Characterizing the U-Pb systematics of baddeleyite through chemical abrasion: application of multi-step digestion methods to baddeleyite geochronology

U-Pb baddeleyite geochronology has become a major tool for dating mafic rocks, especially dikes associated with Large Igneous Provinces. However, in many cases, post-crystallization Pb-loss and intergrowth of baddeleyite and zircon limit the precision and/or accuracy of crystallization ages. We present results from multi-step digestion experiments designed to understand and reduce these effects. Experiments were carried out on Neoproterozoic baddeleyites with zircon inter- and over-growths from the Gannakouriep dike swarm, Namibia, and on fragments of a large Paleoproterozoic baddeleyite from the Phalaborwa carbonatite, South Africa. Multi-step digestion experiments on annealed Phalaborwa baddeleyite were designed to test whether the recently developed chemical abrasion technique for U-Pb zircon geochronology can be applied to baddeleyite. The experiments generated complex results-individual digestion steps were both normally and reversely discordant suggesting that U and Pb were decoupled in the multi-step digestions-and indicate that the current form of multi-step chemical abrasion is not an effective method for reducing the impact of Pb-loss in baddeleyite. A separate set of experiments on the Gannakouriep baddeleyite focused on isolating the zircon and baddeleyite components in composite grains. Conventional single-step digestion experiments for this sample resulted in a discordant suite of analyses with significant scatter attributed to inter- and over-grown zircon and highlight the difficulty of obtaining precise and accurate ages from composite grains. To isolate the baddeleyite and zircon in these grains, a two-step HCl-HF chemical abrasion procedure for annealed grains was developed. This technique was successful at selectively dissolving the baddeleyite and zircon components. Secondary zircon inter- and over-growths of baddeleyite can occur in samples affected by low-temperature alteration to granulite facies metamorphism, and the new HCl-HF chemical abrasion procedure provides a method for resolving both the igneous and metamorphic history of these composite grains.