In situ Sulfur Isotope Analysis of Sphalerite by SIMS: Precision vs. Accuracy

Secondary ion mass spectrometry (SIMS) measurement of sulfur isotope ratios is a potentially powerful technique for in situ studies in many areas of Earth and planetary science. Tests were performed to evaluate accuracy and precision in six natural sulfide minerals using the WiscSIMS ims-1280 large radius multicollector SIMS. The grain-to-grain precision for δ34S is ~0.3‰ for chalcopyrite, galena, pentlandite, and pyrrhotite, and ~0.2‰ (2SD) for pyrite using a 1.6 nA Cs+ primary beam that was focused to 10 µm diameter with a Gaussian beam density distribution. Similar precision is attained for δ34S measurements in Gaussian beam mode within single crystals of sphalerite. However, between individual sphalerite grains, measured δ34S varies by up to 3.4‰ and the grain-to-grain precision is poor (1.7‰, 2SD, n = 20). The grain-to-grain variations in measured δ34S of sphalerite correlate well (R2 = 0.87) with secondary ion yield, which shows a large range (18%). Further, these variations correspond with pit microstructures, ranging from a smooth surface for highest (least fractionated) raw δ34S values, to pronounced ripples and terraces in analysis pits from grains featuring lowest (highest fractionated) raw δ34S values. This relation between pit appearance and measured δ34S was observed in all sphalerite samples, including NBS-123. Crystal orientation determined by Electron Back Scatter Diffraction (EBSD) shows that individual sphalerite grains are single crystals, whereas orientation varies from grain-to-grain. The 3.4‰ variation in measured δ34S between individual grains of sphalerite is attributed to crystal orientation. Highest raw δ34S values in sphalerite correspond to an incident primary Cs+ beam parallel to the set of directions <uuw>, from [111] to [110], which are preferred directions for channeling and focusing in diamond-centered cubic crystals. The negative impact of crystal orientation on analytical precision in sphalerite can be minimized by the combination of a Köhler illuminated (20 µm) primary beam at a low beam intensity of 0.3 nA. This analytical approach reduces the depth of the analysis pits in sphalerite from 4.0 µm to 0.1 µm. The resulting grain-to-grain precision in δ34S improves from 1.7‰ in Gaussian beam mode to 0.75‰ (2SD) using a Köhler illuminated beam. With careful use of appropriate standards and analytical conditions, the accuracy of in situ SIMS analysis for δ34S approaches a precision of 0.3‰ (2SD) for five sulfide minerals and 0.7‰ for sphalerite