Accurate SIMS analysis of Ca in olivine based on high-energy doubly charged secondary ions

A SIMS procedure was developed to quantify Ca in the silicate matrix olivine, which involves 40Ca2+, 24Mg2+and/or 28Si2+(as matrix reference) high-energy ions. The adoption of reference elements Mg and/or Si at mass 12 and 14 (amu), respectively, does not introduce any bias since interferences were shown to be negligible at these mass numbers. Internal analytical repeatability of 40Ca2+/24Mg2+ or 40Ca2+ / 28Si2+ over 10 cycles is typically in the range 1.5-6% (as 1 Sigma k%, k = 10) for 500-20 ppm wt Ca, respectively. Reproducibility of 40Ca2+/24Mg2+ and 40Ca2+ / 28Si2+ ratios in the standards over three analytical sessions is better than 8% rel. (Mg reference) and 6% rel. (Si reference) (as 1 Sigma m%), respectively, where Sigma m represents the standard deviation of the mean. Moreover, Ca can be monitored at the same spot with other trace elements of interest in olivine, such as H, Li, B, HREE, generally studied as high-energy ions. An estimation of matrix effects (< 9% rel.) on Ca2+/Si2+ ionization in silicates not containing Fe has been derived by studying an anorthoclase standard whose chemical composition is significantly different from that of olivine (no Mg and Fe, 20.12 wt% Al2O3 and 66.44 wt% SiO2). With this procedure it is possible to get accurate, rapid and straightforward quantitative data for Ca in low-Ca silicates. Important applications are in geothermobarometry of silicate systems involving olivine coexisting with clinopyroxene phase.