Zoning in zircons

The TL emission in many zircons is zoned. Because regions of bright TL emission are anti-correlated with regions of high uranium content, age determination by conventional methods give TL dates which are too low. We designed an image intensifier based TL system capable of resolving features on a 5 micron scale during heating in order to detect zoned grains early in the dating procedure. Early rejection of zoned grains would have made the conventional dating approach feasible. However, although polished crystals can unambiguously be identified as being zoned from their TL images, untreated, whole crystals often have superficial features which tend to blur images of the internal zoned light emissions. (author)