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Less than 20% of aliquots measured for a set of ‘infinitely old’ quartz samples (i.e. burial time >1 Ma) from the northern Alpine foreland of Switzerland were found to display a natural signal that was in saturation. The signals from small aliquots displayed a range of saturation levels of between ∼300 and 600 Gy and dose recovery tests were successful up to 350 Gy. A comparison of dose response curves and test-dose response for a single-aliquot regenerative-dose (SAR) and a sensitivity-corrected multiple-aliquot regenerative-dose (SC-MAR) protocols, showed similar growth and sensitivity. The addition of a large laboratory dose was able to bring the natural signals into saturation, and no evidence was found to suggest that the unsaturated signals were due to a problem with the SAR protocol. While laboratory isothermal decay measurements confirmed that the lifetime of the luminescence signal was insufficient beyond 1 Ma, a correction for the thermal loss of signal confirmed that natural signals should still be in saturation, and that thermal instability alone cannot explain the unsaturated signals. Fading tests recorded athermal loss of signal, and higher g-values displayed a significant correlation with lower levels of signal saturation. A combination of thermal and athermal loss of the signal is understood to be responsible for the unsaturated signals in these infinitely old samples. Extensive isothermal decay measurements revealed a surprisingly broad range of lifetimes for the quartz signals of over two orders of magnitude within the studied samples, and this was also observed in several batches of calibration quartz. While machine repeatability was shown to explain part of the observed variability, further work is encouraged on samples from other regions to establish the major source of the spread, and caution is recommended before making lifetime comparisons between studies and settings. [ABSTRACT FROM AUTHOR]

Optically Stimulated Luminescence (OSL) is a well-established Quaternary dating method, which has recently been adapted to application in low-temperature thermochronometry. The Infra-Red Stimulated Luminescence (IRSL) of feldspar, which so far is the most promising target signal in thermochronometry, is unfortunately prone to anomalous fading. The fading of feldspar IRSL is at times not only challenging to measure, but also laborious to incorporate within luminescence growth models. Quantification of IRSL fading is therefore a crucial step in OSL thermochronometry, raising questions regarding (i) reproducibility and reliability of laboratory measurements of fading, as well as (ii) the applicability of existing fading models to quantitatively predict the level of IRSL field saturation in nature. Here we investigate the natural luminescence signal and anomalous fading of IRSL measured at 50 °C (IRSL 50 ) in 32 bedrock samples collected from a variety of lithologies and exhumation settings (Alaska and Norway). We report a large span of IRSL 50 fading rates between samples (g 2days ranging from ∼0.5 to ∼45%/decade), which further demonstrates (i) a good reproducibility between two common fading measurement protocols, and (ii) the ability of tunnelling models to predict the level of feldspar IRSL 50 field saturation in nature. We observe higher IRSL 50 fading in feldspar with increasing Ca content, although other factors cannot be dismissed at present. Finally, our dataset confirms that the applicability of feldspar IRSL 50 in OSL thermochronometry is limited to rapidly-exhuming settings or warm subsurface environments. [ABSTRACT FROM AUTHOR]