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

Abstract

The garnet-biotite geothermometer 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 ΔWAl Margules parameters for biotite have been retrieved by combining experimental results on [6]Al-free and [6]Al-bearing biotite using statistical methods. Margules parameters, per mole of biotite are WB4MgFe = 40719 - 30T J/mole, ΔWB4Al = WB4FeAl - WB4MgAl = 210190 - 245.40T J/mole, ΔWB4Ti = WB4FeTi - WB4MgTi = 310990 - 370.39T J/mole. Based on this model, the exchange reaction ΔH is 41952 J/mol and ΔS is 10.35 J/(K mol). Estimated uncertainty for this geothermometer is 25 °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 fo2 with graphite, a limited range of P (∼3 to 4.5 kbar), and a moderate range in T (∼550-650 °C), and which were all analyzed on a single microprobe using the same standards. Results indicate that the Maine staurolite zone averages 574 °C compared with 530 °C previously calculated and that the muscovite-breakdown T is consistent with experimental data. The second set consisted of cordierite-garnet granulites without hypersthene from Ontario. Results here suggest an average T of 662 °C, compared with significantly lower or higher Ts calculated from other geothermometers. This model reproduces the Perchuk and Lavrent'eva (1983) experimental Ts with a standard deviation of 12 °C and discriminates the assemblages in the Maine data set better than other models.