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Volcanic feldspars anomalous fading: Evidence for two different mechanisms
Radiation Measurements xxx (2015) 1e6
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Reprint of: Volcanic feldspars anomalous fading: Evidence for two different mechanisms* rin a, *, Raphae €l Visocekas b Gilles Gue a b
LSCE/IPSL, CEA-CNRS-UVSQ, Avenue de la Terrasse, 91198 Gif sur Yvette Cedex, France Universit e Paris-Sud, GEOPS UMR CNRS-UPS, 91405 Orsay Cedex, France
h i g h l i g h t s Anomalous fading of TL of volcanic feldspars is drastic in a few weeks. Anomalous fading after two decades of storage is twice the fading after one decade. After storage for two decades at 77 K the fading is practically suppressed. Anomalous fading is due at least to two several different mechanisms. Volcanic feldspars have disordered lattice and electrons can hop from trap to trap.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 27 February 2015 Accepted 10 May 2015 Available online xxx
This study presents measurements of anomalous fading of feldspars extracted from volcanic units from the Auvergne (France) and Patmos Island (Greece). We measured the fading rate for samples stored at ambient temperature, and also at liquid nitrogen temperature. A strongly different behaviour is then observed, the fading is reduced to values usually obtained and explained by pure tunnelling recombination of charges to near enough luminescence centres, which is athermal. We suggest that the temperature dependent or “frozen” part of the fading is relevant to a different mechanism, which is “hopping”, already proposed in the mid-sixties, which preserves the experimental logarithmic fading decreasing law. © 2015 Published by Elsevier Ltd.
Keywords: Anomalous fading Volcanic feldspars Sanidine Plagioclase Freezing of fading Hopping Conductivity Crystal disorder Tunnelling Luminescence
1. Introduction Feldspars display an anomalous fading that can lead with volcanic samples to a nearly complete loss of the stored thermoluminescence (TL), OSL or IRSL after a few years storage. Since the
DOI of original article: http://dx.doi.org/10.1016/j.radmeas.2015.05.003. This article forms part of the LED Special Issue but was published in an earlier issue of the journal due to an administrative error. For citation purposes, please use rin, R.Visocekas,79, pp. 1e6. the original publication details; G.Gue * Corresponding author. rin). E-mail address: [email protected] (G. Gue *
first observations (Wintle, 1973, 1977) many attempts have been made to evaluate this decay in contradiction with conventional kinetics of TL, in order to use feldspars in TL or OSL dating (Spooner, 1992, 1994; Lamothe and Auclair, 1999; Huntley and Lamothe, 2001; Lamothe et al., 2012) and this phenomenon has been well documented (Aitken, 1985, 1998). Among various possible mechanisms for the reported anomalous fading tunnelling recombination has been favoured as it accounts for its reported temperature independence and experimental logarithmic decay law (Visocekas, 1979, 1985, 1993). This very strong anomalous fading present in most plagioclases or K-feldspars (sanidine or anorthose) extracted from volcanic
http://dx.doi.org/10.1016/j.radmeas.2015.08.009 1350-4487/© 2015 Published by Elsevier Ltd.
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
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G. Guerin, R. Visocekas / Radiation Measurements xxx (2015) 1e6
rocks appears to us not to be quantitatively justifiable by a simple tunnelling mechanism. Tunnelling is a quantum effect, strictly temperature independent. By tunnelling, fixed ionised centres and trapped electrons recombine through the potential barrier separating them, with kinetics: exp(-at) for a given distance, The life time (1/a) is a function of the distance d between them from a fraction of a second to a near infinite time. Thus electrons trapped at greater distances will not actually recombine by simple tunnelling even for times of storage up to 1 Ma. They could be detrapped only thermally in TL or OSL. Examples are shown at scale on Fig. 1. The decay of tunnel luminescence at time t after excitation has been consistently observed down to 77 K to fit nearly a (1/t) law over many decades of time (Delbecq et al., 1974, Visocekas et al., 1976, Huntley, 2006). This observation is well accounted for by the addition of all tunnel recombinations of electron-centre pairs: R∞ 0 expðatÞda ¼ 1=t Simple is a purely quantum effect by which a trapped electron has some probability to be beyond the potential barrier trapping it. There it may recombine radiatively in a centre. This effect is temperature independent. It may be quantitatively justified (Visocekas, 1979 with calcite), there were similar observations of it in many materials (see Huntley, 2006 for a review). It is all the more expected to display similar intensities in all K-feldspars, volcanic or granitic, since they have quite similar composition and crystallography and display similar glow curves following similar irradiations. The odd thorough anomalous fading observed specifically in volcanic feldspars (at ambient temperature), typically such as sanidine, leads to propose another mechanism for electrons detrapping, namely “hopping” to account for it (Visocekas et al., 2014). It is based on the oddity that in such feldspars the constitutive Al3þ ions are no more ordered in the lattice (Barth, 1934). This disorder generates a huge density of defects or “localised centres” acting as traps (Mott, 1968). Trapped electrons can enjoy some mobility by “hopping” from one trap to a next one, a short range tunnelling with weak thermal stimulation as sketched in Fig. 2 (Poolton et al., 2002). Finally, all of them end up by tunnelling into a luminescent centre when they get near enough to it, with a lifetime depending on the initial distance to reach this centre (Visocekas et al., 2014). The possibility of such a mechanism of fading adding to tunnelling at ambient temperature will be further confirmed by the experimental results proposed hereafter. They will show drastic reduction of the anomalous fading by storage at liquid nitrogen temperature. This is totally at odds with tunnelling, basically an athermal quantum effect.
