U-Pb in garnet by LA-ICPMS: dating extremely U-poor minerals

Traditionally, “laser-ablation-geochronologist” have taken advantage of the developments in analytical techniques; trying to improve the spatial resolution in their analysis; i.e. reducing the spot size as much as possible. On the other hand, those advances in instrumentation have opened not only the possibility of analysing smaller sample volumes but also lower concentrations. Minerals with low U (and Pb) contents (< 1 µg/g) like garnet have become the target of in-situ U-Pb geochronology since it has been demonstrated that isochrons can be potentially obtained. In this contribution, we explored the current limits of in-situ U-Pb geochronology: what are the minimum Uranium concentrations from which an accurate and precise U-Pb age can be obtained?

For that purpose, we have analysed garnets from different localities that were unsuccessfully analysed in the past using single-collector instruments. These garnets have been re-measured using a Neptune Plus MC-ICP-MS coupled to a RESOLution-LR ArF Excimer laser. The analyses were performed in static mode measuring the masses 206Pb and 207Pb with Secondary Electron Multiplier (SEM) and 202Hg, 204Pb and 238U with the Multiple Ion Counters (MIC). The analysed garnets have typically U contents below 10 ng/g, which taking into account the spots sizes used, results in a Uranium amount ca. 3,000 times less than compared to conventional LA-ICPMS zircon analysis. About 15–30 spots are sufficient for a regression line in the Tera-Wasserburg diagram, yielding a precision of typically <3 % for the lower intercept age. The analysed. Challenges and details of the method will be discussed.