An improved radiofluorescence single-aliquot regenerative dose protocol for K-feldspars

Quaternary Geochronology 38 (2017) 13e24

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Research paper

An improved radiofluorescence single-aliquot regenerative dose protocol for K-feldspars
bastien Huot b, Sebastian Kreutzer c, Christelle Lahaye c, Marine Frouin a, *, Se Michel Lamothe d, Anne Philippe e, f, Norbert Mercier c a

Research Laboratory for Archaeology and the History of Art, University of Oxford, South Parks Road, OX1 3QY, United Kingdom Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, 615 E. Peabody, Champaign, IL 61820, United States IRAMAT-CRP2A, UMR 5060 CNRS, Universit
e Bordeaux Montaigne, Maison de l'Arch
eologie, Esplanade des Antilles, 33607 Pessac Cedex, France d
 Montr
Departement des sciences de la Terre et de l'atmosph ere, Universit
e du Qu
ebec a eal, 201 avenue du Pr
esident-Kennedy, H2X 3Y7 Montr
eal, Qu
ebec, Canada e Laboratoire de math
ematiques Jean Leray, UMR 6629 CNRS, Universit
e de Nantes, 2 rue de la Houssini ere, 44322 Nantes Cedex, France f ANJA INRIA, Campus universitaire de Beaulieu, Avenue du General Leclerc, 35042 Rennes Cedex, France b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 January 2016 Received in revised form 13 October 2016 Accepted 16 November 2016 Available online 18 November 2016

Measuring the infrared radiofluorescence (IR-RF) signal of K-feldspars, which is thought to be not affected by any anomalous fading, potentially provides a promising alternative dating approach to the more studied IR and pIRIR signals. Here we report a series of experiments aimed at characterising the IRRF signal, which led us to propose an improved IR-SAR (Infrared Radiofluorescence Single-Aliquot Regeneration) protocol. In comparison with the original protocol from Erfurt and Krbetschek (2003a), the proposed one relies on IR-RF signal measurements and bleaching experiments conducted at elevated temperatures. Stabilising the temperature at 70
C during the measurements helps keeping the very shallow traps empty during the process. Internal protocol tests and dating applications are presented using known age samples. We show that the improved protocol (named RF70) is capable to reproduce known age results within uncertainties. However, considering the slow bleaching rate of the RF70 signal, the multi-grain approach is likely not the most suitable for the De determination of complex samples. © 2016 Elsevier B.V. All rights reserved.

Keywords: Luminescence dating K-feldspars Radiofluorescence Elevated temperature

1. Introduction Since the pioneering work of Wintle (1973), it is well known that the thermally stimulated luminescence (TL) signal of feldspars is affected by anomalous fading, leading to systematic age underestimations. The same drawback is observed when these minerals are stimulated at low temperature (typically < 60
C) with infrared (IR) photons, which led to the development of various methods aimed at correcting the IRSL ages (Huntley and Lamothe, 2001; Lamothe et al., 2003). More recently, post-IR IRSL approaches (henceforth pIRIR) were introduced by Thomsen et al. (2008). The pIRIR signal is measured at elevated temperature (usually, 225
C or 290
C) after an initial IR measurement at low temperature, in order to directly circumvent or at least minimize the fading problem.

* Corresponding author. Research Laboratory for Archaeology and the History of Art, University of Oxford, South Parks Road, OX1 3QY, United Kingdom. E-mail address: [email protected] (M. Frouin). http://dx.doi.org/10.1016/j.quageo.2016.11.004 1871-1014/© 2016 Elsevier B.V. All rights reserved.

In 1998 an alternative approach using irradiation (b- or g-irradiation, X-rays) instead of optical stimulation had been proposed by Trautmann et al. (1998) and Schilles and Habermann (2000). These authors observed and documented an infrared radiofluorescence (IR-RF) emission at 1.44 eV (855 nm) occurring during a continuous b-irradiation, later reported at 1.43 eV (865 nm) by Erfurt (2003). It is believed that this signal corresponds to the trapping of electrons during irradiation (Pb2þ þ b, g, Pbþ; Erfurt, 2003), and because the number of available traps decreases with time, the IR-RF signal follows a decreasing monotonic (stretched) exponential law against the duration of irradiation until a stable level is reached. This behaviour was explained by the model of Erfurt and Krbetschek (2003a; their Fig. 2): the IR-RF intensity is proportional to the probability of trapping an electron. Hence, a high signal intensity is associated to a low density of trapped electrons. With the accumulation of dose, the IR-RF signal decreases in intensity as the electron trap fills up, with natural or laboratory-induced exposure. In both cases, the processes are assumed as being directly comparable, even though the dose rates and the type of irradiations differ

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considerably. Erfurt and Krbetschek (2003a) were the first to propose a singlealiquot regenerative dose protocol to determine an equivalent dose (De) using the IR-RF signal, named IR-SAR (Infrared Radiofluorescence Single-Aliquot Regeneration). With this protocol, the De is obtained in 3 main measurement steps: (1) an additive irradiation is given while the IR-RF signal is measured, (2) the sample is optically bleached in order to empty the traps and after a pause, (3) a second irradiation is performed and the IR-RF regenerated signal measured. Despite its relative simplicity, this protocol has only been used in a small number of dating studies (Trautmann et al., 1999; Erfurt et al., 2003; Degering and Krbetschek, 2007; Wagner et al., 2010; Novothny et al., 2010; Lauer et al., 2011; Buylaert et al., 2012a; Kreutzer et al., 2012a). The purpose of our research is to test the suitability of the IR-RF signal to provide reliable ages by measuring K-feldspars grains from chronologically controlled samples. We recently investigated the behaviour of the IR-RF signal subjected to different lighting conditions in order to define the optimum bleaching parameters (Frouin et al., 2015). We recommended using a complete solar spectrum within the luminescence system. In parallel, it was showed that at least two shallow traps coexist in K-feldspars and contribute to the IR-RF signal (Huot et al., 2015). Then, it was advised to maintain the sample at an elevated temperature during the entire measurement sequence (i.e., additive dose, bleaching, regenerative dose) to exclude these shallow traps from taking part of the IR-RF signal. We present here an improved radiofluorescence SAR protocol, henceforth termed RF70, for dating K-feldspars, which builds on the previously proposed recommendations. We investigate in details the effect of elevated temperature measurements on the IR-RF signal of K-rich feldspar samples. Additionally, an analysis routine was implemented in the R ‘Luminescence’ package (Kreutzer et al., 2012b; R Luminescence Developer Team, 2016) in order to properly estimate IR-RF equivalent doses and their associated standard errors.

