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本文通过对8个澳大利亚TEM标准锆石和近百枚云南梅树村剖面前寒武系—寒武系界线M5层斑脱岩火山成因锆石进行了不同入射方位的激光拉曼光谱分析和对比研究。证实正常锆石拉曼光谱各峰的出现、强度与拉曼光束的入射方位有关,同时也发现晶化度差的异常锆石其拉曼光谱各峰的出现、强度与拉曼光束的入射方位无关或有很小的关系。这类异常锆石在四个不同方位上并不产生特征拉曼谱峰,就是有其强度也非常小。可以认为这些异常锆石的激光拉曼光谱特征与其晶体结构及蜕晶化的程度是有密切关系的。笔者等发现:①产在新鲜的火山灰M5层中约有80%的锆石因为高铀钍含量已发生不同程度蜕晶化,5%的锆石为滚圆状的继承老锆石,均为不适合铀-铅离子探针质谱(SHRIMP)和热电离同位素质谱(ID-TIMS)定年的异常锆石;②因为低铀钍含量该M5层中仅有15%的锆石没有发生蜕晶化,为适合SHRIMP和ID-TIMS同位素定年的正常锆石,只有这类锆石的年龄才能代表前寒武系—寒武系界线的年龄。首次提出火山成因锆石晶化度与锆石拉曼光谱的定量关系。使用这项显微拉曼光谱新技术方法可以对寒武系底界M5层火山灰或其它火成岩和变质岩锆石晶体的晶化状况作出了快速判断。对U-Pb定年来讲是一种简单易行并且十分有效的重要制约手段。
In this paper, the laser Raman spectra of eight zircons from Australian TEM zircon and nearly M45 volcanic intrusive volcanic rocks from the Precambrian-Cambrian boundary of Meishu village in Yunnan Province were analyzed and compared . The results show that the peaks of Raman spectra of normal zircons are related to the incident azimuths of Raman beams and the appearance of Raman spectra peaks of the anomalous zircons with poor crystallinity. The intensity and the incident azimuth of Raman beams Irrelevant or have a small relationship. This type of anomalous zircon does not produce characteristic Raman peaks in four different orientations, that is, its intensity is also very small. It is believed that the laser Raman spectra of these anomalous zircons are closely related to their crystal structure and degree of decrystallization. The authors found that: ① about 80% of the zircons produced in the fresh volcanic ash M5 layer have been degenerated to varying degrees due to the high content of uranium and thorium, and 5% of the zircons are rounded and inherit the old zircons, both of which are not Zircons suitable for dating by uranium-lead ion probe mass spectrometry (SHRIMP) and thermal ionization isotope mass spectrometry (ID-TIMS); (2) because only 15% of the zircons in the M5 layer are not exfoliated due to the low uranium thorium content, For normal zircons dating to SHRIMP and ID-TIMS isotopes, only the age of these zircons can represent the age of the Precambrian-Cambrian boundary. For the first time, quantitative relationship between zircon crystallization degree and zircon Raman spectrum was proposed. Using this new microscopic Raman spectroscopy method, a rapid assessment of the crystallinity of the M5 layer volcanic ash or other igneous and metamorphic zircon crystals in the Cambrian bottom can be made. For U-Pb dating is a simple and very effective and important means of control.