Potential impacts of chemical weathering on feldspar luminescence dating properties

Abstract

Abstract. Chemical weathering alters the chemical composition of mineral grains. As a result, trapped-charge dating signals of primary silicates may be progressively modified. In this study, we artificially weathered three feldspar specimens to understand the effect of proton- and ligand-promoted dissolution on their luminescence properties. We conducted kinetic experiments over 720 h using two solutions: (1) oxalic acid (pH 3, 20 °C), an organic acid with chelating abilities, and (2) aqua regia (pH < 1, 40 °C), a mixture of strong acids creating aggressive acid hydrolysis conditions. These two solutions were chosen to approach over laboratory timescales some of the changes that may occur over geological time scales as minerals weather in nature. The effect of the extracting solutions on mineral dissolution was investigated by following the concentration of elements accumulating in solution, while changes in feldspar surface morphology was assessed by scanning electron microscopy (SEM). Subsequent changes in feldspar luminescence in the near-UV (~340 nm) and blue (~410 nm) thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission bands were assessed at the multi- and/or single-grain levels to gain insight into the emission spectra, dose response, saturation, and anomalous fading characteristics of the feldspars. In all experiments, only minor feldspar dissolution was observed after 720 h with < 5 % of total Al, Si, Na, and Ca appearing in the aqueous phase, while 5–8 % of the total Mn and Fe were extracted. In general, aqua regia, the more chemically-aggressive solution, had a larger effect on feldspar dissolution compared to that of oxalic acid. Additionally, our results showed that although the TL and IRSL intensities changed slightly with increasing artificial weathering time, the feldspar luminescence properties were otherwise unmodified. This suggests that chemical alteration of feldspar surfaces may not affect luminescence dating signals obtained from natural samples.