2. Material and methods 2.1. Selected samples and sample preparation We selected and retained samples originating from different geological contexts and covering a wide range of natural accumulated doses. Most of them were derived from presumably wellbleached environments (aeolian, loess, marine beaches), while some may have been only partially bleached. Moreover, two samples were constrained by independent dating techniques (Radiocarbon and U/Th); for the others, IR-RF results can be compared to quartz OSL ages. Details and references are provided in Table 1. The coarse K-feldspar grains were extracted and purified using commonly accepted methods: wet sieving and hydrochloric acid (10 % HCl) followed by a hydrogen peroxide (30 % H2O2) treatment for 24 h. After separating heavy minerals using heavy liquid solution (LST, density > 2.7 g cm3), the K-feldspar-rich fraction was then isolated using a density lower than 2.58 g cm3. We replicated the heavy liquid separation three times in order to increase the purity. One sample (C5) was further purified with a Frantz magnetic separator. Samples BT714 and BT715 had been further etched in HF (10 %) for 40 min and rinsed in HCl and purified water afterwards; this treatment is likely capable to remove the outer a-irradiation affected layer of the grain (cf., Aitken and Fleming, 1972; Porat et al., 2015). Sample BT706 neither underwent a second density separation (isolating the K-feldspar fraction) nor was it further etched.

2.2. Measurement setup Infrared-radiofluorescence measurements were carried out on a lexsyg research device (Richter et al., 2013) equipped with a 90Sr/90Y ring shaped irradiation source (Richter et al., 2012). This source is specifically designed for IR-RF measurements and its shape grants the most spatially uniform irradiation plane across the sample. The dose rate delivered to K-feldspar grains is 3.9 Gy min1 at the sample position (see section 3.5). For signal detection, the reader is equipped with a near infrared sensitive photomultiplier tube (PMT, Hamamatsu H7421-50), installed with a thermoelectric cooling system. Luminescence is passing through a plano-convex silica lens and a Chroma D850/40 interference filter with a FWHM of 20 nm, thus recording an emission between 830 nm and 870 nm. This filter encompasses the IR-RF peak at 1.43 eV (865 nm) and suppresses the influence of nearby peaks emissions at 1.35 eV (915 nm) and 1.73 eV (715 nm). For optical resetting (bleaching), we relied on a solar simulator fitted inside the lexsyg research system. It facilitates 6 high power, individually controlled, LEDs of different wavelengths: 365 nm (max. 80 mW cm2), 462 nm (max. 160 mW cm2), 525 nm (max. 60 mW cm2), 590 nm (max. 45 mW cm2), 623 nm (max. 115 mW cm2) and 850 nm (max. 280 mW cm2). All measurements were performed in a nitrogen atmosphere. The K-rich feldspar grains were dispensed on 9.95 mm diameter stainless steel cups. All of our experiments were performed by continuously preserving the thermal contact between the heating and the aliquot (Richter et al., 2013). 3. Testing the IR SAR protocol In the dating applications published so far using the IR-SAR protocol, few IR-SAR ages have been compared against independent ages. Nevertheless, these applications showed that the IR-RF signal seems to make possible the dating of sediments up to 300 ka (e.g., Degering and Krbetschek, 2007; Krbetschek et al., 2008). These initial studies, conducted by the German group headed by the late M. Krbetschek, took advantage of a fully automated multispectral radioluminescence system based on a Daybreak 1100 TL reader (Erfurt et al., 2003). More recently, a radioluminescence attachment for IR-RF was also developed for the Risø TL/OSL DA 20 reader (Lapp et al., 2012; Buylaert et al., 2012a). Both systems have strikingly different conceptual designs, but in practice the radiofluorescence measurement is carried out in the same manner: 1. Measure the IR-RF intensity of a natural dose, 2. Bleach the sample and wait, 3. Measure the regenerated IR-RF signal. The main differences between both research groups resided in i) how the De was calculated and ii) how to properly artificially bleach the IR-RF signal. 3.1. Data analysis e which approach? During each IR-RF measurement two curves are recorded: (1) the natural signal (RFnat) and (2) the regenerated signal (RFreg). To obtain the De from these measurements two methods have been described in literature (Fig. 1): 1) fitting a stretched exponential function to the observations (Erfurt and Krbetschek, 2003b) or 2) a sliding both curves onto each other (Buylaert et al., 2012a). With the curve fit approach the best parameters (f0, Df, l and b) are calculated for a stretched exponential function representing the observed data, where f0 represents the initial IR-RF intensity, Df, a dose dependent change in IR-RF intensity, l, an exponential parameter and b, a dispersive factor. The equivalent dose corresponds to the interpolated natural signal (RFnat) into the stretched curve. With the second method, the natural IR-RF curve is horizontally translated on the regenerated dose axis until it overlaps

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Table 1 Samples used for testing the performance of the radiofluorescence protocol. Lab code

Origin

Reference

SAR0

Sardinia - Aeolian

Lamothe, unp. data

TH0

Morocco - Aeolian

Bouab, 2001

IC388

DenmarkeBeach deposit


rin et al., 2015a Gue

TH8a

Morocco - Aeolian

Bouab, 2001

BT706 BT714

Germany e Loess Germany - Loess

Meszner et al., 2013 Meszner et al., 2013

BT715

Germany - Loess

Meszner et al., 2013

RDM11

France - Colluvium


rin et al., 2012 Gue

FER3

France - Colluvium


rin et al., 2015b Gue

C5

PerueBeach deposit

Pedoja et al., 2006

TML1

Fluvial

Luminescence dating

Age control

Grain size [mm]

M.

