Zircon Metamictisation study by Cathodoluminescence and X-ray Imaging

Zircon (ZrSiO4) exhibits an extraordinary memory. Its stability, durability, low solubility and low elemental diffusivities combine to preserve in it a record of most of the important events that have affected it, its host rocks, and the crust of which it is a part. The zonation in zircon grains delineate the boundaries of discrete geochemical processes that occurred at different times. Zircon grains often contain high levels of Th and U either as inclusions of discrete mineral grains, or as a solid solution in the crystal lattice. Radiation generated by these impurities leads to metamictisation which is partial or complete destruction of the optical, physical and chemical characteristics of the zircon crystals over time if they are not subjected to natural healing processes (e.g. heating through metamorphism). If significant modification to the structure of the zircon grains occurs, they can lose their opacity and hence their commercial value to the ceramics industry. To lower the Th and U levels to acceptable levels it is essential that the chemistry and structure of the zircon grains are well understood and also how any potential radiation removal technique may modify the structure. Cathodoluminescence (CL), backscattered electron (BSE) and chemical imaging reveals detailed zonation patterns that are commonly invisible or barely visible with conventional transmitted and reflected light microscopy [1]. Many of the factors that contribute to the CL spectra from zircon are well established. Studies of natural and synthetic zircon have shown that the presence of various CL-active impurity ions, such as trivalent rare earth elements (REE 3+ ), causes narrow luminescence emission peaks at characteristic wavelengths. In natural zircon, these narrow REE 3+ peaks are commonly superimposed on a broad emission peak [2]. However, the origin of the broad CL peak is contentious and has been attributed to point defects, OH-defects, or defects within the silica tetrahedra [3].