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.