1. A primer on apatite fission track dating
Apatite commonly contains a little bit of 238U.
When 238U decays, the ejected alpha particle creates a track of damage inside the apatite grain.

2. How does this occur? A fission track
Use dilute nitric acid to etch the tracks on the apatite surface so they are visible.
Etched fission tracks

3. So, what determines how many tracks are made?
Directly related to two variables:
1) time
2) uranium content
So, we must determine how much U is present in each apatite to eliminate it as a variable.
Then, we can calculate the age.

4. What age are we calculating?
Apatite tracks anneal at elevated temperature!
Cold temperatures stop apatite from recrystallizing.
Let’s look at the age of tracks vs temperature:

5. So, what does this mean:?
If the U content is known, and the tracks are counted, then time since passing through the 100°C ± 20°C isotherm can be calculated. This is an apatite fission track age. Gives information on uplift rate for rocks.
Also, mean track length gives information on how slowly or rapidly the sample came through this isotherm (i.e. fast = all long tracks; slow = many short tracks).