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[SCG57-P03] Simultaneous determination of U–Th ages and trace-element compositions for zircon utilising LA-ICP-MS/MS
Keywords:Zircon, Quaternary, LA-ICP-MS, U–Th radioactive disequilibrium
To understand multiplex magmatic processes, including magma generation, storage, and eruption, constraining the timeline and duration of each stage is essential. Among igneous minerals, zircon can provide both the chronological information from radioisotope dating methods based on the uranium decay series and the critical trace-element data reflecting the conditions of host magma. To derive the information of the separate magmatic processes, statistical analysis of large-scale zircon data is important, and hence, high-throughput acquisition of both the radioisotope ages and the trace-element compositions is desired.
To obtain chronological data required for deciphering the magmatic processes, dating of Quaternary volcanic zircon with the precision of hundreds to thousands of years is required, and measurements of 230Th, which is an intermediate product of the uranium decay series with a long half-life (ca. 75 thousand years), is needed (i.e., 238U-230Th dating method). Recently, ICP mass spectrometry coupled with laser ablation sampling technique (LA-ICP-MS) has been a viable approach. The pioneering research using LA-ICP-MS was conducted with high-sensitivity SF-MS and demonstrated the analytical issue on mass spectrometric interference.[1, 2] The major polyatomic interference on m/z 230 is Zr2O3+ emanating from the zircon matrix, and the peak tail from 232Th+ is also problematic. In the prior research, numerical correction for signal intensities of interference is applied to improve the accuracy of measurements. However, the interference correction can be a source of analytical errors.
To reduce the contribution of interference, collision/reaction cell (CRC) technique is a promising method. The latest study demonstrates the effectiveness of interference reduction by introducing He into the CRC with energy discrimination between the CRC and the quadrupole-based mass analyser (kinetic energy discrimination; KED).[3] By utilising the He KED mode, polyatomic ions having larger cross sections than 230Th+ are preferentially removed from MS, and the abundance sensitivity is also improved through the energy focusing of ion beams. The He KED mode reduces the contribution of interference on m/z 230 to less than one percent, and the accurate age determination of Quaternary zircon is practically achieved. Despite the success of the approach using the CRC, the degree of the contribution from mass spectrometric interference on m/z 230 can be significant when determining ages of low-U Holocene zircon. Another critical issue on the He KED mode is the difficulty of eliminating doubly-charged interference ions, which deteriorate the analytical accuracy of some important trace-element compositions, such as Ti used for the estimation of crystallisation temperature.[4]
In this study, for the complete elimination of interference ions, the O2 mass-shift mode of Tandem MS (MS/MS) with higher selectivity than the He KED is employed. In the O2 mass-shift mode, the first quadrupole-based mass filter separates analyte ions based on their m/z, subsequently, the introduced ion beam reacts with O2 to form oxide ions inside the CRC, and finally, the oxide ions are allowed to pass through the second quadrupole-based mass filter. Through the optimisation of ion optics, the achieved reaction yield of ThO+ via the CRC in this study is ca. 70%, and this loss of ions is comparative to the He KED mode used in the previous study. In the presentation, the accuracy and precision of the present technique will be evaluated through the resulting data of a synthetic zircon without U and Th, reference zircons having the 238U-230Th isotope ratio in radioisotope equilibrium, and Quaternary zircons, and potential applications of the present technique will be discussed.
[1] M. Guillong, A. K. Schmitt and O. Bachmann, Journal of Volcanology and Geothermal Research, 2015, 296, 101–103.
[2] M. Guillong, J. T. Sliwinski, A. Schmitt, F. Forni and O. Bachmann, Geostandards and Geoanalytical Research, 2016, 40, 377–387.
[3] S. Niki, S. Kosugi, H. Iwano, T. Danhara and T. Hirata, Geostandards and Geoanalytical Research, 2022, 46, 589–602.
[4] T. Yuguchi, K. Ishibashi, S. Sakata, T. Yokoyama, D. Itoh, Y. Ogita, K. Yagi and T. Ohno, Lithos, 2020, 372–373, 105682.
To obtain chronological data required for deciphering the magmatic processes, dating of Quaternary volcanic zircon with the precision of hundreds to thousands of years is required, and measurements of 230Th, which is an intermediate product of the uranium decay series with a long half-life (ca. 75 thousand years), is needed (i.e., 238U-230Th dating method). Recently, ICP mass spectrometry coupled with laser ablation sampling technique (LA-ICP-MS) has been a viable approach. The pioneering research using LA-ICP-MS was conducted with high-sensitivity SF-MS and demonstrated the analytical issue on mass spectrometric interference.[1, 2] The major polyatomic interference on m/z 230 is Zr2O3+ emanating from the zircon matrix, and the peak tail from 232Th+ is also problematic. In the prior research, numerical correction for signal intensities of interference is applied to improve the accuracy of measurements. However, the interference correction can be a source of analytical errors.
To reduce the contribution of interference, collision/reaction cell (CRC) technique is a promising method. The latest study demonstrates the effectiveness of interference reduction by introducing He into the CRC with energy discrimination between the CRC and the quadrupole-based mass analyser (kinetic energy discrimination; KED).[3] By utilising the He KED mode, polyatomic ions having larger cross sections than 230Th+ are preferentially removed from MS, and the abundance sensitivity is also improved through the energy focusing of ion beams. The He KED mode reduces the contribution of interference on m/z 230 to less than one percent, and the accurate age determination of Quaternary zircon is practically achieved. Despite the success of the approach using the CRC, the degree of the contribution from mass spectrometric interference on m/z 230 can be significant when determining ages of low-U Holocene zircon. Another critical issue on the He KED mode is the difficulty of eliminating doubly-charged interference ions, which deteriorate the analytical accuracy of some important trace-element compositions, such as Ti used for the estimation of crystallisation temperature.[4]
In this study, for the complete elimination of interference ions, the O2 mass-shift mode of Tandem MS (MS/MS) with higher selectivity than the He KED is employed. In the O2 mass-shift mode, the first quadrupole-based mass filter separates analyte ions based on their m/z, subsequently, the introduced ion beam reacts with O2 to form oxide ions inside the CRC, and finally, the oxide ions are allowed to pass through the second quadrupole-based mass filter. Through the optimisation of ion optics, the achieved reaction yield of ThO+ via the CRC in this study is ca. 70%, and this loss of ions is comparative to the He KED mode used in the previous study. In the presentation, the accuracy and precision of the present technique will be evaluated through the resulting data of a synthetic zircon without U and Th, reference zircons having the 238U-230Th isotope ratio in radioisotope equilibrium, and Quaternary zircons, and potential applications of the present technique will be discussed.
[1] M. Guillong, A. K. Schmitt and O. Bachmann, Journal of Volcanology and Geothermal Research, 2015, 296, 101–103.
[2] M. Guillong, J. T. Sliwinski, A. Schmitt, F. Forni and O. Bachmann, Geostandards and Geoanalytical Research, 2016, 40, 377–387.
[3] S. Niki, S. Kosugi, H. Iwano, T. Danhara and T. Hirata, Geostandards and Geoanalytical Research, 2022, 46, 589–602.
[4] T. Yuguchi, K. Ishibashi, S. Sakata, T. Yokoyama, D. Itoh, Y. Ogita, K. Yagi and T. Ohno, Lithos, 2020, 372–373, 105682.