Annealing kinetics of fission tracks in zircon_an experimental study_1995

1. Introduction Latent fission tracks ( F-F' s) in minerals are annealed with increasing temperature, and the analysis of fossil FT's accumulated for geologic periods provides useful information on a rock's thermal history. Quantitative analysis of track facling on the geological time scale enables us to reliably estimate thermal histories. The fading of F T ' s is chza'acterized by the reduction of both track density and length; especially the lengths of horizontal confined tracks ( H C T ' s ; Laslett et al., 1982)
Received 5 April 1994; accepted after revision 12 January 1995
Abstract A series of laboratory annealing experiments on zircon fission tracks has been carded out under heating conditions of 3507500(2 for 10- t-103 hr (i.e. 4.5 min- ~ 40 days). Variation in the confined lengths of spontaneous fission tracks was determined using zircon grains fn)m Nisatai Dacite. The fading contours of normalized mean track length (r) on the Arrhenius diagram showed as sets of straight lines. We performed a series of model fittings, called the parallel and fanning models, in order to describe the decrease in r with increasing temperature or heating time. The lowest temperature limit of the zircon partial annealing zone (ZPAZ) was defined as r = 0.95, and the highest as r-- 0.4, which approximately corresponded to the total fading of surface tracks. Extrapolation of the results of the laboratory experiments to the geological timescale gives, for a heating duration of 106 yr, estimated values of the ZPAZ of ~ 210-320°C ( + 60°C, 2tr) with the parallel model; ~ 190-350°C ( + 50°C, 2~') with the fanning model (critical temperature, To= ~); and ~ 170-390°(2 ( _ 50°C, 2tr) with the fanning model (TO~ oo). The temperatures of the ZPAZ decrease by ~ 20°C for an annealing duration that is an order of magnitude longer. Because the estimated closure temperature of zircon fission-track analysis approximately corresponds to the middle of the ZPAZ, these results support the previously estimated closure temperature of ~ 240°C. By varying the etching time it was revealed that significant removal of a-radiation damage occurs at r ~--0 et al. / Chemical Geology (Isotope Geoscience Section) 122 (1995) 249-258
observed track density during annealing with measuring angular distribution of etched tracks. Additionally, the effect of overetching is anisotropic and variable for different crystallographic directions according to highly anisotropic etching characteristics. Yamada et al. (1993) precisely assessed this effect for reliable track length analysis. A reference framework of HCT length measurement in zircon was established, focusing on the angular distribution of length and number of HCT's (Hasebe et al., 1993). It is known that various factors control the observed and measured lengths of annealed FT' s (Laslett et al., 1982, and Gleadow, 1984; Tagami et al., 1990). Yamada et al. (1995) assessed the factors affecting the paleotemperature estimate, and recommended realistic experimental criteria for routine HCT length measurements. In order to estimate quantitatively the thermal histories using zircon FT length analysis, we here report data sets of Arrhenius plot experiments in the laboratory. Based on the experimental criteria proposed by Yamada et al. (1995), track annealing experiments were carried out at 350-750°C for 10-L103 hr (i.e. 4.5 m i n - ~ 40 days). The data were fitted by mathematical models that describe the reduction of etched length as a function of time and temperature. We used both parallel and fanning models (e.g., Laslett et al., 1987) for calculation, and estimated the temperatures of the zircon partial annealing zone (ZPAZ) for the geological time scale on the order of 106-108 yr.
Ryuji Yamada a,., Takahiro Tagami a, Susumu Nishimura a, Hisatoshi Ito b
a Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto 606-01, Japan b Central Research Institute of Electric Power Industry, 1646 Abiko 270-11, Japan
* Corresponding author.
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0009-2541/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSD10009 -254 ! ( 95 ) 0 0 0 0 6 - 2
offer an accurate parameter with minimum experimental biases. On the basis of both laboratory and geological annealing data, quantitative analysis of the thermal history has been achieved using observed apatite FT parameters (e.g., Green et al., 1989). In the case of zircon, however, potential difficulties have been recognized on laboratory FT annealing experiments. Gleadow (1978) found that accumulated a-radiation damage in the mineral is reduced during annealing. This annealing process accordingly increases the anisotropy in the procedure of track etching and can result in an apparent reduction of track etching efficiency. Tagami et al. (1990) successfully corrected the effects of varying etching anisotropy on