Precision analysis of multisulfur isotopes in sulfides by femtosecond laser ablation GC-IRMS at high spatial resolution

Abstract A new laser ablation method has been developed for measurements of multiple sulfur isotopes in sulfides using a femtosecond laser ablation system in combination with a reactor for sulfide aerosol conversion into SF 6 gas, a cryogenic and chromatographic purification system, and an isotope ratio mass spectrometer (FsLA-GC-IRMS). In this method, femtosecond laser-generated aerosol sulfide particles were transferred by the helium carrier gas from the sample chamber into the reactor to convert them to SF 6 via reaction with BrF 5 . Cryogenically and chromatographically purified SF 6 was transferred by helium carrier gas to the interface that was developed for introducing SF 6 into the ion source of a mass spectrometer running in high-vacuum operation mode. For the routine analysis of sulfide, a precision of 0.1–0.2‰ (1σ) for δ 34 S and higher than 0.03‰ (1 σ) for ∆ 33 S values has been obtained with an ablation crater size of 80 μm diameter and 40 μm depth. The method was tested using reference materials IAEA-S-1, IAEA-S-2, IAEA-S-3 and NBS-123, as well as some natural pyrite, sphalerite and galena samples. The δ 34 S values obtained by our method are in good agreement (within analytical precision) with the results obtained by the conventional dual inlet method. The FsLA-CG-IRMS method was applied to study sulfur isotope ratios in Archean rocks of the Cheremshanskaya formation (Sharyzhalgai uplift in the southeastern Siberian craton). We found that ∆ 33 S values in sulfides from these rocks ranged +2.30 to +2.68‰. The reliability of the results obtained here was confirmed by comparative analyses of isotope ratios in sulfides from rocks younger than 2.4 Ga in which the anomaly of the 33 S isotope could be unlikely to occur. The calculated δ 34 S’ = ln(δ 34 S/1000 + 1)*1000 and δ 33 S’ = ln(δ 33 S/1000 + 1)*1000 values of this set of samples yielded a straight line passing through the origin with a slope of 0.5152 (±0.002) on a conventional sulfur three-isotope plot that is in good agreement with a slope of 0.515, which was theoretically calculated for the mass-dependent fractionation of sulfur isotopes.