Red emission luminescence from quartz and feldspar for dating applications: an overview

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Radiation Measurements 37 (2003) 383–395www.elsevier.com/locate/radmeas

Red emission luminescence from quartz and feldspar fordating applications: an overview

Stephen Stokesa ;∗, Morteza Fattahia;baOxford Luminescence Research Group, School of Geography and the Environment, University of Oxford, Mans�eld Road,

Oxford OX1 3TB, UKbInstitute of Geophysics, Tehran University, Tehran, Iran

Received 23 August 2002; received in revised form 10 January 2003; accepted 13 February 2003

Abstract

In dating applications, detection of thermal or optically stimulated (IR & visible) luminescence is overwhelmingly biasedtowards the UV-blue portions of the electromagnetic spectrum. This bias is based on a number of factors including thegenerally good performance of UV-blue emission luminescence as an integrating dosimeter for quartz over the time rangeca. 0–150 ka, the historical development of blue-sensitive photomultiplier tubes and problems in the separation of longerwavelength emissions from thermal and other background contributions. Research into the use of orange-red and far-redluminescence emissions has, however, also progressed.

Applications to quartz have been limited to investigating the thermoluminescence (TL) dating of volcanic sediments.Application of red TL to known age volcanic demonstrated that the dosimeter may have potential for routine dating of burntor 6red quartz back to more than 1 Ma. The potential advantages of using feldspathic minerals as integrating dosimeters forutilization in TL and optically stimulated luminescence research have been complicated by the widespread observation ofanomalous fading. Red TL from feldspar (� ∼ 600–750 nm) has been demonstrated not to exhibit anomalous fading. Thefar-red IRSL is bleachable by IR (�=830 nm) or via polychromatic daylight; the measured IRSL being accurately described bythe sum of three exponential components. Experimentation using the single-aliquot regeneration (SAR) protocol demonstrates,in contrast to some studies using the UV-blue IRSL emission, that it is possible to use a test dose to accurately monitorsensitivity changes in the course of experimental procedures, and that synthetic doses may be reconstructed. Short-term fadingexperiments on samples that exhibit signi6cant anomalous fading in the UV-blue portion of the emission spectrum suggestthat anomalous fading is absent or signi6cantly reduced.c© 2003 Published by Elsevier Ltd.

Keywords: Optical dating; Thermoluminescence, Red emission, Quartz; Feldspar

1. Introduction

Over approximately the past decade there have been sig-ni6cant developments in luminescence, and particularly, op-tical dating. These developments have embraced re6nementsin both apparatus and dating methodology (e.g., Banerjeeet al., 2001; B@tter-Jensen et al., 2000; B@tter-Jensen andMurray, 2001; Bulur et al., 2000; Duller et al., 1997, 1999,

∗ Corresponding author. Tel.: +44-1865-271-914;fax: +44-1865-271-929.

E-mail address: [email protected](S. Stokes).

2000). Single-aliquot and single-grain approaches for theevaluation of equivalent doses (De’s) from quartz have, inparticular, radically advanced the 6eld and provide bases forboth moderate to high-precision De estimation and interro-gation of the homogeneity of absorbed dose within a sedi-mentary grain sample population (e.g., (Duller, 1995; Fuchsand Land, 2001; Murray and Wintle, 2000a; Stokes et al.,2000, 2001). While these developments have signi6cantlycontributed to a more widespread acceptance and utilisa-tion of optical dating (e.g., Colls, 1999; Goudie et al., 1999;Juyal et al., 1998; Lian and Shane, 2000; Rich and Stokes,2001; Roberts et al., 2001; Stokes et al., 1998; Thomaset al., 1997; Wintle, 1999), they have not provided a means

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384 S. Stokes, M. Fattahi / Radiation Measurements 37 (2003) 383–395

of overcoming two key outstanding challenges: namely, theroutine extension of the age range of luminescence beyondca. 150 ka, and the circumvention of anomalous fading infeldspars.

