Matrix Corrections and Error Analysis in High-Precision SIMS 18O/16O Measurements of Ca-Mg-Fe Garnet

We report technical and data treatment methods for making accurate, high-precision measurements of O-18/O-16 in Ca-Mg-Fe garnet utilising the Cameca IMS 1280 multi-collector ion microprobe. Matrix effects were similar to those shown by previous work, whereby Ca abundance is correlated with instrumental mass fractionation (IMF). After correction for this effect, there appeared to be no significant secondary effect associated with Mg/Fe2+ for routine operational conditions. In contrast, investigation of the IMF associated with Mn- or Cr-rich garnet showed that these substitutions are significant and require a more complex calibration scheme. The Ca-related calibration applied to low-Cr, low-Mn garnet was reproducible across different sample mounts and under a range of instrument settings and therefore should be applicable to similar instruments of this type. The repeatability of the measurements was often better than 0.2 parts per thousand (2s), a precision that is similar to the repeatability of bulk techniques. At this precision, the uncertainties due to spot-to-spot repeatability were at the same magnitude as those associated with matrix corrections (+/- 0.1-0.3 parts per thousand) and the uncertainties in reference materials (+/- 0.1-0.2 parts per thousand). Therefore, it is necessary to accurately estimate and propagate uncertainties associated with these parameters - in some cases, uncertainties in reference materials or matrix corrections dominate the uncertainty budget.