Testing infrared radiofluorescence dating on polymineral fine-grains from the Luochuan loess-palaeosol sequence, Chinese loess plateau

Abstract

In the past infrared radiofluorescence (IR-RF) dating has been focused on coarse-grain potassium feldspar (K-feldspar). However, polymineral fine-grains (4–11 μm) have proven to be successful dosimeters when dating using conventional infrared stimulated luminescence (IRSL). In this study, we investigated the potential of IR-RF signals generated from polymineral fine-grains for dating on 6 loess samples (∼30–480 ka) from the well documented Luochuan loess-palaeosol sequence on the Chinese Loess Plateau. Both the natural and regenerated IR-RF signals from the polymineral fine-grains were uncharacteristically flat with little or no dynamic range using either a bleaching duration of 1500 s or 20000 s between the measurements. X-ray fluorescence (XRF) analyses showed that the polymineral fine-grains were dominated by quartz and Na-feldspars. K-feldspar and Na-feldspar mid-grain (38–63 μm) sample sets were further prepared, measured and compared. The IR-RF signals from the K-feldspar mid-grains exhibited the characteristic exponentially decaying signal, while the IR-RF signals from the Na-feldspar mid-grains were flat with little dynamic range, indicating that the Na-feldspar dominates the polymineral fine-grain IR-RF signal. The polymineral fine-grain ages (both bleaching settings) were consistent with reference ages up to 300 ka (∼1000 Gy), however, due to the flat nature of the curves the uncertainties were large. The consistency of the results using the two bleaching durations indicates that the bleaching duration has a less significant role in the IR-RF dating with polymineral fine-grains than in the coarse grains. Of the mid-grain ages, the IR-RF ages of Na-feldspar mid-grains were consistent with the reference ages up to 360 ka (∼1200 Gy) while those of the K-feldspar mid-grains were consistent with the reference ages up to 200 ka (∼600 Gy). Dose recovery tests were undertaken on all samples, however, where the dose recovery ratios were considered the most successful (K-feldspar mid-grains) the dating results were the least successful, suggesting that dose recovery tests in this way may be a less useful predictor of the dating performance of the IR-RF dating. IR-RF dating of polymineral fine-grains was possible up to 300 ka for Chinese loess, despite the uncharacteristically flat signals.