Uncertainties in the activities of garnets and their propagation into geothermobarometry

The effect of uncertainties in the activities of garnets on P-T estimates are explored by propagating the (co-)variances of an internally consistent set of least-squares-derived mixing parameters through the thermodynamic formulations of both the garnet-biotite geothermometer and the garnet-Al (sub 2) SiO (sub 5) - plagioclase-quartz geobarometer. In combination with uncertainties related to DELTA H (super 0) (sub R) , DELTA S (super 0) (sub R) and composition, total uncertainties incorporating all types of errors can now be estimated. From data for 80 samples, it is concluded that typical uncertainties of the order of approx 60 degrees C and 1.5 kbar must be assumed for P-T estimates.