Elsevier

Radiation Measurements

Volume 47, Issue 9, September 2012, Pages 696-702
Radiation Measurements

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

https://doi.org/10.1016/j.radmeas.2012.04.005Get rights and content

Abstract

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.

Highlights

► A single-grain IRSL age distribution is derived for a K-feldspar sample from India. ► The range of individual K-feldspar ages is similar to that obtained from individual quartz grains. ► Age overdispersion is attributed mostly to mixing of river-transported grains with older, slumped riverbank deposits. ► Almost half of measured K-feldspar grains exhibit negligible rates of anomalous fading. ► Grain-to-grain variations in K content and fading rate have little effect on K-feldspar age overdispersion.

Introduction

Luminescence dating procedures that make use of multi-grain aliquots of quartz or feldspar, and in which more than one grain contributes significantly to the luminescence signal, implicitly assume that all contributing grains have suitable luminescence properties for dating and have been sufficiently bleached by sunlight before burial, and have not been mixed after burial. Single-grain optically stimulated luminescence (OSL) dating techniques have been available for quartz for more than a decade, enabling the identification of partially bleached grains or intrusive grains from overlying or underlying sedimentary units, as well as the rejection of grains with unsuitable characteristics for reliable dose determination using single-aliquot regenerative dose (SAR) procedures (Galbraith et al., 1999; Jacobs and Roberts, 2007). Similar single-grain dating techniques are rarely used for potassium (K)-rich feldspars but would also entail measurements of the internal dose rates (due principally to 40K) and anomalous (athermal) fading rates of individual grains (Duller et al., 2003). Risø readers equipped with a 150 mW infrared (IR) (830 nm) laser allow for the direct stimulation of individual feldspar grains (Duller et al., 2003). In this paper, we describe the use of the IR laser to obtain single-grain equivalent dose (De) values and fading-corrected ages for K-rich feldspar (KF) grains from a fluvial sample collected from a sand unit underlying Youngest Toba Tuff (YTT) deposits in the Middle Son Valley, Madhya Pradesh, India. This study is part of a larger luminescence dating program to assess the time of deposition of the alluvial deposits and YTT ash in the Middle Son Valley. Experiments were conducted on two additional samples (one from the same geological section, and one from a modern sandbar in the Son River channel) to assess potential sources of overdispersion (OD) in the KF grain age distribution. The impact of single-grain fading rates and K contents on the KF single-grain age distribution is also examined.

Section snippets

Samples

IR stimulated luminescence (IRSL) measurements were made on three samples (GHO-2, GHO-3, and KHUT-10). Sample GHO-2 was collected from a well-drained, medium-coarse fluvial sand unit that underlies YTT deposits in a cliff section on the north bank of the Son River (24° 30′ 7.608″ N, 82° 1′ 2.748″ E) (Jones, 2010). The YTT deposit at this location is thought to have been deposited ∼74 ka ago (Jones, 2010; Gatti et al., 2011; Smith et al., 2011) but the ash here has not been dated directly.

Natural IRSL signals

Most KF grains (60–80%) from sample GHO-2 are characterized by bright initial signals (commonly greater than 40,000 counts in the first 0.134 s) that decay rapidly upon laser stimulation, but fail to reach a constant background (Fig. 1a). Approximately 20–40% of grains from sample GHO-2 exhibit dim and very slowly decaying, IRSL signals (Fig. 1a, inset). Elemental (microprobe) analyses (Section 6.4) show that the bright grains are most commonly orthoclase and some are plagioclase, and the dim

De determination and sources of overdispersion

The single-grain De distribution of sample GHO-2 is shown in Fig. 2b and the decay curves for two extreme values are plotted in Fig. 2c; the latter exhibit no obvious differences in shape. Of 1149 measured grains, 475 (41%) passed all rejection criteria (Table S4). The weighted mean (CAM) De of all accepted grains is 52.7 ± 1.6 Gy, with an OD of 46.5 ± 1.5% (Table 1). The OD and relative spread in values of this sample are much larger than those obtained in the dose recovery test, which we

Fading measurement procedures

Fading tests for both single grains and aliquots in this study followed the procedure of Auclair et al. (2003). After De measurement of single grains, each grain (still located in the same hole) was stimulated repeatedly with the IR laser after being given a laboratory dose of 34 Gy (Lx) and following a series of delay times after irradiation and preheating (Table S5). Each Lx measurement was immediately followed by a test dose (14 Gy) measurement (Tx) to correct the Lx signal for sensitivity

Single-grain and multi-grain KF ages

The fading-corrected single-aliquot and single-grain age distributions for sample GHO-2 are shown in Fig. 4a, and the calculated weighted mean (CAM) ages are shown in Table 1. The single-grain weighted mean age for all grains is 29.3 ± 1.3 ka and this is consistent with the weighted mean age of 29.3 ± 1.7 ka for 24 aliquots of the same sample. The multi-grain aliquot age OD (20.0 ± 3.5%) is smaller than that of the single-grain age (37.3 ± 1.5%), and this is likely due to averaging effects of

Discussion and conclusions

We have reported KF single-grain De and fading-corrected ages for a fluvial sand sample collected from north-central India, and considered some potential sources of OD: namely, grain-to-grain variations in luminescence properties, residual doses at deposition, anomalous fading rates and internal K contents. The dose recovery test suggests that at least 6.9% of the OD in De may be attributed to natural variations in luminescence properties of the grains. The impact of incomplete bleaching of

Acknowledgements

This project was funded by an ARC Discovery Project grant to Roberts and by University of Wollongong scholarships to Neudorf. Sébastien Huot is thanked for advice and Excel macros for fading correction, and Kevin Grant and Norman Pearson are thanked for their advice and assistance with WDS measurements. José Abrantes is thanked for help with sample preparation for microprobe measurements. We also thank Michael Petraglia and Jagannath Pal for logistical support and archaeological advice.

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