Evaluation of zircon from the Pliocene Utaosa rhyolite Japan as
reference material for (U-Th)/He thermochronometry
The zircon (U-Th)/He (ZHe) system with a typical closure temperature of
~160-200°C*1, but lower for higher
radiation damaged grains*2, 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.*3, which can also
contribute to evaluating long-term tectonic stabilities for the geologic
disposal project. So far, the effectivity of ZHe thermochronology has
been verified, however improved age standards for the method are
required. To date, the method has conventionally employed zircon
fission-track age standards such as the Fish Canyon Tuff (FCT)
zircon*4. ZHe grain ages are sometimes over-dispersed
owing to factors such as zoning of parent nuclei, radiation damage,
grain size and He-bearing inclusions*2,5. Considerable
parent isotope zonation was reported in some FCT
crystals*6, inviting a search for alternative
potential ZHe standards*7,8,9. These works reported
robust ZHe data with little age dispersion because of homogeneous U-Th
distribution in zircon megacrysts, making them possible reference
material candidates. However, a practical issue remains because ZHe
analyses of unknown samples are carried out grain-by-grain as opposed to
analyzing large pieces of a single grain. 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)*10, 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*11, 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 e.g., FCT. References 1: Reiners et al. (2004),
Geochim. Cosmochim. Acta, 68, p. 1857–1887 2: Guenthner et al. (2013),
Am. J. Sci., 313, p. 145–198 3: Ault et al. (2019), Tectonics, 38, p.
3705–3739 4: Gleadow et al. (2015), Earth Planet Sci. Lett., 424, p.
95–108 5: Danišík et al. (2017), Sci. Adv., 3, p. 1–9 6: Dobson et al.
(2008), Geochim. Cosmochim. Acta, 72, p4745–4755 7: Li et al. (2017),
Geostand. Geoanal. Res., 41, p. 359–365 8: Yu et al. (2020), Geostand.
Geoanal. Res., 44, p. 763–783 9: Kirkland et al. (2020), Geochim.
Cosmochim. Acta, 274, p. 1–19 10:Tagami et al. (2003), Earth Planet
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382–394 Acknowledgements This study was supported by the Ministry of
Economy, Trade, and Industry (METI) of Japan.