Sources of overdispersion in a K-rich feldspar sample from north-central India: Insights from De,K content and IRSL age distributions for individual grains

Luminescence dating of individual sand-sized grains of quartz is a well-established technique in Quaternary geochronology, but the most ubiquitous mineral on the surface of the Earth—feldspar—has received much less attention at the single-grain level. In this study, we estimated single-grain equivalent dose values and infrared stimulated luminescence (IRSL) ages for K-rich feldspar (KF) grains from a fluvial sample underlying Youngest Toba Tuff (YTT) deposits in north-central India, and compared these ages (corrected for anomalous fading) with those obtained from individual grains of quartz from the same sample. Both minerals have broadly similar single-grain age distributions, but both are greatly overdispersed and most grains have ages substantially younger than the expected age of the YTT deposit (～74 ka). Almost half (45%) of KF grains used for age calculation have fading rates statistically consistent with zero, but the age distribution of these grains is as dispersed as that of the entire population. We obtained a similar distribution of ages calculated for 51 grains using their individually measured internal K contents, which exhibited only minor grain-to-grain variation. Given the lack of dependency of single-grain ages on the measured fading rates and internal K contents, and the overall adequacy of bleaching of grains collected from a sandbar in the modern river channel, we consider the spread in ages is most likely due to mixing, at the time of deposition and after the YTT event, of potentially well-bleached fluvially-transported sediments with older grains derived from slumping of riverbank deposits. Some spread may also be due to natural variations in the IRSL properties of individual KF grains.