Fig. 2. The three routes for luminescent electron-centre recombination. (1) After strong thermal or optical stimulation: from trap to conduction band (CB), independent from the distance, the route of TL and OSL. (2) By tunnelling: without any stimulation, temperature independent, very strongly dependent on the electron-centre distance (distances are shown for lifetimes 1 year and 1 Ma). (3) By hopping from trap to trap with weak thermal stimulation: the flow is greatly increased by a high density of defects and all trapped electrons may eventually detrap in a few decades.
2. Samples and experimental settings 2.1. Sample preparations Three K-feldspars and a plagioclase have been studied. One sanidine comes from Patmos Island (Greece). It is a new sample preparation different from those studied by some authors (Spooner, 1992). We named it Patmos-B to prevent any confusion with other samples coming from the same Island. We selected a single and large (ca 1 cm) phenocryst from a late tertiary trachytic formation at the base of the island. Its age can be attributed from 5 Ma to 5.5 Ma (Barton and Wyers, 1991). Others two K-feldspars are phenocrysts coming from two different outcrops in the Puy de Sancy stratovolcano (France): Roc de Courlande and Puy de la Tache. Based on geological evidence, they are dated between 500 ka and 350 ka (Nomade et al., 2012). For these three samples a crystal the size of a centimetre is crushed and sieved. The 80e160 mm fraction is then cleaned in an ultrasonic bath. Ferromagnetic grains (due to small magnetite inclusions within the crystal) are removed using a permanent magnet. The last sample is a plagioclase extracted from a basaltic lava flow outcrop at Olby in the Chaîne des Puys (France) volcanic province. It is a well-known lava flow because it evidences the last Earth magnetic field excursion (Laschamp Event) and is dated about 41.5 ka (Laj et al., 2014). This sample is made of the plagioclase microliths of the basalt. The rock is crushed, sieved and the fraction 80e125 mm cleaned in distilled water and ethanol in an ultrasonic bath. Then, the fraction of density less than 2.8 is separated using tribromomethane and ethanol. Once dried at 40 C, the ferromagnetic grains are removed. 2.2. Experimental settings
Fig. 1. Sketch of the structure and relation of distance from centre to electron with the half-life of tunnelling.
TL measurements were carried out with a laboratory made TLrin and Lefe vre, 2014). A near OSL reader holding 36 disks (Gue UV glass filter (Sagem DH 380c0 ) is used, with a band from 320 nm to 430 nm (Supp. Figure 1), almost equivalent to a Schott BG 39 associated to a Corning 7e59. The protocol of measurements starts by a thermal bleaching of a batch about 300 mg of each sample. Using an open-air furnace, sanidine samples (Courlande, la Tache and Patmos-B) are heated up to 650 C (at an approximate rate of 15 C/s) and then left to cool slowly for 1 h to ambient temperature. The plagioclase sample (Olby) is bleached in the same way but heated up to 750 C in order to empty the very high temperature traps (revealed by TL experiments in the 325 nm UV domain).
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
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Measurement of the fading has been made for one and two decades of storage time. For each sample, the measurement of the fading occurring after one decade is made by dividing the bleached preparation into 12 aliquots of ca 2 mg (using a calibrated laboratory spoon). These aliquots were disposed on the measurement disks and introduced in the TL-OSL reader. Then irradiations are done successively using the 90Sr beta source of the TL-OSL reader as follows: Aliquot 1 to 6 (“one decade TL”) irradiation 20 min (ca 30 Gy) Aliquot 7 to 12 (“prompt TL”)
irradiation 20 min pause for 20 min preheat 10 se260 C (10 C/s) TL e first glow e (5 C/s up to 625 C) TL e second glow e (background). Back to aliquot 1 to 6.
preheat 10 se260 C (10 C/s) TL e first glow e (5 C/s up to 625 C) TL e second glow e (background). The delay (corresponding to the measurement of aliquots 7e12) between the end of irradiation of aliquots 1 to 6 and their measurements is about 400 min. The software journal of the TL reader records the precise timing of these operations. The good stability of our photomultiplier with no appreciable signal drift allows good accuracy with such large delay between the first and the last measurement. As prompt fading decreases less than the logarithmic law, correction for it shows that between 20 min and 400 min the fading is actually equivalent to one decade in the final logarithm range. For two decades, the protocol used for the measurement of the fading is different. Sample preparations are divided in 50 mg batches and irradiations are performed using successively a 137Cs gamma facility on 50 mg batches for the 4 samples. Batch 1 (two decades): Irradiation 120 min (ca 125 Gy) Storage at ambient temperature (22 C) 23,000 min (16 days) Batch 2 (two decades at liquid nitrogen temperature): Irradiation 120 min (ca 125 Gy) Storage in a cryostat at liquid nitrogen temperature (ca 77 K) ca 23,000 min (16 days) Batch 3 (prompt): Irradiation 120 min (ca 125 Gy) Storage at ambient temperature 120 min At the end of these storage times, each batch is divided in nine 2 mg aliquots disposed on the measurement disks and introduced in the TL-OSL reader. Then TL is measured: preheat 10 se260 C (10 C/s) TL e first glow e (5 C/s up to 625 C) TL e second glow e (background).
