U-Pb dating of zircon by laser ablation ICP-MS: recent improvements and new insights

A robust method for U-Pb dating of zircon by laser ablation ICP-MS was established using a Thermo-Finnigan Element2 sector field ICP-MS coupled to a New Wave UP193HE ArF Excimer laser system. This included the construction of a low-volume sample cell and the establishment of a data reduction routine. A detailed systematic approach for determining optimal ablation settings and ICP-MS parameters is presented, which is also applicable to other instrument set-ups. Laser induced time-dependent elemental fractionation is corrected using the intercept method, whereas instrument drift, mass bias and elemental fractionation caused by ionisation differences are corrected by applying external standardisation using the matrix-matched reference zircon GJ-1. The isobaric interference of Hg-204 on Pb-204 is reduced by using a gold trap enabling the application of an accurate common Pb correction. Typical laser craters are 12, 25 or 35 mu m in diameter and ca. 20 mm deep. A single analysis takes 65 s and the automated analysis of a measurement sequence ensures optimal efficiency. The performance of the established procedure was assessed by analysing zircon ranging in age from Mesoproterozoic to Miocene. Concordia ages obtained from multiple analyses on the various well-characterised zircons are all within uncertainty of the accepted values, demonstrating the high accuracy of the method. The uncertainty on the individual ages ranges from 1 to 4 % (2 sigma), depending on the absolute age, Pb content and homogeneity of the analysed zircon. These results indicate that the precision and accuracy obtained using the technique presented in this study are similar to those of other LA-ICP-MS laboratories and SIMS analyses.