Diffusion kinetics of geochronologically relevant species in baddeleyite
Abstract:Baddeleyite (ZrO2) is commonly present as an accessory mineral in silica under-saturated igneous rocks. In recent years, baddeleyite has gained popularity as a geochronological and petrological tool since it can provide important constraints on the age and geochemical evolution of mafic rocks, a rock type in which zircon, the far more routinely used mineral for geochronology, is often absent. Because baddeleyite has a high budget for U and preferentially excludes Pb from its crystal structure , the initial U/Pb ratio of baddeleyite is typically very high and its overall Pb isotopic composition becomes dominantly radiogenic in a relatively short timeframe. At the same time, baddeleyite has a strong affinity for Hf and a low budget for rare earth elements (REEs); therefore, the small amount of 176Lu present in baddeleyite does not significantly alter the 176Hf/177Hf composition of baddeleyite over time, making it an ideal mineral to fingerprint the initial 176Hf/177Hf ratios of its host rocks . These geochemical characteristics of baddeleyite make it an important tool for studying the genesis and history both terrestrial and extraterrestrial mafic igneous rocks.
Despite the favorable geochemical characteristics of baddeleyite outlined above, the interpretations of baddeleyite U-Pb ages and 176Hf/177Hf ratios are limited by the lack of diffusion data for these species in baddeleyite. The importance of quantifying the diffusion kinetic properties of the parent and daughter nuclides of any geochronological system has been well documented [3, 4], thus making acquisition of these data an important priority for the continued development of baddeleyite as a geochronological tool. We have performed preliminary experiments on U, Th, Pb, Hf and REE diffusion in baddeleyite, and obtained high-quality data utilizing a time-of flight secondary ion mass spectrometer. The results of this study should provide a quantitative framework for the interpretation of baddeleyite U-Pb ages and 176Hf/177Hf ratios.
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