Abstract
Following the successful application of thermoluminescence (TL) dating to fired archaeological materials in the 1960s, similar techniques were applied to geologically-fired volcanic deposits that have had their luminescence ‘clocks’ reset by volcanism. Luminescence dating methods have undergone extensive development and refinement during last 30 years. Today, thermoluminescence techniques are potentially highly useful methods for dating volcanic and related materials and events over timescales ranging from 102-106a. The potential of red thermoluminescence (RTL) emission from quartz and feldspar has attracted some attention during the last decay. RTL from heated quartz has shown important physical characteristics such as signal stability, low sensitivity changes and the slow onset of saturation with dose.
This paper presents the result of a series of experiment exploring the suitability of RTL from unheated quartz for dating geological sediments. Outlines of the single-aliquot regenerative dose (SAR) protocol for equivalent dose determination using RTL signal are discussed. It is shown that, for the samples under study, SAR can successfully and accurately recover a known laboratory dose in RTL from unheated quartz separates. Natural RTL De was also calculated. It is found subtracting RTL De estimated using bleached aliquots, from RTL De estimates using non bleached aliquots provides results that match the UV OSL De estimated from the same sample.
Keywords: luminescence dating; quartz; red emission; Equivalent dose; RTL
1. INTRODUCTION
Red Thermoluminescence (RTL) in quartz (which possesses a broad emission band with a peak around 620 nm) was first observed in samples collected from volcanic ash layers (Hashimoto et al, 1987). The potential of RTL in natural quartz for dating applications was examined after the failure of blue thermoluminescence (BTL) for dating old volcanic quartz (e.g., Miallier et al 1991). Since then the potential of this signal as a dosimeter for dating heated quartz was reported by limited authors (e.g., Hashimoto et al., 1991, 1996; Miallier et al., 1994; Fattahi and Stokes 2001).
The presence of red emissions (650-750 nm) in feldspar TL has been shown in several studies (e.g., Huntley et al., 1988; Krbetschek et al., 1997). Previous studies have shown that red thermoluminescence (RTL) of feldspar does not suffer from anomalous fading whereas blue-UV emissions from the same sample do demonstrate the effect (Zink and Visocekas 1997; Fattahi, 2001). Using this advantageous, Zink and Visocekas (1997) successfully dated three volcanically derived feldspars using the far-red TL and additive dose method.
Despite of these successes the main reason for not using wide spread RTL of quartz and feldspar was related to unwanted incandescence and the technological difficulties involved in blocking these emissions (e.g., Miallier et al., 1991). For temperatures greater than 350°C, the incandescence of the sample and heater plate has an intense emission at wavelengths of > 790 nm (Fattahi 2001); this has limited routine measurements to temperatures below 400°C (Zink and Visocekas, 1997; Scholefield and Prescott, 1999).
Results following technical developments in quartz and feldspar RTL have enabled Fattahi (2001) to observe RTL peaks up to 600°C for quartz and feldspar and to produce successful dates for heated quartz with ages between 300 to1200 ka (Fattahi and Stokes, 2000a, b). Successful dating has been due to the reproducibility of quartz glow curves; stability and long lifetime of its trap components and high dose saturation of its growth curves.
As a part of general study exploring the suitability of RTL from naturally occurring unheated quartz for geological dating applications, the result of a series of experiments is presented in this paper which investigate the suitability of single aliquot regeneration method for equivalent dose determination.
2. EXPERIMENTAL SET UP AND SAMPLES USED
Experiments were carried out using a Risø TA-15 automated TL/OSL system (fitted with a 90Sr/90Y beta source delivering ~ 7 Gy.minute-1). The photomultiplier used is an alternative cooled (~-20°C) extended 9658“red” S20 PMT, that is equipped with an S 600 PHOTOCOOL thermoelectric refrigeration chamber which allows active cooling of the photocathode down to 40°C below room temperature. Luminescence was measured through a range of filter combinations designed to transmit red (= 600-650 nm) emissions. All TL glow curves were measured using a heating rate of 5°C/s. The measurements were made on unheated quartz extracted from samples laboratory number (SQA).
3. DOSE RECOVERY USING AN ORANGE-RED IRSL SAR
A key requirement for testing any De determination method is to accurately measure a dose given in the laboratory. This requirement was examined for SAR protocol (Diagram 1, step 1-6). After depletion of natural luminescence of a set of aliquots using RTL measurement, a laboratory dose of 1150 Gy was administered. This dose was treated as a ‘surrogate’ natural dose, and the De was determined via a SAR protocol using a range of regenerated doses from 350 –1500 Gy (Figure 1 & 2). To monitor the possible sensitivity changes a fixed test dose was applied during the RTL measurement sequence (Figure 3 & 4).
To test the accuracy of the sensitivity correction, after the first three cycles, the first regeneration cycle was repeated. The ratio of the 4th regeneration to the 1st regeneration data point (recycling ratio) is 0.98. This is well within the acceptability criteria proposed by Murray and Wintle (2000). This experiment is repeated for 3 aliquots of sample SA5Q and the result is 1143.3±6.28 Gy.
These experiments suggest that this modified SAR method as applied using RTL meets a necessary requirement for accurate De determination for red emission.
4. ESTIMATION OF NATURAL ABSORBED DOSE USING AN RTL SAR
We have applied the SAR protocol (Diagram 1, step 1-6), to SQA quartz samples. This allows us to estimate RTL De values obtained on RTL emissions. The De was determined by interpolation and the sensitivity was corrected using the procedure explained in the previous section. The result for De values is shown in Table 1. The author compares the result of RTL and UV OSL. Three aliquots of sample SA7Q was bleached by sunlight in Oxford (20 Nov 2002) and the De was estimated to be 553 Gy. As a result the RTL De since the last thermal resetting is 813 Gy, while the De since the last optical resetting is 260±20 Gy that matches with the result produced by UV OSL (237±11 Gy).
5. CONCLUSION
SAR RTL from unheated quartz can be used for the equivalent dose estimation. SAR RTL has provided De for SAQ that are much bigger than the estimated using blue OSL. This difference is mainly due to thermal residual signal that has not been deleted during past exposure of the sample to sun light. However, RTL De estimated from bleached samples was subtracted from the De estimates of unbleached samples. The result is comparable with the De produced by UV OSL of same quartz.
ACKNOWLEDGEMENT
The author would like to thank Research Department of Tehran University and The Institute of Geophysics for their general support. Particular thanks are due to Mr Karimkhani and Asgharzadeh for technical support. I gratefully acknowledge Stephen Stokes, Richard Bailey, Aby Stone and Helen Bray from Oxford Luminescence Research Group, University of Oxford for their close co-operations.
REFERENCES
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List of Figures:
Figure 1. Regenerated RTL Glow curves of sample SA5Q
Figure 2. Regenerated RTL growth curves of sample SA5Q
Figure 3. Test dose (460Gy) RTL Glow curves of sample SA5Q
Figure 4. The response of test dose signal following regenerated RTL measurement vs the cycles.
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