Routine extension of the age range of luminescence dat-ing beyond the typical dose saturation limit of sedimentaryquartz (ca. 150–200 Gy) has been attempted by a num-ber of researchers via a variety of strategies. Yoshida et al.(2000) tried to use bright (so-called ‘supergrains’) grainswith high dose saturation characteristics to date old Aus-tralian samples. While they were able to demonstrate that asmall (¡ 1%) portion of single grains fromwithin their sam-ples did exhibit relatively high dose saturation, they werenot able to reconstruct reliable dose histories and it is possi-ble that the bright grains actually correspond, at least in part,to residual feldspar grains which might therefore be subjectto anomalous fading (Duller, pers. commun.). Murray andWintle (2000b) tried to extend the age range by focussing onthe optically bleachable portion of the 375◦C peak in quartz.They used a strong thermal pretreatment to remove the opti-cally sensitive signal associated with the quartz 325◦C peak,but were unable to obtain results which indicated realisticpotential for this signal as a routine method. Signal deconvo-lution and component striping, using both continuous wave(CW) and linear modulation (LM) optically stimulated lu-minescence OSL from quartz has also been attempted (e.g.,Bailey, 2000; Singarayer et al., 2000). In earlier work, Mi-allier and colleagues (e.g., Miallier et al., 1991, 1994a,b,c;Pilleyre et al., 1992) demonstrated using multiple aliquotprocedures that the near red emission in quartz from burntsediments may extend age range considerably.

The use of feldspathic minerals in optical dating ap-plications has declined due to the apparently ubiquitousoccurrence of anomalous fading in the UV-blue portionsof their thermal and optically stimulated luminescenceemissions (e.g., Duller, 1997; Huntley and Lamothe, 2001;Lamothe et al., 1994). And while research programs seek-ing to identify, correct, or avoid anomalous fading in thisemission band are ongoing (e.g., Huntley and Lamothe,2001, Lamothe and Auclair, 1999), fading remains a ma-jor limitation, and this, in combination with the failureof attempts to apply single-aliquot regeneration (SAR)techniques (e.g., Wallinga et al., 2001) has largely prohib-ited feldspar-based dating applications. A possible novelmethod by which anomalous fading could be avoided wassuggested by Visosekas and co-workers (e.g., Zink andVisocekas, 1997; Visocekas and Zink, 1999; Visocekas,2000). They exploited the red emission band from relativelylow temperatures (i.e., ¡ 300◦C) and demonstrated goodagreement with independent age control for a selection ofvolcanic deposits (Visocekas and Zink, 1999).

While the presence of red luminescence emissions fromboth quartz and feldspar has been known for some time (e.g.,Hashimoto et al., 1986, 1996; Bos et al., 1994; Prescott et al.,1994; Montret et al., 1992; Miallier et al., 1994a; Zink andVisocekas, 1997; Visocekas and Zink, 1999; Franklin et al.,

2000), the major developments of the past decade in appa-ratus and De determination have focussed exclusively on theuse of emission in the UV-blue wavelength band from boththe minerals. The lack of focus on red emissions is gener-ally attributable to a mixture of the historical precedentialuse of UV-blue-sensitive bialkali photomultipliers (PMTs),and the perception that their detection is highly problem-atical (e.g., Duller, 1997). Over the past 3 years, we havesought to generate a more complete understanding of the redemissions from both quartz and feldspar as a potential meanof extending the age range of luminescence techniques forboth 6red and sedimentary deposits, and to con6rm theexistence of a non-fading signal in feldspars. While a fullrealisation of these objectives is some way oP, by incorpo-rating extended range PMTs and novel stimulation sourceand signal 6lter combinations we have demonstrated that it ispossible to observe high-temperature (up to 500◦C) thermaland optical red emission luminescence (Fattahi and Stokes,2000a, b, in press a-d). Furthermore, these emissions appearto be highly suited to long-range dating applications (e.g.,Lai et al., 2003; Arnold et al., 2003).

This overview summarises previous research into the util-isation of red emission luminescence for dating applications.We outline the initial studies which developed and appliedred luminescence from both quartz and feldspar, explain thebasic modi6cations which we have undertaken to allow thedetection of high-temperature and optical anti-Stokes redemission luminescence, and illustrate some of the initial ex-periments which we have undertaken to quantify aspects ofthe behaviour of the signal in quartz and feldspar. We seemuch promise in the utilisation of these signals and hope thatour 6ndings might catalyse other researchers to follow upthese and other experiments. More extensive discussions ofred emissions in quartz and feldspar can be found in Fattahi(2001) and Fattahi and Stokes (2000a, 2003a), and refer-ences therein. Discussion of more detailed aspects of the lu-minescence mechanism in alkali feldspar may be found inKirsch et al. (1987), Finch and Klein (1999), Brooks et al.(2002) and references therein.

