砂岩型铀矿微区原位U-Pb同位素定年技术方法研究

In situ U-Pb Isotope Dating Techniques for Sandstone-type Uranium Deposits

铀矿物定年一直是成矿年代学中的难点,随着微区原位U-Pb同位素定年技术的发展,可以直接针对矿石矿物(铀矿物)进行同位素定年;但是其中的砂岩型铀矿由于其存在状态复杂,在原位定年中剥蚀要求高,也缺乏合适的外部校正标准物质,所以定年准确度有待提高。本文研究了两种微区原位U-Pb同位素测年的方法,对砂岩型铀矿定年进行了尝试,试图解决铀矿测年中的无基体匹配问题并提高砂岩型铀矿定年水平。一是建立了一种激光剥蚀多接收电感耦合等离子体质谱仪联合电子探针进行微区原位U-Pb同位素测年的技术(LA-MC-ICP-MS&EMPA)。通过优化实验方法,对秦岭陈家庄花岗岩型铀矿进行了测试,获得与同位素稀释热电离质谱法(ID-TIMS)一致的年龄结果,证明了微区原位U-Pb同位素测年无基体匹配标准物质分析的可行性;并利用此法获得鄂尔多斯盆地红庆河和塔然高勒砂岩型铀矿的微区原位U-Pb同位素年龄信息。二是尝试了利用飞秒激光剥蚀多接收电感耦合等离子体质谱法(fsLA-MC-ICP-MS)对红庆河和宁夏宁东砂岩型铀矿样品进行微区原位U-Pb同位素定年,并获得了微区原位U-Pb同位素年龄,表明飞秒激光剥蚀技术在砂岩型铀矿定年中有很好的应用前景。本文提出,比较单一且年龄偏老的单矿物样品可以选择LA-MC-ICP-MS&EMPA联合法进行分析,需要高空间分辨率的样品建议使用fsLA-MC-ICP-MS法。

BACKGROUND:Uranium mineral dating has been difficult to ascertain in mineralization chronology.With the development of in situ U-Pb isotope dating technology,it is possible to directly date ore minerals(uranium minerals).However,due to complex occurrences of sandstone-type U deposits,high requirements for laser ablation during in situ dating,and the lack of suitable external calibration standards,the dating accuracy needs to be improved.OBJECTIVES:To solve the problem of shortage of reference materials for in situ U-Pb dating of sandstone-type uranium deposit.METHODS:Two micro in situ U-Pb isotope dating methods are used for sandstone-type uranium deposits,attempting to solve the problem of no matrix-matched standards and improve the dating technology of sandstone-type uranium deposits.A laser ablation multi-collector inductively coupled plasma-mass spectrometer combined with an electron probe for micro-area in situ U-Pb isotope dating technology was established(LA-MC-ICP-MS&EMPA).RESULTS:By optimizing the analytical technique,the Chenjiazhuang granite-type uranium deposit in Qinling was tested,and the ages were consistent with those determined by the isotope dilution-thermoionization mass spectrometry(ID-TIMS),which demonstrated the feasibility of non-matrix matched reference materials for isotope dating.This method was used to data Hongqinghe and Tarangalle sandstone-type uranium deposits in the Ordos Basin.At the same time,in situ U-Pb isotope dating of Hongqinghe and Ningdong sandstone-type uranium samples was carried out by fs-laser ablation multiple-collector inductively coupled plasma-mass spectrometry(fsLA-MC-ICP-MS).In situ U-Pb isotope ages were obtained for these two deposits,indicating that femtosecond laser ablation technology had a good application prospect in the dating of sandstone-type uranium deposits.CONCLUSIONS:Combined LA-MC-ICP-MS and EMPA method can be selected for analysis of simple-texture and relatively old minerals,whereas fsLA-MC-ICP-MS method is recommended for samples requiring high spatial resolution.