Infrared radiofluorescence (IR-RF) dating: A review
Introduction
In the late 1990s, Trautmann et al. (1998) characterized radioluminescence signals, the emission stimulated by ionizing radiation, from various feldspar specimens to investigate their potential for dating applications. Focusing first on radiation-induced emissions in the UV and yellow wavelength ranges, where luminescence signal increases with radiation dose, they incidentally observed the opposite for potassium bearing (K-) feldspar specimen such as microcline and orthoclase: a dose-dependent signal decrease of the emission centred at 854 nm (1.45 eV; based on peak tail fitting only). Later, Trautmann et al. (1999a, b), Schilles (2002) and Erfurt and Krbetschek (2003a) determined that the emission peak was centred at 865 nm (1.43 eV). Trautmann et al. (1998) recognized that this emission energy is similar to the excitation energy used for infrared stimulated luminescence (IRSL; Hütt et al., 1988) and consequently interpreted this process as a luminescent transition (trapping) of electrons. Their pioneer work paved the way to what is known today as infrared radiofluorescence (IR-RF) dating, the method first proposed by Trautmann et al. (1999a, b) and termed by Erfurt and Krbetschek (2003a, b). The IR-RF signal is believed to be a direct measure of the fraction of empty electron traps, unlike conventional luminescence dating methods such as those based on thermoluminescence (TL; cf. Aitken, 1985a), optically stimulated luminescence (OSL, Huntley et al., 1985; Aitken, 1998) and infrared stimulated luminescence (IRSL, Hütt et al., 1988), for which the signals are associated with more complex recombination pathways. Since the IR-RF signal intensity decreases with increasing dose, it can be used for dosimetry and dating purposes. IR-RF may provide advantages over conventional single aliquot regenerative (SAR; Murray and Wintle, 2000) dose, IRSL dating methods (e.g. SAR IRSL; Wallinga et al., 2000) or its derivatives deploying higher reading temperatures (post-IR IRSL, Thomsen et al., 2008; MET-pIRIR, Li and Li, 2011a) in terms of required measurement time (relatively short protocol, cf. Erfurt and Krbetschek, 2003b), resolution of the dose-response curve (continuous recording of data points) and dating range (Sec. 7). Nevertheless, IR-RF as a dating method is still subject to ongoing research, with its general applicability being questioned (Buylaert et al., 2012). On the contrary, recent technological and methodological work, e.g. on the optical resetting of the IR-RF signal, and improved routines for dose estimation have yielded promising results (Frouin, 2014; Frouin et al., 2015, 2017; Huot et al., 2015; Kreutzer et al., 2017; Murari et al., 2018).
This contribution provides an overview of past and recent developments of IR-RF from K-feldspar as a dating method. We summarize current knowledge on existing models on the origin of IR-RF, outline commonly applied measurement procedures and equipment, and highlight shortfalls, challenges and open questions. Understanding what remains unknown may stimulate discussions and lead to improved experimental designs towards a full establishment of IR-RF as a valuable chronological tool.
Section snippets
Origin of the IR-RF signal and relevant models
The 1.43 eV (865 nm) IR-RF emission of K-feldspar (Trautmann et al., 1999a, b; 1.45 eV in Trautmann et al., 1998, see below for an explanation) has the same energy as the typical excitation maximum of IRSL (Hütt et al., 1988; Poolton et al., 2002a, b), which led Trautmann et al. (1998) to suggest that both signals are derived from the same principal electron trap. More recent observations of infrared photoluminescence (IR-PL; Prasad et al., 2017) seem to support this hypothesis (see also Kumar
Measurement devices
The first IR-RF studies were carried out on home-made systems (Trautmann et al., 1998, 1999a; Schilles and Habermann, 2000; Erfurt et al., 2003), which differ from the ready-to-use systems available today (Lapp et al., 2012; Richter et al., 2013). The device described in Trautmann et al. (1998) was equipped with a spectrometer for K-feldspar IR-RF exploration. However, because the spectrometer was limited to 800 nm (Trautmann et al., 1998, 1999b), first peak investigations were based on signal
Sample preparation methods
IR-RF sample preparation methods extract K-feldspar enriched mineral grains following routine procedures (e.g. Preusser et al., 2008). After sieving and chemical treatments with HCl and H2O2, density separation using heavy-liquids (e.g. lithium heteropolytungstate or sodium polytungstate) extracts feldspar grains. Additional (froth) flotation (e.g. Herber 1969; application examples: Miallier et al., 1983; Sulaymonova et al., 2018) can be applied to enrich the K-feldspar concentration further; a
IR-RF spectroscopy: signal identification and measurement optimization
The IR-RF signal composition was extensively studied using a home-made spectrometer in combination with a liquid nitrogen-cooled charged coupled device (CCD) detector (Trautmann et al., 1998, 2000b; Krbetschek and Trautmann, 2000). Their spectroscopic investigations of IR-RF from feldspar revealed many peaks centred at various wavelengths. However, the 865 nm emission peak was found to be stable with its intensity decreasing with increasing dose. K-feldspar showed the highest signal intensity
Measurement protocols and data analysis
Like other luminescence measurement protocols for equivalent dose (De) determination (e.g. for TL, OSL or IRSL), over the years, several measurement protocols and data analysis techniques have been proposed to determine the De for IR-RF.
Application of IR-RF dating
The broader use of IR-RF as a dating method for sediments started mainly after introducing the IRSAR protocol by Erfurt and Krbetschek (2003b). Signal saturation levels of >1000 Gy (e.g. Erfurt and Krbetschek, 2003b and Sec. 5.7) favoured IR-RF dating applications on Middle Pleistocene sediments which are generally beyond the quartz OSL dating limit. The typical dose saturation for quartz OSL measured in the UV is reached around 150–200 Gy (Wintle and Murray, 2006) except for a few cases where
Summary and future directions of IR-RF dating
Overall, regardless of ambivalent dating results in some studies, IR-RF appears to be a promising but somewhat overlooked dating method on K-feldspars. The status quo renders a picture with several, potentially, game-changing advantages but without a significant breakthrough because those benefits are not received as significant enough by dating practitioners. On the other hand, IR-RF poses a bunch of open questions and challenges that are yet to overcome.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to Sébastien Huot, Frank Preusser and two anonymous reviewers for their patience, constructive comments, and strong support for this manuscript. M.K. Murari and M. Fuchs were supported by the German Research Foundation (DFG FU417/19-1). S. Kreutzer and N. Mercier received financial support from the LaScArBx. LaScArBx is a research programme supported by the ANR (ANR-10-LABX-52). The contribution of S. Kreutzer in 2020 and 2021 received funding from the European Union's Horizon
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