Lithological controls on light penetration into rock surfaces-Implications for OSL and IRSL surface exposure dating

Luminescence surface exposure dating is a newly developed geochronological technique that allows the age of geological or archaeological rock surfaces to be accurately constrained. This dating method requires measuring and calibrating the depth-dependent luminescence signal below an exposed surface and relies on the assumption that neither the shape of the daylight spectrum nor the light attenuation change significantly with depth into the rock. However, lithologies with mm-scale heterogeneity in their mineral distribution or those lacking high sensitivity quartz present a challenge, partly because light attenuation with depth is not necessarily constant in such samples. Addressing these challenges is important for further development of the luminescence surface exposure dating technique. Here we investigate the shape of luminescence-depth profiles in lithologies revealing complex fabrics such as coarse-grained granitic gneisses or gneisses with distinct planar metamorphic layering. We also present luminescence-depth profiles from quartzite, a lithology that appears – at first glance – highly homogenous. We find that the spatial distribution of opaque mineral phases in the metamorphic samples and precipitation of iron hydroxides in the quartzite strongly influence the 3-dimensional transparency and, consequently, the light attenuation with depth, and are the main cause for the observed scatter in the OSL- and IRSL-depth profiles in our samples. The data suggest that for rocks of heterogeneous lithology (i) close petrographic analysis of luminescence-depth profiles are required to ensure that the cores used for calibration have similar mineralogical composition (and thus light attenuation with depth) to those used to calculate a luminescence rock surface exposure age, and that (ii) RGB depth profiles appear to provide a useful semi-quantitative tool for such analysis.