High-precision SIMS oxygen, sulfur and iron stable isotope analyses of geological materials: accuracy, surface topography and crystal orientation

A high-precision SIMS analysis technique has been established for oxygen, sulfur, and iron isotope ratios and applied to a wide range of geoscience research areas using a Cameca IMS-1280 at the Wisconsin Secondary Ion Mass Spectrometer Laboratory (WiscSIMS). Precision and accuracy of 0.3 parts per thousand is achieved routinely for the measurement of O-18/O-16 ratio using multicollection Faraday Cup (FC) detectors and primary Cs+ beam size of 10 mu m. Smaller beam sizes of 3 mu m to <1 mu m yield precisions of 0.7-2 parts per thousand using a multicollection Electron Multiplier (EM) in pulse-counting mode for O-18. We evaluate small SIMS analytical biases at the level of a few parts per thousand or less using standard minerals with homogeneous oxygen isotope ratios: (i) topography of samples related to polishing relief of grains and location of analysis in a sample holder; and (ii) crystal orientation effects in magnetite (Fe3O4). The latter effect has not been detected for oxygen isotope ratio measurements in other minerals including a variety of silicate, oxide, and carbonate minerals at WiscSIMS. However, similar analytical biases that are correlated with crystal orientation have been identified from Fe isotope analyses in magnetite and S isotope analysis in sphalerite (ZnS), and many minerals have not yet been evaluated. The total range of analytical bias among randomly oriented magnetite grains becomes smaller by reducing the sputtering energy of the primary ions (from 20 to 10 keV), which may help reduce crystal orientation effects. Copyright (C) 2010 John Wiley & Sons, Ltd.