THERMAL EFFECTS IN THE OPTICALLY STIMULATED LUMINESCENCE OF QUARTZ AND MIXED FELDSPARS FROM SEDIMENTS

Because recombination luminescence stimulated optically yields no information about the activation energy whence the recombining charges originate, the effect of any treatments, or lack thereof, relating to thermal stability of the luminescence must be carefully examined. Long-term fading observations of optically stimulated luminescence of quartz and mixed feldspar extracts after irradiation show that the luminescence is delay dependent in both minerals. As it relates to dating, this is highly unsatisfactory. Preheating is the simplest energetically justifiable approach taken to ensure that the charges yielding luminescence have the same activation energies in natural and irradiated aliquots. Luminescence depletion due to preheating of Holocene and Quaternary sediments with natural and artificial doses was examined and found to behave as expected. Preheating was found to have the following undesirable effects. In aliquots with very high doses, preheating lowered the onset of saturation by 20 to 40%. This effect is thought to be due to thermal quenching of recombination centres. In optically zeroed quartz and feldspar aliquots, preheating caused the luminescence signal to increase. In aliquots with geologically very low absorbed doses, preheating to 230-degrees-C caused dose response growth curves to become nonlinear. This effect can be explained in terms of second-order kinetic behaviour. A procedure is suggested which corrects for the undesirable thermal effects in young samples. When verified against thermoluminescence measurements, this procedure yielded correct optical dose estimates for sediments less-than-or-equal-to 12 000 years in age.