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Fission-Tracks in Apatite

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AFT thermochronology has become a standard technique for investigating thermal and burial histories of sedimentary basins (Miller and Duddy 1989; Ravenhurst et al. 1994; Carter et al. 1998), and the unroofing histories of ancient (Crowley 1991; Corrigan et al. 1998) and active orogens (Brandon et al. 1998; Blythe and Kleinspehn 1998). AFT analysis includes both the determination of an AFT age and modeling of time-temperature histories based on the measured fission-track length distributions. The fission-track technique is based on the formation of damage zones resulting from the spontaneous fission of naturally-occurring 238U in U-bearing minerals such as apatite. Fission-tracks are retained in apatite below a closure temperature of 100 20C (Naeser and Faul 1969; Naeser 1981; Wagner 1968). If apatite grains are heated above their closure temperature for times on the order of 1 Ma, then all existing fission-tracks will be annealed and the fission-track age will be reset. In the case of complete annealing, the apatite fission-track (AFT) age provides a cooling age recording the time the rock containing the apatite passed through its closure temperature. For the most common type of apatite, F-rich apatite, the temperature range of ~60-100C is referred to as the partial annealing zone or PAZ. By assuming a geothermal gradient and a surface temperature, the apatite PAZ can be used to infer burial depth and the magnitude of denudation (Brown 1991; Brandon et al. 1998).

Confined track length measurements in apatite, combined with the AFT age, help constrain the cooling history of a rock below 100C. Track length distributions for apatite from rapidly cooled rocks, such as volcanics, yield characteristic long mean track lengths and small standard deviations ranging from 14-15 0.8-1.3 mm (Gleadow et al. 1986). In contrast, apatite from rocks that have cooled slowly through the apatite PAZ will yield shorter mean track lengths and larger standard deviations in the range of 12-14 1-2 mm (Gleadow et al. 1986). Experimental annealing studies of fission-tracks in apatite (Laslett et al. 1987; Carlson 1990; Crowley et al. 1991) have yielded the basis for algorithms used to calculate model thermal histories for samples within the apatite PAZ (Green et al. 1989; Crowley 1993; Issler 1996). An AFT age and track length distribution can be used to constrain the low temperature portion of the rock’s thermal history by calculating model time-temperature paths from the rock’s cooling age to the present.

 

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