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[SCG57-10] Development of an analytical method for 238U–230Th isotope ratios of whole-rock powder samples utilising multiple-spot LA-CRC-ICP-MS
Keywords:LA-ICP-MS, U–Th radioactive disequilibrium, Quaternary
Igneous processes from generation of magma to eruption can be decoded from the abundance ratios of radioactive nuclides belonging to the uranium decay series. Among the nuclides, 230Th with a long half-life (ca. 75000 years) has key information. The 230Th/238U value of magma can change due to fractionation of Th/U along with magma generation or fractional crystallisation, and addition of high-U fluid.[1, 2] The fractionated 230Th/238U value approaches the isotope ratio in secular equilibrium over hundreds of thousands of years, and the remaining deviation from the secular equilibrium value can reflect chronological information of geological events causing the fractionation of Th/U. Hence, 238U–230Th isotope data for volcanic rock samples is used for understanding magmatic processes.
Conventionally, 238U–230Th isotope ratios of rock samples have been analysed by using solution-based methods.[e.g., 3] Despite the high precision of the solution analysis, required preparation processes of chemical digestion and separation of elements are time-consuming, and fast analytical techniques are desired. In this study, a high-throughput method for 238U–230Th isotope analysis of whole-rock powder samples is developed utilising multiple-spot laser ablation collision/reaction cell ICP mass spectrometry (msLA-CRC-ICP-MS). The msLA system is composed of two high-speed scanning mirrors and high-repetition-rate femtosecond laser (>1 kHz), and the combined system enable to ablate a large area (> 1 mm2) within a few seconds by fast scanning of the laser beam with the size of 10 µm (i.e., solid nebulisation).[4] The resulting sample introduction rate of solid nebulisation into ICP is more than two-order-of-magnitude higher compared to conventional laser ablation or solution introduction systems, and the large-volume sample introduction enhances the signal intensity of 230Th.
A critical issue on direct sample introduction using LA is mass spectrometric interference derived from sample matrices because of the absence of chemical separation. In this study, kinetic energy discrimination (KED) of collision/reaction cell (CRC) techniques using He gas, which is originally used for the zircon 238U–230Th dating method,[5] is applied to 238U–230Th isotope analysis of powdered samples. Through the CRC, the introduced ion beam collides with He and polyatomic ions having larger cross sections have less kinetic energy than 230Th+. Subsequently, prior to mass separation, energy discrimination is employed for eliminating polyatomic ions, and the contribution of polyatomic interference on m/z 230 can be reduced. An additional advantage of the He KED mode is energy focusing of the introduced ion beam, resulting in the improvement of the abundance sensitivity. With the achieved abundance sensitivity, the contribution of the peak tail from 232Th+ is less than 1%.
For the evaluation of the data quality obtained through the present technique, pelletised whole-rock powder references (JR-1, JA-1, JB-1a, JB-2, and JB-3) were analysed by msLA-CRC-ICP-MS. The acquired 230Th/232Th values are in good agreement with the previously reported values determined by solution analysis[6] and the high analytical accuracy is demonstrated. The developed method has a critical advantage in the analysis throughput, and, in the presentation, potential geological applications will be discussed.
[1] H. Iwamori, Earth and Planetary Science Letters, 1994, 125, 1–16.
[2] S. Turner, C. Hawkesworth, P. van Calsteren, E. Heath, R. Macdonald and S. Black, Earth and Planetary Science Letters, 1996, 142, 191–207.
[3] T. Yokoyama, A. Makishima and E. Nakamura, Anal. Chem., 1999, 71, 135–141.
[4] Y. Makino, Y. Kuroki and T. Hirata, J. Anal. At. Spectrom., 2019, 34, 1794–1799.
[5] S. Niki, S. Kosugi, H. Iwano, T. Danhara and T. Hirata, Geostandards and Geoanalytical Research, 2022, 46, 589–602.
[6] S. Fukuda and S. Nakai, Geochem. J., 2002, 36, 465–473.
Conventionally, 238U–230Th isotope ratios of rock samples have been analysed by using solution-based methods.[e.g., 3] Despite the high precision of the solution analysis, required preparation processes of chemical digestion and separation of elements are time-consuming, and fast analytical techniques are desired. In this study, a high-throughput method for 238U–230Th isotope analysis of whole-rock powder samples is developed utilising multiple-spot laser ablation collision/reaction cell ICP mass spectrometry (msLA-CRC-ICP-MS). The msLA system is composed of two high-speed scanning mirrors and high-repetition-rate femtosecond laser (>1 kHz), and the combined system enable to ablate a large area (> 1 mm2) within a few seconds by fast scanning of the laser beam with the size of 10 µm (i.e., solid nebulisation).[4] The resulting sample introduction rate of solid nebulisation into ICP is more than two-order-of-magnitude higher compared to conventional laser ablation or solution introduction systems, and the large-volume sample introduction enhances the signal intensity of 230Th.
A critical issue on direct sample introduction using LA is mass spectrometric interference derived from sample matrices because of the absence of chemical separation. In this study, kinetic energy discrimination (KED) of collision/reaction cell (CRC) techniques using He gas, which is originally used for the zircon 238U–230Th dating method,[5] is applied to 238U–230Th isotope analysis of powdered samples. Through the CRC, the introduced ion beam collides with He and polyatomic ions having larger cross sections have less kinetic energy than 230Th+. Subsequently, prior to mass separation, energy discrimination is employed for eliminating polyatomic ions, and the contribution of polyatomic interference on m/z 230 can be reduced. An additional advantage of the He KED mode is energy focusing of the introduced ion beam, resulting in the improvement of the abundance sensitivity. With the achieved abundance sensitivity, the contribution of the peak tail from 232Th+ is less than 1%.
For the evaluation of the data quality obtained through the present technique, pelletised whole-rock powder references (JR-1, JA-1, JB-1a, JB-2, and JB-3) were analysed by msLA-CRC-ICP-MS. The acquired 230Th/232Th values are in good agreement with the previously reported values determined by solution analysis[6] and the high analytical accuracy is demonstrated. The developed method has a critical advantage in the analysis throughput, and, in the presentation, potential geological applications will be discussed.
[1] H. Iwamori, Earth and Planetary Science Letters, 1994, 125, 1–16.
[2] S. Turner, C. Hawkesworth, P. van Calsteren, E. Heath, R. Macdonald and S. Black, Earth and Planetary Science Letters, 1996, 142, 191–207.
[3] T. Yokoyama, A. Makishima and E. Nakamura, Anal. Chem., 1999, 71, 135–141.
[4] Y. Makino, Y. Kuroki and T. Hirata, J. Anal. At. Spectrom., 2019, 34, 1794–1799.
[5] S. Niki, S. Kosugi, H. Iwano, T. Danhara and T. Hirata, Geostandards and Geoanalytical Research, 2022, 46, 589–602.
[6] S. Fukuda and S. Nakai, Geochem. J., 2002, 36, 465–473.