ISOTOPE DISTURBANCE IN BADDELEYITE - ISOTOPE MOBILITY, DECAY DAMAGE AND THE LACK OF CHEMICAL ABRASION PROCEDURES

Baddeleyite (ZrO₂) occurs as a trace phase in silica under-saturated igneous rocks. Its high parent/daughter ratio potentially makes it a suitable mineral for high precision U-Pb geochronology, often targeted to date mafic rocks, especially those associated with large igneous provinces. Earlier geochronological studies have shown its potential, however, when compared to chemically abraded zircon from the same sample, baddeleyite dates often are normally or inversely discordant or spread along the concordia, while zircon dates are older and often more consistent. The chemical abrasion technique, which has been transformative for U-Pb dating of zircon, has been shown to be ineffective in removing discordance from baddeleyite crystals (Rioux et al. 2010). Since we currently lack understanding of what is controlling Pb mobility in baddeleyite, developing routines to either avoid it or mitigate its effect are not possible. Therefore, at present, baddeleyite, is considered a lower quality cousin of zircon and U-Pb ages from this mineral need to be treated with caution without robust CA-ID-TIMS zircon U-Pb ages to confirm them.

We will present new high precision U-Pb dates from baddeleyite and zircon from the same mafic samples and discuss the origin of their non uniform U and Pb isotopic compositions. Complexities in the datasets suggest problems associated with inheritance and Pb loss in both data sets. In combination with these high-precision dates, we explore possible proxies for Pb disturbance in baddeleyite, including trace element concentrations, Hf isotope composition, Raman spectral analysis and cathodoluminescence (CL) imaging. Analysis of our own and literature data show that baddeleyite responds to radiation differently than zircon and does not incorporate resolvable non-stochiometric trace elements at high alpha doses. However, CL images show complex features and alteration, which are invisible using an optical binocular microscope. Hafnium isotope analysis and CL imaging can contribute to identify the presence of inherited material (cores). While we do not have specific recommendations for future baddeleyite studies, our results suggest that further detailed work on baddeleyite crystals before geochronologic analysis may hold the clues to understanding the U-Pb system in this mineral.