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

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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.