FE-EPMAによるCHIMEモナザイト年代測定

CHIME monazite dating using FE-EPMA

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

先新第三紀の岩盤の構造・温度履歴を明らかにすることは、地殻変動が活発な日本列島における高レベル放射性廃棄物の地層処分にとって重要であり、これらの解明には砕屑物の年代スペクトルが有効となる場合がある。そこで東濃地科学センターでは、FE-EPMAを用いたCHIME(Chemical Th-U total Pb Isochron Method, Suzuki and Adachi, 1991)年代測定技術を整備した。本手法は数の高い空間分解能を有しており、例えば、微小な砕屑性モナザイト粒子の累帯構造に対応した年代測定を行うことが可能である。本研究では、年代既知の花崗岩中のモナザイトのCHIME年代測定を実施しFE-EPMAによる本測定法の信頼性を確認した。

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 PbM 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.