3
As above, correction for the prompt fading shows that between 120 min and 23,000 min the fading is actually equal to two decades in the logarithmic range. 3. Results 3.1. Anomalous fading after one decade storage at ambient temperature Fig. 3 shows the results of above mentioned “one decade” TL measurements. For every sample, two TL glow curves are shown, one for prompt TL, the other for the “one decade TL”. Each of these average TL minus photomultiplier and thermal noise intensities for 6 aliquots. Intensity standard error is close to 2.5% at the maximum of emission. Higher than 550 C the TL intensity is poorly evaluated due to thermal emission. TL glow curves of sanidine samples, Patmos-B, Courlande and La Tache are very similar. They show an intense peak around 305 C followed by a broad emission ca 375 C and no significant emission above 500 C. With the plagioclase sample it is different: the glow curve shows a first peak close to 320 C, then a broad and intense emission around 550 C obviously followed by a higher temperature emission that cannot be measured above 600 C because of the thermal background emission. The fading rate (expressed in percent) is calculated as the ratio of the difference between the prompt TL and the “one-decade” TL divided by the prompt TL. At the maximum of the 305 C peak, the fading rate for the Patmos-B sample is less than 3%. This value is within experimental errors implying that this sample displays a very small fading. The other three samples display large fading rates between 20 and 30%. These values are those that are commonly derived from measurements of volcanic alkali-feldspars. The activation energy of the first emission have been evaluated using the initial rise method (Garlick and Gibson, 1948; McKeever, 1985). They are summarised Table 1 (see also Supp. Figure 2 for details). The four samples lead to similar values; this confirms that the observed fading is anomalous as it cannot be attributed to thermal fading corresponding to these peaks. 3.2. Anomalous fading after two decades storages at ambient and liquid nitrogen temperatures In a second experiment, as shown above, the four samples have been measured after approximately two decades of storage at ambient temperature, a complementary batch of each sample having been stored in liquid nitrogen. The timing of the experiment, in particular the delay to prepare measurement aliquots and introduce them in the TL-reader have been chosen to insure that the liquid nitrogen aliquots have spent the same time (or less) at ambient temperature as the “prompt TL” aliquots before their measurement. Fig. 4 shows glow curves for prompt, and 2 decades storage in liquid nitrogen and at ambient temperature. These curves have been obtained in the same way as the curves for Fig. 3 but in this case each curve is the average of 9 aliquots. The Table 2 summarized values obtained for one and 2 decades of storage in these conditions. The fading rate has been normalized to the effective number of decades thus is an approximation of the gfactor. As in the case of the one-decade study, the Patmos-B sample displays almost no measurable fading. The other three samples display typical strong anomalous fading. They show g-factor about 30%, consistently similar to values reported in IRSL studies (Spooner, 1994). Determinations of g-factor made for one -decade are slightly lower than for two decades because when calculated
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
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Fig. 3. TL glow curves of the 4 samples measured promptly after irradiation (a) and after ca one decade of storage (b). Fading rate, shown as a series of hatching for each temperature is the ratio of the value of the lost signal to the prompt one (in percent).
Table 1 Estimated values of the activation energy and frequency factor for the first emission peak of the glow curve.
Patmos-B Courlande La Tache Olby
Peak temperature ( C)
Energy (eV)
315 305 305 325
1.23 1.30 1.30 1.33
± ± ± ±
0.01 0.01 0.01 0.01
s (GHz) 7.9 ± 0.3 48.4 ± 1.4 50.2 ± 4.8 34.3 ± 0.8
using a small number of decades the g-factor has not yet reached its asymptotic value. With storage time changed from one decade to two decades, the fading of ambient TL is observed to double roughly, in accordance with a logarithmic law. Another fact, clearly visible for the Olby sample, is the strong decrease of the fading with TL temperature. However, the unexpected result brought by this study is that by
Fig. 4. TL glow curves of the 4 samples measured promptly after irradiation (a), after ca two decades of storage at liquid nitrogen temperature (b) and after the same storage at ambient temperature (c). Fading rate, shown as a series of hatching for each temperature is the ratio of the value of the lost signal to the prompt one (in percent).
