Circumvention of anomalous fading in feldspar luminescence dating using Post-Isothermal IRSL

In luminescence dating of feldspar, laboratory induced luminescence necessary to assess the naturally acquired dose in the environment suffers from a poorly understood instability over time, known as anomalous fading (AF). AF is the source of commonly observed age underestimation in optical dating of feldspar. Approaches to circumvent AF include correction methods as well as attempts to measure directly an unfading component of feldspar luminescence, e.g. post-IR IRSL protocols. The former has the advantage of measuring the easily bleachable traps but requires extrapolations of fading rates. Post-IR IRSL methods can be limited by the use of more difficult-to-bleach traps and hence by potential age overestimations. In this paper, we show how post-isothermal laboratory induced luminescence (pIt-IR) allows measuring an equivalent dose (D-e) that is not dependent on time elapsed since irradiation. In this procedure, one measures two IRSL signals, first from an IR stimulation at low temperature (i.e. IR50) followed by a second one, at a higher temperature (i.e. IR225). Since both signals fade at different rates, D-e from IR50 is lower than that of IR225. In pIt-IR, a succession of thermal treatments is carried out before the measurement of laboratory-induced IR50 and IR225 luminescence. The dependence of D-e on thermal annealing is different for each IR signal so one can find which thermal treatment will yield the same D-e for both signals. This D-e is assumed to be the true total radiation dose received by the feldspar minerals in nature, more properly known as the paleodose (P). This new methodology is herein applied to three samples of different ages and different geological contexts. Post-isothermal luminescence is thus proposed as a way to circumvent AF, even though extended isothermal annealing treatment is not shown to eradicate AF.