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.
Highlights
Cathodoluminescence (CL) and backscattered electron (BSE) imaging of zircon (ZrSiO4) commonly records finescale composition zoning [1] that demonstrates its ability to retain geochemically important trace and rare earth elements (REE) over a range of geological conditions
Electron backscatter diffraction (EBSD) data The zircon grain is orientated such that the c-axis of the grain is broadly parallel with the sample surface, and approximately lies in the x direction, while the symmetrically equivalent a-directions closely correspond to y and z (Fig. 2a,b)
MFiagpusroef 3area A shown in Fig. 2c Maps of area A shown in Fig. 2c. a) Cumulative misorientation map derived from EBSD data showing disorientation relative to reference orientation shown by a cross
Summary
Cathodoluminescence (CL) and backscattered electron (BSE) imaging of zircon (ZrSiO4) commonly records finescale composition zoning [1] that demonstrates its ability to retain geochemically important trace and rare earth elements (REE) over a range of geological conditions. Quantitative microstructural studies have found that zircon may deform by crystal-plastic processes, manifest by heterogeneously distributed, discrete low-angle boundaries (
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