低铀方解石高空间分辨率LA-MC-ICP-MS U-Pb测年方法

High Spatial Resolution U-Pb Dating of Low-Uranium Calcite Using LA-MC-ICP-MS

  • 摘要: 碳酸盐矿物特别是方解石分布广泛,作为原生和次生矿物在多种地质环境中形成,结合U-Pb同位素测年体系,能为地球科学应用提供直接的时间约束,具有广阔的应用前景。但方解石矿物的铀含量通常较低(小于5μg/g),测年难度大,限制了该方法的发展。激光剥蚀多接收电感耦合等离子体质谱仪(LA-MC-ICP-MS)具有高灵敏度和高精度的特点,已成功应用于锆石、方解石等副矿物的高空间分辨率U-Pb测年。本文针对LA-MC-ICP-MS的载气和氮气流速、屏蔽炬状态等实验参数进行了详细优化,建立了适用于低铀方解石矿物的高空间分辨率U-Pb测年方法。为提升仪器灵敏度和等离子体状态,详细讨论了锥组合、屏蔽炬接地状态、N2引入量和Ar载气流速对U、Pb信号强度及氧化物产率(UO/U)的影响。结果表明,在Jet+X锥组合、屏蔽炬接地、8mL/min N2引入量和0.9L/min载气Ar流速条件下,Pb灵敏度达到最高,同时氧化物产率(UO/U)低于1%。为验证该方法的准确性,采用三个低铀(0.04~0.63μg/g)方解石U-Pb定年参考物质进行测试,使用高铀含量的WC-1标准物质作为校正238U/206Pb的外标,同时减小束斑大小和剥蚀频率,保持标准物质和待测样品的剥蚀坑纵横比一致。实验结果表明,在44μm束斑条件下,LD-5、PTKD-2和TARIM的U-Pb年龄测试值分别为73.20±0.56Ma、152.7±2.5Ma和206.2±1.3Ma,与ID-TIMS/ID-MC-ICP-MS的定值结果在误差范围内一致,验证了LA-MC-ICP-MS高空间分辨率测定低铀方解石U-Pb年龄的可靠性。

     

    Abstract: The combination of carbonate minerals with the U-Pb isotope dating system provides direct temporal constraints for geoscientific applications. LA-MC-ICP-MS with its high sensitivity and accuracy, has been successfully applied to the high spatial resolution U-Pb dating of zircon, calcite and other minerals. The experimental parameters of LA-MC-ICP-MS were optimized in detail, and a high spatial resolution U-Pb dating method was established for low-uranium calcite minerals. The effects of cone combination, GE mode, N2 introduction and Ar carrier gas flow rate on the U and Pb signal intensity and oxide yield (UO/U) were discussed in detail. The results showed that the highest sensitivity of Pb was achieved under the conditions of Jet+X cone combination, guard electrode (GE) grounded state, 8mL/min N2 introduction and 0.9L/min Ar carrier gas flow rate, while the oxide yield (UO/U) was lower than 1%. Three low uranium (0.04−0.63μg/g), calcite U-Pb dating reference materials were used to validate the reliability of the method. Samples LD-5, PTKD-2 and TARIM yielded a lower intercept age of 73.20±0.56Ma, 152.7±2.5Ma and 206.2±1.3Ma, respectively, under the condition of 44μm beam spot, which were in agreement with the calibrated results of ID-TIMS/ID-MC-ICP-MS within the error range. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202403160057.

     

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