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

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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. 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 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 leucogranites from the Gaudeanmus in Namibia was analyzed by a fs-LA-ICP-MS equipped with a Signal Smooth Device (2σ, n = 21) and 503 ± 3 Ma (2σ, n = 21), using laser spot and frequency of 10μm and 1 Hz, respectively. (2σ, n = 22), and only one of two samples yields a con 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 (2σ, n = 21) 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 generally suffered significant Pb loss after formation. Our results confirm that GBW04420 is an ideal matrix matching standard for in-situ U-Pb dating of uraninite.
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