Signal

De ± se [Gy]

Dose rate [Gy ka1]

Age [ka]

Method

Age [ka]

180e250 180e250 125e250 125e250 180e250 180e250 180e250 125e250 125e250 125e250 4e11 4e11 4e11 4e11 4e11 4e11 4e11 180e250 180e250 180e250 180e250 180e250 180e250 180e250 180e250 125e250 125e250 125e250

KF KF KF KF Q KF KF Q KF KF Q Q PM PM Q PM PM Q KF KF KF Q Q KF KF KF KF KF

IR50 fc pIRIR290 IR50 fc pIRIR290 blue OSL IR50 fc pIRIR290 blue OSL IR50 fc pIRIR290 blue OSL blue OSL IR50 fc pIRIR225 fc blue OSL IR50 fc pIRIR225 fc blue OSL IR50 fc pIRIR225 fc pIRIR290 blue OSL blue OSL sg pIRIR160 pIRIR290 IR50 fc pIRIR290 IR50

~0 4±1 0.18 ± 0.01 2±1 4.7 ± 0.2 6.9 ± 0.3 14 ± 1 76 ± 9 83 ± 4 119 ± 22 66 ± 2 104 ± 1 79 ± 1 95 ± 1 250 ± 5 195 ± 15 245 ± 1 97 ± 1 67 ± 3 86 ± 6 103 ± 6 60 ± 2 65 ± 4 105 ± 6 173 ± 15 76 ± 11 222 ± 35 >3000

3.5 ± 3.5 ± 1.9 ± 1.9 ± 1.2 ± 1.6 ± 2.1 ± 1.3 ± 1.7 ± 1.7 ± 3.6 ± 3.5 ± 3.9 ± 4.2 ± 3.4 ± 3.9 ± 4.3 ± 1.4 ± 2.2 ± 2.2 ± 2.2 ± 1.6 ± 1.6 ± 2.4 ± 2.4 ± 1.4 ± 1.4 ± n.d.

~0 ~0 ~0 ~0 3.8 ± 0.3 4.3 ± 0.3 6.7 ± 0.4 58 ± 8 50 ± 4 70 ± 10 18 ± 3 29 ± 4 28 ± 3 28 ± 4 73 ± 8 70 ± 8 72 ± 10 68 ± 4 39 ± 4 47 ± 4 48 ± 3 38 ± 2 41 ± 3 44 ± 3 73 ± 7 76 ± 18 163 ± 24 e

e e e e e e e e e e e e e e e e e TL TL e e AMS 14C e e e U/Th e e

e e e e e e e e e e e e e e e e e 45 ± 4 61 ± 7 e e 47e44b e e e 85 ± 1 e e

1.0 1.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

M. ¼ Mineral (Q ¼ quartz, KF ¼ K-feldspar, PM ¼ polymineral). fc ¼ fading corrected; sg ¼ single grain. a Some investigations have been done on this sample but the RF age has not been measured due to a too small amount of available material. b Three samples (S-EVA-26506- 26507 -26508) cal. ka BP the lowest and the higher estimation at 95.4 %.

Fig. 1. The principle of the sliding method using synthetic IR-RF curves. Curves of this figure are produced according to the equation given in Erfurt and Krbetschek (2003b): f ðtÞxF0  DF½1  expð  ltb . Parameters are set as follows: F0 ¼ 1; l ¼ 2:274 $ 103 ; b ¼ 7:6 $ 101 ; DF ¼ 1 with F0 the initial RF flux, DF the dose dependent change of the RF flux, l the decay parameter and b the dispersion factor (cf. Erfurt and Krbetschek, 2003a for further details on the physical meaning of the parameters). In this example the natural curve (in red) is deduced from the regenerative curve (in green). Noise was added to both curves independently using the R function base::jitter() for illustrative reasons only. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. De determination using two bleaching settings A) using a external solar simulator SOL 500 during 4 h and B) using a solar spectrum during 30 min in the lexsyg research system.

