U–Pb age determination for zircons using laser ablation-ICP-mass spectrometry equipped with six multiple-ion counting detectors

Abstract

Precise zircon U–Pb age determinations have been made on Plešovice zircon using laser ablation-multiple ion counting-inductively coupled plasma-mass spectrometry (LA-MIC-ICP-MS). To achieve high precision and high spatial resolution age determination, multiple ion counting using six electron multipliers was employed. The intensities of Hg–Pb–U isotope (²⁰²Hg, ²⁰⁴(Hg + Pb), ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, and ²³⁸U) signals were monitored simultaneously without mass scanning. In static acquisition mode, the resultant ²³⁸U–²⁰⁶Pb concordia age for Plešovice was 336.3 ± 1.9 Ma, demonstrating improved precision over that achieved using a magnetic sector-based single-collector-ICP-MS, which was 340.3 ± 3.5 Ma for Plešovice. A high duty cycle can be achieved, along with a short integration time or a small sample volume for analysis, allowing high spatial resolution. More importantly, downhole fractionation can be reduced with a shallow ablation pit. To take full advantage of the setup, a one-second LA analysis (8 laser shots with an 8 Hz repetition rate) was adopted for U–Pb age determination. The resultant concordia age for Plešovice was 339.5 ± 6.7 Ma, demonstrating that the repeatability and laboratory bias precision of the resultant age data were comparable to conventional ablation with a single-collector-ICP-MS. The depths and crater diameters of the ablation pits were, respectively, about >1 μm and 25 μm. The data presented herein demonstrate clearly that multiple ion counting-ICP-MS can become a fast and user-friendly tool for use in U–Pb zircon geochronology.