MC-ICP-MS标样-样品交叉测试法测定石笋样品的230Th/U年龄

借鉴国际上已经建立的方法，本研究设计了一套便捷的UTEVA树脂分离纯化U与Th的化学流程，以及多接收电感耦合等离子体质谱测量U/Th同位素的方法，用于精确测定石笋样品的²³⁰Th/U年代。U/Th同位素的测量采用标样-样品交叉测试法（standard-sample bracketing），即在测量未知样品之前和之后分别测量标准样品的同位素，通过外部归一化和线性内插法估算测量未知样品时仪器的校正因子，如质量歧视和二次电子倍增器相对于法拉第杯的增益。为检验该方法的可靠性，本研究分析了国际U标准样品Harwell uraninite-1（HU-1）、U/Th内用标准样品MFT和已知年代石笋样品的U/Th同位素。结果显示：HU-1的δ²³⁴U=（-1.27±2.14）‰（±2σ，n=29），该结果与国际上不同实验室的测量值在误差范围内一致。U/Th内用标准样品MFT（±2σ，n=22）的δ²³⁴U、²³⁰Th/²³⁸U放射性活度比和年代平均值分别为（408.2±3.2）‰、1.002±0.003和(124.3±0.5) ka，具有较好的重现性。47个石笋样品的²³⁴U/²³⁸U、²³⁰Th/²³⁸U放射性活度比和²³⁰Th/U校正年代的相对误差（±2σ）分别约为0.6‰、2.4‰和0.5%，与已发表数据在误差范围内吻合。

Measurement of U/Th Isotopic Compositions in Stalagmites for 230Th/U Geochronology Using MC-ICP-MS by Standard-Sample Bracketing Method

Over the past two decades, multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has gradually become the most commonly used technique for high-precision U/Th dating studies in the world. Based on the previously established methods, this study presents a simplified chemical procedure for U/Th separation and purification using the UTEVA resin columns and a standard-sample bracketing method for U/Th isotope measurements by MC-ICP-MS. In routing measurements, the daily variations of instrumental mass discrimination and SEM/Faraday cup gain are less than 0.1 and 1%, respectively. By the use of rigorous arid washout, potential contribution of instrumental background to U/Th isotopes measurements is less than 0.2‰, or even negligible. The precision and accuracy of this method were tested by the repeated measurements of Harwell uraninite (HU-1) as an unknown sample, the averaged δ²³⁴U in HU-1 is (-1.27±2.14)‰ (±2σ，n=29), which is in agreement with the values that reported previously by different international laboratories with various analytical techniques. The repeatability of this method was assessed by a large number of replicate analyses using the in-house U/Th standard MFT, which has δ²³⁴U of (408.2±3.2)‰,²³⁰Th/²³⁸U activity ratio of 1.002±0.003 and ²³⁰Th/U age of (124.3±0.5) ka, respectively. We also analyzed 47 stalagmite samples with known ages to test the accuracy of our method. For these stalagmite samples, the measured 234U/²³⁸U and²³⁰Th/²³⁸U activity ratios and the age have averaged uncertainties (±2σ) of 0.6‰, 2.4‰ and 0.5%, respectively, and they are consistent well with the previously published data. In addition, this study shows that the reliability of standard-sample bracketing method is largely influenced by the stability of SEM/Faraday cup yield, which in turn is sensitive to the stability of room temperature and humidity and is sensitive to the presence of organics or any other matrix materials in the U/Th fractions.