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Fission Tracks
Science.
Radioactive decay of uranium isotopes in the mineral grain releases
alpha particles that pass through the lattice of the crystal, leaving a
trace of their path, these paths are “fission tracks”
Annealing point, distance from annealing point
The number of Uranium fission tracks corresponds to the amount of
uranium in the rock
Used in Zircon and Apatite, accessory minerals in metamorphic and
igneous rocks.
AFTA, ZFTA. Termochronologicical technique.
Techniques
How are fission tracks measured?
In the case of apatite, if the grains are
heated above their closure
temperature, all existing fission tracks
will be annealed and the fission track
age will be reset.
Uranium-238 decays at a known rate
and is the only isotope with a decay
rate that is relevant to the production
of fission tracks.
Chemically etch the fission tracks and
use of an optical microscopy to count
the fission tracks (generally 1-15
micrometers).
Fs= (λf/λα)238U(e λt -1)
Applications
Used to figure out what time in the past a mineral was at a certain
temperature.
Calculate erosion rates, faulting rates
If a rock cools quickly as in the case of a volcanic rock or a shallow igneous
intrusion, the fission-track ages will date this initial cooling.
Has been to applied to diverse areas of earth sciences, such as volcanology,
stratigraphy, basin analysis and extraterrestrial bodies.
“Fissure Track Dating of Meteorite impacts”
The age of the impact craters was estimated by Storzer and
Wagner (1977) at approximately 6400 yr, using fission tracks in
impact melt droplets.
Sources of Errors?
Laboratory Analysis: Relying on looking and counting
of the fission tracks under microscopy.
Due to annealing points, tracks can be erased.
The value of the fission decay constant or U-238 is
currently in contention.
“not too many, not too few” fission tracks