2. Red emission luminescence from quartz and feldspar

Spectral analysis readily identi6es the existence of redthermoluminescence (TL) emissions for both quartz andfeldspar (Fig. 1). Irradiated quartz grains show TL peaksof variable intensity in various emission bands (Fig. 2). Ofthese bands, only three have been used for luminescencedating: a band between 360 and 440 nm (near UV to vio-let), another between 460 and 500 nm (blue to green) andthe last between 620 and 650 nm (orange to red). The blueTL of quartz possesses a broad emission band with a peakaround 470 nm, and is derived from phenocrystals ofvolcanic origin or from pegmatite or plutonic rockscontaining �-quartz that presumably mineralised at lowtemperatures. The red TL of quartz, which posses a broad

S. Stokes, M. Fattahi / Radiation Measurements 37 (2003) 383–395 385

Fig. 1. Examples of 3-D emission spectra of quartz (a) and sanidine(potassium feldspar) (b). Redrawn from Zink et al. (1995)(a) and Krbetschek et al. (1997) (b). Both spectra clearly showthe presence of a signi6cant higher temperature TL emission aboveca. 600 nm.

emission band with a peak at around 620 nm, has beenobserved in quartz from volcanic ash layers containing�-quartz mineralised beyond the �–� transition point at573◦C. Mixtures of both red and blue luminescing grainshave been observed in quartz extracted from sediments(e.g., Hashimoto et al., 1987, 1993b, 1996; Krbetscheket al., 1997).

Laboratory irradiated TL glow curves of feldspar displaya composite wide curve (e.g., Fig. 1b). The nature of TL

peaks is highly variable from sample to sample, reRectingtheir mineralogy and feldspar structure (Fig. 3). For exam-ple, while mixed K/Na and Na/Ca feldspars are characterisedby low intensities, and TL decreases with increasing Caconcentrations, potassium-rich and sodium-rich feldsparsusually show the highest TL peaks (Prescott and Fox, 1993;Krbetschek et al., 1997). The brightest luminescence emis-sions have been found in disordered feldspars (Krbetscheket al., 1997). There are a variety of diPerent TL emissionsin feldspars (e.g. 275–290, 320–340, 390–440, 450–490,580–660, 700–760 nm) that display composite TL peaksat diPerent temperatures. Red emission is a prominentcomponent in many feldspar TL spectra and may often beobserved as the strongest component of a TL spectrum inmaterials such as sanidine or intermediate K–Na or Na–Cafeldspars (Krbetschek et al., 1997). The presence of orange(� ca. 580–660 nm), red-IR (� ca. 700–760 nm) and IR(�¿ 800 nm) emissions in feldspar TL has been shown inseveral studies (e.g., Kirsch et al, 1987; Huntley et al., 1988;Krbetschek et al., 1997). The far-red band has a Gaussianspectrum near IR around 700–710 nm with a maximum at1:7 eV (ca. 710 nm) (Krbetschek et al., 1997). Other stud-ies (e.g., Kirsch et al, 1987) have noted that the red peakemission wavelength may vary and consist of up to threediscrete sub-bands at 700, 738 and 765 nm; shifts in peakposition being related to major element (K–Na–Ca) com-position of the feldspar species and the inRuence (includingbinding location) of Fe3+ impurities.

It is important to note that while researchers have referredto the longer visible wavelength emissions from both quartz(�peak ca.∼ 620–640 nm) and feldspar (�peak ∼ 710 nm) as‘red’, it is only the emission band observed within feldsparwhich lies fully within the red region of the electromagneticspectrum. As detection of these contrasting long-wavelengthemission bands prompt the use of separate PMT/6lter com-binations, we have adopted a more appropriate terminol-ogy which recognises this contrast. Hereafter, following thenomenclature of Krbetschek et al. (1997), we refer to the620 nm emission from quartz as orange-red luminescence,and the ca. �=710 nm emission from feldspar as the far-redluminescence emission.

