Short communication: Synchrotron-based elemental mapping of single grains to investigate variable infrared-radiofluorescence emissions for luminescence dating

Abstract

Abstract. During ionising irradiation, potassium (K)-rich feldspar grains emit infrared (IR) light, which is used for infrared-radiofluorescence (IR-RF) dating. The late-saturating IR-RF emission centred at ~880 nm represents a promising tool to date Quaternary sediments. However, in the present work, we report the presence of individual grains of K-feldspar displaying an aberrant IR-RF signal shape, whose combined intensity contaminates the sum signal of an aliquot composed of dozens of grains. Our experiments were carried out at the National Synchrotron Light Source (NSLS-II) on coarse (> 90 µm) K-feldspar grains of five samples of different ages, nature and origin in order to characterise the composition of grains yielding the desired or contaminated IR-RF emission. Using micro-X-ray-fluorescence (µXRF), we successfully acquired element distribution maps of fifteen elements (<1 µm resolution) of the surface of grains previously used for luminescence dating. In keeping with current theories of IR-RF signal production, we observed a correlation between the relative proportions of Pb and Fe and the shape of the luminescence signal: most grains with the desired IR-RF signal shape had high Pb and low Fe contents. Interestingly, these grains were also defined by high Ba and low Ca contents. Additionally, this study represents a proof-of-concept for mapping the oxidation states of Fe-ions using micro-X-ray absorption near-edge structure spectroscopy (µXANES) on individual grains. The high spatial resolution enabled by synchrotron spectroscopy makes it a powerful tool for future experiments to elucidate long-standing issues concerning the nature and type of defect(s) associated with the main dosimetric trap in feldspar.