U–Pb dating of detrital zircons for sediment provenance studies—a comparison of laser ablation ICPMS and SIMS techniques

Abstract

New developments in U–Pb dating of zircons by laser ablation (LA) ICPMS are described and, for the first time, a direct comparison of detrital zircons dated by LA ICPMS and SIMS methods is presented. True real-time mass bias correction is made by aspirating a Tl/U tracer at the same time as laser ablation. The method is similar to that described in Horn et al. (2000), except that enriched 233U rather than 235U is used in the tracer solution. Correction for laser-induced Pb/U elemental fractionation is based on a mathematical treatment of time-resolved data that is independent of laser ablation characteristics and does not require external standardisation. Internal corrections for mass bias and elemental fractionation eliminate the effects of variable sample matrix on isotopic ratios and improve the accuracy of U–Pb dating by laser ablation ICPMS. With the proper error propagation, the precision of U–Pb age determinations is only slightly worse than SIMS-based ion probe dating. However, LA ICPMS is capable of much more rapid analysis of the large number of zircons required for sediment provenance studies. There is excellent agreement between concordant laser ablation ICPMS and SIMS analyses of detrital zircons extracted from lower Silurian metasandstone from the Ulven Group (Skarfjell Formation) in the west Norwegian Caledonian nappes. Both LA ICPMS and SIMS U–Pb zircon ages indicate that sedimentary detritus of the Ulven Group was supplied from a terrain containing zircons of Archean, Proterozoic and early Ordovician age.