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compositions of apatites affect the track annealing kinetics. In general, Cl-apatites
anneal at higher temperatures than F-apatites (e.g., Green et al. 1986, 1989). For
precise reconstructions of T-t paths measurements the chemical compositions of
apatites are therefore required. These have been carried out at the Mineralogisches
Institut, Universität Heidelberg, using a Cameca SX 51 microprobe. The measurements
were done with an acceleration voltage of 15 kV, a probe current of 20 nA, and a
defocused spot of 20 �m. Mineral standards for machine calibration were manufactured
by Micro-Analysis Consultant Ltd (MAC), Cambridgeshire, UK. The chemical
composition of each dated apatite grain in selected fission track samples was
determined. The Durango apatite standard was used to calibrate P, Ca, and F. A
scapolite mineral standard was used for the calibration of chlorine, and a wollastonite
standard for the calibration of silica. The OH-content has been calculated
stochiometrically. Electron microprobe analyses of the apatites are summarised in
Appendix C, Tab. C5.
2.12 Thermal modelling of the fission track age and length distribution in apatites
Apatite fission track data provide not only age information but also an estimate of
the thermal history of host rocks. The annealing of fission tracks is an important aspect
of the fission track thermochronometer. Thermal histories can be reconstructed from
forward modelling of time-temperature histories and through comparison of predicted
and measured fission track ages and lengths based on the understanding of annealing
kinetics in apatite (Green et al. 1986 and 1989, Laslett et al. 1987, Crowley et al. 1991).
The fission track age and track length distribution determine the cooling below the
respective closure temperature of each mineral and the cooling rate. In case of fission
tracks, the closure temperature is defined as effective retention temperature at which
50% of the original number of fission tracks are preserved. For apatites, a closure
temperature of 100±20° C is generally assumed (Wagner 1968, Naeser & Faul 1969,
Hurford 1986), whereas the zircon closure temperature has been determined as
240±50° C for slow cooling rates (Hurford 1986). For cooling rates >10° C/Ma, as in
alpine regions, a value of 225±25° C has been proposed by Hurford et al. (1989, 1991).
The computer program AFTSolve (Ketcham et al. 2000) was used to model the thermal
histories of selected samples with sufficient track length data. The program combines
“forward modelling”, in which fission track parameters for hypothetical thermal
histories are predicted, with “inverse modelling” that reconstructs thermal histories
based on input of measured fission track data. AFTSolve (1) generates a large number
of forward models, (2) compares predicted apatite fission track parameters including
apparent age and length distribution with measured values for each forward model,
and (3) uses the good and acceptable forward models to provide quantitative
constraints on time-temperature histories that are permitted by the measurements.
Input parameters for the program are the apparent fission track age, the fission track
length distribution, and the chlorine content or d par (etch pit diameter) of the apatites.
Time-temperature constraints, such as geochronologic data of other minerals, or known
burial and erosion history can be incorporated in the model. The program uses a Monte
Carlo algorithm to generate a best fit thermal evolution path for an apatite sample with
known apparent age and track length distribution. The operator can determine the
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