TL dating of feldspars using their far-red emission to deal with anomalous fading

Abstract

New studies of thermoluminescence (TL) dating of feldspars involving their far-red emission are proposed. The intense TL of feldspars is affected by the pervasive effect of anomalous fading, a strong deterrent to their use for TL dating. With volcanic feldspars, the natural TL can fade completely. With other feldspars, such as found in sediments, anomalous fading may turn out to be weaker and correct TL dates could be obtained. This effect of fading may be dealt with methods based on the little studied far-red emission, a band centred around 710 nm present in the TL of all feldspars. Apparently this emission does not fade during geological storage, or at least much less than the visible and UV (‘blue’) emissions used by conventional TL, OSL or IRSL readers. This emission has enabled dating by TL and has been expanded to IRSL. These TL experiments are resumed with improved methods for filtering, collection of data and protocols. Firstly, the stability of TL in some sediments is tested by the B ir protocol. B ir is the ratio of the conventional ‘blue’ emission to the far-red emission measured simultaneously in the course of a TL measurement by use of filters on a carousel. Comparison of the values of B ir between prompt TL and the natural TL provides a sensitive gauge of the stability of conventional TL, which is verified notably with some sediments dated by Preusser [IRSL dating of K-rich feldspars using the SAR protocol: comparison with independent age control. Ancient TL 21, 17–23.]. Secondly, two volcanic plagioclases of known age from Olby and Royat are considered. In their natural TL, the conventional ‘blue’ emission has drastically faded away but not the far-red one. Additive TL using this far-red emission leads to a satisfactory evaluation of the natural dose of these samples, hereby confirming the stability of this far-red emission during geological storage.