【摘要】 方解石在成岩过程、生物作用、热液过程等多种地质环境中广泛形成，激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）方解石U-Pb定年技术在解决关键地质问题中具有广阔的应用前景。方解石中U含量相对低(<5.0×10^-6),普通Pb占比高(²³⁸U/²⁰⁶Pb<1),这对仪器灵敏度提出了高要求。相比于Q-ICP-MS和SF-ICP-MS,MC-ICP-MS具有同位素同时测定优势，分析精度更高。前人优化了仪器参数，对提高灵敏度进行了一些尝试，为了最大化的提高灵敏度，本研究基于Neptune Plus型号MC-ICP-MS,系统优化锥组合和辅助氮气，使得U和Pb元素灵敏度最优。研究表明采用Jet+X锥结合氮气增敏(N₂=4.0mL·min^-1)条件下仪器灵敏度最高，相比于常规条件（S+H锥未结合氮气增敏）,U和Pb的信号强度分别提高4倍和3倍。同时我们发现氮气增敏情况与锥组合有很大的关系，当采用X型号截取锥时，氮气的引入可以使得U的信号强度提高，而采用H型号截取锥时，氮气的引入并没有改变信号强度。基于优化后的仪器参数，本文建立了高空间分辨率（～85μm）LA-MC-ICP-MS方解石U-Pb定年方法。采用三个方解石U-Pb定年标准物质对方法进行了验证，所得结果与ID-TIMS定值结果在误差范围内一致，证明我们的方法对低U(～0.03×10^-6)样品的分析是有效的。初步测定了两个实验室内部标准物质：TLM和LSJ07。一次测量TLM的结果为219.8±2.3Ma（2s, n=30）,两次测量LSJ07的结果为26.4±0.2Ma（2s, n=30）和26.7±0.2Ma（2s, n=30）。这两个标准物质的普通铅占比相对较低，是潜在的方解石U-Pb定年标准物质。我们发现激光束斑大小的改变明显影响到结果的准确度，这可能与不同束斑下的分馏效应差异性有关，特别是当束斑小于60μm时。因此我们建议在进行方解石U-Pb定年时，标样和未知样品要严格匹配激光参数，最后初步讨论了方解石U-Pb体系的封闭情况。更多还原

【Abstract】 Calcite is widely formed in various geological environments such as diagenesis, biological processes and hydrothermal processes. Laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS） calcite U-Pb dating technique has a broad application prospect in solving key geological problems. Calcite has relatively low U content(<5.0×10^-6) and high common lead proportion(²³⁸U/²⁰⁶Pb<1), which requires high sensitivity of the instrument. Compared with Q-ICP-MS and SF-ICP-MS, MC-ICP-MS has the advantage of simultaneous isotope determination and higher analysis accuracy. Predecessors optimized instrument parameters and made some attempts to improve sensitivity. To maximize sensitivity, based on Neptune Plus model MC-ICP-MS, this study gets the best sensitivity of U and Pb by optimizing cone and auxiliary nitrogen systematically. The research shows that the instrument has the highest sensitivity under the condition of Jet+X cone combined with nitrogen(N₂=4.0mL·min^-1). Compared with the conventional condition（S+H cone combined without nitrogen）, the signal intensities of U and Pb are increased by about 4 times and 3 times respectively. At the same time, we find that the nitrogen sensitization has a great relationship with the cone combination. When X sample cone is used, the introduction of nitrogen can improve the signal intensity of U, while when H sample cone is used, the introduction of nitrogen does not change the signal intensity. Based on the optimized instrument parameters, we established a high spatial resolution（～85μm） LA-MC-ICP-MS calcite U-Pb dating technique. Three calcite U-Pb dating reference materials are used to verify our technique, and the results are consistence with ID-TIMS value within error, which proves that our method is effective for the analysis of low U(～0.03×10^-6) sample. Two reference materials in the laboratory are preliminary determined: TLM and LSJ07. The results of one measurement of TLM are 219.8±2.3Ma（2s, n=30）, and the results of two measurements of LSJ07 are 26.4±0.2 Ma（2s, n=30） and 26.7±0.2Ma（2s, n=30）. The proportion of common lead in these two reference materials is relatively low, and they are potential calcite U-Pb dating reference materials. We find that the change of laser spot size significantly affected the accuracy of the results, which may be related to the difference of fractionation effect under different laser spots, especially when the laser spot is smaller than 60μm. Therefore, we suggest that the laser parameters should be strictly matched between the reference materials and the unknown samples in the U-Pb dating of calcite. Finally, we preliminarily discussed the closure of calcite U-Pb system.更多还原