JAEA東濃地科学センターのJEOL JXA-8530F FE-EPMA を用いた鉱物分析と年代測定

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  • Mineralogical analysis and dating using the JEOL JXA-8530F FE-EPMA at the JAEA Tono Geoscience Center
  • JAEA トウノウチ カガク センター ノ JEOL JXA-8530F FE-EPMA オ モチイタ コウブツ ブンセキ ト ネンダイ ソクテイ

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We established a CHIME monazite dating method using RR=100 mm spectrometers and a provenance analysis method using heavy minerals for the dating and analysis of minerals. CHIME dating has traditionally been performed with conventional RR=140 mm spectrometers for measurement of U, Th, and Pb. These spectrometers, however, have low count rates in measurements of Pb. We used a JEOL JXA-8530F FE-EPMA equipped with RR=100 mm spectrometers, a system installed at Tono Geoscience Center in 2014, for the CHIME dating of monazite. The RR=100 mm spectrometers permit measurement at high count rates, enabling us to shorten measurement times compared to before. We corrected the spectral interferences peculiar to RR=100 mm spectrometers by the method of Åmli and Griffin (1975) for monazite analyses. We calculated the correction factors by a new interference correction method using natural monazites. We also established a quick quantitative analysis and identification method for heavy minerals using the EPMA. The chemical compositions and abundance ratios of heavy minerals can be used as indexes of provenance rocks. A clear characterization of the rock features requires the measurement of as many grains as possible. We therefore prioritized the swiftness of measurement over accuracy in this study. We applied a sequence of the method to sediment samples whose geological features are already known. The results are consistent with the provenance change suggested by previous studies based on the electron spin resonance of the quartz grains.

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