Common-Pb corrected in situ U–Pb accessory mineral geochronology by LA-MC-ICP-MS

Abstract

LA-MC-ICP-MS is shown to be a rapid, precise and accurate method for determination of U–Pb ages of accessory minerals. For the protocol described, total analysis time is <3 min with a main acquisition sequence of only 30 s. Using a raster ablation protocol, within-run inter-element fractionation can be effectively eliminated and an external ablation standard used to quantify an overall error for the analysis. Reproducibilities of 206Pb/238U = ca. 3% and 207Pb/206Pb = <1% (2 σ) are achieved, with the resulting age accurate to within 1% as determined using in-house samples previously characterised by TIMS. A key control on the Pb/Pb reproducibility is shown to be the size of the 207Pb peak and an error propagation curve is determined for the accurate representation of this data. Propagation of these errors allows each individual sample analysis to be considered a stand-alone result, removing the need for statistical averaging of multiple data points. Simultaneous collection of flat-topped peaks enables precise measurement and correction of isobaric interference from 204Hg and a procedure for the consistent correction of common-Pb using 204Pb is described. Determination and correction of the common-Pb component is shown to be critical to the reliable interpretation of the data for certain minerals including those phases where a correction is often deemed unnecessary. Combined with time-resolved analysis of the data, this allows the Pb-loss history and nature of discordance within individual crystal domains to be ascertained. Successful analyses of zircons using a non-matrix matched (monazite) standard are also demonstrated suggesting that particle size distribution, ionisation efficiency and plasma loading, are more important issues in controlling inter-element fractionation in the plasma than absolute matrix matching.