Garnet-biotite geothermometry revised; new Margules parameters and a natural specimen data set from Maine

Abstract
The garnet-biotite thermometer has been recalibrated using recently obtained Margules parameters for iron-magnesium-calcium garnet, Mn interactions in garnet, and Al interactions in biotite, as well as the Fe oxidation state of both minerals. Fe-Mg and Delta W Al Margules parameters for biotite have been retrieved by combining experimental results on (super [6]) Al-free and (super [6]) Al-bearing biotite using statistical methods. Margules parameter, per mole of biotite are W BtMgFe = 40719-30T J/mole. Delta W BtAl = W BtFeAl -W BtMgAl = 210190-245.40T J/mole. Delta W BtTi = W BtFeTi -W BtMgTi = 310990-370.39T J/mole. Based on this model, the exchange reaction Delta H is 41952 J/mol and Delta S is 10.35 J/(K mol). Estimated uncertainty for this geothermometer is 25 degrees C. This geothermometer was tested on two data sets. The first consisted of 98 specimens containing garnet and biotite from west-central Maine, which formed under reducing f O2 with graphite, a limited range of P ( approximately 3 to 4.5 kbar), and a moderate range in T ( approximately 550-650 degrees C), and which were all analyzed on a single microprobe using the experimental data. The second set consisted of cordierite-garnet granulites without hypersthene from Ontario. Results here suggest an average T of 662 degrees C, compared with significantly lower or higher T S calculated from other geothermometers. This model reproduces the PErchuk and Lavrent'eva (1983) experimental T S with a standard deviation of 12 degrees C and discriminates the assemblages in the Maine data set better than other models.

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