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 34 S is high (∼4.5%) relative to the major isotope 32 S. 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 δ 34 S 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)

Abstract Diagenetic calcite, dolomite, pyrite, marcasite, and anhydrite of the Upper Devonian Nisku Formation in central Alberta, Canada, were analyzed for trace elements and sulfur isotopes by ion microprobe with sample spots <20 nm in size. Calcite samples exhibit significant trace element variations (of up to about 3 orders of magnitude), and disseminated sulfides display large sulfur isotope variations (8 34 S of up to about 60%o CDT) on a scale of < 100 nm. These variations cannot be resolved using conventional analysis of powdered samples. The data suggest that most of the analyzed calcites formed from marine pore fluids that evolved toward lower redox-potential. The sulfur isotope variations indicate intensive bacterial sulfate reduction and closed-system Rayleigh fractionation as the dominant diagenetic processes leading to iron sulfide formation. Together, carbonate and sulfide data indicate that the system was at least partially closed during shallow to intermediate burial diagenesis (up to about 1100 m) in the Late Devonian to Early Carboniferous. During the subsequent 300 my, the Nisku reefs in the study area were buried to more than 4000 m, with maximum burial in the Late Cretaceous to Early Tertiary. The existence and preservation of the observed geochemical variations further imply that water-rock interaction during this prolonged and deep burial was quite limited (i.e., the analyzed calcites and sulfides are remarkably resistant to compositional modification via recrystallization).