M. Frouin et al. / Quaternary Geochronology 38 (2017) 13e24

the regenerated curve. The length of the sliding vector is taken as the equivalent dose (Prescott et al., 1993; Lapp et al., 2012). Buylaert et al. (2012a) and Varma et al. (2013) favoured this method over the fitting approach, as it does not rely on a specific physical assumption as to which model best describes the IR-RF vs applied dose relationship. Furthermore, due to the amount of individual data used, the results can also be considered as statistically more robust. The details on the estimation of the De and its standard error are described in supplementary data. In the present study, most of the data analyses and graphical displays were done using the free and open source statistical programming language R (R Development Core Team, 2016). In doing so, we aim to achieve a transparent and reproducible procedure. The analysis routine for estimating the De and its corresponding standard error (se(De)) was implemented in the R function analyse_IRSAR.RF()as part of the R package ‘Luminescence’ (Kreutzer et al., 2012a; 2012b; R Luminescence Developer Team, 2016; version: >¼ 0.6.0). Within this function, several additional test parameters (e.g., f0, Df, l and b obtained from fitting using a stretched exponential function) are calculated during each analysis and can be used to define rejection criteria. For details on the available test parameters the reader is referred to the manual of the R package ‘Luminescence’ published under General Public Licence (GPL) conditions. The reader is referred to the supplementary material for further details concerning the data analysis. 3.2. Which bleaching setting? In the first IR-RF bleaching experiments (Trautmann et al., 1999), natural direct sunlight (VIS þ UV component) was used. The authors reported that few hours (between 2 and 5 h) are sufficient to reach the maximum IR-RF signal level (corresponding to the total bleach). Later, further bleaching experiments have been conducted using a 300 W OSRAM ‘Ultravitalux’ sunlamp placed at a distance of 35 cm from the aliquot for 6.5 h (Krbetschek et al., 2000) or an onboard lamp (250 W OSRAM metal halide), connected to a fiber optic, which delivered ~100 mW cm2 to the aliquot for 30 min (Erfurt et al., 2003). Recently, Buylaert et al. (2012a); (202b) used a UV LED (delivering 700 mW cm2) with an exposure duration of 25 min. However, they showed that the IR-RF signal could be more €nle SOL 2 solar simulator for completely bleached using a 4 h Ho light exposure instead of the UV LED bleach. We decided to compare two approaches using: i) the onboard € nle SOL solar simulator of the lexsyg research device and ii) an Ho 500 solar simulator (having the same spectral distribution than the €nle SOL2). Bleaching for 30 min with the lexsyg's solar simulator, Ho using settings for a complete solar spectrum, should delivery the same amount of energy as 5 h of natural sunlight as used by Trautmann et al. (1999). A series of 6 samples were tested, whose natural dose is expected to range from zero dose (SAR0, TH0) to the “field saturation” (TML1), in passing through intermediate doses (TH8, C5, FER3). More details on these samples are given in Table 1. The IR-RF measurement consisted of recording the signal emitted during irradiation for a total of 200 Gy (with a 0.7 Gy data resolution per channel) e called additive IR-RF signal - followed by bleaching and a 1 h pause (to avoid any phosphorescence) and finally recording the regenerated IR-RF signal while irradiating up to 500 Gy. €nle SOL 500 Fig. 2a and b shows the results obtained with the Ho simulator for 4 h and the onboard solar spectrum for 30 min, respectively. Using the external solar simulator resulted in a mismatch between the shapes of both curves for 2 out of the 6 tested samples (SAR0 and TH0). In such a case, it is not possible to calculate a De. Moreover, for three samples (TH8, C5 and FER3) the obtained De values overestimate largely the expected dose. This

17

strongly indicates that the chosen bleaching time is likely not sufficient to fully reset the IR-RF signal. Conversely, using the solar spectrum provided by the onboard equipment, the curve shapes of the additive and regenerative signals match for all samples, which suggests non-kinetic changes. Moreover, the IR-RF De values of the modern samples are zero, as expected, but they are slightly overestimated for 3 older samples (TH8, C5, FER3). It is worth noting that in both cases for the saturated sample (TML1), the additive signals appear to have a far slower decay than the one of other samples; i.e. the sample is in dose saturation. Finally, we recommended using a complete solar spectrum within the luminescence system with a longer time exposure and proposed the following setting: 365 nm (10 mW cm2), 462 nm (63 mW cm2), 525 nm (54 mW cm2), 590 nm (37 mW cm2), 623 nm (115 mW cm2) and 850 nm (96 mW cm2) for 3 h (see Frouin et al., 2015). 4. Improving the IR-SAR protocol 4.1. Investigating preheat effects The thermal stability of the IR-RF signal was investigated for three samples (TH0, TH8 and C5). The same experiment was also carried out on another fraction of TH0 (a modern sample that has a ~0 Gy natural dose), irradiated beforehand with a 200 Gy dose in the laboratory. In the first part of the experiment, pulsed annealing measurements were carried out by applying successive and increasing thermal treatments, from 30
C up to 500
C (with 30
Ce60
C increments), while holding the temperature for 10 s at each step (filled circles in Fig. 3). The samples were cooled down to 30
C before measuring the IR-RF signal. Each signal reported here was recorded during a short b-irradiation (5 s, i.e. ca. 0.3 Gy), hence keeping the delivered dose at a minimum while providing a statistically representative signal, named here sIR-RF (for short IR-RF). At the end of this series (filled circles; i.e. after reaching 500
C), the aliquots were optically bleached (for 3 h according to Frouin et al., 2015), followed by a 1 h pause (to avoid unwanted signal contribution due to phosphorescence; Erfurt, 2003), and the sIR-RF signal was subsequently measured at 30
C (crosses in Fig. 3). Then, a 200 Gy regenerated dose was given and another pulsed annealing series was measured (open black circles; Fig. 3). Finally, these measurements were repeated (open grey light circles; Fig. 3). Fig. 3 shows that for temperatures between 30
C and 230
C, the sIR-RF signal measured during the first part of the experiment (Fig. 3, filled circles) remained stable for the naturally dosed samples, indicating that the signal is not dependent on the thermal treatment between these limits. These observations are consistent with previous published experiments by Erfurt (2003), who also showed a thermally stable IR-RF signal at least up to 250
C. For the TH0 þ 200 Gy sample (Fig. 3b), the same pattern was observed (i.e. stable up to 230
C during the first pulse annealing cycle). After that plateau, the sIR-RF intensities decrease both for the natural and dosed samples. Comparing signal measurements taken after the second step of this experiment (crosses) with those obtained after a pulse annealing treatment performed at 500
C indicates that the bleaching (followed by the 1 h pause) did not change the intensities of the sIR-RF signals. During the third part, the sIR-RF signal was measured after a 200 Gy dose had been delivered and was found to be lower than the previous one (crosses), except for sample C5. Next, the following sIR-RF signals tend to increase slightly with temperature and the latest, measured after a pulse annealing at 500
C, was found to be identical to those measured during the first part of this experiment. This behaviour is observed for all samples.

