Anomalous U–Pb systematics in mantle-derived baddeleyite xenocrysts from Île Bizard: evidence for high temperature radon diffusion?

Mantle baddeleyite inclusions in titanomagnetite megacrysts from the Île Bizard alnöite intrusion display both an unusual morphology and anomalous U–Pb systematics. These baddeleyite crystals have a cubic to octahedral form, possibly indicating crystallization with tetragonal symmetry under upper mantle conditions. The U–Pb results show a very consistent discordance pattern that is independent of grain size, grain shape, or geochemistry. Compared to the intrusion age of 139.9±2.6 Ma, based on U–Pb groundmass perovskite, the weighted mean 207Pb/235U age of 140.7±0.2 Ma is interpreted as the best estimate for the time of baddeleyite crystallization. The 206Pb/238U age of 138.7±0.3 Ma is interpreted as the time when these baddeleyite crystals remained closed to high temperature diffusion of the intermediate daughter product 222Rn. The association of titanomagnetite–baddeleyite–kimzeyite–zirconolite in the main Île Bizard intrusion breccia could be linked to a precursor carbonatite event that occurred at least 2 million years prior to formation of the Île Bizard alnöite magma. A significantly younger, less precise U–Pb age of 109 Ma was obtained for melanite, which could reflect a more complicated post-crystallization and subsolidus mineral growth history.