LA-MC-ICPMS分析古老熔体包裹体Pb同位素组成中的误差评价

目前应用LA-MC-ICPMS分析熔体包裹体Pb同位素,由于没有同时测试U和Th的信号,导致熔体包裹体Pb同位素的研究仅局限于中生代以来的样品。本文应用LA-MC-ICPMS分析了玻璃样品以及存在显著U-Th衰变影响的古老熔体包裹体的Pb同位素组成,评价了U/Pb和Th/Pb分析误差对初始Pb同位素比值校正的影响。实验中以国际玻璃标样NKT-1G为外部标样,采用"标样-样品-标样法"进行仪器漂移和质量歧视校正,结果表明,国际玻璃标样BHVO-2G、TB-1G的208Pb/206Pb和207Pb/206Pb分析精度优于0.30%(2RSD),与推荐值的偏差小于0.30%,然而232Th/206Pb和238U/206Pb分析结果显示了较大分散性(外精度约5.0%)。根据误差传递计算,样品的年龄对初始铅的误差有很大影响。对于古生代以来的样品(年龄小于540 Ma),即使测试的232Th/206Pb和238U/206Pb与真值偏差达到10%,经过U-Th衰变校正后的Pb同位素比值与真值的偏差依然小于0.80%。因此本方法可以将熔体包裹体等地质样品的Pb同位素研究由新生代样品(年龄小于65 Ma)扩展到古生代样品。

Evaluation of Error Propagation in Lead Isotope Analysis of Ancient Melt Inclusions by LA-MC-ICP-MS

Determination of Pb isotopes in melt inclusions by Laser Ablation -Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICP-MS) is limited to samples formed later than Mesozoic because U and Th signals are not monitored. In this study, Pb isotopic compositions of two reference glasses and ancient melt inclusions which have undergone significant U-Th decay are determined by LA-MC-ICP-MS. The effect of the U/Pb and Th/Pb analytical errors on the calculation of initial Pb isotopic ratios was also evaluated. In the experiment USGS NKT-1G is used as the external calibrant and a standard-sample-standard bracketing method is used to correct instrument drift and mass bias. Analytical precisions of ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb of USGS standards, BHVO-2G and TB-1G, are better than 0.30% (2RSD) and both agree within 0.30% of the preferred values. However, ²³²Th/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios are highly variable and yield an external precision of 5%. Error propagation calculations indicate that the ages of the samples have significant effects on the errors of the initial lead isotopic ratios. For Phanerozoic samples that are earlier than 540 Ma, even the differences between the measured ²³²Th/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios and the real values reach 10%, yet the age-corrected initial lead isotopic ratios are still within 0.80% deviations relative to the real values. Therefore, the proposed method in this study can determine the lead isotopic compositions of melt inclusions in samples from Cenozoic to Paleozoic, expanding the geological application.