3. Development of improved red emission luminescencedetection systems

Previous attempts to observe red TL have been compli-cated by two factors. Firstly, the presence of considerablethermal background noise (for temperatures greater than350◦C, the incandescence of the sample and heater platehas an intense emission at wavelengths of ¿ 790 nm) hasresulted in glow curve temperatures being limited to amaximum temperature of approximately 400◦C when us-ing a bialkali PMT (Miallier et al., 1994c; Schole6eld andPrescott, 1999). Considerably lower temperatures whenfeldspar samples have been heated and observed using an


Fig. 2. Common TL emissions (0–300◦C) in quartz (modi6ed from Krbetschek et al., 1997).

Fig. 3. Common TL emissions (0–300◦C) in feldspar (modi6ed from Krbetschek et al., 1997).

extended range trialkali PMT (e.g., Zink and Visocekas,1997). Secondly, the relatively low-luminescence sensitiv-ity of RTL has typically limited its detection to doses inexcess of 50 Gy (e.g., Miallier et al., 1991; Visocekas andZink, 1999), limiting its applicability for Late Pleistoceneand Holocene deposits.

These problems have been countered by the developmentof a high-sensitivity detection system enabling Fattahi andStokes (2000a, b) to observe quartz and feldspar RTL up to500◦C, and to detect doses below 10 Gy (Table 1). They em-ployed an actively cooled (−15◦C) Electron Tubes extendedrange S-20 PMT attached to a standard RISO TL/OSLreader (Fig. 4) and combinations of either Schott OG-590and Corion FR400S or Omega 625D50 6lters to observethe orange-red emission in quartz. Detection of the far-redTL emission from feldspars using the S-20 PMT is compli-cated by its high quantum eTciency at longer wavelengths(Fig. 5) and accordingly an adapted, Electron Tubes mid-range D716A (‘green’) may results in lower backgrounds

and improved signal-to-noise ratios for some samples(Table 1).

4. Orange-red luminescence from quartz

Since Fleming (1970) established the quartz inclusiontechnique, quartz has been widely used for TL dating ofunheated and heated sediment (Aitken 1998; Stokes, 1999;Wintle, 1997). The UV-violet emissions of quartz com-monly used for dating applications constitute only a mi-nor fraction of the total TL signal (e.g. Kuhn et al., 2000).The orange-red TL of quartz in the range 600–650 nm hasbeen detected in almost all synthetic and natural quartz (e.g.,Krbetschek et al., 1997; Schole6eld and Prescott, 1999).This signal has primarily been used for dating volcanic (i.e.thermally-reset) samples (e.g., Miallier et al., 1994a) andhas been exploited for provenance studies (e.g., Hashimotoet al., 1986).


Table 1Summary of PMT characteristics

Advantages/application Disadvantages

‘Blue’ Low QE in IR (¡ 0:01% at Low QE in red (0:1% at 630 nm)Electron tubes 9635 830 nm) Poss. red TL -low to v. low count rates‘Red’ High QE in Red (15% at High QE in IR (2.5% at 830 nm)Electron tubes 630 nm). -High dark noise at room temperature (requires cooling)9658 (S-20) Good for red TL -High R-IR background‘Green’ Electron tubes Med QE in IR (ca. 0.01% at Med QE in red (2.5% at 630 nm)D716A 830 nm). -High dark noise at room temperature (requires cooling)(S-11) Good for far-red IRSL

Fig. 4. Photograph of extended S-20 PMT and cooling jacket at-tached to an RISU TL/OSL reader.

Published, principally low temperature (T ¡ 350◦C),spectral data of orange-red TL exhibit considerableinter-sample variability. Schole6eld and Prescott (1999)reported frequent red peaks in 14 Australian samples ataround 135◦C=1:96 eV, 205◦C=1:97 eV, 305◦C=2:00 eVand 355◦C=2:05 eV (at a heating rate of 5◦C=s1) after labo-ratory irradiation. In the context of TL dating, Miallier et al.(1991, 1994b, c) studied the properties of one orange-redTL peak, lying in the range 380–395◦C (heating rate of5◦C=s1), using an EMI 9635Qa PMT and a narrow band 6l-ter (ORIEL 610 FS) or a long pass 6lter (RG 610). Miallieret al. (1991) observed orange-red TL peaks at around 600–620 nm, while heating to approximately 400◦C, in about

Fig. 5. PMT quantum eTciencies.

