Tutorial 1

The computed ages are summarized in Table A9.1. The ages obtained with Mad_He are all very close to 40 Myr, demonstrating that inverting an age dataset to obtain a temperature history is a non-unique problem. Many thermal histories can lead to the same age. Note also that Dodson's method is valid only for simple cooling histories. The absolute-closure-temperature method is almost always inaccurate.

Fission-track-length distributions as computed from MadTrax.f are shown in Figure A9.1. Rapid cooling (scenario 1) leads to a narrow track distribution whereas slow cooling (scenario 2) leads to a broad track distribution.

Tutorial 2

Muscovite 40Ar/39 Ar ages are 40 Myr for the first scenario, and > 100 Myr for the four others. Except for the first scenario of very rapid cooling from high temperatures, muscovite 40Ar/39Ar is not the appropriate system to study – its closure temperature is too high to discriminate among the different low-temperature thermal histories.

The apatite fission-track ages calculated using MadTrax.f are given in Table A9.1, together with the (U–Th)/He ages calculated in Tutorial 1. The combination of apatite (U–Th)/He and fission-track thermochronometers adds additional constraint to the scenarios and removes some of the ambiguities noted in Tutorial 1. The fission-track ages for the first three scenarios are different.

Fission-track length distributions as computed from MadTrax.f are shown in Figure A9.1.