Allanite is a common REE-rich accessory mineral found in various igneous and metamorphic rocks, and can record a variety of geological processes. Therefore, allanite geochronology has the potential to answer a range of important geological questions. Allanite U–Th–Pb geochronology is hampered by common open-system behavior in the system. In this study, we demonstrate the feasibility of allanite Lu–Hf dating by LA–ICP–MS/MS. A total of nine allanite samples from different rock types (, granite, pegmatite) were investigated. These allanite samples have ages ranging from 2650 to 100 Ma, and Lu contents ranging from several to hundreds of μg g levels and relatively high Lu/Hf ratios (> 30). At a mass shift of +82, the isobaric interferences Lu and Yb have extremely low reaction rates of ∼0.003 % and ∼ 0.0003 %, respectively, indicating the isobaric interference corrections are insignificant for the allanite samples (mean Lu/Hf = ∼814; mean Yb/Hf = ∼719). A two-step calibration strategy was proposed for the Lu/Hf and Hf/Hf ratio corrections using NIST SRM 610 (for instrument drift) and LE2808 (for matrix effect). The nine allanite samples contain common Hf contents () of ∼5 % to >90 %, and the data are plotted in inverse isochron diagrams. Unanchored inverse isochron ages exhibit a large deviation in accuracy (5–10 %), whilst anchored isochron ages have a better accuracy of <5 %. The uncertainty of anchor initial Hf/Hf values was investigated and, in general, was insignificant (< 2.8 %) for samples with < 80 %. Our results demonstrate that allanite Lu–Hf dating by LA-ICP-MS/MS is feasible and can yield precise and accurate ages (< 5 %). Lu–Hf geochronometer captures the high-temperature process and may be more resistant during the late thermal event. Thus, it provides an alternative solution for those samples which are suffered by open-system behavior in U-Th-Pb system.

中文翻译：

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采用 LA-ICP-MS/MS 进行铝长石原位 Lu-Hf 定年：对地质年代学的启示


铜榴石是一种常见的富含稀土元素的副矿物，存在于各种火成岩和变质岩中，可以记录多种地质过程。因此，铜榴石地质年代学有可能回答一系列重要的地质问题。铜铀矿 U-Th-Pb 地质年代学受到系统中常见的开放系统行为的阻碍。在本研究中，我们证明了通过 LA-ICP-MS/MS 进行铝榴石 Lu-Hf 定年的可行性。总共研究了来自不同岩石类型（花岗岩、伟晶岩）的九个铜榴石样品。这些铝长石样品的年龄范围为2650至100 Ma，Lu含量范围为数至数百μg g级，且Lu/Hf比值较高（> 30）。在质量位移为+82时，同量异位干扰物 Lu 和 Yb 的反应速率极低，分别为 ∼0.003 % 和 ∼ 0.0003 %，这表明同量异位素干扰校正对于铜铍样品来说是微不足道的（平均 Lu/Hf = ∼814；平均 Yb/Hf = ∼719)。提出了使用 NIST SRM 610（针对仪器漂移）和 LE2808（针对基体效应）进行 Lu/Hf 和 Hf/Hf 比率校正的两步校准策略。九个铝镁石样品的常见 Hf 含量 () 为 ∼5 % 至 >90 %，数据绘制在逆等时线图中。非锚定逆等时线年龄的准确度存在较大偏差 (5–10%)，而锚定等时线年龄的准确度更高，为 <5%。研究了锚定初始 Hf/Hf 值的不确定性，一般来说，对于 < 80 % 的样品来说，不确定性不显着 (< 2.8 %)。我们的结果表明，通过 LA-ICP-MS/MS 进行铝镁石 Lu-Hf 定年是可行的，并且可以得出精确且准确的年龄 (< 5 %)。 Lu-Hf地质计时仪捕获高温过程，并且在后期热事件期间可能更具抵抗力。 因此，它为那些在U-Th-Pb系统中遭受开放系统行为的样品提供了替代解决方案。