[S-GC54] [JJ] 地球化学の最前線

[SGC54-03] 隕石の年代サイトメトリー解析に向けたマルチイオンカウンティング-ICP質量分析法の開発

*平田 岳史¹、大林 秀行¹、槇納 好岐¹、折橋 裕二² (1.東京大学大学院理学系研究科地殻化学実験施設、2.東京大学地震研究所)

キーワード：年代サイトメトリー、マルチイオンカウンティング、レーザーアブレーション、ウランー鉛年代測定法、デイリー検出器

Age distribution (age cytometry) is one of the most principal and versatile information to understand the geological events through the Earth’s evolutional history. To take full advantage of the age cytometry, both the high analytical throughput and the better precision in the age determination is severely desired. Combination of laser ablation sampling technique and the high-sensitivity ICP-mass spectrometer (LA-ICPMS) enables us to measure precise U-Pb ages directly from the small area in the solid samples. Recently, we have developed an MC-ICPMS system equipped with multiple-ion counting using three Daly detectors (Obayashi et al., 2017). With the multiple-ion counting system, the analysis time for the U-Pb age determination could be dramatically shortened down to 1 - 5 sec, which was almost 1/4 - 1/10 levels over the conventional U-Pb age determinations using the single collector ICPMS instruments. This suggests that both the higher analytical throughput and the better precision in the Pb/U ratio measurements could be achieved. Moreover, with the Daly detectors, higher long-term gain stability can be obtained, obviating the risk of systematical error in the measured U-Pb isotope ratios. After the correction for the counting loss due to dead time, the Daly detector is capable to accept the signal intensities as high as 10⁷ cps. This indicates that the overlap of the analysis range, between the Daly detector (10⁰ – 10⁷ cps) and the Faraday detector (10⁴ – 10¹⁰ cps), would be at least two orders of magnitude, suggestive of easier cross calibration of the collector gain between the detectors. With the multiple ion counting using three Daly detectors, better precision and smaller contribution of the time-dependent changes in the gain and background counts could be achieved. We will discus the wide versatility of the present technique for the geochronology on the solar system evolution.