Internal dose rates of single feldspar and composite mineral grains: Methodological developments and optical dating applications

Optical dating of feldspar grains is playing a pivotal role in establishing timelines for hominin occupations in many parts of the world, including the Altai and Wallacea—two regions that are research hotspots for the study of human evolution, ancient dispersals and inter-group hominin interactions. A key challenge of optical dating of feldspar grains is determining the radioactivity within individual grains that are used for dating. Feldspar grains can contain up to ~14 wt% potassium (K) and considerable concentrations of rubidium (Rb), thorium (Th) and uranium (U), all of which have radioactive isotopes that give rise to an internal dose rate component. The internal dose rate can have a major impact on both the precision and accuracy of optical age estimates. Feldspar grains are often composed of multiple mineral phases of variable compositions. Previous techniques used to determine K concentrations of feldspar grains are time-consuming, and either lack the spatial resolution to classify discrete mineral phases within grains or the coverage to obtain whole-of-grain average K concentrations. Samples from two sites (Ust’-Karakol-1 in the Altai and Leang Bulu Bettue in Wallacea), located in contrasting geological settings (plutonic versus volcanic terranes), are used to develop an approach where quantitative evaluation of minerals using energy-dispersive spectroscopy (QEM-EDS) is used to rapidly determine whole-of-grain average K concentrations of individual luminescent feldspar grains. This approach is also applied to samples from two iconic archaeological sites: Denisova Cave in the Altai and Liang Bua in Wallacea. Individual luminescent grains from Denisova Cave are dominated by low-temperature feldspar varieties, which are characteristic of plutonic terranes; most grains are K-rich. Individual luminescent grains from Liang Bua are composite mineral grains composed of a range of feldspar varieties, quartz, clay minerals, heavy minerals and volcanic glass. These grains have a broad range of whole-of-grain average K concentrations—most are low-K. A novel approach, using QEM-EDS and laser ablation inductively coupled plasma mass spectrometry, is developed to investigate the K, Rb, Th and U concentrations of these grains, and determine single-grain and sample-average internal dose rates. Samples from different sedimentary contexts at the site have different internal dose rate distributions. The potential of using infrared stimulated luminescence (IRSL) and post-infrared IRSL signal behaviours as proxies for K concentrations, as an alternative to directly measuring K concentrations, is investigated for one sample with a broad range of K concentrations. Signal intensity and fading rates are poor proxies, whereas thermal stability shows good potential for selecting the most K-rich grains with the most thermally stable IRSL signals. Finally, optical dating of K-rich feldspar grains from 32 sediment samples is used to construct a better-resolved chronology for the sedimentary deposits of the South Chamber of Denisova Cave. The internal and external dose rates and equivalent dose distributions of samples from three sedimentary profiles are scrutinised. The resulting chronology is compared to those previously obtained for Main and East chambers. Together, the data provide further insights into the timing of occupation of this iconic site by Denisovans, Neanderthals and modern humans.