Characterising the luminescence behaviour of ‘infinitely old’ quartz samples from Switzerland

Lowick, Sally; Valla, Pierre (2018). Characterising the luminescence behaviour of ‘infinitely old’ quartz samples from Switzerland. Quaternary geochronology, 43, pp. 1-11. Elsevier 10.1016/j.quageo.2017.09.004

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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.

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