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
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5
Table 2 Estimated values of the g-factor (percent per decade) at the first maximum of TL emission. The number of decades is calculated including half of the irradiation time. Emission peak T C
Nb of decades
g-factor ambient
Courlande
305
Olby
325
La Tache
305
Patmos-B
315
1.09 2.01 1.12 1.90 1.12 1.97 1.12 1.90
30.0 36.7 19.2 29.4 24.1 32.1 1.7 1.1
storage at liquid nitrogen temperature, the anomalous fading is mostly “frozen”, the fading rates g are lowered to 5e9% per decade. These are values close to those measured for non-volcanic feldspars stored at room temperature, by TL (Huntley and Lamothe, 2001) or by OSL (Huntley and Lian, 2006). 4. Discussion A feature of volcanic feldspars is their rapid cooling to ambient temperature. Thus they keep their high temperature arrangement and crystallize typically as monoclinic sanidine, which is metastable at ambient temperature. It contrasts with the stable low temperature arrangement form of common feldspars such as found in granites, which is typically triclinic microcline (Goldsmith and Laves, 1954), and display very few macroscopic differences with sanidine. At a microscopic point of view, in the crystal sanidine the 25% Al atoms associated with 75% Si in KAlSi3O8 have a “disordered” arrangement (Barth, 1934). This results in the presence of a very high percentage of crystal radiation centres (such as AleOeAl, interstitials), contrasting with “ordered” feldspars. All these geometrical defects, likely to trap electrons, are termed “localized states” with 4 dimensions: 3 are geometrical, in the lattice; the 4th one is the energy level. N. Mott (1968) proposed a possibility of transport of trapped charge in amorphous or disordered lattice by diffusion or “hopping” from one localized state to a neighbouring one. The electrons are no more fixed but can tunnel from a state to the next one, at a distance R and a difference in energy W. The probability of hopping is:
g-factor liquid N2 5.5 8.8 5.2 1.2
activation energy has to be demonstrated. 5. Conclusions The most unexpected observation of our studies is the thermal “freezing” of the anomalous fading shown above (Fig. 4), after storage for two decades at liquid nitrogen temperature. It is a decrease of g-factor next to values observed with non-volcanic feldspars. Tunnelling alone being a pure quantum mechanical athermal process, it cannot account for this freezing of anomalous fading. Conversely electrical charge transport by “hopping” does call for phonon activation energy through the factor: exp(W/kT). Thus, at lower temperatures, “hopping” is constrained to localised states less remote in energy (as shown in Fig. 2), i.e. the transport of charges is much reduced. This is why “hopping” accounts properly for the observed freezing of anomalous fading. Storage at even lower temperatures such as liquid He may possibly further eliminate remains of contribution of “hopping” and leave ‘’pure’’ tunnelling alone. This observation has also an implication on the accuracy of the g-factor used in feldspar dating as it is measured at room temperature and may not correctly represent the “geological” value which depends on temperature history of the sample. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.radmeas.2015.05.003. References
P ¼ expð 2aRÞexpðW=kTÞ This was further applied to feldspars (Poolton et al., 2002). This mechanism appears to us as quite adapted to the situation of volcanic feldspars with their disordered lattice which provides the required basis of high density of defects i.e. “localized states”. In such a situation, all trapped electrons in the course of their “random walk” from one state to the next one will happen to be at a tunnelling distance from a radiative centre and then recombine in a reduced time spell. The above probability involving exponentials like the “pure” tunnelling does, it is expected that the overall law of fading should be exponential as well, as was observed initially (Visocekas, 1979). Our observations indicates a variation of the g-factor measured at 300 K and at 77 K to be a factor 6 and 7 for Courlande and La Tache feldspars (Table 2). If we suppose this variation is governed by a Boltzmann law, its activation energy can be roughly estimated to be a small 0.018 eV. The magnitude of this value suggests to address mechanisms related to phonon interactions, thermal diffusion in the band tail states as suggested in a recent work (Jain and Ankjærgaard, 2011). However, this can be a coincidence as our set of experimental data is limited and a behaviour ruled by an
Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press. ISBN 0-12046381-4. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press. ISBN 0-19-854092-2. Barth, T.F.W., 1934. Polymorphic phenomena and crystal structure. Am. J. Sci. 27 (160), 273e286, 5 series. Barton, M., Wyers, G.P., 1991. Estimates of P, T, PH20 and fo2 for lavas from Patmos (Greece) and implications for magmatic evolution. J. Volcanol. Geotherm. Res. 47, 265e297. Delbecq, C.J., Toyozawa, Y., Yuster, P.H., 1974. Tunnelling recombination of trapped electrons and holes in KCl:AgCl and KC1:TlCl. Phys. Rev. B 9, 4497e4505. Garlick, G.F.J., Gibson, A.F., 1948. The electron trap mechanism of luminescence in sulphide and silicate phosphors. Proc. Phys. Soc. 60, 574e590. Goldsmith, J.R., Laves, F., 1954. Potassium feldspars structurally intermediate between microcline and sanidine. Geochim. Cosmochim. Acta 6, 100e118. rin, G., Lefe vre, J.-C., 2014. A low cost TLeOSL reader dedicated to high temGue perature studies. Measurement 49, 26e33. Huntley, D.J., 2006. An explanation of the power-law decay of luminescence. J. Phys. Condens. Matter 18, 1359e1365. Huntley, D.J., Lamothe, M., 2001. Ubiquity of anomalous fading in K-feldspars and the measurement and correction for it inoptical dating. Can. J. Earth Sci. 38, 1093e1106. Huntley, D.J., Lian, O.B., 2006. Some observations on tunnelling of trapped electrons in feldspars and their implications for optical dating. Quat. Sci. Rev. 25, 2503e2512. Jain, M., Ankjærgaard, C., 2011. Towards a non-fading signal in feldspar: insight into charge transport and tunnelling from time-resolved optically stimulated
