CHIME monazite dating using FE-EPMA equipped with R=100 mm spectrometers

ローランド円半径100mmの分光器を備えたFE-EPMAによるモナザイトCHIME年代測定

柴田 健二; 清水 麻由子   ; 鈴木 和博; 末岡 茂   ; 丹羽 正和   

Shibata, Kenji; Shimizu, Mayuko; Suzuki, Kazuhiro; Sueoka, Shigeru; Niwa, Masakazu

日本原子力研究開発機構東濃地科学センターに導入されたJEOL JXA-8530F FE-EPMAを用いてモナザイトのCHIME年代測定を試みた。U-Pb年代が報告されているCooma花崗岩(432.83.5Ma)と江若花崗岩(68.50.7Ma)を用いて、当センターで得られたCHIME年代値の信頼性を確認した。その結果、Cooma花崗岩は42525Maであり、江若花崗岩は677Maで、いずれも誤差の範囲で一致した年代値が得られた。以上の結果より、JEOL JXA-8530F FE-EPMAを用いたCHIME年代測定法が確立できたと判断できる。

The CHIME (Chemical Th-U-total Pb isochron method) dating has been performed on JEOL JXA-8530F FE-EPMA introduced in Tono Geoscience Center of Japan Atomic Energy Agency. Analysis of Pb on conventional spectrometers have disadvantage in count rate. JXA-8530F equipped with three R=100 provides significant shortening in analyzing time. Spectral interferences peculiar to R=100 mm spectrometers are found to be collected on the monazite analyses. The intrinsic responses of the R=100 mm counting systems with a PET crystal and a sealed Xe detector for PbMb are around ten times higher than that of the JCXA-733 counting systems. The measurement time required for a spot is ca. 35 minutes. The advanced procedure carried out 42525 Ma for monazite from Cooma granite in southeastern Australia, which had dated by SHRIMP as 432.83.5 Ma. The CHIME monazite age of the Kojaku granite in southwestern Japan is 685 Ma that accords well with the LA-ICP-MS U-Pb zircon ages of 68.50.7 Ma.