GARNET-BIOTITE GEOTHERMOMETRY REVISED - NEW MARGULES PARAMETERS AND ANATURAL SPECIMEN DATA SET FROM MAINE

The garnet-biotite geothermometer has been recalibrated using recently obtained Margules parameters for iron-magnesium-calcium garnet, Mn in teractions in garnet, and Al interactions in biotite, as well as the F e oxidation state of both minerals. Fe-Mg and Delta W-Al Margules para meters for biotite have been retrieved by combining experimental resul ts on Al-[6]-free and Al-[6]-bearing biotite using statistical methods . Margules parameters, per mole of biotite are W-MgFe(Bt) = 40719 - 30 T J/mole, Delta W-Al(Bt) = W-FeAl(Bt) - W-MgAl(Bt) =210190 - 245.40T J /mole, Delta W-Ti(Bt) = W-FeTi(Bt) - W-MgTi(Bt) = 310990 - 370.39T J/m ole. Based on this model, the exchange reaction Delta H is 41952 J/mol and Delta S is 10.35 J/(K mel). Estimated uncertainty for this geothe rmometer is 25 degrees C. This geothermometer was tested on two data s ets. The first consisted of 98 specimens containing garnet and biotite from west-central Maine, which formed under reducing f(O2) with graph ite, a limited range of P (similar to 3 to 4.5 kbar), and a moderate r ange in T (similar to 550-650 degrees C), and which were all analyzed on a single microprobe using the same standards. Results indicate that the Maine staurolite zone averages 574 degrees C compared with 530 de grees C previously calculated and that the muscovite-breakdown Tis con sistent with experimental data. The second set consisted of cordierite -garnet granulites without hypersthene from Ontario. Results here sugg est an average T of 662 degrees C, compared with significantly lower o r higher Ts calculated from other geothermometers. This model reproduc es the Perchuk and Lavrent'eva (1983) experimental Ts with a standard deviation of 12 degrees C and discriminates the assemblages in the Mai ne data set better than other models.