15 quartz samples from various geological origins (mostlyxenolithic quartz grains heated by volcanic activities). Theyfound that a sharp TL peak appeared after preheating at∼ 320◦C for 10 s (e.g., Miallier et al., 1994b, Fig. 1). Theyobserved that the peak was reproducible and usually shifteda few degrees towards lower temperatures as the dose wasincreased (e.g., Miallier et al., 1991, 1994b). Miallier et al.(1991) found no saturation level for their samples’ growthcurve for doses up to 50 kGy and no evidence of fading.

The above promising results, led to quartz orange-red TLbeing used for dating sediments heated by lava Rows and py-roclastic products (e.g., Pilleyre et al., 1992; Montret et al.,1992; Miallier et al., 1994b, c; Fattahi and Stokes, 2000b).Pilleyre et al. (1992) successfully used the multiple aliquotadditive dose technique to date a number of samples fromvolcanoes in the Massif Central, France, with age ranges (of14–150 ka) in agreement with the geological evidence. Twoolder samples (age estimate ca. 2 and 14 Ma) were not dat-able because their growth curves could not be reproduced in


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the laboratory. Using the additive dose technique, Miallieret al. (1994b) obtained ages of 42–544 ka for other samplesof French quartz heated by pumice eruptions. These resultswere in good agreement with independent age control. To-gether these results suggested that quartz RTL can be usedfor dating over an age range from 10–600 ka.

Fattahi and Stokes (2000b) have recently undertakena systematic analysis of RTL properties of older (300ka–1:4 Ma) quartz-bearing, known age, silicic volcanic de-posits from New Zealand. Isothermal analysis of their sam-ples indicated a stable dating trap at an ambient temperatureof ¿ 109 a and they observed high-temperature orange-redTL dose response in excess of 20 kGy (Fig. 6). These char-acteristics led them to develop and apply a modi6ed-SARmethod (Murray and Wintle, 2000a). They observed theslow onset of growth curve saturation with dose, good re-producibility of glow curves and only limited sensitivitychanges occurring (Fig. 6). Their results were in agree-ment with ages based on other methods over time scales of

105–106 a, in doing so extending the demonstrated dat-able time range of the luminescence method to ca. 1:3 Ma(Fig. 7).

The bleachability of the orange-red TL band has receivedonly limited attention. Miallier et al. (1994a) exploredorange-red TL bleaching by sunlight using 100–200 �mquartz grains extracted from a lava-baked sediment. Theyexposed the quartz grains to sunlight for various time peri-ods and then measured the TL. They found that the 390◦Corange-red TL peak bleached very slowly to around 82%of its initial intensity during the 6rst 3 days of bleaching bysunlight. Its lower temperature shoulder at around 325◦C,however, bleached rapidly. Schole6eld and Prescott (1999)explored the ePect of bleaching on the natural spectrumof the same and 18 other samples. They con6rmed theslow bleachability of the 360◦C=2:05 eV=605 nm RTLpeak, and also found rapid bleaching of red TL peaks at∼ 270◦C and 305◦C(2:0 eV; 620 nm) in some of the sam-ples (Schole6eld and Prescott 1999, Fig. 2). They reportedthat the 305◦C red peak bleaches more slowly than the bluerapidly bleaching peak.

An alternative strategy to evaluate bleaching would be totest the completeness of resetting for sample collected frommodern depositional environments. This has not yet beenundertaken in a systematic fashion for orange-red TL. How-ever, preliminary data for a single sample collected fromthe Sahara is suggestive of complete resetting under idealbleaching conditions and high dose response characteristicsin unburnt sediments (Fig. 8). If optical resetting can becon6rmed, there would be much scope for application oforange-red TL from quartz (or a related optically stimulatedsignal) for extending the age range of luminescence in sed-iments.

