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利用热电离质谱(TIMS)和207Pb-204Pb双稀释法测定低含量Pb岩石样品的同位素比值,可以有效校正由质谱分析造成的同位素分馏效应,提高铅同位素比值测定的精度和准确度。本文利用中国科学院广州地球化学研究所Triton TIMS,建立了207Pb-204Pb双稀释剂测试方法,并对Pb含量为5~20 ng的国际标样SRM981进行3个月的TIMS仪器测定,双稀释剂法外部校正后得到的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb外部精度分别为±0.0034、±0.0046和±0.0100(2sd,n=28)。达到或优于其他国际高水平TIMS实验室双稀释剂法测定精度。同时,运用新方法对国际岩石标样AGV-2和BCR-2进行测定,得到的比值分别为:206Pb/204Pb=18.870±0.006、207Pb/204Pb=15.619±0.003、208Pb/204Pb=38.546±0.012和206Pb/204Pb=18.752±0.011、207Pb/204Pb=15.619±0.005、208Pb/204Pb=38.723±0.020,与国际同类实验室测定结果和精度一致,表明该新建方法是可靠的。运用207Pb-204Pb双稀释剂法对岩石样品进行测定,可获得高精度铅同位素比值,尤其对低含量Pb的基性、超基性地质样品测定有很大的优势。
The determination of the isotopic ratio of low-Pb Pb rock samples by thermal ionization mass spectrometry (TIMS) and 207Pb-204Pb dual-dilution method can effectively correct the isotopic fractionation effect caused by mass spectrometry and improve the accuracy and accuracy of determination of Pb isotope ratio. In this paper, 207Pb-204Pb dual-diluent test method was established by Triton TIMS of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The TIMS instrument of international standard SRM981 with Pb content of 5 ~ 20 ng was measured for 3 months. The external accuracy of the 206Pb / 204Pb, 207Pb / 204Pb and 208Pb / 204Pb are ± 0.0034, ± 0.0046 and ± 0.0100 (2sd, n = 28) after external calibration. To meet or better than other high-level international TIMS dual-diluent method for the determination of precision. In the meantime, the new method was used to determine the international rock samples AGV-2 and BCR-2. The obtained ratios were: 206Pb / 204Pb = 18.870 ± 0.006,207Pb / 204Pb = 15.619 ± 0.003,208Pb / 204Pb = 38.546 ± 0.012 and 206Pb / 204Pb = 18.752 ± 0.011,207Pb / 204Pb = 15.619 ± 0.005,208Pb / 204Pb = 38.723 ± 0.020, which is consistent with the results and accuracy of similar international laboratories, indicating that the new method is reliable. The determination of rock samples by 207Pb-204Pb dual-diluent method can obtain high-precision Pb isotope ratios, especially for the determination of basic and ultrabasic Pb samples with low Pb contents.