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

Abstract

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 μm) 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 μm at 5 μm and ≤15 μm at 40 μm spot sizes). Standardization and normalization factors for the 206Pb/ 238U and 207Pb/ 235U values are calculated based on well-characterized external mineral standards previously dated by high precision ID-TIMS analysis. During an analytical session, the 2 σ relative standard deviation (i.e., external reproducibility) for the 206Pb/ 238U and 207Pb/ 235U values is ≤3%. The measured 207Pb/ 206Pb value is simultaneously corrected for instrumental mass bias by the aspiration of a Tl solution resulting in a 2 σ relative standard deviation of between ∼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.