Then, by comparing the found KD value to the calculated garnet-biotite geothermometer, the peak temperature of rock formation can be determined.
Geothermobarometry allows for the calculation of equilibrium temperatures and pressures from measured distributions of elements between coexisting phases.
Ferry and Spear's calculation of the garnet-biotite geothermometer (1978),[2] involved running experimental reactions between biotite and garnet at a constant pressure (0.207 GPa) and varying temperatures between 500 °C and 800 °C.
Using an excess of garnet forced the biotite do the majority of the work to reach the equilibrium value (KD), expediting the reaction process.
[2] The resulting data from the experiments (final biotite composition and Fe / Mg concentration) were used to calculate an equilibrium constant (KD) at each reaction temperature.
So, given a rock containing garnet and biotite, the temperature of formation can be determined by finding the KD value, and comparing it to the calculated trend line.
However, Spear argues that the garnet-biotite geothermometer is relatively insensitive to pressure changes, and can apply to rocks at any mid-crustal level.