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
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luminescence. Radiat. Meas. 46, 292e309. Laj, C., Guillou, H., Kissel, C., 2014. Dynamics of the earth magnetic field in the 10e75 kyr period comprising the Laschamp and Mono Lake excursions: new results from the French Chaîne des Puys in a global perspective. Earth Planet. Sci. Lett. 387, 184e197. Lamothe, M., Auclair, M., 1999. A solution to anomalous fading and age shortfalls in optical dating of feldspar minerals. Earth Planet. Sci. Lett. 171, 319e323. , M., Huot, S., Ouimet, S., 2012. Natural luminescence and Lamothe, M., Barre anomalous fading in K-feldspar. Radiat. Meas. 47, 682e687. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. Mott, N.F., 1968. Conduction in non-crystalline materials. III. Localised states in a pseudogap and near extremities of conduction and valence bands. Phil. Mag. 19, 835e852. Nomade, S., Scaillet, S., Pastre, J.-F., Nehlig, P., 2012. Pyroclastic chronology of the sancy stratovolcano (Mont-Dore, French Massif Central): new high-precision 40Ar/39Ar constraints. J. Volcanol. Geotherm. Res. 225e226, 1e12. Poolton, N.R.J., Ozanyan, K.B., Wallinga, J., Murray, A.S., Botter-Jensen, I., 2002. Electrons in feldspar II: a consideration of the influence of conduction band-tail states in luminescence processes. Phys. Chem. Miner. 29, 217e225. Spooner, N.A., 1992. Optical dating: preliminary results on the anomalous fading of
luminescence from feldspars. Quat. Sci. Rev. 11, 139e145. Spooner, N.A., 1994. The anomalous fading of infrared-stimulated luminescence from feldspars. Radiat. Meas. 23, 625e632. Visocekas, R., Ceva, T., Marti, C., Lefaucheux, F., Robert, M.C., 1976. Tunnelling processes in afterglow of Calcite. Phys. Stat. Sol. (A) 35, 315e527. s irradiation cathodique: Visocekas, R., 1979. La luminescence de la calcite apre thermoluminescence et luminescence par effect tunnel. Thesis. Universite Pierre et Marie Curie, Paris. Visocekas, R., 1979. Miscellaneous aspects of artificial TL of calcite: emission spectra, athermal detrapping and anomalous fading. PACT 3, 258e265. Visocekas, R., 1985. Tunnelling radiative recombination in labradorite: its association with anomalous fading of thermoluminescence. Nucl. Tracks Radiat. Meas. 10, 521e529. Visocekas, R., 1993. Tunnelling radiative recombination in K-feldspar sanidine. Nucl. Tracks Radiat. Meas. 21, 175e178. Visocekas, R., Barthou, C., Blanc, Ph, 2014. Thermal quenching of far-red Fe3þ thermoluminescence of volcanic K-feldspars. Radiat. Meas. 61, 52e73. Wintle, A.G., 1973. Anomalous fading of thermoluminescence in Mineral samples. Nature 245 (5421), 143e144. Wintle, A.G., 1977. Detailed study of a thermoluminescent mineral exhibiting anomalous fading. J. Luminescence 15, 385e393.
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Reprint of: Volcanic feldspars anomalous fading: Evidence for two different mechanisms* rin a, *, Raphae €l Visocekas b Gilles Gue a b
LSCE/IPSL, CEA-CNRS-UVSQ, Avenue de la Terrasse, 91198 Gif sur Yvette Cedex, France Universit e Paris-Sud, GEOPS UMR CNRS-UPS, 91405 Orsay Cedex, France
h i g h l i g h t s Anomalous fading of TL of volcanic feldspars is drastic in a few weeks. Anomalous fading after two decades of storage is twice the fading after one decade. After storage for two decades at 77 K the fading is practically suppressed. Anomalous fading is due at least to two several different mechanisms. Volcanic feldspars have disordered lattice and electrons can hop from trap to trap.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 27 February 2015 Accepted 10 May 2015 Available online xxx
This study presents measurements of anomalous fading of feldspars extracted from volcanic units from the Auvergne (France) and Patmos Island (Greece). We measured the fading rate for samples stored at ambient temperature, and also at liquid nitrogen temperature. A strongly different behaviour is then observed, the fading is reduced to values usually obtained and explained by pure tunnelling recombination of charges to near enough luminescence centres, which is athermal. We suggest that the temperature dependent or “frozen” part of the fading is relevant to a different mechanism, which is “hopping”, already proposed in the mid-sixties, which preserves the experimental logarithmic fading decreasing law. © 2015 Published by Elsevier Ltd.
Keywords: Anomalous fading Volcanic feldspars Sanidine Plagioclase Freezing of fading Hopping Conductivity Crystal disorder Tunnelling Luminescence
1. Introduction Feldspars display an anomalous fading that can lead with volcanic samples to a nearly complete loss of the stored thermoluminescence (TL), OSL or IRSL after a few years storage. Since the
DOI of original article: http://dx.doi.org/10.1016/j.radmeas.2015.05.003. This article forms part of the LED Special Issue but was published in an earlier issue of the journal due to an administrative error. For citation purposes, please use rin, R.Visocekas,79, pp. 1e6. the original publication details; G.Gue * Corresponding author. rin). E-mail address: [email protected] (G. Gue *
first observations (Wintle, 1973, 1977) many attempts have been made to evaluate this decay in contradiction with conventional kinetics of TL, in order to use feldspars in TL or OSL dating (Spooner, 1992, 1994; Lamothe and Auclair, 1999; Huntley and Lamothe, 2001; Lamothe et al., 2012) and this phenomenon has been well documented (Aitken, 1985, 1998). Among various possible mechanisms for the reported anomalous fading tunnelling recombination has been favoured as it accounts for its reported temperature independence and experimental logarithmic decay law (Visocekas, 1979, 1985, 1993). This very strong anomalous fading present in most plagioclases or K-feldspars (sanidine or anorthose) extracted from volcanic
http://dx.doi.org/10.1016/j.radmeas.2015.08.009 1350-4487/© 2015 Published by Elsevier Ltd.
