MC-ICP-MS两步静态法测量U-Th同位素

本研究建立了多接收电感耦合等离子体质谱（MC-ICP-MS）两步静态法测量U-Th同位素的分析步骤和数据处理流程。对于U同位素，第一步用二次电子倍增器（SEM）测量²³⁴U，同时用法拉第杯测量²³³U、²³⁵U、²³⁶U和²³⁸U；第二步用SEM测量²³⁶U，同时用法拉第杯接收²³³U、²³⁴U、²³⁵U和²³⁸U。对于Th同位素，第一步用SEM测量²²⁹Th；第二步用SEM测量²³⁰Th，同时用法拉第杯接收²²⁹Th和²³²Th。U和Th的测量均通过两个步骤的多次循环采集同位素数据。该方法的关键在于采用SEM与法拉第杯测量的²³⁶U_SEM/²³³U_FC比值（经过质量歧视校正）与²³⁶U/²³³U真值的比较以校正SEM与法拉第杯的相对增益。本研究采用两步静态法重复分析了U同位素的国际标准样品Harwell uraninite（HU-1）。测量结果表明，HU-1的δ²³⁴U平均值为（-0.48±1.92）‰，误差为单次测量结果的标准偏差（±2σ，n=55）。该结果与国内外不同实验室发表的数据在误差范围内一致。此外，本研究将两步静态测试法与SEM跳峰扫描法分别用于分析次生碳酸盐岩实验室标准样品的U-Th同位素和²³⁰Th/U年代（SB-530、SB-8600、SB-108K、SB-240K、SB-335K和SB-435K）。对比实验结果表明，MC-ICP-MS两步静态法测量U-Th同位素行之有效，²³⁴U/²³⁸U和²³⁰Th/²³⁸U比值的分析精度可分别达到1‰~2‰和1‰~3‰，然而该方法的可靠性还需要更多国际标准样品的检验。

U-Th Isotopic Measurements by MC-ICP-MS with Multi-Static Method

This study presented a newly developed multi-static U-Th isotopic measurement method. For U isotopes, firstly, the ²³⁴U was measured on the SEM and simultaneously, the signals of ²³³U,²³⁵U, ²³⁶U and²³⁸U were measured in Faraday cups. Secondly, ²³⁶U was collected on the SEM, and the ²³³U,²³⁴U, ²³⁵U and²³⁸U were measured in Faraday cups. The ratio of²³⁶U_SEM/²³³U_FC (the ratio of SEM to Faraday cup measurement corrected for mass fractionation) was used to monitor the SEM/Faraday cup relative yield. The²³⁸U/²³⁵U ratio was used for instrumental mass fractionation correction by comparing the measured value with the natural value. For Th isotopes, the ²²⁹Th and ²³⁰Th were alternately measured on SEM and ²³²Th in a Faraday cup. Th isotopic measurements were carried out immediately after the U measurements from a same sample. The correction factors of SEM/Faraday cup relative yield and instrumental mass fractionation determined in U measurements were used for Th isotopic data corrections. The ideal ion beam intensities for measurements are 3-6 mV for ²³⁶U, 5-15 V for²³⁸U (equivalent to 10¹¹ Ω amplifier), <10⁵ cps for ²²⁹Th and²³⁰Th. The U and Th isotope data are usually acquired 200-400 cycles taking 20-30 min. To test the reliability of this method, the U isotopic standard Harwell uraninite (HU-1) was repeatedly measured and yielded a mean δ²³⁴U value of (-0.48±1.92)‰ (±2σ, n =55), which is in agreement with the data previously published by different laboratories. In this study, the multi-static method was applied to six in-house secondary carbonate standards, SB-530,SB-8600, SB-108K, SB-240K, SB-335K and SB-435K. The repeated measurements suggest that the six standards have U content of 500-3 000 ng/g and Th content of 0.1-1.0 ng/g. Their δ²³⁴U values range from 95 to 1030, and ²³⁰Th/²³⁸U ratios range from 0.01 to 1.33. Their error weighted mean ages are (0.53±0.01), (8.61±0.01), (107.6±0.2), (239.4±0.8), (332.8±1.9) and (437±5) ka, respectively. These results were confirmed by the comparative study using the SEM method carried out in the Institute of Geology and Geophysics, Chinese Academy of Sciences. This study demonstrates that the newly developed U-Th isotopic measurement method can routinely measure the²³⁴U/²³⁸U and²³⁰Th/²³⁸U ratios at the precision of 1‰-3‰, but its reliability needs to be further validated by more international standards.