OPTICALLY STIMULATED LUMINESCENCE DATING USING SINGLE MINERAL GRAINS: POTENTIAL AND CHALLENGES

Luminescence measurements of geological sediments allow the date of the last exposure of mineral grains to be calculated, and has been applied in the age range from a few decades to over 100 ka. The method has proven especially suitable for aeolian sediments where exposure to daylight prior to deposition is very likely. Conventional luminescence measurements involve analysis of sub-samples consisting of many hundreds or thousands of mineral grains and assumes that all of the grains have been exposed to sufficient daylight at the time of deposition to reset the luminescence signal to a low level. However, for many types of Quaternary sediment, it is difficult to be confident that this assumption will be met and this makes them problematic for luminescence dating. If the sediment contains some grains that were exposed to insufficient daylight to remove their signal then the age calculated will be too old. Such sediments are common, especially in glacio-fluvial, colluvial and to a lesser degree fluvial settings. Many of these sediments consist of complex mixtures of grains, some of which were exposed to daylight at the time of deposition, and some which were not.

Recent technological developments have made it easier to measure the luminescence signal from individual sand-sized (100-300 µm diameter) grains of quartz. In theory this single grain approach makes it possible to explicitly assess whether all the grains within a sample have the same apparent age. This would give greater confidence in the ages obtained. Equally, where a mixture of grains is present, some of which were exposed to daylight at deposition and others which were not, it should be possible to select only those fully-exposed grains. However, it is now clear that micro-dosimetry problems during measurement and numerical modelling of the distributions obtained are complex, and these provide major challenges for this single grain method. Results of single grain measurements on aeolian, glacio-fluvial and fluvial samples from Australia, Scotland and South Africa are shown, illustrating the potential of this approach.