In optical dating of feldspars, it is commonly supposed that potassium (K)-rich feldspar grains have been isolated for luminescence measurements by means of density separation. The K and rubidium (Rb) concentrations of these grains deliver a significant internal beta dose rate. K and Rb concentrations of 12.5 ± 0.5 wt% and 400 ± 100 ppm, respectively, are typically assumed for dose rate and age calculation, but this assumption may be wrong. Thorium (Th) and uranium (U) also contribute to the beta and alpha components of the internal dose rate. We investigated the K, Rb, Th and U concentrations of individual luminescent feldspar grains in four sediment samples from Liang Bua in Indonesia to determine single-grain internal dose rates and appropriate sample-average internal dose rates. The composition of the ‘K-rich’ feldspar fraction was investigated using X-ray diffraction spectrometry (XRD), which found that this fraction was heavily contaminated in each sample by quartz and plagioclase feldspars. Grains with detectable post-infrared infrared stimulated luminescence (pIRIR) signals were characterised using a range of techniques, including energy-dispersive spectroscopy (EDS), wavelength-dispersive spectroscopy (WDS) and quantitative evaluation of minerals using EDS (QEM-EDS). The grains were found to be composed of a range of minerals, including feldspars, quartz, clay minerals and heavy minerals, as well as volcanic glass. The K concentrations of these composite grains were determined using QEM-EDS, and their Rb, Th and U concentrations were determined using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The calculated internal dose rates were very low for the majority of grains in each sample (<0.2 Gy.ka⁻¹ for 180–212 μm-diameter grains), although a wide spread of internal dose rates was also observed. None of the samples showed any relationship between internal dose rate and either pIRIR signal intensity or equivalent dose (D_e). Two samples of clastic sediment were found to have different distributions of internal dose rate to those of two samples of tephra. To obtain reliable ages for sediment samples from Liang Bua thus requires the use of appropriate whole-of-grain and sample-average internal dose rates for grains with composite mineralogies deposited in these contrasting sedimentary contexts. More generally, greater consideration needs to be given to the internal dose rates of feldspar grains in volcanic regions such as Island Southeast Asia, especially composite grains with complex mineralogies, to ensure that the calculated ages are accurate.

在长石的光学定年中，通常认为富含钾（K）的长石颗粒已经通过密度分离的方法进行了发光测量。这些晶粒的K和rub（Rb）浓度提供了显着的内部β剂量率。剂量率和年龄计算通常假设分别为12.5±0.5 wt％和400±100 ppm的K和Rb浓度，但是这种假设可能是错误的。（Th）和铀（U）也有助于内部剂量率的β和α成分。我们调查了印度尼西亚梁布亚的四个沉积物样品中各个发光长石颗粒的K，Rb，Th和U浓度，以确定单颗粒内部剂量率和适当的样品平均内部剂量率。使用X射线衍射光谱仪（XRD）对“富含钾”的长石成分进行了研究，结果发现该成分在每个样品中均受到石英和斜长石长石的严重污染。使用多种技术对具有可检测到的红外后红外激发光（pIRIR）信号的谷物进行了表征，包括能量色散光谱（EDS），波长色散光谱（WDS）以及使用EDS（QEM-EDS）对矿物进行定量评估。发现谷物由多种矿物组成，包括长石，石英，粘土矿物和重矿物以及火山玻璃。使用QEM-EDS测定了这些复合晶粒的K浓度，以及它们的Rb，使用激光烧蚀电感耦合等离子体质谱法（LA-ICP-MS）确定Th和U浓度。每个样品中大多数谷物的计算内部剂量率都非常低（<0.2 Gy.ka对于180-212μm直径的晶粒为^-1），尽管内部剂量率也分布广泛。没有一个样品显示内部剂量率与pIRIR信号强度或等效剂量（D _e）。发现两个碎屑沉积物样品的内部剂量率分布与两个特非拉样品的内部剂量率分布不同。因此，要使梁布瓦的沉积物样品具有可靠的年龄，就需要对这些沉积环境中沉积有复合矿物的谷物使用适当的全粒和样品平均内部剂量率。更普遍地，需要更多地考虑火山岛如东南亚地区的长石颗粒的内部剂量率，特别是具有复杂矿物学的复合颗粒，以确保计算出的年龄是准确的。