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Activity of Cangbaishan Tianchi Volcano since Late Pleistocene: The constrain from geochronology of high precision U- series TIMS method

WANG Fei1, CHEN Wen- ji 1, PENG Zi- cheng2, LI Qi1 (1. Institute of Geology, China Seismological Bureau, Beijing 100029, China; 2. Department of Earth and Space Science, Science and Technology University of China, Hefei 230026, China)  
Eleven samples of lava and pumice from the cone of Changbaishan Tianchi Volcano, Jiling, China, were dated by using high precision U- series TIMS method. The results show that the bottom of the cone formed before 350 ka, the middle part in 70~ 80 ka, the upper during 20~ 1 ka, and the top less than 1 ka, and the age based periods of the volcano eruption since Late Pleistocene is given as follows: 350 ka, 70 ka, 18~ 25 ka, 10 ka, 4~ 5 ka, 1~ 0.75 ka, which may offer the basis for the study of volcanic disaster in future. In addition, the principle of dating young volcanic rocks by using U- series TIMS method is introduced briefly. Differentiation characteristics of U and Th in different minerals of the volcanic rocks are discussed, and the ability producing isochrons, based on U and Th differentiation, are discussed. In the last part of the paper, the closure of samples to the elements U and Th, which is important for age results, is discussed by using (234U/238U) radioactivity ratio which can be used to monitor if the samples have been weathered or eroded or leached since the time they formed. In this study, all samples have (234U/238U) activity ratios within 1% of secular equilibrium [(234U/238U) radioactivity ratios are unity), indicating no disturbance of the 234U- 238U system. All of these discussions show that the TIMS method is good to date Tianchi volcanics and the results are reliable.
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