5. Far-red luminescence from feldspar

Despite using high-temperature (¿ 250◦C) UV throughyellow TL emissions that should be stable over geologicalperiod, feldspars have shown age underestimation (Duller,1997). The consensus is that the fading of signal is caused byloss of charge over the burial period from the charge traps,which should, theoretically, be stable. Research into ‘anoma-lous fading’ in the UV-visible part of the emission spec-trum indicates that with rare exception, all known TL peaks(¡ 500◦C) exhibit this anomalous behaviour.Wintle (1977)investigated anomalous fading in plagioclase feldspar, andobserved that when a sample was stored at room temper-ature, the traps progressively lost their charges at a rateindependent of temperature. The ePects was attributed tothe non-thermal, tunnelling radiative recombination process(Wintle, 1977). Her explanation was strengthened by show-ing that the anomalous fading follows a logarithmic decaylaw. Subsequently, Visocekas (1985) con6rmed this modelby direct observation of tunnelling afterglow in the samematerial.


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Fig. 7. Orange-red TL SAR De estimates for NZ volcanic quartz samples. (a, c) glow curve data (inset response of aliquot to test dosesduring SAR procedure). (b, d) Corrected SAR growth curves (note good match of corrected TL for initial and repeated 6rst dose point).

Systematic studies of the relationship between anoma-lous fading of TL in feldspars and tunnelling radiativerecombination enabled Visocekas to characterize the far-redemissions. The emissions when measured above 470◦K (ca.200◦C) seem to be stable and appropriate for dating vol-canic feldspars up to 300 ka (Visocekas and Zink, 1999).They proposed comparing the ratio of blue to far-red TLof feldspar during storage time as a gauge for monitoringanomalous fading of feldspars (Visocekas, 2000) (e.g.,Fig. 9). The two main bands are similar in terms of trapactivation energies (after correcting for the ePect of thermalquenching), dose variation and bleaching, but diPerent interms of their kinetic and glow peak temperatures (Zink andVisocekas, 1997). In particular, the red emission bandappears to be unaPected by anomalous fading and there-fore suitable for dating. Zink and Visocekas (1997)successfully dated three volcanic feldspars using thered TL.

Despite the advantage of the non-fading of red TLbands compared with blue emissions, for temperaturesgreater than 350◦C, the incandescence of the sample andheater plate result in high background levels which havepreviously prohibited extensive investigations. Recentresearch by Fattahi and Stokes (in press, c) using themodi6ed apparatus described above has further demon-strated the relative stability of the red emission for glow

curve temperatures between 300 and 600◦C (Fig. 10).While much further research is required, this is a highlypromising, high-temperature non-fading feldspar signal.

5.1. Optical bleaching of Red TL

Red TL of feldspar is optically bleachable and this prop-erty could be exploited to date sediments (e.g., Bos et al.,1994; Prescott et al., 1994; Zink and Visocekas, 1997). Boset al. (1994) reported that arti6cially irradiated oligo-clase was bleachable, and that the bleaching eTciency ofboth the blue and red emissions increased with decreasingwavelength. While their sample remained unbleached atwavelengths between 703 and 800 nm, they made no studyof the ePect of stimulation wavelengths exceeding 800 nm.Prescott et al. (1994) and Zink and Visocekas (1997) re-ported that sunlight bleaches red emissions (¿ 650 nm) ofTL in some feldspars. For the somewhat broad orange-redthrough red TL emissions (590–750 nm) studied by Zinkand Visocekas, the intensity was found to reduced by∼ 70% after 2 h exposure to sunlight. Fattahi and Stokes(2003c) have explored the ePect of IR (¿ 800 nm) expo-sure on red TL emissions. They observed that both high-and low-temperature red TL peaks are bleachable by IR(� = 830 nm) (Fig. 11).


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Fig. 8. Orange-red TL from quartz extracted from a modern Saharadune (mal/15/5). (a) Examples of natural and arti6cial TL. (b)SAR growth curve indicating growth of signal above regenerationdoses of 500 Gy (data after Bailey and Stokes unpublished).