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Fig. 3. Pulsed annealing experiments on four samples. The temperature was held successively from 30
C to 500
C for 10 s. The IR-RF signal was measured for a short duration (5 s; sIR-RF). One aliquot is used here, for each sample. It was first measured on naturally-dosed samples (filled circles) and repeated twice on a 200 Gy regenerated dose (open circles). At the end of each series, the aliquots were measured again after an optical bleach followed by a 1 h delay (crosses). The measurements are normalized to the intensity of the first sIR-RF signal (integrated over 5 s). The dashed horizontal lines delineate the plateau regions.

However, for sample C5, it can be observed an initial decrease in the sIR-RF intensities, from 30
C up to 150
C, for the natural sample (Fig. 3d) and similarly but to a least extent, after the sample had been irradiated with a 200 Gy dose. This behaviour, not observed for the other samples, is not yet understood and will require further investigation to explain. These experiments indicate that an annealing at high temperature (here 500
C) seems to have an irreversible effect on the measured IR-RF intensities. This is in accordance to what was commonly reported in the thermoluminescence dating of feldspars (e.g., Aitken, 1985, 1998). Following a high temperature treatment, a drastic decrease in luminescence sensitivity is observed between the natural (or additive) TL signal and the regenerated one, which is not repeatable on successive regenerated dose measurements. Bearing in mind that the IR-RF signal is attributed to a recombination within the electron trapping defects (e.g., Erfurt, 2003), any changes in terms of IR-RF signal intensities imply changes within the density of available electron traps. 4.2. Investigating stimulation temperatures Whereas in classic OSL protocols, a low preheat temperature is easy to perform after the irradiation and before the OSL measurement, allowing the eviction of electrons from shallow traps (~200
C), this cannot be applied in a IR-RF protocol since the irradiation and measurement of the radiofluorescence signal occur at the same time. Moreover, a scheme where one would convert a long and continuous irradiation into a sum of smaller increments bisected by low temperature preheats, such as the method of pulsed irradiations (e.g. for quartz, Bailey et al., 2005), could be

implemented, but would be excessively time consuming and impracticable. Recent developments of the lexsyg research system makes it possible to irradiate a aliquot on the heating plate with a continuous temperature control (cf., Poolton et al., 2001). We performed IR-RF measurements from the ambient (~30
C) up to 250
C (Fig. 4). The temperature was stabilised for 10 s before opening the b-source shutter. The data are normalized either with the highest signal (Fig. 4a) or with the IR-RF intensity recorded between 100 Gy and 120 Gy (Fig. 4b). The IR-RF signals and the overall curvature measured with low temperatures (below 100
C) are very similar. At 150
C and 200
C, the curves are still a close similar but the signals are slightly higher during the first seconds of stimulation. At 250
C, the IR-RF curve shows a slower decrease, after the initial steep and rapid decrease. At every stimulation temperature we observe a rapid rise in the IR-RF intensity at the onset of irradiation, which becomes increasingly more apparent with higher temperatures. The curvature in the beginning of irradiation was also progressively steeper with higher temperatures. This is reminiscent of the peak-shaped observation of Huot et al. (2015). These rapid rise and fall would therefore be due to a rapid thermal depletion from the shallow traps. Actually, at 250
C, the thermal lifetime of electrons held in traps seem to be considerably reduced. The dosimetric trap depths of the widely used signals (TL/IRSL) of K-feldspars are still relatively loosely defined and range from 1.4 eV up to 2.5 eV (Duller, 1997; Trautmann et al., 2000; Jain et al., 2015). This large range is due, in part, to our inability to properly define the process of electron eviction: (1) via an excited state, (2) a localized transition or (3) direct tunnelling. Additionally, it is unclear of whether the conduction band is involved or if the electron

M. Frouin et al. / Quaternary Geochronology 38 (2017) 13e24

19

immediately release electrons. This flat signal was observed for all samples at 250
C (Fig. S4), which suggests the inability of the IR-RF signal to decrease or to reach a steady intensity as it is for a sediment having received a very large, saturating dose, in the environment (Trautmann, 2000; Frouin et al., 2015). As suggested by Huot et al. (2015), it would be advisable to irradiate at an elevated temperature (>70
C) to maintain empty very shallow traps (~0.3 eV and 0.8e0.9 eV) during irradiation. Also, from the current elevated temperature measurements, we suggest maintaining temperature below 150
C in order to avoid significant thermal depletion during IR-RF measurements.

4.3. De plateau determinations and RF70 protocol

Fig. 4. The RF signal decay curve for different stimulation temperatures, for sample SAR0. A) Signals are normalized with the maximal and B) with the intensity between 100 and 120 Gy.

hops along donoreacceptor recombination pairs or band-tail states (McKeever and Chen, 1997; Poolton et al., 1995). In the case of the IR-RF signal however, we can rely on a 1.43 eV value, since it corresponds to its luminescence peak emission at 865 nm. It was tentatively assigned to the dissociation of Pb2þ to Pbþ during interaction with ionising radiation occurring in the Kfeldspar lattice (Nagli and Dyachenko, 1986, 1988; Erfurt, 2003). With a trapping depth of 1.43 eV and a conservative frequency factor of 1013 s1 (Aitken, 1985), the thermal lifetime of 6 s when the sample is held at 250
C, which is too short for exhibiting any significant retention of electrons within the trap. It suggests that we had approached a balance between the charges ‘accumulation’ rate in the trap, due to continuous irradiation, and the charges ‘escaping’ rate from it. Consequently, even at very high doses, there is still a large IR-RF intensity as the dosimetric traps capture and

