A REFINED GARNET - BIOTITE FE - MG EXCHANGE GEOTHERMOMETER AND ITS APPLICATION IN AMPHIBOLITES AND GRANULITES

A new formulation of garnet-biotite Fe-Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe-Mg-Ca-Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe-Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality with W(FeMg)bi = 1073 +/- 490 cal/mol. DELTA-H(r) and DELTA-S(r) for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.