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    Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon

    19934_downloaded_stream_452.pdf (5.237Mb)
    Access Status
    Open access
    Authors
    Timms, Nicholas Eric
    Kinny, Peter
    Reddy, Steven
    Date
    2006
    Type
    Journal Article
    
    Metadata
    Show full item record
    Citation
    Timms, Nicholas E. and Kinny, Peter D and Reddy, Steven M, 2006, Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon, Geochemical Transactions (10).
    Source Title
    Geochemical Transactions
    DOI
    10.1186/1467-4866-7-10
    Additional URLs
    http://www.geochemicaltransactions.com/content/7/1/10
    Faculty
    Department of Applied Geology
    Division of Resources and Environment
    School
    TIGeR (Department of Applied Geology)
    URI
    http://hdl.handle.net/20.500.11937/46360
    Collection
    • Curtin Research Publications
    Abstract

    The effects of crystal-plasticity on the U-Th-Pb system in zircon is studied by quantitative microstructural and microchemical analysis of a large zircon grain collected from pyroxenite of the Lewisian Complex, Scotland. Electron backscatter diffraction (EBSD) mapping reveals a c.18 variation in crystallographic orientation that comprises both a gradual change in orientation and a series of discrete low-angle (<4) boundaries. These microstructural data are consistent with crystal-plastic deformation of zircon associated with the formation and migration of dislocations.A heterogeneous pattern of dark cathodoluminescence, with the darkest domains coinciding with low-angle boundaries, mimics the deformation microstructure identified by EBSD. Geochemical data collected using the Sensitive High Resolution Ion MicroProbe (SHRIMP) shows a positive correlation between concentrations of the elements U, Th and Pb (ranging from 20 60 ppm, 30 110 ppm, and 14 36 ppm, respectively) and Th/U ratio (1.13 1.8) with the deformation microstructure. The highest measured concentrations and Th/U coincide with low-angle boundaries. This enrichment is interpreted to reflect enhanced bulk diffusion of U and Th due to the formation and migration of high-diffusivity dislocations. 207Pb/206Pb ages for individual analyses show no significant variation across the grain, and define a concordant, combined mean age of 2451 14 Ma. This indicates that the grain was deformed shortly after initial crystallization, most probably during retrograde Inverian metamorphism at amphibolite facies conditions. The elevated Th over U and consistent 207Pb/206Pb ages indicates that deformation most likely occurred in the presence of a late-stage magmatic fluid that drove an increase in the Th/U during deformation. The relative enrichment of Th over U implies that Th/U ratio may not always be a robust indicator of crystallization environment. This study provides the first evidence of deformation-related modification of the U-Th system in zircon and has fundamental implications for the application and interpretation of zircon trace element data.

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