Revealing hidden structures: The application of cathodoluminescence and back-scattered electron imaging to dating zircons from lower crustal xenoliths

Abstract

The use of cathodoluminescence (CL) and/or back-scattered electron (BSE) imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive. Using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by SHRIMP (sensitive high-resolution ion microprobe). In all three cases, the zircons are featureless in transmitted light but CL and BSE reveal internal structures that correlate with distinct growth events. Generally, CL and BSE images reveal similar structures, with CL showing finer detail. Neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques. Igneous cores in zircons commonly emit a different color CL emission. In many cases zoning in the cores is truncated, indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event (s) or that part of the core zircon was resorbed during the subsequent metamorphic event. Metamorphic rims, when present, are commonly 10 to 30 μm thick, and are nearly always unzoned and featureless.Igneous cores and metamorphic overgrowths commonly have distinctive CL emission spectra and trace element concentrations. However, the CL spectra can only be used to qualify chemical differences, as a linear relationship has not been shown to exist between CL intensity and trace element concentration in natural zircons. In many cases the Hf, Y, P, U and the heavy rare-earth elements (HREEs) concentrations can be correlated to igneous and metamorphic growth using a combination of CL and BSE imaging techniques and in situ trace element analyses with either the electron microprobe or PIXE (particle induced X-ray emission).