LA-MC-ICP-MS方解石U-Pb定年技术  

LA-MC-ICP-MS calcite U-Pb dating technique

作　　者：谢博航 吴石头 杨岳衡[1,2,3] 王浩 赵子福 黄超[1,2] 谢烈文[1,2] XIE BoHang;WU ShiTou;YANG YueHeng;WANG Hao;ZHAO ZiFu;HUANG Chao;XIE LieWen(State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;Innovation Academy for Earth Science,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Science,University of Chinese Academy of Sciences,Beijing 100049,China;CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Science,University of Science and Technology of China,Hefei 230026,China;Center for Excellence in Comparative Planetology,Chinese Academy of Sciences,Hefei 230026,China)

机构地区：[1]中国科学院地质与地球物理研究所,岩石圈演化国家重点实验室,北京100029 [2]中国科学院地球科学创新研究院,北京100029 [3]中国科学院大学地球与行星科学学院,北京100049 [4]中国科学院壳幔物质与环境重点实验室,中国科学技术大学地球和空间科学学院,合肥230026 [5]中国科学院比较行星学卓越创新中心,合肥230026

出　　处：《岩石学报》2023年第1期236-248,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41903024、41525012、42173034);中国科学院青年创新促进会项目(2022066);中国科学院地质与地球物理研究所重点部署项目(IGGCAS-202204)联合资助.

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(238 U/206 Pb<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(N 2=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。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(238 U/206 Pb<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(N 2=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±0.2

关 键 词：方解石U-Pb定年 LA-MC-ICP-MS 低U副矿物 高灵敏度 Jet采样锥 

分 类 号：P578.61[天文地球—矿物学] P597.3[天文地球—地质学]