Relationship among titanium, rare earth elements, U-Pb ages and deformation microstructures in zircon: Implications for Ti-in-zircon thermometry
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A zircon grain in an orthopyroxene-garnet-phlogopite-zircon-rutile-bearing xenolith from Udachnaya, Siberia, preserves a pattern of crystallographic misorientation and subgrain microstructure associated with crystal-plastic deformation. The zircon grain records significant variations in titanium (Ti) from 2.6 to 30 ppm that corresponds to a difference in calculated Ti-in-zircon temperatures of over several hundred degrees Celsius. The highest Ti concentration is measured at subgrain centres (30 ppm), and Ti is variably depleted at low-angle boundaries (down to 2.6 ppm). Variations in cathodoluminescence coincide with the deformation microstructure and indicate localised, differential enrichment of rare earth elements (REE) at low-angle boundaries. Variable enrichment of U and Th and systematic increase of Th/U from 1.61 to 3.52 occurs at low-angle boundaries. Individual SHRIMP-derived U-Pb ages from more deformed zones (mean age of 1799 40, n = 22) are systematically younger than subgrain cores (mean age of 1851 65 Ma, n = 7), and indicate that open system behaviour of Ti-Th-U occurred shortly after zircon growth, prior to the accumulation of significant radiogenic Pb. Modelling of trace-element diffusion distances for geologically reasonable thermal histories indicates that the observed variations are ~ 5 orders of magnitude greater than can be accounted for by volume diffusion. The data are best explained by enhanced diffusion of U, Th and Ti along deformation-related fast-diffusion pathways, such as dislocations and low-angle (<5°) boundaries. These results indicate chemical exchange between zircon and the surrounding matrix and show that Ti-in-zircon thermometry and U-Pb geochronology from deformed zircon may not yield information relating to the conditions and timing of primary crystallisation.
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