Dating Of Volcanic Eruptions And Estimation Of Paleo-firing Temperature Using Luminescence Signals For Samples From Baked Layers In Datong, China

Datong volcano groups are located in the east of Datong Basin. The volcanic eruptions lie between Quaternary loess layers or lacustrine sediments, which provides unique and valuable geological records for both volcanic and Quaternary studies. Since1930s, much research on Datong volcano groups has been carried out, thus making a lot of achievements on volcanic active ages, tectonic and geomorphologic background of volcanoes, chemical component types of volcanic rocks and deep dynamic characteristics affected by volcanoes. However, different results of volcanic eruption ages in Datong were reported. Besides, the study on temperatures of lava was rare, which blocks the understanding of cycles and the trends of volcanic activities.The luminescence signals of minerals in baked layers can be zeroed because of being heated by the lava above, which is essential for dating by luminescence method. In the meantime, luminescence signals can provide the historical heating information of samples; but up to now, there is no appropriate method for determining the firing temperatures. In this paper, fine grain quartz extracted from baked layer samples was studied by using luminescence methods. On one hand, samples from baked layers from different sites were dated for determining volcanic eruption histories in Datong. On the other hand, methods for determining firing temperatures were built by studying the relations between sensitivity changes and re-firing temperature. Then, the newly-built methods were applied to the fine grain quartz extracted from baked layer samples. Based on the results of firing temperatures of baked layer, the temperature of lava above was deduced.As for dating, Optical Stimulated Luminescence (OSL), Recupated OSL(Re-OSL)/Thermal Transferred OSL (TT-OSL) and Thermoluminescence (TL) dating protocols were applied. The results showed that the TL dating method gave more reliable results, for the reasons that doses could be well recovered and that the response curves could be well fitted with a saturating exponential function, showing no signs of saturation in the regenerative dose range. The difference between the TL method we used and conventional TL method was mainly exhibited in2aspects. Firstly, instead of poly-minerals, quartz was used for dating. Secondly, instead of additive dose procedure, regenerative protocol with sensitivity calibration by the response of test dose was applied. It indicated that there were more than5phases of volcanic eruptions in the volcanic area. There were eruptions in Nanshishan area dating back to about380ka, in Xigelaoshan and Yujiazhai area about260ka ago, in Yujiaxiaobao and xishawo area dating back to about200ka, in Dongshuitou dating back to85ka, in Huangjiawa area about50ka ago.As for determining temperatures, the relations between sensitivity changes and re-firing temperature were observed by using samples of different known temperatures. The findings from the re-firing experiments indicated that110℃TL and150℃TL sensitivity had the potential to indicate whether the sample had been fired to temperatures above500℃、900℃, respectively. And more specific firing temperatures could be estimated by comparing the OSL/150℃TL ratio against re-firing temperature between the lava-baked samples and the unfired samples. A temperature profile of the8lava-baked samples was established by using the methods built. The paleo-firing temperature decreased exponentially as the distance from the lava increased. According to this trend, the temperature of the lava was about1100℃.