Towards a prediction of long-term anomalous fading of feldspar IRSL
Introduction
The luminescence of feldspar, thermally or optically stimulated, is known to decay without stimulation after a laboratory irradiation, contrary to kinetic expectations. This loss of luminescence is generally thought as resulting from quantum mechanical tunnelling (Visocekas, 1979). In order to derive true ages for geological or archaeological events using feldspar IRSL, different approaches have been suggested to correct for the observed fading. Among those, two apparently different methods have been proposed, namely the fadia (Lamothe and Auclair, 1999) and g (Huntley and Lamothe, 2001) methods.
The fadia approach uses differential decay of single grains or single aliquots from the same sample. If the feldspar grains or aliquots have received the same dose in the natural environment, one can derive the equivalent dose for a non-fading feldspar hence the true paleodose. This fadia method was the first approach to offer potential corrections for fading in sediment samples of any age up to saturation. However, difficulties arise when the sample is composed of more than one grain population, as for partially bleached sediments, or when there is not enough variability in anomalous fading rates between different aliquots.
The correction method of Huntley and Lamothe (2001) is based on the measurement of the percentage fading loss of luminescence per decade of time, a parameter termed g by Aitken (1985, Appendix F). The correction is obtained by integrating the loss of luminescence over geological time, the resulting luminescence intensity being If. This is compared with the intensity of luminescence observed in the laboratory before fading (Io). This approach introduces a correction by assuming that If/Io=Tf/T, where T is the true age and Tf is the age measured before correction. The advantage of this approach is that fading can be corrected on a single aliquot. However, it can be applied only to the linear part of the growth curve, and hence to young samples.
The purpose of this paper is to introduce and test a new idea for correcting fading in feldspar minerals that would circumvent the limitations described above. In other words, it should be applicable to a single aliquot and over the whole range of radiation dose, up to saturation. We first describe a new equation stating that the loss of luminescence over geological time, which is F(κ,T/tc) according to Huntley and Lamothe (2001), could be similarly quantified by . An interesting consequence of this result is that the rate of decay of luminescence with time can be theoretically predicted. Therefore, the luminescence intensities, and hence the induced regeneration growth curve, could be corrected. This new idea is tested on the same samples used in Huntley and Lamothe (2001) paper, as well as on two glacial sediments that are in field saturation.
Section snippets
An empirical approach to anomalous fading
The pioneer work of Visocekas 1979, Visocekas 1985 and the exhaustive review by Aitken 1985, Aitken 1998 describe anomalous fading as resulting from quantum mechanical tunnelling. If this is the case, then theoretical predictions could be made on how a sample is affected by it over geological time from the observation of the decay plotted as a function of log (time). Making quantitative corrections using logarithmic predictions can be hazardous (read Aitken, 1985, p. 279) but has however proven
Comparing irradiation over geological and laboratory time scales
Huntley and Lamothe (2001) have published the following equation that quantifies the predicted effect of fading on the intensity of a feldspar dominated sample over time T:where T is the age of burial (in ka), κ is the fractional decay of luminescence during the time interval tc to 2.3tc, I is the intensity of luminescence, and tc is a fixed time constant. The values with the f subscript are those affected by fading. The true age (T) is found by iteration, and the correction
Correcting young ages using the dose-rate correction method
Huntley and Lamothe (2001) have proposed Eq. (1) for correcting IRSL ages for feldspar-dominated young sediments affected by anomalous fading. In this range of applicability, the filling of electron traps is more or less independent of the dose already acquired and growth of luminescence with dose is linear. Interestingly enough, this H+L equation can be transformed into a similar form as the DRC equation (6) by substituting tc by , and κ by . Therefore, we tried to correct the
Correcting anomalous fading for feldspar in luminescence field saturation
In this dose–response range, it can be postulated that only unstable traps are being filled by the laboratory irradiation. This second test is to predict the time necessary for the induced luminescence to fade away for feldspars in field saturation as they are subjected to an intense radiation dose. Here, we use polymineralic 4– fine grains extracted from two glacial sediments. Sample LFA is a lodgment till from Québec, Canada (Bécancour Till; Lamothe, 1989). This sediment is produced in
Conclusion
The DRC equation and the H+L correction use logarithmic predictions to assess the extent of anomalous fading over geological time. This could be seen as a demonstration that quantum mechanical tunnelling of trapped electrons is the most probable process responsible for the observed IRSL age underestimations. The Huntley and Lamothe (2001) correction is straightforward for young sediments but the limitation to the linear part of the growth curve precludes its use for most Pleistocene sediments.
Acknowledgements
This work has been funded by NSERC Canada. The authors wish to thank MDS Nordion for access to their food irradiation facilities as well as A. Jennane for the use of his Troll sample.
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