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
2
G. Guerin, R. Visocekas / Radiation Measurements xxx (2015) 1e6
rocks appears to us not to be quantitatively justifiable by a simple tunnelling mechanism. Tunnelling is a quantum effect, strictly temperature independent. By tunnelling, fixed ionised centres and trapped electrons recombine through the potential barrier separating them, with kinetics: exp(-at) for a given distance, The life time (1/a) is a function of the distance d between them from a fraction of a second to a near infinite time. Thus electrons trapped at greater distances will not actually recombine by simple tunnelling even for times of storage up to 1 Ma. They could be detrapped only thermally in TL or OSL. Examples are shown at scale on Fig. 1. The decay of tunnel luminescence at time t after excitation has been consistently observed down to 77 K to fit nearly a (1/t) law over many decades of time (Delbecq et al., 1974, Visocekas et al., 1976, Huntley, 2006). This observation is well accounted for by the addition of all tunnel recombinations of electron-centre pairs: R∞ 0 expðatÞda ¼ 1=t Simple is a purely quantum effect by which a trapped electron has some probability to be beyond the potential barrier trapping it. There it may recombine radiatively in a centre. This effect is temperature independent. It may be quantitatively justified (Visocekas, 1979 with calcite), there were similar observations of it in many materials (see Huntley, 2006 for a review). It is all the more expected to display similar intensities in all K-feldspars, volcanic or granitic, since they have quite similar composition and crystallography and display similar glow curves following similar irradiations. The odd thorough anomalous fading observed specifically in volcanic feldspars (at ambient temperature), typically such as sanidine, leads to propose another mechanism for electrons detrapping, namely “hopping” to account for it (Visocekas et al., 2014). It is based on the oddity that in such feldspars the constitutive Al3þ ions are no more ordered in the lattice (Barth, 1934). This disorder generates a huge density of defects or “localised centres” acting as traps (Mott, 1968). Trapped electrons can enjoy some mobility by “hopping” from one trap to a next one, a short range tunnelling with weak thermal stimulation as sketched in Fig. 2 (Poolton et al., 2002). Finally, all of them end up by tunnelling into a luminescent centre when they get near enough to it, with a lifetime depending on the initial distance to reach this centre (Visocekas et al., 2014). The possibility of such a mechanism of fading adding to tunnelling at ambient temperature will be further confirmed by the experimental results proposed hereafter. They will show drastic reduction of the anomalous fading by storage at liquid nitrogen temperature. This is totally at odds with tunnelling, basically an athermal quantum effect.
Fig. 2. The three routes for luminescent electron-centre recombination. (1) After strong thermal or optical stimulation: from trap to conduction band (CB), independent from the distance, the route of TL and OSL. (2) By tunnelling: without any stimulation, temperature independent, very strongly dependent on the electron-centre distance (distances are shown for lifetimes 1 year and 1 Ma). (3) By hopping from trap to trap with weak thermal stimulation: the flow is greatly increased by a high density of defects and all trapped electrons may eventually detrap in a few decades.
2. Samples and experimental settings 2.1. Sample preparations Three K-feldspars and a plagioclase have been studied. One sanidine comes from Patmos Island (Greece). It is a new sample preparation different from those studied by some authors (Spooner, 1992). We named it Patmos-B to prevent any confusion with other samples coming from the same Island. We selected a single and large (ca 1 cm) phenocryst from a late tertiary trachytic formation at the base of the island. Its age can be attributed from 5 Ma to 5.5 Ma (Barton and Wyers, 1991). Others two K-feldspars are phenocrysts coming from two different outcrops in the Puy de Sancy stratovolcano (France): Roc de Courlande and Puy de la Tache. Based on geological evidence, they are dated between 500 ka and 350 ka (Nomade et al., 2012). For these three samples a crystal the size of a centimetre is crushed and sieved. The 80e160 mm fraction is then cleaned in an ultrasonic bath. Ferromagnetic grains (due to small magnetite inclusions within the crystal) are removed using a permanent magnet. The last sample is a plagioclase extracted from a basaltic lava flow outcrop at Olby in the Chaîne des Puys (France) volcanic province. It is a well-known lava flow because it evidences the last Earth magnetic field excursion (Laschamp Event) and is dated about 41.5 ka (Laj et al., 2014). This sample is made of the plagioclase microliths of the basalt. The rock is crushed, sieved and the fraction 80e125 mm cleaned in distilled water and ethanol in an ultrasonic bath. Then, the fraction of density less than 2.8 is separated using tribromomethane and ethanol. Once dried at 40 C, the ferromagnetic grains are removed. 2.2. Experimental settings
Fig. 1. Sketch of the structure and relation of distance from centre to electron with the half-life of tunnelling.
TL measurements were carried out with a laboratory made TLrin and Lefe vre, 2014). A near OSL reader holding 36 disks (Gue UV glass filter (Sagem DH 380c0 ) is used, with a band from 320 nm to 430 nm (Supp. Figure 1), almost equivalent to a Schott BG 39 associated to a Corning 7e59. The protocol of measurements starts by a thermal bleaching of a batch about 300 mg of each sample. Using an open-air furnace, sanidine samples (Courlande, la Tache and Patmos-B) are heated up to 650 C (at an approximate rate of 15 C/s) and then left to cool slowly for 1 h to ambient temperature. The plagioclase sample (Olby) is bleached in the same way but heated up to 750 C in order to empty the very high temperature traps (revealed by TL experiments in the 325 nm UV domain).