Following our observations of heating effects, we modified the original IR-SAR protocol by Erfurt and Krbetschek (2003a) to include thermal stabilization of the sample prior to irradiating at an elevated temperature (Table 2). The sample was held for 10 min before the opening of the shutter, and the equivalent doses obtained for different irradiation temperatures between 30
C and 150
C (Fig. 5) were measured for both the natural and the regenerated signal. The natural signal was measured for 5000 s (~320 Gy) and the regenerated signal for 10000 s (~640 Gy). For comparison, we also report the mean De value obtained from the standard IR50 fading-corrected signal (black line) or from the pIRIR160 signal (for sample FER3) along with the pIRIR290 signal (grey line). In every case, the pIRIR290 Des overestimate the De values obtained with other IR/pIRIR methods, indicating that the pIRIR290 signal of these samples has probably not been bleached enough at the time of deposition. For the modern sample TH0 (aeolian sand deposit), the RF De is consistent with zero between 30
C and 130
C but is slightly underestimated at 150
C. For sample TH8 (aeolian), the De values are in good agreement with the fading-corrected IR50 De for temperatures between 30
C and 70
C. At higher temperature, the RF De values trend toward the pIRIR290 results. For FER3 (colluvium) and C5 (sandy marine beach), the IR-RF came out as a close match to the pIRIR290 De which substantially over-estimate their respective pIRIR160 and IR50 Des. Nevertheless, these RF De values seem to be independent on the irradiation temperature, between 30
C and 110
C. This effect is less visible for sample C5. As the temperature has been demonstrated to have an influence on the IR-RF curve shape, we suggest that temperature be stabilized during the entire measurement process. The measurement temperature should be sufficient to keep the very shallow traps empty. In view of the thermal stability of the IR-RF signal and plateau test results, the range of 70e110
C appear to be most suitable. We choose to retain an irradiation temperature of 70
C, which also corresponds to the temperature spontaneously reached by the thermocouple during the bleaching step (Huot et al., 2015). For simplicity, and to distinguish it from Erfurt and Krbetschek's

Table 2 The RF70 measurement protocol. During bleaching a simulated spectrum was utilised (i.e. 365 nm: 10 mW cm2, 462 nm: 63 mW cm2, 525 nm: 54 mW cm2, 590 nm: 37 mW cm2, 623 nm: 115 mW cm2 and 850 nm: 96 mW cm2). Step

Treatment

1. 2. 3. 4. 5. 6.

Preheat (70
C for 900 s) IR-RF at 70
C Bleaching for 3 h Pause for 1 h Preheat (70
C for 900 s) IR-RF at 70
C

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M. Frouin et al. / Quaternary Geochronology 38 (2017) 13e24

Fig. 5. De plateau test. The equivalent doses are from a three aliquots average per temperature. Before each IR-RF mesurement, the temperature was held for 10 s in order to allow thermal stabilization. The dashed lines delineate the plateau regions.

protocol, we name this approach “RF70” (Table 2). The protocol comprises three main steps and the equivalent dose is obtained by sliding the added growth curve, obtained during the first step, onto the regenerated one (third step). This protocol includes as well in step two the bleaching of the sample with a simulated solar spectrum, for 3 h, followed by a pause of at least 1 h at ambient temperature to make sure all post-bleach phosphorescence had time to decay away (Erfurt, 2003). Both added and regenerated IR-RF measurements should be measured at the same elevated temperature, e.g., 70
C. Alternatively, one could elevate the temperature to 110
C during each measurement step, but it would increase the measurement duration, without improving the De's estimation. 4.4. “Bleachability” of the RF70 Luminescence signals used in optical dating must bleachable in natural conditions, such that the signal is completely reset after a reasonable duration of sunlight exposure. Here, we characterize the bleaching rates of different luminescence signals (IR50, pIRIR225, pIRIR290 and RF70) emitted by K-feldspar grains from a representative sample (C5). To compare these rates, a series of aliquots was exposed to a solar simulator SOL 500 for different durations, from 1 min to 5 h, before the residual dose of those signals was measured using the stated protocols: for the IR50 signal, the SAR protocol comprises measuring the luminescence signals at 50
C for 100 s after a 250
C preheat for 60 s following each irradiation, and an IR bleaching (at 30
C for 300 s) was used at the end of each cycle to reset any signal recuperation. For the measurement of the pIRIR signals at 225
C and 290
C, an additional IR stimulation at the chosen elevated temperature has been added after the first IR50 stimulation following Thomsen et al. (2008) and Buylaert et al. (2012b). The details of each protocol used in this experiment are

given in Table S1. For each bleaching duration and each SAR protocol, the De's have been measured on three aliquots. Fig. 6 demonstrates that the RF70 signal is considerably reduced during light exposure: after 1 h, the intensity is reduced to ~20 % of its original value, and after 3 h, there is 2 % remaining only. Overall, the RF70 seems to bleach faster than the pIRIR290 but much slower than the classical IR50 signal. Then, it is likely that the RF70 could then be of much use in a depositional environment providing total bleaching of the grains. 4.5. Source calibration During the course of this study, the 90Sr/90Y b-source employed in the lexsyg research reader was explicitly calibrated for the IR-RF measurements. To the best of our knowledge, this is the first time this has been done for IR-RF measurements of coarse grain Kfeldspars. 4.5.1. Sample preparation and irradiation For the calibration, the modern analogue sample TH0 (K-feldspar, 125e250 mm) was retained. Its chemical composition was measured using a scanning electron microscope (supplementary data). The sample was split into two subsamples prior to irradiation with an external g-source: TH0natural and TH0bleached. The first batch (TH0natural) was prepared for irradiation without any bleaching pretreatment. By contrast, subsample TH0bleached was placed on a series of stainless steel cups routinely used for luminescence measurements without mounting in silicone oil in order to obtain a monolayer of grain. Subsequently, each cup was treated with the same bleaching step (3 h using the onboard solar simulator) as used within the RF70 protocol for the De estimation. The material was then carefully removed from each cups and packed in a small and

