Garnet-biotite thermometry revisited; the effect of Al (super VI) and Ti in biotite

Eleven published versions of the garnet-biotite thermometer have been appraised by applying them to the experimental data of Ferry & Spear and Perchuk & Lavrent'eva; several calibrations show extreme deviations of up to approximately 260 degrees C from the experimental T. A new calibration has been developed which uses the garnet activity model of Berman [A.M. 75-328] and a new activity model for biotite. For the new thermometer, mean deviation and mean proportional deviation with reference to the experimental T are -1 degrees C/1.5% for the Perchuk & Lavrent'eva dataset and -6 degrees C/4.5% for the Ferry & Spear dataset. Precision and accuracy have also been tested on natural rocks from well-studied metamorphic sequences; the results show that the influence of octahedral Al and Ti in biotite on the T calculation is significant and cannot be ignored.