Thermoluminescence of ZrSiO 4 (zircon): A new dating method?

Abstract

Zircon appears to be a suitable medium for thermoluminescence (TL) dating of sediments from the Quaternary. TL of zircon results predominantly from internal irradiation, due to the relatively high internal concentrations of α-emitting U and Th. The internal dose predominates over the external one that is caused by the surrounding geological layers and cosmic rays. Measurement of the TL buildup forms the basis for the development of a geochronometer, to measure the time elapsed since burial of the sediment by more recent layers. The separation and selection procedures, which are used to concentrate the high quality, transparent and colorless part of the zircon fraction of the sediments are an important part of the zircon TL measurements methodology. By improving the procedures, the colored (i.e. light absorbing) grains are excluded from the measurements. For all sand samples, the 3D TL spectra show Dy 3+ peaks at low temperatures and Tb 3+ bands at high temperatures. The Dy 3+ peaks fade rapidly but we have found that after storage for 16 weeks in the dark, the peaks associated with Tb 3+ are stable at room temperature for at least two years. Zircons were formed many millions to several billions years ago and therefore we suspected that the problems with zircons are related with “old” radiation damage. In this paper we will focus on two major problems of zircon dating: fading and zoning. We will show that if suitable procedures are used during the preparation stage and the dating experiments, these problems can be solved to a large extent.