U–Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array

Abstract

This study reports U–Pb geochronological data for zircon obtained by laser ablation-multi-collector-ICP-MS using a new collector block design that includes three ion counters and twelve Faraday buckets. The collector configuration allows for simultaneous detection of ion signals from mass ²³⁸U to ²⁰³Tl, an important factor for the achievement of highly precise and reproducible Pb–Pb and Pb–U ratios. The main advantage of the multiple ion counting system is the capability to readily measure low Pb ion signals (<1 × 10⁶ counts per second) resulting from single spot analyses of ≤40 microns (and corresponding small sample volumes). The latter is an extremely important feature when deciphering multiple domains in complexly zoned zircon populations. A comparative study was conducted between analytical protocols involving non-Tl- and Tl-doped laser ablation runs with regards to evaluating the external reproducibility (i.e., relative standard deviation). The results indicate that the (2σ) external reproducibility for both analytical protocols varies between ∼0.3–1.0% for the ²⁰⁷Pb/²⁰⁶Pb, and ca. 1.3 to <3.0% for the ²⁰⁶Pb/²³⁸U (and ²⁰⁷Pb/²³⁵U) values. The external reproducibility for the Pb/U values are relatively constant for the Tl-doped analyses, whereas those for the non-Tl runs are inversely correlated with the total absolute (Pb) count rates. The precision and accuracy of both analytical protocols were verified with the analysis of BR266 and 91500 zircon standards and zircons previously dated by ID-TIMS. The instrument configuration and Tl-doping protocol employed here are ideally suited for studies requiring the rapid analysis of a large number (n > 50) of zircon grains.