SMALL-VOLUME BADDELEYITE U-PB GEOCHRONOLOGY AND LU-HF ISOTOPE CHEMISTRY BY LA-ICP-MS – TECHNIQUES AND APPLICATIONS

Baddeleyite is the monoclinic polymorph of ZrO₂ that naturally occurs in many silica-undersaturated igneous rocks both of terrestrial and extraterrestrial origin. It strongly partitions both U and Hf into the crystal structure, while, at the same time, excludes Pb and Lu, thus making it an ideal candidate for U-Pb and Lu-Hf geochronology. It is especially important for basic rock compositions, a lithologic group where the more common phase used for precise geochronology, zircon, is very rarely present. In this contribution, we present recent technique developments that allow us to accurately and precisely date baddeleyite crystals by LA-MC-ICP-MS, as well as measure their Hf isotopic composition. We show that crystallization ages for Precambrian baddeleyites can routinely be reproduced to within 1% total age uncertainty and accuracy (²⁰⁶Pb/²⁰⁷Pb weighted mean ages), while Phanerozoic crystals can be dated to within ~2.0 to 2.5% age uncertainty depending on their U concentrations (²⁰⁶Pb/²³⁸U weighted mean ages), even with ablation-spot diameters as small as 10µm or less. Previous attempts to date baddeleyite by SIMS methods have shown a strong variability on the measured U/Pb fractionation factors based on crystallographic orientation with respect to the incident primary-ion beam. We have conducted a systematic study using randomly-oriented crystal fragments from samples with known ages. The age of these grains have previously determined by TIMS and their orientations were measured using EBSD. LA-ICP-MS U-Pb analyses were conducted in order to test for the potential effect of crystal orientation on the age determinations. Our results show that no significant orientation-dependent variability in the U/Pb data can be observed beyond the normal scatter expected from our analytical uncertainties. We believe, therefore, that our method of LA-MC-ICP-MS dating of baddeleyite using a matrix-matched reference standard represents a very robust way of spot-dating crystals of a wide range of ages with analytical uncertainties that are appropriate to solve many issues in the Earth Sciences.