Evaluation of zircon from the Pliocene Utaosa rhyolite Japan as reference material for (U-Th)/He thermochronometry

(U-Th)/He熱年代法の標準試料としての鮮新統歌長流紋岩のジルコンの評価

福田 将眞   ; Kohn, B.*; 末岡 茂   ; 鏡味 沙耶   ; 梶田 侑弥*; 南 沙樹*; 岡本 晃*; 田上 高広*

Fukuda, Shoma; Kohn, B.*; Sueoka, Shigeru; Kagami, Saya; Kajita, Yuya*; Minami, Saki*; Okamoto, Akira*; Tagami, Takahiro*

ジルコン(U-Th)/He法(以下、ZHe法)において、年代標準試料となるジルコンの確立を目指すため、形成年代が若い(100Ma以下)岩石を対象にZHe年代分析を実施した。100Ma以下の試料では、ZHe年代のばらつきと放射線損傷に相関がほとんど見られないことが経験的に知られている。そこで、鮮新統の歌長流紋岩(TRG-04, TRG-07)及び、他の年代測定法の標準試料であるBuluk TuffやOD-3を対象としてZHe年代分析を行った。本講演では、歌長流紋岩の予察的ZHe年代を報告し、各試料の年代のばらつきや再現性を評価するためZHeデータの比較を行う。

The zircon (U-Th)/He (ZHe) system with a typical closure temperature of ~160-200 degree Celsius (Reiners et al., 2004), but lower for higher radiation damaged grains (Guenthner et al., 2013), offers the potential for evaluating thermal histories in the uppermost ~10 km of the crust. ZHe thermochronometry has been applied to different geological settings in order to estimate tectonics, uplift and denudation, basin evolution, etc. (Ault et al., 2019), which can also contribute to evaluating long-term tectonic stabilities for the geologic disposal project. We have attempted to assess suitable zircon samples as ZHe age standards by using rapid cooling rock samples of relatively young (100 Ma) age. This is because such rock samples are expected to empirically exhibit simple thermal histories and little radiation damage. Therefore, age dispersion caused by radiation damage can be relatively small. In order to reassess previous data obtained by Tagami et al. (2003), ZHe analyses of the Pliocene Utaosa rhyolite (TRG-04 and -07) and the Miocene Buluk Tuff have been carried out. In addition, OD-3 zircon (Iwano et al., 2013), a zircon U-Pb age standard, was also analyzed. In this presentation, preliminary ZHe age data from these samples will be presented and compared to evaluate their suitability as ZHe reference materials.