Geoscience Reference
In-Depth Information
400
Figure 6.8.
Closure
temperatures for the
retention of fission tracks
as a function of cooling
rate, for a variety of
minerals. (From Faure
(1986).)
Sphene
Garnet
Epidote
Zircon
300
Sphene
Allanite
200
Vesuvianite
Muscovite
Hornblende
Phlogopite
Vermiculite
Apatite
100
Biotite
0.1
1.0
10
100
Cooling Rates in
°
C Ma
--1
high temperatures over geological periods of time, the damaged zones in the
crystals along the particle track anneal (heal). The rate of annealing differs for
every mineral and is temperature-dependent. At room temperatures the tracks
are stable. Thus, two minerals of the same age that have been at the same high
temperature for the same length of time can yield two different fission-track
ages. For example, after 1 Ma at 50
◦
Casmall number of fission tracks in apatite
will have annealed, but to anneal all fission tracks within 1 Ma the apatite would
need to be at 175
◦
C. If the heating time is only 10 000 yr, the temperatures
required for annealing increase to 75 and 190
◦
C, respectively. Tracks in the
mineral
sphene
can withstand much higher temperatures: annealing will start
if the mineral has been at 250
◦
C for 1 Ma, but not all tracks will completely
anneal unless the mineral has been at 420
◦
C for 1 Ma. The corresponding tem-
peratures for 10 000 yr are 295 and 450
◦
C. This means that, although fission-
track dates can be completely reset by heating, the temperature history of a
