Geol. 655 Isotope Geochemistry
Lecture 14 Spring 2003
90 February 19, 2003
GEOCHRONOLOGY X: FISSON TRACK DATING
INTRODUCTION
As we have already noted, a frac-
tion of uranium atoms undergo
spontaneous fission rather than al-
pha decay. The sum of the masses
of the fragments is less than that of
the parent U atom: this difference re-
flects the greater binding energy of
the fragments. The missing mass has
been converted to kinetic energy of
the fission fragments. Typically, this
energy totals about 200 MeV, a con-
siderable amount of energy on the
atomic scale. The energy is depos-
ited in the crystal lattice through
which the fission fragments pass by stripping electrons from atoms in the crystal lattice. The
ionized atoms repel each other, disordering the lattice and producing a small channel and a wider
stressed region in the crystal. The damage is visible as tracks seen with an electron microscope
operating at magnifications of 50,000¥ or greater. However, the stressed region is more readily
attacked and dissolved by acid; so by acid etching the tracks can be enlarged to the point where
they are visible under the optical microscope. Figure 14.1 is an example.
Because fission is a rare event in any case, fission track dating generally uses uranium rich miner-
als. Most work has been done on apatites, but sphene and zircon are also commonly used.
Fission tracks will anneal, or self-repair, over time. The rate of annealing is vanishingly small at
room temperature, but increases with temperature and becomes significant at geologically low to
moderate temperatures. In the absence of such annealing, the number of tracks is a simple function
of time and the uranium content of the sample:
Fs = (lƒ/la) 238U (elat – 1) 14.1
where Fs is the number of tracks produced by spontaneous fission, 238U is the number of atoms of
238U, la is the a decay constant for 238U, and lƒ is the spontaneous fission decay constant, the best
estimate for which is 8.46 ± 0.06 ¥ 10-17 yr-1. Thus about 5 ¥ 10-7 U atoms undergo spontaneous fis-
sion for every one that undergoes a-decay. Equation 14.1 can be solved directly for t simply by de-
termining the number of tracks and number of U atoms per volume of sample. In this case, t is the
time elapsed since temperatures were high enough for all tracks to anneal. This is the basis of fis-
sion track dating. The temperatures required to anneal fission damage to a crystal are lower than
those required to isotopically homogenize one. Thus fission track dating is typically used to “date”
lower temperature events than conventional geochronometers.
ANALYTICAL PROCEDURES
Determining fission track density involves a relatively straightforward procedure of polishing and
etching a thin section or grain mount, and then counting the number of tracks per unit area. A
number of etching procedures have been developed for various substances. These are listed in Ta-
ble 14.1. Track densities of up to several thousand per cm2 have been recorded. A minimum den-
sity of 10 tracks per cm2 is required for the results to be statistically meaningful. A fission track,
which is typically 10 µ long, must intersect the surface to be counted. Thus equation 14.1 becomes:
rs= Fsq = (lƒ/la) 238U (elat – 1)q 14.2
Figure 14.1. Fission tracks in a polished and etched
zircon. Photo courtesy J. M. Bird.