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
G. Guerin, R. Visocekas / Radiation Measurements xxx (2015) 1e6
Measurement of the fading has been made for one and two decades of storage time. For each sample, the measurement of the fading occurring after one decade is made by dividing the bleached preparation into 12 aliquots of ca 2 mg (using a calibrated laboratory spoon). These aliquots were disposed on the measurement disks and introduced in the TL-OSL reader. Then irradiations are done successively using the 90Sr beta source of the TL-OSL reader as follows: Aliquot 1 to 6 (“one decade TL”) irradiation 20 min (ca 30 Gy) Aliquot 7 to 12 (“prompt TL”)
irradiation 20 min pause for 20 min preheat 10 se260 C (10 C/s) TL e first glow e (5 C/s up to 625 C) TL e second glow e (background). Back to aliquot 1 to 6.
preheat 10 se260 C (10 C/s) TL e first glow e (5 C/s up to 625 C) TL e second glow e (background). The delay (corresponding to the measurement of aliquots 7e12) between the end of irradiation of aliquots 1 to 6 and their measurements is about 400 min. The software journal of the TL reader records the precise timing of these operations. The good stability of our photomultiplier with no appreciable signal drift allows good accuracy with such large delay between the first and the last measurement. As prompt fading decreases less than the logarithmic law, correction for it shows that between 20 min and 400 min the fading is actually equivalent to one decade in the final logarithm range. For two decades, the protocol used for the measurement of the fading is different. Sample preparations are divided in 50 mg batches and irradiations are performed using successively a 137Cs gamma facility on 50 mg batches for the 4 samples. Batch 1 (two decades): Irradiation 120 min (ca 125 Gy) Storage at ambient temperature (22 C) 23,000 min (16 days) Batch 2 (two decades at liquid nitrogen temperature): Irradiation 120 min (ca 125 Gy) Storage in a cryostat at liquid nitrogen temperature (ca 77 K) ca 23,000 min (16 days) Batch 3 (prompt): Irradiation 120 min (ca 125 Gy) Storage at ambient temperature 120 min At the end of these storage times, each batch is divided in nine 2 mg aliquots disposed on the measurement disks and introduced in the TL-OSL reader. Then TL is measured: preheat 10 se260 C (10 C/s) TL e first glow e (5 C/s up to 625 C) TL e second glow e (background).
3
As above, correction for the prompt fading shows that between 120 min and 23,000 min the fading is actually equal to two decades in the logarithmic range. 3. Results 3.1. Anomalous fading after one decade storage at ambient temperature Fig. 3 shows the results of above mentioned “one decade” TL measurements. For every sample, two TL glow curves are shown, one for prompt TL, the other for the “one decade TL”. Each of these average TL minus photomultiplier and thermal noise intensities for 6 aliquots. Intensity standard error is close to 2.5% at the maximum of emission. Higher than 550 C the TL intensity is poorly evaluated due to thermal emission. TL glow curves of sanidine samples, Patmos-B, Courlande and La Tache are very similar. They show an intense peak around 305 C followed by a broad emission ca 375 C and no significant emission above 500 C. With the plagioclase sample it is different: the glow curve shows a first peak close to 320 C, then a broad and intense emission around 550 C obviously followed by a higher temperature emission that cannot be measured above 600 C because of the thermal background emission. The fading rate (expressed in percent) is calculated as the ratio of the difference between the prompt TL and the “one-decade” TL divided by the prompt TL. At the maximum of the 305 C peak, the fading rate for the Patmos-B sample is less than 3%. This value is within experimental errors implying that this sample displays a very small fading. The other three samples display large fading rates between 20 and 30%. These values are those that are commonly derived from measurements of volcanic alkali-feldspars. The activation energy of the first emission have been evaluated using the initial rise method (Garlick and Gibson, 1948; McKeever, 1985). They are summarised Table 1 (see also Supp. Figure 2 for details). The four samples lead to similar values; this confirms that the observed fading is anomalous as it cannot be attributed to thermal fading corresponding to these peaks. 3.2. Anomalous fading after two decades storages at ambient and liquid nitrogen temperatures In a second experiment, as shown above, the four samples have been measured after approximately two decades of storage at ambient temperature, a complementary batch of each sample having been stored in liquid nitrogen. The timing of the experiment, in particular the delay to prepare measurement aliquots and introduce them in the TL-reader have been chosen to insure that the liquid nitrogen aliquots have spent the same time (or less) at ambient temperature as the “prompt TL” aliquots before their measurement. Fig. 4 shows glow curves for prompt, and 2 decades storage in liquid nitrogen and at ambient temperature. These curves have been obtained in the same way as the curves for Fig. 3 but in this case each curve is the average of 9 aliquots. The Table 2 summarized values obtained for one and 2 decades of storage in these conditions. The fading rate has been normalized to the effective number of decades thus is an approximation of the gfactor. As in the case of the one-decade study, the Patmos-B sample displays almost no measurable fading. The other three samples display typical strong anomalous fading. They show g-factor about 30%, consistently similar to values reported in IRSL studies (Spooner, 1994). Determinations of g-factor made for one -decade are slightly lower than for two decades because when calculated
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
G. Guerin, R. Visocekas / Radiation Measurements xxx (2015) 1e6
4
Fig. 3. TL glow curves of the 4 samples measured promptly after irradiation (a) and after ca one decade of storage (b). Fading rate, shown as a series of hatching for each temperature is the ratio of the value of the lost signal to the prompt one (in percent).
