DEFORMATION OF ZIRCON AND IMPLICATIONS FOR GEOCHEMISTRY AND GEOCHRONOLOGY

Orientation mapping of zircon (ZrSiO₄) from a range of different geological environments, reveals progressive variations in intragrain crystallographic orientations associated with dislocation creep. The progressive nature of orientation change is documented by crystallographic pole figures that show systematic small circle distributions, and disorientation axes which lie parallel to rational low index crystallographic axes. These data allow the identification of deformation slip systems and calculation of the geometrically necessary dislocation density required to accommodate these lattice distortions. Transmission electron microscopy analysis of FIB-prepared foils across low angle boundaries confirm the dislocation geometries and densities inferred from EBSD data. Microstructural data therefore verify that zircon can deform by crystal plastic processes under a variety of geological conditions.

Analyses of deformed zircon grains using high-spatial resolution geochemical techniques (panchromatic and hyperspectral cathodoluminescence and secondary ion mass spectrometry) indicates that the defects associated with dislocation creep facilitate the modification of the REE, Ti, U, Th and Pb geochemistry of zircon by fast-path diffusion. The extent of this chemical modification is dependent upon a number of factors, including the element concentration in the original zircon, the presence of a reservoir for geochemical exchange and the geometry and migration rate of dislocations and low-angle boundaries. However, a significant discovery is that under some circumstances the deformation is closely linked to the isotopic resetting of zircon’s radiometric clock; a finding that allows the direct dating of deformation events in the geological record.