Assistant Professor, Earth Physics Department, Institute of Geophysics, University of Tehran, Iran
M.Sc. Student of Geophysics, Earth Physics Department, Institute of Geophysics, Universityof Tehran, Iran
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Gowk fault which is located west of the Lut-block east of Iran is one of the main and active structures in the region. To estimate the activity of this fault, we need to calculate its’ slip rate. The South part of the Gowk fault passes through the Golbaf Lake and has displaced the rivers which have cut the lake. The displacement is around 30 meters and if we can find the time of displacement we can then calculate the slip rate. One of the most useful methods to date the Quaternary sediments directly, is Luminescence dating. This method has been widely used and the range of materials that can be dated, using different procedures of luminescence dating, is being developed. In this paper, we are presenting part of the research which was done to sort out the problems that we had for dating the samples that were collected from the Golbaf lake. To measure the age of the samples, two parameters are needed; the equivalent dose and the dose rate. To determine the equivalent dose, the single aliquot regenerative dose (SAR) protocol was used. In this method, after measuring the natural signal, the sample is exposed to different known doses in the laboratory and the related luminescence signal is measured. Then, a standard growthcurve is built, which the equivalent dose can be calculated from.
To examine the suitability of the SAR protocol in dating the collected samples, the capability of this method in order to recover a known laboratory dose was investigated. We confronted some unexpected results while recovering the dose because the determined dose overestimated the certain given dose in the laboratory by around20 percent. Three possible reasons were considered: methodological factors which can influence the determination of the equivalent dose, physical processes affecting the luminescence signal and technological factors(the measurement device).The first two reasons were assessed experimentally (according to tables 1 and 2). Thirty aliquots were built from the GB3 feldspar sample and the Risoe automated TL/OSL DA-15 reader was employed for all experiments. The first 15 aliquots were used for determinations according to the left column in table1 (the cut heat is fixed). The natural signal was depleted and after 10 hours gap, the signal was measured again (Figure1). Then, all samples were exposed to 26 Gy dose (assuming as the natural dose), the procedure in the left hand side of table1 was followed to test the capability of SAR in recovering this dose. In table1, Lx shows Ln (natural luminescence signal) and LR (regenerative luminescence signal) and Tx shows Tn (natural test dose signal) and TR(regenerative test dose signal).The results in figure2 demonstrate that, the recovered dose is more than the given dose by approximately 20% (about 30 Gy).After that, the same procedure was followed for the next 15 aliquots according to the right column of table1, but with equal preheat and cutheat. The results are shown in figure3 which is the same as figure2.We then repeated one stage of the SAR protocol five times for each aliquot using a fixed dose and expected that the results for all to be similar. This stage was completed according to table2 for four aliquots. The results are shown in figures 4, 5 and 6. As is shown in figure 4, the first signal consists of more photons in comparison with the other signals. Figure 5 shows the regenerative dose and the test dose curves for the four aliquots in five cycles. As is clear in the diagram, the first point of the regenerative dose curve for all the four aliquots are more than the other points in the curve, for both IR and Post-IR methods. Figure 6 shows the Lx/Tx ratio. It can be explicitly seen that the first stage is 40% more than other stages. As all the luminescence signals following the test doses (Tx) are almost similar, this 40% difference cannot be due to sensitivity change.
The above experiments show that electron exchange from heat-sensitive traps to light-sensitive ones because of the preheat effect cannot be the reason for the dose recovery overestimate, since both IRSL and OSL signals measured 10 hours after natural measurements, are negligible in compare to natural signals. Electron exchange from the light-sensitive traps to the heat-sensitive ones due to laser light and subsequently in the opposite pattern cannot be a possible reason since all the standard growth curves pass through the origin (figure 7).Incomplete evacuation of the electrons because of inadequate intensity and time of the laser beamis not responsible for the dose recovery overestimate because laser beam has been similar for all the stages through the experiments. Inability of the SAR protocol to correct the sensitivity according to heat, dose and light is not the case since the SAR method has provided almost similar result for the last four stage of figure 6.
As the methodological and physical factors could not be the reason for overestimating the recovered dose in the laboratory, we focused on the third factor, technology. Although the device (TL/OSL DA-15 reader) is claimed to be complete and without any defect, some reports of its failure has been presented. The effect of the dose and light on adjacent aliquots as reported by Bray et. al (2002), was found to be responsible for dose overestimation. This is because of the packed arrangement of aliquots on a disk. Since we wanted to investigate and eliminate the possible effect of the radioactive dose and the laser beam on adjacent aliquots, we decided to rearrange the aliquots. So the same procedure presented in tables 1 and 2 was followed for aliquots 1, 4, 7, 10 and 13. The results have been shown in figures 10 and 11. Figure 10 shows the result of repeating a cycle for these aliquots. This figure demonstrates that there was not much sensitivity change for the sample in this case. Figure 11 shows that the SAR method was capable of recovering the dose properly at different temperatures, but 2400 and 2800 are the most suitable temperatures in this case.
So the problem was solved by putting the aliquots in every two other positions, and the SAR protocol could recover the given dose properly. We repeated this procedure by setting the aliquots in every other position too and the results were the same. Based on above result we put the natural aliquots in every other position and determined the age of the GB3 sample. This procedure provided an age of 4100 years for this sample. Similar approach was considered for calculating the age of other samples from the same site. Detailed information about the process of dating and the slip rate of Gowk fault could be found in the paper ‘Determination of the slip rate on the Gowk fault’ by Fattahi et al. (submitted).