The Method: Thermochronology
What is Thermochronology?
Thermochronology comes from the greek
- Thermos: Hot (temperature)
- Chronos: Time (dating)
and implies dating of temperature related events.
In geology, thermochronologists investigate temperature-histories of rocks. Research aims can range from
- intrusion/eruption ages (cooling of a magma/lava),
- hydrothermal activity (hot fluids circulating in the crust),
- metamorphism of rocks (alteration of rocks through deformation, pressure and/or temperature)
- exhumation (bringing rocks to the surface by lifting them out of the hot mantle/crust (mountainbuilding through tectonics) or erosion)
One way to investigate cooling of rocks is Helium-dating. The radioactive isotopes 238Uranium, 235Uranium, 232Thorium and 147Samarium, which can be found in many minerals, decay via alpha-decay and thus produce alpha-particles (4He atoms) in the process.
Because this process is linked to the decay constant of the parent isotopes (the ones mentioned above) the number of He-atoms in the crystal increases with time. But only if the crystal (or rock in which the crystal is found) is below a certain temperature (the so called 'closure temperature'). If the temperature is higher vibrations of the crystal lattice (framework of atoms that make up the crystal) allows the Helium to be lost through diffusion.
The minerals I am working with are apatite and zircon and their closure temperatures for the (U-Th)/He method are 70-100oC and 180-200oC respectively (the exact values depend on many factors which shall not be explained in detail here). So: if I measure both apatites and zircons found in the same rock with this method I will be able to tell when the rock was last at a temperature of 70oC and 180oC - or in other words at depths of about 2km and 6km beneath the surface.
Fission Track Dating
The second method I am using uses spontaneous fission of 238U into two smaller atoms with masses between 140-150 and 87-93. This process creates a small fissure in the crystal lattice which is typically about 15-20 micrometers long. This tiny little 'path of destruction' can be made visible within the crystals by etching them. As this is also related to a decay constant the number of 'fission tracks' increases with time - so the older a grain is the more tracks we will see.
As with the Helium-method this techniques is also depending on temperature. If it is too hot vibrations of the crystal lattice will 'heal' the fission tracks by allowing atoms to jump back into their original position. The closure temperatures for apatite and zircon in this method range from 90-120oC and 200-350oC respectively and correspond to depths of around 5km and 14km.