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Yuanyuan Wang, Northeast Agricultural University (China)
Yinxi Wang, Center of Modern Analysis (China)
Huiming Li, Center of Modern Analysis (China)
Jianxin Zhou, Center of Modern Analysis (China)
Qiang Chen, Center of Modern Analysis (China)
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This paper reports the laser Raman spectra of zircons at different scatter angles, including 8 standard TEM zircons in Australia and about 100 volcanic zircons from the bentonites of the M5 Precambrian- Cambrian boundary at the Meishucun section in Yunnan , China. It was confirmed that the emergence and relative intensity of Raman peaks for the normal zircons are related to the difference angle of incidence. While these characteristic Raman peaks of abnormal zircons disappeared or were very weak, they didn't show any relevant relationship with incidence angle as that of normal zircons. Raman characteristic of the abnormal zircons germane relevant to its crystal structures with the metamictization. We have discovered taht:(1) Because of the content of the high contents of uranium and thorium, about 80% of the zircons from the fresh volcano ash M5 layer had already bcome metamict to different degrees. About 5% of the zircons from the fresh volcano ash M5 layer are inherited. These zircons are all abnormal and not suitable for U-Pb isotope dating by SHRIMP or ID-TIMS.(2) Because of the of the low uranium and thorium content, only about 15% of zircons in the M5 layer are not metamicizationt. These zircons are normal and suitable for U-Pb isotope dating by SHRIMP or ID-TIMS. Only the ages of these zircons can represent that real age of the Precambrian- Cambrian boundary. We have firstly pointed out a quantitative relationship between the degree of crystallization and Raman spectra for the volcano zircons. This new technique, using Raman spectroscopic analysis to study the degree of crystallization of zircon from the geological samples, has significant value for requiring the absolute age of zircons from the Procambrian -Cambrian boundary. It is also an important ,simple and effective way to constrain the results of the U-Pb dating method.
Reference
Compston W, Williams I S, Kirschvink J L, Zhang Z C, Ma G G. 1992. Zircon U-Pb ages for the Early Cambrian time-scale. Journal of the Geological Society, London , 149: 171∼184
Jenkins R J F, Cooper J A, Compston W. 2002.Age and biostratigraphy of Early Cambrian tuffs from SE Australlia and southern China. J. Geol. Soc. London , 159: 645∼658
Condon D, Zhu M Y, Bowring S A, Wang W, Yang A H, Jin Y G. 2005.U-Pb Ages from the Neoproterozoic Doushantuo Formation, China. Science, 308: 95∼98
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•Wang Yinxi : Corresponding author
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