Fission Track Analysis (AFTA®)
on analysis of radiation damage trails ("fission tracks")
within detrital apatite grains (extracted from sandstones and other
fission track is created by spontaneous fission of a single atom
Fission tracks can be
selectively dissolved and enlarged by etching in dilute nitric acid
|Etched spontaneous fission tracks in an apatite crystal.
|The number of tracks
in an apatite grain depends on:
- uranium content
Therefore in principle, if we measure the uranium content and the
number of tracks, we can measure the time over which tracks have
accumulated - the fission track age
|But if we measure fission track ages in
sub-surface samples, we find that at temperatures greater than ~70°C,
the ages are progressively reduced to zero at around 120°C.
reduction in fission track age arises because the radiation damage
constituting each fission track is progressively repaired, at a
rate which increases with temperature. This repair is manifested
as a decrease in the length of individual tracks.
The proportion of tracks
intersecting the polished surface of an apatite grain depends on
the track length, and therefore as track length is progressively
reduced, so is the fission track age.
|Confined tracks in apatite.
grain fission track ages in a sample from a present-day temperature
of ~95°C show a clear and consistent variation with Cl content:
Chlorine content exerts
a significant control on fission track annealing rates in apatite,
as also seen in laboratory experiments:
fission track analysis (AFTA®)
AFTA is based on three types of measurement:
- fission track age
- confined track lengths
- chlorine content
content is measured in every grain in which either age or length
data is collected. Grain locations are recorded using computer-controlled
microscope stages, and archived for future reference.
modelling AFTA parameters through likely thermal history scenarios,
we can define the range of temperature-time conditions giving predictions
which are consistent with the observed data.
of chlorine content
Common detrital apatite grains in sedimentary rocks
from around the world show significant variation in chlorine content,
both within individual samples and between different samples
- Ternary plot 4172 grains |
The kinetic model used to extract temperature-time solutions from our
AFTA data explicitly takes into account the effect of Cl content on annealing
Calibration of this kinetic model against data from simple geological
situations confirms the validity of the model.
Use of a kinetic model that does not take compositional effects into
account (above right) provides a very poor match with the calibration