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

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 Pb-206/U-238 = ca. 3% and Pb-207/Pb-206 = <1% (2 sigma) 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 Pb-207 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 Hg-204 and a procedure for the consistent correction of common-Pb using Pb-204 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.