Like apatites, zircons are a common
detrital constituent of medium to coarse grained clastic sediments.
The zircon grains originate as accessory minerals in igneous or
metamorphic rocks, and as they are very stable during erosion
and transport, they are a common sedimentary constituent. Zircons
contain trace amounts of uranium (up to 1000 ppm) and thus fission
tracks form within the crystal lattice over time, the density
of which is proportional to the uranium content of the zircon.
As in apatite, fission tracks in zircon shorten
when heated, but significant effects require much higher temperatures
than in apatite. The annealing kinetics of fission tracks in zircon
are less well known than for apatite, but recent experimental
annealing studies have shown that the kinetics in the two minerals
are of the same general form.
Technical difficulties relating to anisotropic
etching and the effects of alpha-recoil damage in zircon mean
that interpretation of experimental data is not straightforward.
Studies of annealing kinetics in geological situations offer some
promise, but are limited by the rarity of present-day settings
where temperatures are high enough to produce observable effects.
Data from very deep wells, including the Kola Peninsula Superdeep
Drillhole SG-3 which penetrates over 12 km of Precambrian rocks,
and reaches a temperature of 216°C at TD, show that temperatures
well over 200°C are required to produce detectable annealing.
information on fission track annealing kinetics in zircon is available
from comparison of ZFTA data with vitrinite reflectance (VR) data
from sedimentary sequences which have been subjected to profound
Our research in South Island, New
Zealand, has shown that the onset of significant fission track
age reduction in zircon correlates with VR values of ~6%, while
total annealing is equivalent to ~8% Romax or higher (see Figure),
implying temperatures of around 300°C (i.e. Greenschist grade
Thermal history applications of ZFTA are currently
limited to the evaluation of fission track age data. The nature
of most zircon grains is such that track length measurements cannot
be routinely carried out, as they are in apatite. Combined with
uncertainty over the exact form of the annealing kinetics, detailed
thermal history modelling of ZFTA data is not, as yet, possible.
ZFTA can be applied to sediments in which the
vitrinite reflectance is very high (> 6%) to determine the
timing of cooling from a high temperature event. In conjunction
with AFTA® and a vertical vitrinite reflectance profile, ZFTA
may provide unique information on the timing of high temperature
events due to igneous activity, elevated heat flow or deep burial.
Due to the high thermal stability of tracks in zircon, ZFTA
can also be used as a simple dating tool in stratigraphic applications
where conventional radiometric techniques are inadequate. For
example, direct determinations of the age of highly weathered
igneous rocks and tuffs are possible because zircon is resistant
to severe weathering conditions. Altered basaltic intrusions that
do not contain zircon may be dated by analysing zircons from the
adjacent contact metamorphic zone in suitable rock types (e.g.
sediment provenance studies, ZFTA may be used to identify regions
contributing detritus to a basin. As a conservative guide, if
the regional vitrinite reflectance level is less than 2% in the
formation of interest, then the ZFTA results from a sediment can
be interpreted simply in terms of the time of cooling below ~300°C
in the provenance terrain from which the zircons were eroded.
Different components within a spectrum of fission track ages can
then be related to different source terrains.
Geotrack offers a zircon fission track analytical
program and interpretation as a part of its thermal history reconstruction
services. Consultation at the commencement of a project will enable
us to assess the validity of the analysis in the specific geological
setting, and to recommend a cost-effective sampling strategy appropriate
to the problems to be addressed.