The Effects of Crystal-plastic Deformation on Zircon Geochemical Systems

The widespread usage of trace element, radiogenic isotope and stable isotope geochemistry of zircon (ZrSiO4) for constraining the nature and timing of Earth processes throughout geological history is underpinned by a thorough understanding of the mechanisms by which zircon geochemistry may be modified. One potentially important but generally overlooked mechanism, is the modification of zircon diffusivity via the formation of deformation-related fast-diffusion pathways throughout the mineral lattice. Electron backscatter diffraction (EBSD) allows quantitative resolution of intra- and intergrain variations in crystallographic orientation, and has facilitated the recognition of low-angle subgrain boundaries and lattice misorientation microstructure in zircon. Dislocation slip systems have been determined for zircon via analysis of EBSD data using simple geometric models for low-angle boundary formation. Assessment of the mean local misorientation has allowed direct comparison of the deformation microstructure with trace element and/or isotopic analyses for zircon via lower spatial resolution ion probe (SHRIMP) techniques. Analysis of several naturally deformed zircons from different environments indicates localized deformation- related geochemical modification with a variety of enrichment/depletion relationships. The most extreme changes occur along low-angle boundaries, with a strong boundary misorientation angle control. In all examples given, the deformation-related geochemical modification of zircon requires trace element mobility via fast diffusion pathways at much higher rates than predicted by known volume diffusion parameters. The exact nature of geochemical modification in zircon depends on many factors, such as trace element compatibility in the zircon lattice, exchange with an external reservoir, and/or possible trace element stabilization of low-angle boundaries, P-T conditions of deformation, and, for the U-Pb system, the timing of deformation relative to initial crystallisation. These results provide some new constraints on geological processes, for example allowing U-Pb systematics to be directly linked to the timing of impact deformation. Therefore, the analysis of deformed zircon provides a new approach to unlocking zircon as a geological tool. However, quantification of the rates and exact mechanisms of deformation-related fast-pathway diffusion in zircon remains a challenge for future research.