Establishment of high-accuracy dating method of tiny zircon (<10μm) : Implication for age determination of the oldest rock on the Earth

锆石原位测年为我们认识复杂地质事件提供了重要信息，本研究的目的是改进新的高质量分辨率离子探针Cameca IMS-1270测年方法。建立这一研究项目的关键设备之一是用于Camaca离子探针的高空间分辨率光学显微镜。作者与OLIMPUS公司合作开发了新的光学显微镜系统(PMLOpt-1)，并将该系统安装在我们的离子探针系统中。通过离子探针分析的现场观测，确定了预期的光学分辨率小于4μm。该体系得到的特殊分辨率与扫描电子显微镜-能谱分析和化学发光分析的结果相当。结合新安装的光学显微镜的观察和处理样品架上目标坐标的数据库系统“VisualStage”系统，我们可以根据样品的详细描述来非常精确地获得年龄数据。作者还利用可靠的锆石标准建立了精确的现场年龄测定方法，并得到了常规TIMS技术的证实。这些高质量的锆石标准通过离子探针分析证实了年龄数据的准确性。作者将开发的技术应用于主要来自变质岩套的锆石。变质岩中锆石的典型粒度小于几十微米，因此，我们开展的高空间分辨率测年是必不可少的。已公布与其他地球化学讨论有关的分析结果，并/或为出版做准备。

In-situ age dating of zircon provides us the important information to understand complicated geological events, and the purpose of this research is to improve the new age dating method using high mass resolution ion probe, Cameca ims-1270. One of the key devices to establish this research project was high-spatial resolution optical microscope for the Camaca ion probe. The authors developed the new optical microscope system (PMLOpt-1) jointly with OLIMPUS Co.Ltd, and installed the system in our ion probe system. As a result, the expected optical resolution, less than 4μm, has been established, on in-situ observation by ion probe analysis. The obtained special resolution by this system is comparable with that by SEM-EDX and CL analysis. Combining observation by the newly installed optical microscope and "Visual Stage" system, which is database system handling the target's coordinates on the sample mount, we could very precise age data based on the detail description of sample. The authors also had been developed the precise in-situ age dating method using the reliable zircon standards, which had been confirmed by the conventional TIMS technique. These high-quality zircon standards confirm the accuracy of age data by the ion probe analysis. The authors applied the developed technique for the zircon mainly from metamorphic rock suites. Typical grain size of zircon in metamorphic rock is less than several tens micrometer, therefore, the high-spatial resolution dating, we developed, is essential. Analytical results with other geochemical discussions are published and/or preparing for the publication.