An ultraviolet laser microprobe for the in situ analysis of multisulfur isotopes and its use in measuring Archean sulfur isotope mass-independent anomalies

Abstract

A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO 2 laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO 2 laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ 34S and δ 33S compositions of reference materials measured with a CO 2 laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency (Ding et al., 2001; Taylor, in press). The precision of replicate analyses of powdered sulfide minerals with the CO 2 laser is typically ±0.2‰ (1σ) for δ 34S. The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO 2 laser. In situ laser craters and drill hole powders give the same δ 34S V-CDT and δ 33S V-CDT values within 0.2‰. The δ 34S V-CDT and δ 33S V-CDT values of both powders and in situ analyses are independent of F 2 gas pressure over a range of 15 to 65 torr. No dependence of δ 34S V-CDT and δ 33S V-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios. A range of Δ 33S anomalies of from – 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.