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 chemical abrasion experiments designed to understand and reduce these effects. Experiments were carried out on Neoproterozoic baddeleyites with zircon inter- and overgrowths from the Gannakouriep dike swarm, Namibia, and on fragments of a large Paleoproterozoic baddeleyite from the Phalaborwa carbonatite, South Africa. Conventional single-step digestion experiments on the Namibian sample resulted in a discordant suite of analyses with significant scatter attributed to inter- and overgrown zircon. To isolate the baddeleyite and zircon in these grains, we developed a two-step HCl-HF chemical abrasion procedure for annealed grains. Initial 48hr-210°C HCl digestions completely dissolved baddeleyite, leaving behind intact zircon rims, which were subsequently dissolved in HF. Analyses of the HCl digestion steps plot along a statistically significant discordia, with an upper intercept interpreted to reflect the igneous crystallization age of the sample. In contrast, analyses from the HF digestions show significant scatter reflecting multiple periods of Pb loss or zircon growth. The data indicate that the two step digestion procedure can be used to resolve both the igneous and metamorphic history of composite grains. Finally, multi-step HCl and HNO3 chemical abrasion experiments were carried out on Phalaborwa baddeleyite. The baddeleyite data are more complex than typical zircon chemical abrasion analyses, but the method shows potential for reducing the impact of Pb-loss in discordant grains.