Enhanced diffusion of Uranium and Thorium linked to crystal plasticity in zircon |
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Authors: | Email author" target="_blank">Nicholas?E?TimmsEmail author Peter?D?Kinny Steven?M?Reddy |
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Institution: | (1) Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA, 6845, Australia |
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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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