected before the calibration of instrumental mass fractionation by a standard–sample–standard bracket method using the standards measured together with the samples. High analytical precision was achieved by counting ³²S, ³³S and ³⁴S with the FCs and ³⁶S with the EM (i.e. the FC–FC–FC–EM mode) using a 0.7 μm diameter ∼350 pA Cs⁺ primary beam and scanning over areas of 5 × 5 μm². The standard deviations of spot-to-spot and grain-to-grain (external reproducibility 1 SD) measurements were less than 0.3, 0.3 and 0.7‰ for δ³³S, δ³⁴S and δ³⁶S, respectively. To achieve a higher lateral resolution of ≤2 × 2 μm², the Cs⁺ beam was reduced to 7–10 pA with a diameter of ∼200 nm; ³²S was measured with the FC and the other signals were measured with the EMs. The external reproducibility (1 SD) was better than 0.5‰ for both δ³³S and δ³⁴S and was 3‰ for δ³⁶S. To achieve the highest lateral resolution for the analysis of submicron-sized sulfides, a ∼0.7 pA Cs⁺ beam of ∼100 nm diameter was used, scanning over areas of 0.5 × 0.5 μm², and all ³²S, ³³S and ³⁴S were counted with the EMs. The external reproducibility (1 SD) was better than 1.5‰ for both δ³³S and δ³⁴S. These three modes have important applications in the isotope analysis of micron-sized sulfur samples, such as pyrite framboids and areas of complex zoning in sulfide minerals.