Impacts of Common Lead on Apatite U-Pb Geochronology by LA-ICP-MS: Assessment and Correction Strategies
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摘要:
磷灰石是火成岩、变质岩和沉积岩中广泛分布的含铀矿物，开展磷灰石U-Pb年代学研究对揭示岩浆演化过程、示踪溯源等方面具有重要意义。激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)是开展磷灰石U-Pb年龄微区分析的重要手段之一。当前，基体匹配磷灰石U-Pb定年矿物标样缺乏和标样中不可避免的普通铅是制约LA-ICP-MS高精度磷灰石U-Pb年龄分析的主要瓶颈。本文对比研究了标样中普通铅对LA-ICP-MS磷灰石U-Pb定年结果的影响，采用含普通铅的磷灰石MAD作外标直接开展U-Pb年龄校正，获得的被测样品年龄会产生显著的系统偏差(最大约6%)；采用207Pb法或Tera-Wasserburg图解法先校正标样中普通铅，再利用校正后的数据进行元素分馏和仪器漂移校正则可获得准确的磷灰石U-Pb年龄，与推荐值偏差在2%以内。另一方面，为消除标样中普通铅对分析结果的影响，本文还采用水蒸气辅助激光剥蚀方法，实现以NIST612玻璃作为外标准确分析磷灰石U-Pb年龄，解决了磷灰石U-Pb定年微区分析高质量标样缺乏的难题。本研究通过对标样中普通铅进行预校正或采用非基体匹配分析，建立了高精度LA-ICP-MS磷灰石U-Pb定年新方法，将促进磷灰石U-Pb年代学在地球科学研究中的应用。
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关键词:
- 激光剥蚀电感耦合等离子体质谱法 /
- 磷灰石 /
- U-Pb定年 /
- 普通铅校正 /
- 非基体匹配分析
Abstract:Apatite is a widespread U-bearing mineral in igneous, metamorphic, and sedimentary rocks. U-Pb geochronology of apatite can provide significant information for constraining magmatic evolution processes and tracing provenance. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a crucial technique for in situ U-Pb age analysis of apatite. However, the lack of suitable matrix-matched apatite reference materials and the inevitable presence of common lead in reference materials are major obstacles restricting high-precision determination of apatite U-Pb ages. This study investigates the impact of common lead on LA-ICP-MS apatite U-Pb dating results. The significant systematic biases (6%−30%) in both measured lower intercept ages and initial lead compositions are observed when calibrating against MAD apatite without common lead correction. However, accurate apatite U-Pb ages (within 2% systematic bias) can be obtained by correcting for common lead in MAD using the 207Pb method or Tera-Wasserburg plot method prior to Pb/U fractionation calibration. Furthermore, a vapor-assisted laser ablation method is employed in conjunction with NIST612 glass as an external standard to accurately analyze apatite U-Pb ages. This non-matrix matched method eliminates the need to consider the influence of common lead in the reference materials. Novel high-precision LA-ICP-MS apatite U-Pb dating methods are established with both matrix-matched and non-matrix-matched analyses, which greatly promote the application of apatite U-Pb geochronology in Earth science. The BRIEF REPORT is available for this paper at
http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202404070079 . -
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表 1 LA-ICP-MS磷灰石U-Pb定年实验仪器参数
Table 1. Instrumental parameters for LA-ICP-MS U-Pb dating of apatite.
LA参数 GeoLas HD 193nm准分子激光 波长 193nm 激光能量密度 10J/cm2 剥蚀频率 5Hz 剥蚀时间 50s 背景时间 20s He流速 650mL/min 束斑直径 60～90μm ICP-MS参数 Agilent 7900型 等离子体功率 1400W 样品气 0.86L/min 检测元素 29Si， 42Ca，49Ti，51V，89Y， 93Nb， 139La，140Ce，
141Pr，146Nd，147Sm，151Eu，157Gd，159Tb，163Dy，
165Ho，166Er，169Tm，173Yb，175Lu，179Hf，181Ta，
201Hg，204Pb，206Pb，207Pb，208Pb，232Th，238U
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表 2 不同测量条件下获得的磷灰石Tera-Wasserburg谐和图下交点年龄与推荐值的偏差
Table 2. Deviations of Tera-Wasserburg values and recommended values for apatite U-Pb age obtained under different measurement conditions.
激光条件 外标 普通铅校正方法 分析样品 Tera-Wasserburg谐和图下交点年龄与
推荐值的偏差(%)60μm，5Hz MAD 无普通铅校正 Otter Lake 2.5 Durango 6.0 60μm，5Hz MAD 207Pb Otter Lake 0.03 Durango 1.7 60μm，5Hz MAD Tera-Wasserburg图解法 Otter Lake 1.7 Durango 6.6 60μm，5Hz NIST612 无普通铅校正 MAD 0.1 Otter Lake 2.3 Durango 0.8
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