[S-CG51] Hybrid Geochronology beyond Zircon Geochronology

[SCG51-P04] Simultaneous determination of zircon U−Th disequilibrium and fission-track ages using LA−ICP−MS

*Hideki Iwano^1,2, Sota Niki², Tohru Danhara¹, Takafumi Hirata² (1.Kyoto Fission-Track Co., Ltd., 2.Geochemical Research Center The University of Tokyo)

Keywords:zircon, U-Th disequilibrium, fission track, dating, Quaternary, LA-ICP-MS

Between the carbon-14 (¹⁴C) dating method, that is applicable to post-Late Pleistocene geological samples, and other radiometric methods (zircon U-Pb and sanidine Ar/Ar methods), their applicable age ranges do not overlap in the range of 50,000 to 100,000 years ago. This issue is an obstacle to Quaternary research, especially volcanic research and volcanic eruption control. The key to this problem is the ²³⁸U-²³⁰Th disequilibrium dating method (hereafter the U-Th method), which utilizes the intermediate product of the uranium series, thorium-230 (half-life 75,000 years). The U-Th method is a dating one that extracts information on the time since mineral crystallization by utilizing the disequilibrium between thorium-230 and uranium-238 that occurs during mineral crystallization to accurately determine their isotopic ratios (²³⁰Th/²³⁸U) and the primary thorium/uranium ratio of the magma (Th/U). The U-Th method is applicable to uranium-bearing minerals such as zircon, apatite, ilmenite, and monazite. The U-Th method was developed in the 1960s [1] and initially required the use of special facilities and/or equipment such as activation analysis and thermal ionization mass spectrometry. Recently, this dating method using LA-ICP-MS was developed [2] and reconstructed to be more versatile.
This study aims to develop a zircon U-Th disequilibrium and fission track (FT) double dating method using LA-ICP-MS as a method that can simultaneously determine the age of crystallization in magma and the age of volcanic eruptions. Although the double dating of zircon using LA-ICP-MS with the U-Pb and FT methods has become popular in the last decade, the U-Pb method is difficult to apply to samples younger than 100,000 years, and a new U-Th method is considered necessary. Niki et al. (2022)[3] developed a new spectral interference removal technique for ²³⁰Th using a quadrupole ICP mass spectrometer equipped with a collision-reaction cell. They reduced tailing of ²³²Th and polyatomic ions such as Zr₂O₃⁺, which are interferences for ²³⁰Th, to 1/100 of the conventional ICP-MS method, enabling accurate analysis of these ions. In addition, the analytical throughput is 20 times faster than that of secondary ion mass spectrometry (SIMS), one of the conventional methods.
In this presentation, we report the results of the following three-step experiments to perform zircon U-Th/FT double dating. (1) We carry out U-concentration measurement under the LA-ICP-MS U-Th dating conditions developed by Niki et al. and compare it with the conventional U measurement technique using thermal neutron induced track method. (2) We perform FT dating of zircons from the older age standard samples including Fish Canyon Tuff and Bishop Tuff, and (3) we try U-Th/FT double dating of zircon from Sambe Kisuki pumice (100 ka), a sample with a known eruption age.
[1] Kigoshi K. (1967) Science 156, 932-934. [2] Guillong M., Sliwinski J.T., Schmitt A., Forni F. and Bachmann O. (2016) Geostandards and Geoanalytical Research, 40, 377-387. [3] Niki S., Kosugi S., Iwano H., Danhara T. and Hirata T. (2022) Geostandards and Geoanalytical Research. doi: 10.1111/ggr.12458