Recrystallisation and trace element mobility in zircons: Implications for U-Pb dating

Abstract

The resistance of zircons to physical and chemical weathering, their high Pb closure temperature and its ubiquitous presence in a variety of rock types have made zircons the mineral of choice in many geochronological studies. Despite these characteristics, zircons have been found which show textures evident of zircon recrystallisation. Understanding the mechanisms responsible for zircon recrystallisation and their effect on trace element mobility is of vital importance for correctly interpreting U-Pb ages of zircons. In this study both igneous and metamorphic zircons form the Jack Hills Metasedimentary Belt in Western Australia, and the granitic injection complex of Harris in North-west Scotland have been studied. Cathodoluminescence (CL), electron backscatter diffraction (EBSD) and wavelength dispersive spectroscopy (WDS) imaging reveal that recrystallisation in the studied zircons is often characterised by 1) a translation, bending and fading of the oscillatory zoning, 2) a structural recovery of the zircon, 3) an enrichment in Hf and depletion in Y in recrystallised zircon and 4) the formation of a recrystallisation interface with a minor (< 2°) misorientation. A mechanism of trace element and dislocation migration is expected to be acting shortly after crystallisation during the cooling of the magma, as similarly described by Hoskin and Black (2000). However, an addition is made to this mechanism as the accumulation of dislocations is expected to form dislocation arrays which act as fast-diffusion pathways supporting pipe diffusion as proposed by (Piazolo et al., 2016). During this process, fluids ingress along these fast-diffusion pathways supplying Hf which is incorporated in the recrystallised zircon. After this recrystallisation event, subsequent metamictization of the primary oscillatory zoning may be structurally recovered by a mechanism of diffusion-reactionre-equilibration (Geisler et al., 2007). The implications of zircon recrystallisation on U-Pb dating are that because of the heterogeneous distribution of trace elements, spots for SHRIMP analysis must be carefully chosen. It must be considered that the recrystallisation expulsed U, which results in a lesser degree of metamictization within recrystallised zircon. For this reason recrystallised zircon may reflect the age of crystallisation better than zoned zircon, which may have experienced a Pb loss event via diffusion through metamictization induced fast-diffusion pathways.