5:15 PM - 7:15 PM
[SCG44-P05] Towards ultra-low temperature thermochronology:
step etching and fission track length measurements on monazite with various formation ages
Keywords:Monazite fission-track, Fission-track length analysis, Ultra low temperature thermochronology
Recently, basic research has been conducted to develop and implement the monazite fission track (MFT) method as a new thermochronometer (e.g., [1]). The MFT method is expected to have an extremely low (<50°C) closure temperature [2,3] and be useful for elucidating the thermal history of the upper crust. However, the MFT method faces various issues for practical use, including the establishment of etching termination conditions, clarification of annealing characteristics, and determination of the initial length of the FT. Although previous studies have conducted experiments with alternative ion tracks, the annealing properties of spontaneous tracks are poorly understood. In addition, few FT length measurements have been reported except one case study [4], but its target is only the Cretaceous monazite.
In this study, we performed step etching on natural monazites of various formation ages and thermal histories, and measured and compared the FT lengths and widths for further understanding of the annealing properties. Three samples were used: pre-Cambrian granite (LANG1) and Cambrian pegmatite (ARP04) from Antarctica, and middle Miocene Yakushima granite (YYG02). In the presentation, we will provide also the results of FT densities in the same grains. Consequently, the etching time varied among individual samples as well as crystal grains. On the comparison between the etching time and the formation ages of the samples, the following trend was observed; the older age, the shorter the etching time. Given that the chemical compositions are the same between these samples, this observation is consistent with the relationship of the accumulated radiation damage and etching time proposed by the previous study [5]. The mean FT length of LANG1 was measured as 9.4±0.3 µm (all with standard error 1SE) for 60 min, and that of ARP04 was measured as 9.4±0.3 µm for 60 min, both showing unimodal shapes with broadening toward the shorter side. Regarding YYG02, the mean FT length was 9.6±0.4 µm for 210 min. Although the mean FT length and standard deviation are almost equal to those of the two Antarctica samples, further measurements are required because of the unclear distribution shape due to the lack of enough number of FTs. In the previous study of the Cretaceous monazite [4], the mean FT length of EY135A and EY137A are 8.94 ± 0.14 µm and 8.02 ± 0.19 µm, respectively. Compared to this study, these mean FT lengths are slightly shorter, while length distributions are similar. In addition, the previous study adopted the value of 10.6 µm as an initial length based on the calculation of the entire length of the 252Cf fragment track [3]. However, dozens of FTs over 11 µm were identified in this study, suggesting that the initial length may be longer than 10.6 µm.
Future work includes establishing etching termination conditions using confined tracks’ widths and other parameters. In addition, long-term annealing kinetics by using borehole samples, and short-term annealing experiments of spontaneous FTs are desirable to investigate annealing characteristics.
Reference:
[1]:Jones et al. (2019), Terra Nova 31(3), 179-188; [2]: Weise et al. (2009), Chrm. Geol.2606, 129-137; [3] Jones et al. (2021), Geochron. 3(1), 89-102; [4]: Jones et al. (2023), Tectonophys. 864, 229998; [5]: Nakajima et al. (2024), Geochron. 6(3), 313-323.
Acknowledgments: This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting programs entitled “Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2024)” (Grant number: JPJ007597). The Antarctic samples (LANG1 and ARP04) were provided by Prof. Naoto Ishikawa of Toyama University and Professor Noriko Hasebe of Kanazawa University, and The Yakushima sample (YYG02) was by Prof. Ryo Anma of Tokushima University.
In this study, we performed step etching on natural monazites of various formation ages and thermal histories, and measured and compared the FT lengths and widths for further understanding of the annealing properties. Three samples were used: pre-Cambrian granite (LANG1) and Cambrian pegmatite (ARP04) from Antarctica, and middle Miocene Yakushima granite (YYG02). In the presentation, we will provide also the results of FT densities in the same grains. Consequently, the etching time varied among individual samples as well as crystal grains. On the comparison between the etching time and the formation ages of the samples, the following trend was observed; the older age, the shorter the etching time. Given that the chemical compositions are the same between these samples, this observation is consistent with the relationship of the accumulated radiation damage and etching time proposed by the previous study [5]. The mean FT length of LANG1 was measured as 9.4±0.3 µm (all with standard error 1SE) for 60 min, and that of ARP04 was measured as 9.4±0.3 µm for 60 min, both showing unimodal shapes with broadening toward the shorter side. Regarding YYG02, the mean FT length was 9.6±0.4 µm for 210 min. Although the mean FT length and standard deviation are almost equal to those of the two Antarctica samples, further measurements are required because of the unclear distribution shape due to the lack of enough number of FTs. In the previous study of the Cretaceous monazite [4], the mean FT length of EY135A and EY137A are 8.94 ± 0.14 µm and 8.02 ± 0.19 µm, respectively. Compared to this study, these mean FT lengths are slightly shorter, while length distributions are similar. In addition, the previous study adopted the value of 10.6 µm as an initial length based on the calculation of the entire length of the 252Cf fragment track [3]. However, dozens of FTs over 11 µm were identified in this study, suggesting that the initial length may be longer than 10.6 µm.
Future work includes establishing etching termination conditions using confined tracks’ widths and other parameters. In addition, long-term annealing kinetics by using borehole samples, and short-term annealing experiments of spontaneous FTs are desirable to investigate annealing characteristics.
Reference:
[1]:Jones et al. (2019), Terra Nova 31(3), 179-188; [2]: Weise et al. (2009), Chrm. Geol.2606, 129-137; [3] Jones et al. (2021), Geochron. 3(1), 89-102; [4]: Jones et al. (2023), Tectonophys. 864, 229998; [5]: Nakajima et al. (2024), Geochron. 6(3), 313-323.
Acknowledgments: This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting programs entitled “Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2024)” (Grant number: JPJ007597). The Antarctic samples (LANG1 and ARP04) were provided by Prof. Naoto Ishikawa of Toyama University and Professor Noriko Hasebe of Kanazawa University, and The Yakushima sample (YYG02) was by Prof. Ryo Anma of Tokushima University.