42 Thermochronological systems
More sophisticated models have been developed and coded for this purpose
by other authors, such as the MacArgon program which is available by
downloading from http://www.earth.monash.edu.au/macargon/ and described in
Lister and Baldwin (1996), which computes Ar ages and spectra for biotite, mus-
covite and hornblende from a given thermal history. Another useful program is
Arvert, which was developed by P. Zeitler and is available by downloading from
http://www.ees.lehigh.edu/EESdocs/geochronology.shtml This program inverts
K-feldspar Ar ages and spectra for thermal history, based on the multi-domain
method described in Lovera et al. (1989).
3.2 (U–Th)/He thermochronology
The production of
4
He ( particles) from uranium (U) and thorium (Th) series
decay in rocks and minerals was the first geochronological dating method to be
proposed early in the twentieth century (Rutherford, 1907; Soddy, 1911–1914).
However, at that time geoscientists were pursuing only ‘absolute’ or formation
ages of rocks, notably in a quest to constrain the age of the Earth (Holmes, 1913).
Since He diffuses easily out of the mineral lattice (a phenomenon that was not
understood at the time), ages determined using U, Th and He measurements were
consistently much younger than those calculated using the U–Pb couple. The use
of He as a geochronometer consequently came to be considered unreliable and
was abandoned (Hurley et al., 1956).
Interest in the technique has been revived since Zeitler et al. (1987) proposed
that the diffusive loss of He could be quantified and that He ages could be
used to constrain cooling through very low temperatures. Subsequent diffusion
experiments (Wolf et al., 1996; Farley, 2000) have demonstrated that the apatite
(U–Th)/He thermochronometer is sensitive to temperatures as low as 40
C, with
effective closure occurring around 70
C, depending on the cooling rate and min-
eral grain size. Comparisons of apatite (U–Th)/He and fission-track ages (Warnock
et al., 1997; House et al., 1999; Stockli et al., 2000) have confirmed this range of
relative temperature sensitivity on geological timescales (see Figure 3.10 later).
More recent experiments to determine the thermal response of other accessory
minerals have shown that the effective closure temperature is ∼160–200
C in zir-
con (Reiners et al., 2002, 2004) and ∼190–220
C in titanite (Reiners and Farley,
1999) (see Table 1.1). The low closure temperatures of the (U–Th)/He system,
especially in apatite, make it particularly sensitive to near-surface cooling and
thermal perturbations. This increased resolving power towards the low end of the
temperature spectrum able to be constrained by thermochronology has gained the
method considerable interest from the Earth-science community over the last few
years, especially from those in the field of geomorphology.