Not fade away - The persistence of fading in feldspar luminescence

Feldspars are widely used as natural luminescence dosimeters to constrain past geological, geomorphological and archaeological events and processes. Unfortunately, the luminescence of feldspars suffers from an unwanted signal loss over time, termed fading, which affects the reliability, precision, and accuracy of all these applications. This review presents an overview of the research conducted into the cause of and the physical processes behind fading, as well as of research focussed on circumventing, minimising or correction for fading. Fading has been shown to be ubiquitous in feldspars, affecting both thermoluminescence as well as optically (infrared) stimulated luminescence signals. The most widely accepted physical explanation for fading is quantum mechanical tunnelling of trapped electrons from the ground state of the electron trapping centre to a nearby recombination centre, however, other mechanisms have been proposed, and some of these different explanations are outlined here. Since fading causes an underestimation of the luminescence age, it is necessary to accurately constrain the rate of fading for a given sample, as well as to develop robust methods for correcting the obtained luminescence signal for fading. This review explains how the rate of fading can be determined in the laboratory, and how this can be used to correct the obtained luminescence signal or luminescence age for fading. This review aims at presenting key findings and selected studies as a means to introduce the topic to new researchers in the field of luminescence dating, while hoping that more experienced luminescence researchers might also discover some new information.