Investigating the possibility of finding a solution for dating the sedimentary quartzes producing a weak luminescence signal

Optically stimulated luminescence (OSL) method estimates the age of the last exposure time of sediments, OSL has been developed as a dating tool for sediments and nowadays it is used as a suitable method to determine the age of Quaternary sediments. This age determination is done using quartz grains of sedimentary samples. The two basic factors in estimating the OSL age of sediments are the dose equivalent to the natural dose (De) and the annual dose (Da). In order to estimate the De, the luminescence signal obtained from light stimulation of quartz grains is used. For most sedimentary quartz, during exposure to the stimulating light (visible) the OSL signal is seen to decay to a low level as a function of illumination time, (normally termed the OSL decay curve). In general, the decay is usually non-exponential, typically exhibiting a long "tail" to the decay at long illumination time. This initial part of the signal has a maximum value and is known as the fast component and is used to estimate the De. The next parts of the signal slowly decrease and are not suitable for dating and their effect is removed. Therefore, the intensity and shape of the luminescence signal decay curve have a fundamental effect on the accuracy of measuring the equivalent dose and, as a result, the accuracy of the resulting age.

The weakness of the sensitivity of quartz sediment grains in Iran to produce the OSL signal and also the bad behavior of the resulting luminescence signal to produce the signal growth curve (in terms of dose) has been repeatedly seen by the second author and has sometimes been published (Fattahi 2015, Appendix; Fattahi et al., 2019 Figure 3). As, this weakness can cause the inability of the luminescence method to determine the reliable age of young samples, investigating and solving this problem of quartz OSL characteristic in Iran is of great importance. However, in many studies, feldspar signal has been used due to the lack of quartz or weak sensitivity or bad behavior of quartz signal (Fattahi et al., 2007).

In order to solve this problem, comprehensive research was conducted, and this article presents part of its results. In this study, following extracting quartz grains from 3 kinds of sedimentary quartz their luminescence characteristics were investigated. The result of designed experiments showed that by increasing the intensity of the stimulating source (blue-470 nm) while sample is hold at 125°C, the intensity and the decay rate of the OSL decay curves increase. It also showed that by increasing the laboratory dose a more accurate growth curve (luminescence vs dose) can be created and the specified laboratory dose can be restored. These finding confirm the finding of previous workers on the quartz OSL characteristic from other part of the world and will provide the potential for dating young samples.