6. Red emission IRSL from feldspar

Given that the Red TL of feldspar bleaches under opticalstimulation, a logical extension of research would be to seekout an optical emission relating to the resetting process. Byutilising extended range cooled PMTs, Fattahi and Stokes(2000a, b, 2003a–c) have demonstrated that it is possibleto detect an Anti-Stokes orange-red (� ca. 600–650 nm)or far-red (� ca. 665–720 nm) emissions from feldspar(Fig. 12). The former of these emissions may be inRuencedby the frequently broad yellow (� ca. 560 nm) emissionpreviously observed from orthoclase (M. Krbetschek, pers.commun.). While technically challenging, a range of IR re-jection 6lters are now available which allow discriminationof the far-red emission from the closely positioned IR stim-

Fig. 9. TL curves (up to ca. 230◦C) for a sanidine feldspar follow-ing irradiation (250 Gy) and storage. (a) Orange-red—red emis-sion (590–750 nm), (b) blue emission (360–590 nm). Redrawnfrom Visocekas and Zink (1999).

ulation source (Fattahi and Stokes, 2000b, 2003b). Furtherenhancement of the frequently weak signal above back-ground can be achieved by blocking shorter wavelengthemissions from the IR source (Lai et al., 2002).

Fattahi and Stokes (2000b, 2003b) found that the far-redIRSL signal in feldspar is readily bleached by monochro-matic or polychromatic light (reaching negligible levelsabove background in a few minutes (Fig. 12b)) and is ther-mally assisted, with the initial signal intensity increasing byca. 33% when shifting the optical stimulation temperaturefrom 20◦C to 120◦C. The samples they studies exhibitedrelatively little sensitivity change over repeated dose—TLand dose-pre-heat-IRSL cycles, and dose response exper-iments indicated that a high dose saturation response iscommon, characteristic saturation doses (Do’s) for somesamples exceeding 1800 Gy (Fig. 13).

A more detailed analysis of physical properties of thefar-red IRSL and TL from feldspar for dating applicationswas undertaken by Fattahi and Stokes (2003c). They foundthat the form of the far-red IRSL decay could be well mod-


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Fig. 10. TL glow curves (heating rate 5◦C=s, sampleWW4A—potassium feldspar). Continuous line- Natural TL glowcurves. Dotted line—TL glow curves obtained following 120 Gyirradiation. Triangles—glow curves following 120 Gy irradiationand preheating at 250◦C for 1 min. (a) and (b) observed using theD716 PMT and an Omega 625df25 6lter and a standard blue bial-kali PMT and Hoya 340 6lters, respectively. (c) Shows the ratioof the natural vs. regenerated TL signals for both emissions. FromFattahi and Stokes (in press, c).

elled by a sum of three exponential components. Whenheated at 2◦C=s, far-red TL glow curves generally a com-bination of a broad low temperature (ca. 100–200◦C) peakand a continuum of peaks from ca. 300◦C to 500◦C, allof which are optically bleachable (Fig. 11). Comparison offar-red TL loss due to IR stimulation and the resulting IRSLemission indicated a linear relationship with IRSL emissionbeing greater than high-temperature (350–450◦C) TL lossby a factor of 2.6. They also undertook a series of relativelyshort-duration fading tests that were conducted on samplesknown to exhibit signi6cant anomalous fading. They foundthat the orange-red emission exhibits fading, potentially re-

lating to the inRuence of the yellow emission centre. How-ever, experiments using the far-red IRSL emission indicatedno fading over a storage period of 2 weeks at elevated(100◦C) temperature, whereas the UV emission from thesame sample faded by more than 30% over the same storageperiod (Fig. 14).

These promising 6ndings lead Fattahi and Stokes (2003d)to explore the possibility of exploiting the SAR protocol(Murray and Wintle, 2000a). While the SAR procedure hasbeen successfully applied to quartz, its application to UVemission IRSL has proven problematic (Wallinga et al.,2001). Fattahi and Stokes demonstrated a good linear rela-tionship between far-red IRSL resulting from a regenerativedose and the far-red IRSL resulting from a subsequent testdose. They additionally demonstrated that it was possible torecover a known laboratory dose using a variant of the SARprocedure which incorporated a long (8000 s) IR bleach ora thermal (0–500◦C) wash at the end of each SAR cycle.They further demonstrated that equivalent dose estimatesbased on the far-red IRSL emission for geological sampleswere consistently greater than estimates based on UV emis-sion IRSL, lending further support to the hypothesis that thestability of the far-red emission is considerably greater thanthe UV emission.

