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

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 lambda = 248 nm and (b) a CO2 laser for whole-grain analysis of powdered samples by infrared heating at lambda = 10.6 mum. A CO2 laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The delta(34)S and delta(33)S compositions of reference materials measured with a CO2 laser fluorination system agree (+/-0.2parts per thousand, 1sigma) 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, laser is typically +/-0.2parts per thousand (1sigma) for delta(34)S. The in situ fluorination of sulfides with a KrF excimer laser (lambda = 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 CO2 laser. In situ laser craters and drill hole powders give the same delta(34)S(V-CDT) and delta(33)S(V-CDT) values within 0.2parts per thousand. The delta(34)S(V-CDT) and delta(33)S(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 delta(34)S(V-CDT) and delta(33)S(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 (lambda = 248 nm). Test analyses with an ArF excimer laser (lambda = 193 nm), however, gave fractionated sulfur isotope ratios. A range of Delta(33)S anomalies of from -1.5 to +3.0parts per thousand 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. Copyright (C) 2003 Elsevier Ltd.