投稿时间：2019-01-04
修订日期：2019-01-10
网络发布日期：2019-03-02
中文摘要:元素分馏是影响LA-ICPMS锆石U-Pb定年准确度的重要因素之一，通常利用标样进行校正。激光聚焦位置变化会引起剥蚀坑形貌及U-Pb分馏的改变，标样和样品聚焦条件不一致将导致标样难以准确校正样品，并最终影响定年结果的准确性，但影响的程度、机制及可容忍范围目前尚不清楚。为此，文章以91500为标样、GJ-1为样品，详细研究了聚焦偏离30 μm范围内剥蚀标样与样品锆石的剥蚀坑形貌变化以及由此导致的U-Pb定年误差。实验表明，在距离锆石表面30 μm范围内，标样和样品焦平面同步变化时，二者U-Pb分馏形式及程度基本一致，激光焦平面偏离所引起的样品年龄与TIMS推荐值的偏差小于1%；当二者聚焦不同步时，标样与样品的U-Pb分馏差别显著，年龄偏差最大可超过3%。激光聚焦不同步导致的标样与样品剥蚀坑纵横比差异是引起年龄误差的根本原因，激光焦平面偏离锆石表面超过15 μm，剥蚀坑坑口明显变大，纵横比减小，U-Pb分馏形式及程度发生改变。通过预剥蚀锆石，观察剥蚀坑轮廓，使激光焦平面在距离锆石样品表面15 μm范围内，可确保标样与样品剥蚀坑形貌及U-Pb分馏状态一致，提高LA-ICPMS定年的准确度。
Abstract:U-Pb zircon geochronology by LA-ICPMS has been widely applied in the geological field due to its relatively high spatial resolution, high efficiency and low cost. Accordingly, there is growing considerations on the accuracy of the method. The fractionation between U and Pb during laser ablation is the predominant factor affecting the accuracy. The analysis parameters such as laser wavelength, pulse width, energy density, carrier gas and the oxygen contentin the carrier gas create a complex fractionation mode. In addition, the inconsistent laser focusing states of reference material and sample, which has usually been neglected by the analyst, also have a significant impact on the pattern of U-Pb fractionation and eventually degrade the accuracy. It is still unclear in which way does the focusing inconsistency influence the accuracy. In this paper, Zircon 91500 and GJ-1 were analyzed as primary reference material and sample respectively under the different laser focusing conditions with focal plane deviating from the zircon surface in the range of 30 μm below and above the zircon surface. The depth and size of craters were measured by the confocal laser scanning microscopy and field emission scanning electron microscope. The impact of laser focus on the geometry of the crater and the dating accuracy was systematically investigated. The results indicate that the U-Pb fractionating pattern and degree of reference material and samples agree well with each other when their laser focal planes deviate from the zircon surface at the same level. In this case, the age bias was less than 1%. Conversely, U-Pb fractionating states of reference material and sample significantly differ from each other when their laser focal plane deviate at different level. The resulting age bias was more than 3%. The age bias are fundamentally attributed to the aspect ratio inconsistency between reference material and sample ablation crater. When the laser focal plane deviated from the zircon surface more than 15 μm, the crater size increased, the aspect ratio decreased and the pattern and degree of fractionation changed significantly. Focusing the laser properly could be achieved by observing the morphology of the pre-ablation crater. The pre-ablation crater should be clear and even when the laser focal plane is within 15 μm near the zircon surface. This can make the U-Pb fractionation mode of reference material and sample be the same and thus improve the dating accuracy.
keywords:geochemistry LA-ICPMS U-Pb zircon geochronology accuracy laser focal plane crater morphology
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中图分类号：P597
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基金项目:重点研发计划项目（编号：2016YFC0600305）和国家自然科学基金项目（编号：41825005，41473041）联合资助
引用文本：
于超,杨志明,周利敏,张亮亮,李振清,赵苗,张佳钰,陈微言,索明石.2019.激光焦平面变化对LA-ICPMS锆石U-Pb定年准确度的影响[J].矿床地质,38(1):21~28YU Chao,YANG ZhiMing,ZHOU LiMin,ZHANG LiangLiang,LI ZhenQing,ZHAO Miao,ZHANG JiaYu,CHEN WeiYan,SUO MingShi.2019.Impact of laser focus on accuracy of U-Pb dating of zircons by LA-ICPMS[J].Mineral Deposits38(1):21~28
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