High spatial resolution in situ U–Pb dating using laser ablation multiple ion counting inductively coupled plasma mass spectrometry (LA-MIC-ICP-MS)

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a powerful micro-analytical tool that has been widely used in geoscience because of its ability to rapidly and precisely analyze isotopes in situ with a typical spatial resolution of 30–80 microns. Here we present a new method for U–Pb dating of zircon with a higher spatial resolution at a scale of 5.8–7.4 μm in diameter and <3 μm in sampling depth (<0.5 ng of zircon material consumption), using a 193 nm ArF excimer laser ablation system coupled with multiple ion counting inductively coupled plasma mass spectrometry (LA-MIC-ICP-MS). Ion counters have an obvious advantage when using a small ion beam (<12.5 mV) and make it possible to date zircons with a ²³⁸U signal of <0.5 mV with improved spatial resolution. The new technique was evaluated by investigating six zircon standards with various known ages ranging from 32 Ma to 2060 Ma. Using this technique, we obtained a precision (2s) for the weighted mean ²⁰⁶Pb/²³⁸U age within 1% and an accuracy of <1% offset to the nominal reference age. We then further demonstrated the utility and robustness of the new high spatial resolution technique for its application to the real geological problem of complexly zoned zircons, to determine the age of the famous Dabie ultrahigh pressure metamorphism. We anticipate that the method will have a wide range of applications in high-resolution U–Pb dating of tiny or complex zircon grains with small cores or relatively narrow zones, with high efficiency and throughput in the geological research community.