高精度测量岩石样品Ce同位素组成的TIMS方法

介绍了对岩石样品的Ce同位素组成进行高精度测量的TIMS方法.采用α-羟基异丁酸(α-HIBA) 离子交换法分离和纯化岩石样品中的La、Ce元素, Ce同位素组成和La、Ce含量测定在Triton热电离质谱仪上完成.质谱标样JMC304的138Ce/142Ce比值统计值为0.0225762±0.0000015, 与大部分文献值一致.应用该方法对USGS玄武岩标样BCR-2和峨眉山玄武岩EQB的138Ce/142Ce比值和La、Ce含量进行了分析, 其结果分别为: 0.0225572±0.0000010、25.2±0.3μg/g、55.8±0.9μg/g和0.0225755±0.0000003、54.2±0.4μg/g、117.4±1.3μg/g.BCR-2的La、Ce含量测定结果与其证书值在误差范围内一致, 其分析精度为0.001%~0.005%.所有样品的140Ce/142Ce比值测定结果统计值为7.9439±0.0002, 介于文献最低值7.941与最高值7.947之间, 可能代表了该比值的最佳估计值. 

Precious Ce Isotope Analysis of Rock Samples by TIMS Method

A high-precision analysis method of La-Ce isotope ratios for rock samples using TIMS is reported. Separation and purification of La and Ce from matrix elements is carried out using α-HIBA ion exchange procedure. Ce Isotope ratios and La, Ce contents are measured by the mass spectrometry of Triton. A long-term analysis result of standard JMC304 gives a weighted 138Ce/142Ce ratio 0.0225762±0.0000015, which agrees well with the reference values from the literatures. Using the method reported in this study, the Ce isotope ratios and La, Ce contents are analyzed for USGS basalt reference material BCR-2 and an alkali basalt from the Emeishan terrain. The 138Ce/142Ce ratios of BCR-2 and Emeishan basalt (EQB) are 0.0225572±0.0000010 and 0.0225755±0.0000003 with corresponding La contents of 25.2±0.3 and 55.8±0.9μg/g, and Ce contents of 54.2±0.4 and 117.4±1.3μg/g, respectively. It shows that the measured La and Ce contents by isotopic dilution of BCR-2 agree with their certificate values within the analysis error, and this is the first report of Ce isotope for both the standards, which displays a precision range of 0.001% to 0.005%. In addition, a weighted average 140Ce/142Ce ratio of 7.9439±0.0002 is obtained based on 105 analyses, which is likely the representative of a best estimated value for the ratio.