Sulfur Isotopes by Ion Microprobe

Abstract Over the last decade, secondary ionization mass spectrometry (SIMS, or the ion microprobe) has been applied to a wide variety of areas in the geosciences. In this technique, a collimated beam of primary ions is accelerated onto the sample, and the secondary ions that result from the sputtering of the sample are extracted for analysis. It offers the advantages of in situ analysis of both elemental concentrations and isotope ratios, with high spatial resolution (typically 10-50 pm) and excellent sensitivity (often ppb level). This technique has been used to study a variety of different light stable isotope systems, including H, B, C, N, and O. However, application of the ion microprobe to sulfur isotope studies has, to this point, been more common than for any of the other stable isotopes, particularly in terrestrial systems. In comparison with other commonly analyzed light stable isotopes (H,C,O), analysis of sulfur isotopes by ion microprobe has several advantages. The abundance of the important minor isotope 34S is high (∼4.5%) relative to the major isotope 32S. This allows favorable counting statistics for the minor isotope, resulting in good precision during isotopic analysis. Many sulfide phases are electrically conductive, which eliminates problems associated with surface charging. In many natural low-temperature systems, the variations in δ34S values are large, so that meaningful results can be obtained even if precision is limited to 1 to 2 per mil, as is the case in many sulfur isotope studies using ion microprobes. The spatial resolution (typically a crater 15-30 pm wide by 2-5 pm deep)