Table 1 Estimated values of the activation energy and frequency factor for the first emission peak of the glow curve.
Patmos-B Courlande La Tache Olby
Peak temperature ( C)
Energy (eV)
315 305 305 325
1.23 1.30 1.30 1.33
± ± ± ±
0.01 0.01 0.01 0.01
s (GHz) 7.9 ± 0.3 48.4 ± 1.4 50.2 ± 4.8 34.3 ± 0.8
using a small number of decades the g-factor has not yet reached its asymptotic value. With storage time changed from one decade to two decades, the fading of ambient TL is observed to double roughly, in accordance with a logarithmic law. Another fact, clearly visible for the Olby sample, is the strong decrease of the fading with TL temperature. However, the unexpected result brought by this study is that by
Fig. 4. TL glow curves of the 4 samples measured promptly after irradiation (a), after ca two decades of storage at liquid nitrogen temperature (b) and after the same storage at ambient temperature (c). Fading rate, shown as a series of hatching for each temperature is the ratio of the value of the lost signal to the prompt one (in percent).
rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009
G. Guerin, R. Visocekas / Radiation Measurements xxx (2015) 1e6
5
Table 2 Estimated values of the g-factor (percent per decade) at the first maximum of TL emission. The number of decades is calculated including half of the irradiation time. Emission peak T C
Nb of decades
g-factor ambient
Courlande
305
Olby
325
La Tache
305
Patmos-B
315
1.09 2.01 1.12 1.90 1.12 1.97 1.12 1.90
30.0 36.7 19.2 29.4 24.1 32.1 1.7 1.1
storage at liquid nitrogen temperature, the anomalous fading is mostly “frozen”, the fading rates g are lowered to 5e9% per decade. These are values close to those measured for non-volcanic feldspars stored at room temperature, by TL (Huntley and Lamothe, 2001) or by OSL (Huntley and Lian, 2006). 4. Discussion A feature of volcanic feldspars is their rapid cooling to ambient temperature. Thus they keep their high temperature arrangement and crystallize typically as monoclinic sanidine, which is metastable at ambient temperature. It contrasts with the stable low temperature arrangement form of common feldspars such as found in granites, which is typically triclinic microcline (Goldsmith and Laves, 1954), and display very few macroscopic differences with sanidine. At a microscopic point of view, in the crystal sanidine the 25% Al atoms associated with 75% Si in KAlSi3O8 have a “disordered” arrangement (Barth, 1934). This results in the presence of a very high percentage of crystal radiation centres (such as AleOeAl, interstitials), contrasting with “ordered” feldspars. All these geometrical defects, likely to trap electrons, are termed “localized states” with 4 dimensions: 3 are geometrical, in the lattice; the 4th one is the energy level. N. Mott (1968) proposed a possibility of transport of trapped charge in amorphous or disordered lattice by diffusion or “hopping” from one localized state to a neighbouring one. The electrons are no more fixed but can tunnel from a state to the next one, at a distance R and a difference in energy W. The probability of hopping is:
g-factor liquid N2 5.5 8.8 5.2 1.2
activation energy has to be demonstrated. 5. Conclusions The most unexpected observation of our studies is the thermal “freezing” of the anomalous fading shown above (Fig. 4), after storage for two decades at liquid nitrogen temperature. It is a decrease of g-factor next to values observed with non-volcanic feldspars. Tunnelling alone being a pure quantum mechanical athermal process, it cannot account for this freezing of anomalous fading. Conversely electrical charge transport by “hopping” does call for phonon activation energy through the factor: exp(W/kT). Thus, at lower temperatures, “hopping” is constrained to localised states less remote in energy (as shown in Fig. 2), i.e. the transport of charges is much reduced. This is why “hopping” accounts properly for the observed freezing of anomalous fading. Storage at even lower temperatures such as liquid He may possibly further eliminate remains of contribution of “hopping” and leave ‘’pure’’ tunnelling alone. This observation has also an implication on the accuracy of the g-factor used in feldspar dating as it is measured at room temperature and may not correctly represent the “geological” value which depends on temperature history of the sample. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.radmeas.2015.05.003. References
P ¼ expð 2aRÞexpðW=kTÞ This was further applied to feldspars (Poolton et al., 2002). This mechanism appears to us as quite adapted to the situation of volcanic feldspars with their disordered lattice which provides the required basis of high density of defects i.e. “localized states”. In such a situation, all trapped electrons in the course of their “random walk” from one state to the next one will happen to be at a tunnelling distance from a radiative centre and then recombine in a reduced time spell. The above probability involving exponentials like the “pure” tunnelling does, it is expected that the overall law of fading should be exponential as well, as was observed initially (Visocekas, 1979). Our observations indicates a variation of the g-factor measured at 300 K and at 77 K to be a factor 6 and 7 for Courlande and La Tache feldspars (Table 2). If we suppose this variation is governed by a Boltzmann law, its activation energy can be roughly estimated to be a small 0.018 eV. The magnitude of this value suggests to address mechanisms related to phonon interactions, thermal diffusion in the band tail states as suggested in a recent work (Jain and Ankjærgaard, 2011). However, this can be a coincidence as our set of experimental data is limited and a behaviour ruled by an
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rin, G., Visocekas, R., Reprint of: Volcanic feldspars anomalous fading: Evidence for two different Please cite this article in press as: Gue mechanisms, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.08.009