M. Frouin et al. / Quaternary Geochronology 38 (2017) 13e24

21

from the R ‘Luminescence’ package. For example, Fig. 7 shows the results of the RF measurements for ten samples. In Fig. S7 all normalised RFnat and RFreg are shown. The shape of the RFnat curves appear to be sample dependent and directly correlated with the obtained De. RFreg curves show a lower variability in curve shapes for all samples, although single IR-RF curves scatter considerably for one sample. The reason for this observation is so far unknown,

Fig. 6. Bleaching curves of the IR50, pIRIR225, pIRIR290 and RF70 signals of the sample C5. The doses have been normalized to the first measured dose in the inset. The € nle SOL 500. bleaching has been done with a solar simulator Ho

opaque capsule for irradiation. With the chosen procedure we intended to test for unwanted effects (e.g., sensitivity changes) caused by the bleaching itself and to have at least one batch of irradiated material where the IR-RF signal of all grains have been fully reset. The irradiation was carried out at the Laboratoire des Sciences du Climat et de l’Environement at Gif-sur-Yvette (France), using a 137Cs g-source delivering a dose rate of ~0.9 Gy min1. Both batches (TH0natural, 56.43 mg and TH0bleached, 44.44 mg) were exposed to a g-dose of 56.02 Gy (cv ~2 %) at room temperature following standard procedures (Valladas, 1978). 4.5.2. Analysis For both subsamples, 10 aliquots were measured and the results are presented in the supplement of this contribution (Fig. S6). The delay between irradiation and measurement was 13 days for sample TH0bleached and 16 days for sample TH0natural. The measurements performed using the RF70 protocol indicated no significant differences (two-sided t-test, p-value: 0.3671) between the two batches and the whole values were merged for calculating the b-source dose rate. It was found to be 0.065 ± 0.006 Gy s1 (mean ± standard deviation) at the date for calibration for the feldspar sample, a value indistinguishable from the quartz dose rate (0.059 ± 0.001 Gy s1) within errors measured with the same source in applying the usual SAR protocol to coarse quartz grains (Risø calibration quartz batch 90; Hansen et al., 2015). 5. Results and discussion Equivalent doses have been determined using the RF70 protocol on 10 samples. Between 5 (TH0) and 25 aliquots (BT714, BT715), each one consisting of hundreds of grains (fixed on a stainless steel cup with silicon spray) were measured depending on the amount of material available. Depending on the sample, the natural luminescence signal was recorded for 1000 s up to 3600 s (corresponding to a dose of ~64 Gye~230 Gy, respectively, with a resolution of 15 s per channel) and the regenerative signal for 7200 s up to 10000 s (~460 Gye~640 Gy respectively). Only for TML1, one aliquot has been measured using an additive signal of ~650 Gy and a regenerated signal to ~4150 Gy. For each aliquot, the De value has been obtained using the function analyse_IRSAR.RF()

Fig. 7. De determination using the proposed RF70 protocol.

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M. Frouin et al. / Quaternary Geochronology 38 (2017) 13e24

Table 3 Estimated ages with the RF70 protocol. Aliquots were reject if RFnat >> RFreg. Lab code

Grain size [mm]

n

De ± se [Gy]

Dose rate [Gy ka1]

SAR0 TH0 IC388 BT706 BT714 BT715 RDM11 FER3 C5 TML1

180e250 125e250 180e250 90e200 90e200 90e200 180e250 180e250 125e250 125e250

6/10 4/5 3/20 10/10 22/25 20/25 9/10 10/10 10/10 1/1

1.2 ± 1.9 0.5 ± 1.0 2.3 ± 3.5 107 ± 32 86 ± 38 158 ± 97 139 ± 72 308 ± 24 272 ± 44 2127 ± 5

3.5 1.9 2.1 3.8 3.5 3.5 2.2 2.4 1.4 e

± ± ± ± ± ± ± ± ±

1.0 0.1 0.1 0.3 0.3 0.2 0.1 0.1 0.1

Age RF70 [ka] 0.3 ± 0.6 0.3 ± 0.5 1.1 ± 1.7 28 ± 9 25 ± 11 45 ± 28 63 ± 33 128 ± 11 194 ± 34 e

n ¼ number of aliquots.

but it is possible that a superimposition of two competing processes (trap filling and depletion) may contribute to the observed signal behaviour (cf. Trautmann, 2000; Kreutzer et al., 2015). However, this effect requires further investigation. The unweighted mean De with their associated standard error and final ages are reported in Table 3. For each sample (excepted TML1), the single aliquot dose distribution is reported in Fig. S8. For the 2 modern samples, TH0 (n ¼ 4/5) and SAR0 (n ¼ 6/10), it is notable that most of their De values are ranging between 0 and 2 Gy (100 % and 80 %, respectively) confirming that the IR-RF signal is well bleachable in natural conditions. Conversely, the field-saturation sample TML1 gave a De of ~2000 Gy (n ¼ 1/1) lying at 96 % of the lab saturation level. This

example may indicate that the IR-RF signal should allow the dating of old deposits, up to 1e2 Ma (considering a dose rate between 1 and 2 Gy ka1). For samples FER3 and C5, the relative standard error around the mean is low, less than 20 %, as it is for sample BT706 as well if one excludes the aliquot having a De value significantly higher than the mean. This observation suggests that when a sample has been homogeneously bleached, the RF70 protocol provides reproducible measurements. However, the other samples (BT714, BT715 and RDM11) yielded strongly scattered De values, ranging from almost 0 Gye300 Gy. Samples with highly scattered De values show a high variety of curve dynamic ranges for the RFreg curves, where samples with a more narrow De distribution (e.g., FER3, C5, BT706) do not.

Fig. 8. A) Plot of RF70 ages against control ages. The inset shows the data from 0 to 40 ka for clarity. B) Plot of the RF70 ages against OSL quartz ages and (C) against feldspar IR, pIRIR ages.

