非基体匹配分馏校正的LA-ICP-MS锡石微区U-Pb定年方法研究

锡石原位微区U-Pb同位素测年,样品制备简单,具有高空间分辨率、高性价比、高效率等显著优势,是锡多金属矿床直接定年的有效手段。然而,基体效应的影响和标样的缺乏是制约该方法发展的关键。本次研究在He做载气以及加入不同辅助气(N_2、水蒸气)条件下使用锆石作为外部标准物质对锡石标准和样品进行元素、同位素分馏校准,并采用TeraWasserburg图解法和~(207)Pb法对普通铅进行校正。分析结果显示,氮气和水蒸气的引入,可以不同程度的提升Pb、U信号强度;3个不同的锡石标准和样品在He做载气及不同辅助气(N_2、水蒸气)条件下获得的Tera-Wasserburg U-Pb下交点年龄及~(207)Pb法普通铅校正后的~(206)Pb/~(238)U加权平均年龄在误差范围内都与文献报道值一致,并未发现明显基体效应的影响,显示采用锆石标准物质对不同类型锡矿床的锡石样品进行原位LA-ICP-MS U-Pb定年是可行的。

In situ U-Pb dating of cassiterite by LA-ICP-MS without a matrix-matched standard

In situ cassiterite U-Pb dating is an efficient method to directly determine the formation age of Sn-polymetallic deposits due to its simple pretreatment, high spatial resolution, high cost-efficient and fast analysis speed. The application of this method, however, is constrained by the influence of matrix effect and the lack of standard reference materials. In this study, the U-Pb age of cassiterite standard and unknow cassiterite samples were determined using different carrier gas and aux gas (N₂, water vapor). Zircon was used as the external reference material for the calibration of element and isotope fractionation, Tera-Wasserburg diagram method and ²⁰⁷Pb method were used to correct the common lead. The results show that the introduction of N₂ and water vapor during measurement can improve the sensitivity of U and Pb. Under different gas conditions, the age obtained from three different types of cassiterite samples were consistent with the previous report values within error, and no obvious matrix effect was found. This study shows that it is possible to use zircon as external standard for in situ LA-ICP-MS U-Pb dating of cassiterite.