15:30 〜 17:00
[SCG54-P06] 第四紀モナザイト試料のフィッション・トラックのエッチング
キーワード：フィッション・トラック、熱年代学、モナザイト、第四紀
We conducted monazite fission-track (MFT) etching experiments for Quaternary monazite dating. The MFT system is vulnerable to thermal annealing with a low closure temperature (~45-25℃: [1]), and thus, the MFT system is potentially applied to an ultra-low-temperature thermochronometer for age determinations on geological events occurring at ultra-low temperatures, such as shallow crustal denudation and faulting. [2] examined the etching conditions and suggested that the etching rate of monazite varies between grains depending mainly on the accumulated radiation damage. Etching conditions were examined by etching experiment of implanted tracks in pre-Mesozoic monazite [1][3], and re-examination of conditions using younger monazites with less radiation damage is required to establish more general conditions. In this study, we attempted to etch FTs of the Quaternary monazites, which have less radiation damage, to investigate appropriate etching conditions and discuss a relationship between the etching rate and radiation damage. The analyzed samples in this study were younger monazites from the Toya Ignimbrite (ca. 0.1 Ma: [4]) and the Kurobegawa Granite (ca. 0.8 Ma: [5]). For comparison, the Cretaceous monazite from the Kibe Granite (ca. 98 Ma: [6]) was also etched.
The FTs of these monazites were etched with 6 M HCl at 90℃ [2]. Spontaneous MFTs of the Kibe Granite appeared in the first 60 min, whereas no FTs were observed in monazites from the Toya Ignimbrite and the Kurobegawa Granite. Step-etching was performed until the diameter of an etch pit of MFT reached 1 µm, which took 90 min for the Kibe Granite and 600 min for the Kurobegawa Granite. Although FT-like structures were observed in the monazite from the Toya Ignimbrite, an etch pit did not reach 1 µm even after 600 min. This result indicates that the etching conditions proposed by [2] are insufficient for Quaternary monazite samples. On the other hand, except for extremely young samples such as the Toya Ignimbrite, it is possible to etch MFTs of young samples by performing additional etching for a longer time. As the time required for etching varies greatly from sample to sample, it is necessary to perform step etching to achieve proper etching and to determine the objective criteria for etching termination, similar to the zircon fission-track method [7].
REFERENCES
[1] Jones S, Gleadow A, Kohn B (2021) Geochronology 3:89–102.
[2] Jones S, Gleadow A, Kohn B, Reddy SM (2019) Terra Nova 2018:179–188.
[3] Weise C, van den Boogaart KG, Jonckheere R, Ratschbacher L (2009) Chem Geol 260:129–137.
[4] Niki S, Kosugi S, Iwano H, et al (2022) Geostand Geoanalytical Res 46:589–602.
[5] Ito H, Yamada R, Tamura A, et al (2013) Sci Rep 3:1–5.
[6] Skrzypek E, Kawakami T, Hirajima T, et al (2016) Lithos 260:9–27.
[7] Yamada R, Tagami T, Nishimura S (1995) Chem Geol 119:293–306.
The FTs of these monazites were etched with 6 M HCl at 90℃ [2]. Spontaneous MFTs of the Kibe Granite appeared in the first 60 min, whereas no FTs were observed in monazites from the Toya Ignimbrite and the Kurobegawa Granite. Step-etching was performed until the diameter of an etch pit of MFT reached 1 µm, which took 90 min for the Kibe Granite and 600 min for the Kurobegawa Granite. Although FT-like structures were observed in the monazite from the Toya Ignimbrite, an etch pit did not reach 1 µm even after 600 min. This result indicates that the etching conditions proposed by [2] are insufficient for Quaternary monazite samples. On the other hand, except for extremely young samples such as the Toya Ignimbrite, it is possible to etch MFTs of young samples by performing additional etching for a longer time. As the time required for etching varies greatly from sample to sample, it is necessary to perform step etching to achieve proper etching and to determine the objective criteria for etching termination, similar to the zircon fission-track method [7].
REFERENCES
[1] Jones S, Gleadow A, Kohn B (2021) Geochronology 3:89–102.
[2] Jones S, Gleadow A, Kohn B, Reddy SM (2019) Terra Nova 2018:179–188.
[3] Weise C, van den Boogaart KG, Jonckheere R, Ratschbacher L (2009) Chem Geol 260:129–137.
[4] Niki S, Kosugi S, Iwano H, et al (2022) Geostand Geoanalytical Res 46:589–602.
[5] Ito H, Yamada R, Tamura A, et al (2013) Sci Rep 3:1–5.
[6] Skrzypek E, Kawakami T, Hirajima T, et al (2016) Lithos 260:9–27.
[7] Yamada R, Tagami T, Nishimura S (1995) Chem Geol 119:293–306.