In-situ U-Pb dating of uraninite by fs-LA-ICP-MS  

作　　者：ZONG KeQing CHEN JinYong HU ZhaoChu LIU YongSheng LI Ming FAN HongHai MENG YanNing 

机构地区：[1]State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences,China University of Geosciences [2]Beijing Research Institute of Uranium Geology

出　　处：《Science China Earth Sciences》2015年第10期1731-1740,共10页中国科学（地球科学英文版）

基　　金：supported by the National Natural Science Foundation of China(Grant Nos.41203027 and 41473031);the State Administration of Foreign Expert Affairs of China(Grant No.B07039);the Special Fund for Basic Scientific Research of Central Colleges,China University of Geosciences(Wuhan)(Grant No.CUGL140403)

摘　　要：In this study, the Pb/U fractionation between zircon and uraninite during femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry （fs-LA-ICP-MS） analysis was studied in detail. The results show significant Pb/U fractionation between zircon and uraninite during fs-LA-ICP-MS analysis that when calibrated against the zircon standard M257, the obtained U-Pb age of the Chinese national uraninite standard GBW04420 is 17% older than the recommended value. Thus, the accurate in-situ U-Pb dating of uraninite by LA-ICP-MS requires matrix-matched external standards for calibration. Uraninite in thin sections of two U-mineralized leucogranite from the Gaudeanmus in Namibia was analyzed by a fs-LA-ICP-MS equipped with a Signal Smooth Device （SSD）, using laser spot and frequency of 10 μm and 1 Hz, respectively. When calibrated using GBW04420 as the external standard, two samples give weighted mean 2066pb/238U ages of 504±3 Ma （2σ, n=21） and 503±3 Ma （2σ, n=22）, and only one of two samples yields a concordia U-Pb age of 507±1 Ma （2or, n=21）. These results are consistent with ID-TIMS U-Pb ages of 509±1 and 508±12 Ma and are also indistinguishable from zircon U-Pb upper intercept ages of 506±33 Ma （2σ, n=29） and 501±51 Ma （2σ, n=29）. The present study shows that in-situ U-Pb dating of uraninite can deliver more reliable formation ages of the deposit than dating coeval high-U zircon because the latter commonly suffer significant Pb loss after formation. Our results confirm that GBW04420 is an ideal matrix matching standard for in-situ U-Pb dating of uraninite.In this study,the Pb/U fractionation between zircon and uraninite during femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry(fs-LA-ICP-MS)analysis was studied in detail.The results show significant Pb/U fractionation between zircon and uraninite during fs-LA-ICP-MS analysis that when calibrated against the zircon standard M257,the obtained U-Pb age of the Chinese national uraninite standard GBW04420 is 17%older than the recommended value.Thus,the accurate in-situ U-Pb dating of uraninite by LA-ICP-MS requires matrix-matched external standards for calibration.Uraninite in thin sections of two U-mineralized leucogranite from the Gaudeanmus in Namibia was analyzed by a fs-LA-ICP-MS equipped with a Signal Smooth Device(SSD),using laser spot and frequency of 10μm and 1 Hz,respectively.When calibrated using GBW04420 as the external standard,two samples give weighted mean 206Pb/238U ages of 504±3 Ma(2σ,n=21)and503±3 Ma(2σ,n=22),and only one of two samples yields a concordia U-Pb age of 507±1 Ma(2σ,n=21).These results are consistent with ID-TIMS U-Pb ages of 509±1 and 508±12 Ma and are also indistinguishable from zircon U-Pb upper intercept ages of 506±33 Ma(2σ,n=29)and 501±51 Ma(2σ,n=29).The present study shows that in-situ U-Pb dating of uraninite can deliver more reliable formation ages of the deposit than dating coeval high-U zircon because the latter commonly suffer significant Pb loss after formation.Our results confirm that GBW04420 is an ideal matrix matching standard for in-situ U-Pb dating of uraninite.

关 键 词：URANINITE fs-LA-ICP-MS U-Pb dating element fractionation Rossing GBW04420 

分 类 号：P619.14[天文地球—矿床学] O657.63[天文地球—地质学]