Abstract

A SIMS procedure was developed to obtain rapid and straightforward quantitative data for Ca in olivine, involving the use of ⁴⁰Ca²⁺ and ²⁸Si²⁺ high-energy ions. The adoption of ion signals at 20 and 14 u did not introduce any bias, since interferences were shown to be negligible at these mass numbers. Internal analytical repeatability of ⁴⁰Ca²⁺/²⁸Si²⁺ over 10 cycles was typically in the range ∼1.5–6% (as 1σ_k%, k = 10) for ∼500–20 ppm wt Ca, respectively. Reproducibility of ⁴⁰Ca²⁺/²⁸Si²⁺ ratios in the standards over three analytical sessions was better than 6% relative (as 1σ_m%), where σ_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 and heavy REE, generally studied as high-energy ions. This procedure was tested in olivine with FeO contents ranging from ∼7 to 10 wt%, with an accuracy of better than 10% rel. The low matrix effects (∼8% rel.) on Ca²⁺/Si²⁺ ionisation in an anorthoclase standard suggests that the procedure would be suitable for olivine containing less than 7 wt% FeO. Due to the extremely high variability of the Fe content in olivine and the ensuing non-negligible matrix effects, a calibration with suitable Fe-matched standards represents the best approach for determining the composition of each unknown. Important applications of the method are in the geothermobarometry of silicate systems involving olivine co-existing with a clinopyroxene phase.