Improving dose rate determinations for rock surface luminescence dating: measuring potassium content within gneissic rocks with μ-XRF

Successful luminescence dating of potassium feldspar grains is dependent on reliable estimates of the dosing from ionising radiation from cosmic sources, surrounding sediments, and from the grains themselves. The internal radiation within potassium feldspar grains originates mainly from beta decay (discharge of an electron from the atom nucleus) of 40K, a long-lived radioisotope of potassium. Overestimating of the internal potassium content in the dated grains will result in age underestimation. Conventionally, a potassium content of 12.5 ± 0.5 % (roughly the potassium content of orthoclase/microcline) within feldspar grains is assumed for most dating applications. Rades et al. (2018), used micro-X-ray fluorescence (μ-XRF) to map and measure the potassium content of rock slices (Fig. 1). They noted variability in the potassium content within feldspars grains from granites from Austria. Attempting to improve dose rate determination for gneisses collected for rock surface luminescence dating, we were given the opportunity to map the potassium content in rock slices from gneisses recovered from the archaeological excavation of a bronze age enclosure. The element mapping was done with μ-XRF at the Nordic Luminescence Laboratory at the Risø Campus, Roskilde, Denmark. Relative potassium content mapping indicates that the high-potassium areas within the rock slices correlates with mica mineral grains and not with the feldspar grains. Point measurements of feldspar grains indicate low potassium content, with averages ranging from 0.5 to 2.5 %. Using the measured potassium content when calculating the effective dose rate for the rocks will improve the dating of the enclosures.

References:
Rades, E., Sohbati, R., Lüthgens, C., Jain, M., Murray, A., 2018. First luminescence-depth profiles from boulders from moraine deposits: Insights into glaciation chronology and transport dynamics in Malta valley, Austria. Radiation Measurements 120, 281 – 289. doi:https://doi.org/10.1016/j.radmeas.2018.08.011

 

Text: Lucas Ageby
Lucas Ageby is a PhD candidate in the F2-project. He is interested in using rock surface luminescence dating to date archaeological structures and gravel deposits.

 

pic 1 Blog group photo WS Ageby Okt 18 350pxFig.1: Images of gneiss slices from Val di Sole, Italy and the relative mapping of potassium content. The high-potassium areas mainly coincide with areas of mica minerals.
Photo: Lucas Ageby

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