フィッション・トラック

ニュースレター第32号 4 - 5

2019年

ジルコンFT熱年代計の現状と展望
田上 高広*The present and future of zircon FT thermochronometry
Takahiro Tagami** 京都大学理学部地球惑星, Earth Planet Sciences, Kyoto Univ.

Fission track (FT) thermochronometry using zircon has widely been applied tounravel geological records of terrestrial materials, particularly in orogenic belts toreconstruct uplift-denudation-cooling of mountains, thermal processes of fault zones,etc. Thermal annealing characteristics of FTs in zircon has primarily been studied
using spontaneous FTs. This is because (a) in contrast to apatite, the mean length of
spontaneous FTs in zircon from rapidly cooled volcanics are indistinguishable fromthat of induced FTs, suggesting the absence of natural shortening and (b)thermochronologic analyses of geological samples are carried out using spontaneous
FTs in natural zircons, for which thermal annealing behaviors should be described.
Hence, the thermal annealing kinetics was ﬁrst determined on spontaneous FTs of
the Nisatai Dacite (NST) zircon.
Yet, it is not well known about how the kinetics can vary among natural zircons,for example, as a possible consequence of radiation damage accumulation.
Conﬁned track length reductions during 1 hr isochronal laboratory annealing weremeasured for both spontaneous and induced FTs in zircons (Kauya and Naeser,1988), and found that (a) induced FTs in pre-annealed zircons are more resistant tothermal annealing than spontaneous FTs, (b) annealing behaviors of spontaneous
FTs are indistinguishable between four samples of Paleogene to Miocene ages, withthe range of spontaneous track densities of 10 to 0.9 x 10⁶ cm-2, and (c) induced FTsin non pre-annealed zircons behave like spontaneous tracks. Conﬁned track length
reductions during 1 hr isochronal laboratory annealing were also compared between
the spontaneous and induced FTs in the NST zircon (Yamada et al., 1995). The
experimental procedures were improved by applying analytical criteria andmeasuring crystallographic orientation of conﬁned tracks. It was found that theinduced FTs are more resistant to thermal annealing than spontaneous tracks at theadvanced stage of annealing, even after the correction of track length anisotropy
that reﬂects the anisotropic etching and annealing.

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On these basis, laboratory annealing experiments were performed using ninezircon samples of diﬀerent ages and spontaneous track densities, in order to better

constrain the possible variation of annealing behaviors among natural zircons(Tagami and Matsuura, 2019). Conﬁned track lengths in the nine natural zircons,

separated from rapidly cooled volcanic rocks, showed consistent and systematic
reductions against annealing temperature after

1 hr isochronal heating. No

systematic diﬀerence of FT annealing characteristics was found among naturalzircons of ~70 to ~0.6 Ma age (and spontaneous track density of 7 to ~0.05 x 10⁶cm-2). Coupled with previous data, it implies that an identical annealing kinetics canwork for many of the late Mesozoic to Cenozoic zircons.

References
Kasuya, M. and Naeser, C.W., 1988. The eﬀect of a-damage on ﬁssion-track annealing inzircon. Nucl. Tracks Radiat. Meas., 14, 477-480.
Tagami, T. and Matsuura, S., 2019. Thermal annealing characteristics of ﬁssion tracks innatural zircons of diﬀerent ages. Terra Nova, 1-6, 10.1111/ter.12394.
Yamada, R., Tagami, T., Nishimura, S. and Ito, H., 1995. Annealing kinetics of ﬁssion tracks
in zircon: an experimental study. Chem. Geol. (Isot. Geosci. Sect.), 122, 249-258.

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