U-Pb DATING AND Hf ISOTOPIC COMPOSITION OF ZIRCON BY LASER ABLATION-MC-ICP-MS
U-Pb DATING AND Hf ISOTOPIC COMPOSITION OF ZIRCON BY LASER ABLATION-MC-ICP-MS
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发表时间：
2001
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通讯作者：
N. Machado;A. Simonetti
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作者：
N. Machado;A. Simonetti
引言 在过去的四十年中，岩石 U-Pb 测年方法经历了几次重大革命。 Silver & Deutsch (1963) 的早期工作报告了对 200-300 毫克锆石的分析，相当于 >10 粒；研究发现，空气磨损减少或消除了不一致，锆石可以水热溶解，同时化学程序小型化，获得皮克范围内的铅坯料（Krogh 1973，1982a，b）突出了这些根本性的变化。构建高分辨率离子微探针来测年锆石（Compston 等人，1984）通过证明单个锆石晶体可以比包含它的岩石记录更多的地质事件，并且某些锆石晶体是公认的最古老的陆地材料，从而预示着 U-Pb 测年的新纪元。尽管这些发展为测定各种地质过程的年代创造了令人兴奋的机会，但 U-Pb 地质年代学并不适合对这些过程进行同位素追踪。激光烧蚀电感耦合等离子体质谱 (LA-ICPMS) 的出现有望成为下一个革命性的步骤，因为它允许对同一矿物进行年代测定和获取同位素数据，而无需大量额外的样品制备（即 TIMS 分析所需的 Pb、U 和 Hf 的离子交换色谱分离）。锆石就是一个很好的例子，因为它可以得出 U-Pb 年龄和 Hf 同位素特征。可以说，LA-ICPMS 方法由于其在 U-Pb 测年和放射性同位素研究中的应用的新颖性而处于不断变化的状态。在本章中，我们描述了锆石测年中遇到的主要问题、解决这些问题的不断发展的策略以及一些有前景的应用。我们预计该方法的主要贡献将是定年锆石的 Hf 同位素表征，因此我们介绍了 UQAM 实验室迄今获得的 Hf 同位素测定方法和结果。
INTRODUCTION In the last four decades, the U-Pb method of dating rocks has undergone several major revolutions. The early work of Silver & Deutsch (1963) reported the analysis of 200-300 milligrams of zircon, corresponding to >10 grains; the finds that air abrasion reduced or eliminated discordancy and that zircon could be hydrothermally dissolved together with the miniaturization of chemical procedures attaining Pb blanks in the picogram range (Krogh 1973, 1982a, b) highlight these radical changes. The construction of a high resolution ion microprobe to date zircon (Compston et al. 1984) heralded a new epoch in U-Pb dating by demonstrating that a single zircon crystal may record many more geological events than the rock containing it and that some zircon crystals are the oldest recognised terrestrial material. In spite of the exciting opportunities created by these developments for dating a variety of geological processes, U-Pb geochronology does not lend itself to isotopic tracing of these processes. The advent of laser ablationinductively coupled plasma mass spectrometry (LA-ICPMS) promises to be the next revolutionary step as it permits dating and acquiring isotopic data on the same mineral without extensive, additional sample preparation (i.e. ion exchange chromatographic separation for Pb, U and Hf required for TIMS analyses). Zircon is a case in point as it yields U-Pb ages and Hf isotopic characteristics. Arguably, the LA-ICPMS method is in a state of flux due to its novelty in U-Pb dating and application to radiogenic isotope studies. In this chapter, we describe the main problems encountered in zircon dating, the evolving strategies to solve them and some promising applications. As we envision that a major contribution of the method will be Hf isotopic characterization of dated zircons, we present the methodology for Hf isotopic determinations and results obtained so far at UQAM’s laboratory.