M. Frouin et al. / Quaternary Geochronology 38 (2017) 13e24

Although this effect is clearly visible, rejection criteria based on this observation was not applied because differences in the curves are not well understood. However, comparing these results with the mean De values obtained with the OSL, IR50 and pIRIR protocols (see Table 1), the low doses obtained with the RF70 protocol are likely meaningless and, since three of these are loess deposits samples (BT706, BT714 and BT715), it seems difficult to invoke the possibility that a part of the grain population had been exposed to a very low dose rate. As a consequence, these low doses can only be explained by the fact that the RF70 protocol is not well adapted to these samples. At the moment, we do not have any straightforward explanation for this, but we observed that the aliquots giving low doses were systematically dim in comparison to the other aliquots and, as stated above, the dynamic range of RFreg is small (cf. Fig. 7). It is possible that the difference in brightness between aliquots could reflect differences in mineralogy. In these cases, the mean De value has been calculated and the corresponding age evaluated considering the available information in the publications. For all our samples, a K-content of 12.5 ± 0.5 % (Huntley and Baril, 1997) was applied to account for the internal dose rate. This chosen K-content value is in agreement with element concentration results obtained by energy dispersive X-ray spectroscopy (EDX) for three samples (TH0, FER3, BT706; cf. Supplement). In this study, the dose rate calculation was carried out using the fixed a-value of 0.08 ± 0.01 (e.g., Rees-Jones and Tite, 1997). Nevertheless, this a-value might not be representative for the investigated IR-RF signal. Studies by Biswas et al. (2013) and Kreutzer et al. (2014) showed that the a-value likely varies for different signals from feldspar, and there is no obvious reason to believe that the a-value is similar to that reported for the IR50 signal on polymineral fine grain. However, this issue is beyond the scope of this study. Mean De values, dose rates and ages are reported in Table 3. The RF70 ages are plotted against control agesfor the 9 samples in this study in Fig. 8a. For most of the samples the RF70 age agrees with at least one widely use protocol. For clarity, the RF70 ages are plotted against the OSL quartz ages in Fig. 8b and against the IR, pIRIR ages in Fig. 8c. For all samples, the RF70 ages are similar (within 2s) to the expected ages previously obtained on quartz or feldspars using different signals. In particular, there is good agreement for the young samples (<10 ka). For samples BT714 and BT715, the RF70 ages are slightly lower than the other ages but, as discussed previously, this might be explained by the fact we did not exclude the low De values obtained for these samples. For FER3 which is known
rin et al., 2015b), the RF70 age is to be a poorly bleached sample (Gue higher than the quartz OSL and feldspar IR50 ages and surprinsingly higher than the pIRIR290 age. For C5 sample however, the RF70 and pIRIR290 ages are in good agreement. These observations would support the idea that, as the pIRIR290 signal, the RF70 signal does not need any fading correction as reported in previous dating studies. A direct fading measurement would be welcome however. 6. Conclusions and perspectives In this study we performed experiments aimed at optimizing the IR-SAR protocol proposed by Erfurt and Krbetschek (2003a). A measurement temperature of 70
C was chosen and bleaching experiments allowed to compare the bleachibility of the IR50, pIRIR225, pIRIR290 and RF70 signals. We show that the RF70 signal is reset by sunlight at a rate lower than the IR50 signal but greater than the pIRIR290 signal. Moreover, a low residual dose is achievable €nle solar simulator, and measurements in 3 h when using a Ho performed on two modern samples confirm that the RF70 signal can also be fully reset in natural conditions.

23

The dating results using samples of known ages suggest that the RF70 signal gives reliable ages without any evidence of systematic age underestimation. Compared to other methods currently available (IR, pIRIR) requiring systematic fading correction and/or residual dose subtraction, the RF70 signal appears as a more simple, statistically robust and accurate way for K-feldspars dating and opens up new investigation fields. However, a systematic trap parameter study should be initiated in order to compare the relation between IR-RF, IRSL and pIRIR signals and their origin in the minerals and try to explain the observed differences in the curve shapes. Nevertheless, considering the slow bleaching rate of the RF70 signal, the multi-grain approach is likely not the most suitable approach for the De determination using this signal and future investigations should focus on systems equipped with devices able to identify signals coming from single grains, such as EMCCD camera, in order to determine individual RF De values. Acknowledgments The authors wish to thank Gilles Guerin (LSCE, Gif-sur-Yvette, France) for carrying out the g-irradiation on the samples used for the b-source calibration. Yannick Lefrais (IRAMAT-CRP2A, Pessac, France) is thanked for his help with the EDX analysis, Guillaume
rin for giving us access to the inter-comparison sample IC388 Gue and Jean-Luc Schwenninger is thanked for his support. The authors are grateful to the Aquitaine Region Council for funding this research, through the program entitled “La radioluminescence des feldspaths: un nouvel outil de datation des gisements arch
eologiques et des s
equence quaternaires d’Aquitaine”. This work was also supported by the French National Research Agency via the LaScArBx Labex (project number ANR-10-LABX-52). MF thanks the “PalaeoChron”project via the European Research Council (grant 324139) for her support. SH acknowledged funding from the CFI-FEI (Canada Foundation for Innovation) for funding this research. SK thanks the DFG (German Research Foundation, SCHM 3051/3-1) for keeping the R Luminescence Developer Team so well connected. ML thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for continued support in his research. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quageo.2016.11.004. References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Aitken, M.J., Fleming, S.J., 1972. Thermoluminescence dosimetry in archaeological dating. Radiat. Dosim. 1, 1e78. Bailey, R.M., Armitage, S.J., Stokes, S., 2005. An investigation of pulsed-irradiation regeneration of quartz OSL and its implications for the precision and accuracy of optical dating (Paper II). Radiat. Meas. 39, 347e359. Biswas, R.H., Williams, M.A.J., Raj, R., Juyal, N., Singhvi, A.K., 2013. Methodological studies on luminescence dating of volcanic ashes. Quat. Geochronol. 17, 14e25.
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