Establishing a pIRIR procedure for De determination of composite mineral grains from volcanic terranes: A case study of sediments from Liang Bua, Indonesia

Abstract Volcanic sediments are generally difficult to date using luminescence dating methods, but many important archaeological and palaeoanthropological sites are located in volcanic regions. Here we present an improved dating procedure for grains with composite mineralogies deposited at Liang Bua, the type locality of Homo floresiensis in Indonesia, using the post-infrared infrared stimulated luminescence (pIRIR) signal. Individual mineral grains that emit detectable pIRIR signals are composed of a range of feldspar varieties, quartz, clay minerals, heavy minerals and volcanic glass, rendering the isolation of individual potassium-rich feldspar grains infeasible. We investigate the luminescence behaviour of these composite mineral grains in detail, including their thermal stability, anomalous fading and dose-response characteristics. A standardised growth curve is developed to enable more time-efficient measurements, together with a ‘micro-aliquot’ approach in which each hole on a disc contains approximately 8–10 grains. Less than 1% of grains yield detectable pIRIR signals when measured individually, so the use of micro-aliquots provides, in effect, a means of estimating the equivalent dose (De) at single-grain resolution. Our results show that the pIRIR signal measured at 275 °C is suitable for estimating De values of these composite grains, without the need for residual dose or fading corrections.