[S-CG57] ハイブリッド年代学ー年代値の意味とは？ー

[SCG57-P02] Comparison of 193 nm and 213 nm laser abrasion systems for LA-ICP-MS zircon U-Th dating

*伊藤 久敏¹、Lee Hao-Yang²、Lee Yuan-Hsi³、Hung Chien-Hui²、Hsin Yi-Ju²、Lo Yun-Chieh³、Tsai Wan-Ling³ (1.Central Research Institute of Electric Power Industry、2.Academia Sinica、3.National Chung Cheng University)

キーワード：zircon、LA-ICP-MS、U-Th dating

Quaternary zircon U-Th dating using LA-ICP-MS has become a method of choice because it is cost-effective and easy-to-apply, and in some cases, it is comparable or superior to more costly and time-consuming SIMS method (e.g., Guillong et al., 2016; Marsden et al., 2022). Recently, Ito (2024) reproposed a simultaneous U-Pb and U-Th zircon dating using LA-ICP-MS. However, as noted in Ito (2024), the U-Th data of Ito (2024) were not as good as that of Marsden et al. (2024) because LA system (213 nm Nd-YAG) of Ito (2024) is inferior to that (193 nm excimer) of Marsden et al. (2022). Here, we show once again that the 213 nm Nd-YAG has no advantage over the 193 nm excimer using zircon reference materials (91500, Plešovice, Fish Canyon Tuff) and ~0.8 Ma Oldest Toba Tuff zircon. This is because (²³⁰Th/²³⁸U) for reference zircons and Oldest Toba Tuff zircon (all >0.4 Ma, and hence should be unity) are more tightly scattered around unity using 193 nm laser than using 213 nm laser (Fig. 1). Laser pits are more uniformly created by 193 nm laser than 213 nm laser (Fig. 2) and hence aerosols (particles) are likely more uniformly scattered by 193 nm laser, reducing a chance to cause elemental fractionation.

Guillong, M.et al, 2016. U-Th zircon dating by laser ablation single collector inductively coupled plasma-mass spectrometry (LA-ICP-MS). Geostand. Geoanal. Res., 40, 377–387.
Ito, H., 2024. Simultaneous U–Pb and U–Th dating using LA-ICP-MS for young (<0.4 Ma) minerals: A reappraisal of the double dating approach. Minerals, 14, 436.
Marsden, R. et al., 2022. SS14-28: An age reference material for zircon U-Th disequilibrium dating. Geostandards Geoanalytical Res., 46, 57–69.