7. Conclusions

Red luminescence is a potentially powerful tool that mayoPer radical alternatives to existing luminescence datingstrategies, and complements and extend the utility of manyothers. In particular, recent research indicates scope forusing red luminescence from potassium feldspars to circum-vent the anomalous fading observed in UV-blue lumines-cence. Using either this or orange-red (� ca. 600–650 nm)TL signals from quartz, it may be possible to signi6cantlyextend the age rage of luminescence dating. The technolog-ical problem of excluding thermal incandescence from RTLemission has hindered previous work and has discouragedattempts to observe any IR-stimulated red emission. It hasrecently been demonstrated that the use of cooled extendedrange PMTs and novel 6lter types make detection of suchsignals possible at high (up to 600◦C) temperatures.

Orange-red (� ca. 600–650 nm) TL in quartz is readilyobservable using extended, and in some cases conventional‘blue’-sensitive bialkali PMTs. It has been have demon-strated that it is possible to use orange-red TL from volcanicquartz back to beyond 1 Ma. Likewise, the zero residual andhigh dose saturation characteristics of red TL from Saharanquartz sand indicates that it may have considerable potentialas an alternative, long-range, chronometer for well-bleachedsedimentary facies.

Additionally, far-red IRSL would appears to be an im-portant signal requiring further systematic investigation.Preliminary investigations indicate that the signal is in partderived from high-temperature (geologically stable) traps;


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Fig. 11. Red TL (0–500◦C) glow curves (sample WW1A) after 120 Gy irradiation and either no preheat or 1 min preheat at 200◦C (6lleddiamond and continuous line, respectively). Empty diamond and dashed lines show RTL glow curve obtained following 120 Gy irradiationand either no preheat or 1 min preheat at 200◦C, respectively, and then exposed to an 830 nm IR source (P = 300 mW=cm2 at sample) at70◦C for 100 s prior to measurement.

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Fig. 12. (a) Examples of natural red IRSL emission for a suiteof Arabian aeolian feldspar samples. (b) Changes in decay formof feldspar sample WW2A following partial (monochromatic IR)bleaching. A laboratory irradiated (240 Gy) aliquot was pre-heated at 250◦C for 2 min and then exposed to IR source(�= 830± 5 nm) for diPerent periods prior to far-red IRSL mea-surements. IRSRL signals observed through a ‘Green’ D716A PMTand using an Omega 625 6lter (data from Fattahi and Stokesin press, b, c).

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Fig. 13. Regenerated red IRSL decay curves of potassium feldsparextracted from sample 99/5/1 (NZ volcanics). Inset shows growthcurve (based on net initial (1 s) signal). Data measured using the‘green’ D716A PMT 6ltered with Omega625. Data from Fattahiand Stokes (2000b, 2003b).

it bleaches rapidly when exposed to monochromatic orpolychromatic light; it exhibits only limited and predictablesensitivity changes in response to irradiation and preheating;application of SAR procedures indicate that it is capableof recovering a known dose and results in De estimates inexcess of those based on UV emissions; and, it has not yetbeen found to exhibit signi6cant anomalous fading.


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Fig. 14. Comparing corrected net initial (6rst sec minus average of 90–100 s) of IRSL, sum of TL (200–300◦C), and sum of TL(300–400◦C) for repeated measurement of sample Y7A after varying delays. The relative intensity of second/6rst (no delay), third/second(2 week delay at 100◦C) and fourth/second (no delay) set of measurements are shown for both OSL and TL. The rate of second to thirdset of measurements indicate the presence of anomalous fading. The rate of second to 6rst, and fourth to second measurement shows theaccuracy of sensitivity correction method. (a) UV. (b) Red (modi6ed from Fattahi and Stokes in press, c).

Acknowledgements

We are extremely grateful to the French (Didier Mi-allier and colleagues, Raphael Visocekas, Antoine Zinkand colleagues) and Japanese (Professor Hashimoto andcolleagues), who undertook key pioneering studies on redluminescence which have guided much of our own redluminescence research direction. We thank an anonymousreviewer for constructive comments on an earlier versionof this manuscript. Ministry of Science, Research andTechnology of Iran kindly supported the funding of MF.

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Red emission luminescence from quartz and feldspar for dating applications: an overview

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