Published on 08.07.2020

New publication

Our group member Johannes Pohlner and co-workers from the universities of Heidelberg, Wyoming and Geneva/Montréal have published an article in Geochronology:

Baddeleyite (ZrO2) is an accessory mineral that is often used for U-Pb dating, especially of mafic rocks (gabbros etc.) which are traditionally difficult to date. This method is known to yield robust intrusion ages for well-preserved samples, but difficulties increase for non-ideal baddeleyite crystals from altered and metamorphosed rocks. A case study of mafic dikes in Newfoundland and Sierra Leone now provides constraints that help to better understand the U-Pb systematics of baddeleyite in complex samples. This required for a multimethod approach combining scanning electron microscope petrography, in situ U-Pb analysis of baddeleyite micro-domains by secondary ionization mass spectrometry, and high-precision single crystal U-Pb dating by thermal ionization mass spectrometry.

Baddeleyite can be intergrown with zircon (ZrSiO4) in at least seven different textures, demonstrating the importance of micro-petrographic pre-investigation. Pure baddeleyite analyses were obtained in this study, but discordance (deviation of the crystal from an ideal U-Pb isotope evolution in a closed system) complicates data interpretation. Diffusional Pb loss is the dominant discordance mechanism in the investigated samples, but additionally, there seems to be excess 206Pb loss, possibly due to the short-lived isotope 222Rn that may leave baddeleyite crystals easily. Applying these constraints on baddeleyite discordance to the case study samples, the resulting ages are in agreement with independent age informations of the dikes. This study highlights how difficult it can be to obtain reliable intrusion ages for mafic rocks.