In situ petrographic thin section U-Pb dating of zircon, monazite, and titanite using laser ablation-MC-ICP-MS

A laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analytical protocol is used to date accessory minerals (zircon, monazite, and titanite) at high spatial resolution (5-40 mu m) using standard petrographic thin sections. The MC-ICP-MS instrument is equipped with a modified collector array containing a combination of Faraday buckets and multiple ion counters, which produces accurate and precise geochronological data using small sample volumes (pit depth <= 2 mu m at 5 mu m and <= 15 mu m at 40 mu m spot sizes). Standardization and normalization factors for the Pb-206/U-238 and Pb-207/U-235 values are calculated based on well-characterized external mineral standards previously dated by high precision ID-TIMS analysis. During an analytical session, the 2 sigma relative standard deviation (i.e., external reproducibility) for the Pb-206/U-238 and Pb-207/U-235 values is <= 3%. The measured Pb-207/Pb-206 value is simultaneously corrected for instrumental mass bias by the aspiration of a Tl solution resulting in a 2 sigma relative standard deviation of between similar to 0.3 and 1%. The accuracy of the analytical protocol was verified on petrographic thin sections of several samples previously dated by ID-TIMS. The capacity of this new, 'small volume' in situ dating technique to provide contextual, relatively rapid and accurate age information is a substantial improvement in reconnaissance-style studies of geological areas with scarce geochronological age information. (c) 2006 Elsevier B.V. All rights reserved.