Abstract

Minerals with well-defined TL peaks are likely candidates for archaeological dating materials; they are short-listed by a trap depth measurement using the initial rise method. But experience shows that this test alone in inadequate to guarantee stability. It is vital to subject apparently satisfactory materials to further kinetic analysis. For example, for a sample of labradorite feldspar which has a broad TL peak centred at about 320°C, the initial rise method gave associated trap depths of 1.6 ± 0.2 eV. However, after stograge at 10°C for 25 hours, only 50% of the TL obtained immediately after irradation was measured. This instability, termed anomalous fading, was found to occur for feldspars in recent lava, and resulted in extremely low ages being determined for dated flows. Anomalous fading was also found in samples of zircon and fluorapatite. Initial rise analysis of the 325°C peak in a quartz sample gave a trap depth of 1.05 ±0.03 eV indicating a meanlife at 20°C of only 200 years. However, the dating programme carried out on quartz inclusions with this TL peak show that it has experienced neglible loss of electrons during the 1900 years since firing. Isothermal decay and the study of peak temperature with heating rate gave trap depths of 1.7 ± 0.1 eV and 1.69 ± 0.02 eV, respectively. The dicrepancy between these values and that obtained by the initial rise method was conclusively shown to be due to thermal quenching of the luminescence centres. The criterion for acceptable dating materials should be good agreement between several methods of analysis, as is exemplified by corroborated dating, for a well-defined peak in calcite. A peak occuring at 275°C occurs in most samples of crystalline calcite, the initial rise method and the peak shift with temperature trap-depth procedure give 1.75 ± 0.03 eV for all the samples.

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