ORIGIN AND SIGNIFICANCE OF THE SEM CATHODOLUMINESCENCE FROM ZIRCON

In geochronological investigations, zircons are used frequently as a geological clock because of the small amounts of radioactive U isotopes and their decay products, e.g. Pb, incorporated into the crystal structure. However, the U or Pb concentration of zircon is not only dependent on the radioactive decay. One factor is the presence of old, inherited zircon cores in younger igneous zircons. This polyphase structure often cannot be recognized by conventional imaging methods, such as light microscopy. Dating such zircons, can therefore lead to ambiguous age results. However, zircons can be selected for dating purposes using cathodoluminescence (CL) in the scanning electron microscope. It is shown that zircon phases of different ages can be identified by their specific CL properties. Moreover, conclusions on the origin and on the development of the inherited phases can be drawn by comparison with detrital zircons. The zircon CL is formed by the superpostion of several broad bands. The shape of the spectra is dependent on the zircon genesis. By fitting the spectra with Gaussian curves, the individual CL bands can be separated. Using this methodology, it is possible to trace back the essential parts of the zircon CL to a superpostion of the quartz and zirconia (ZrO2) CL. In addition, CL phenomena due to dysprosium impurities can be separated from intrinsic zircon CL properties.