Holdaway_p582-595_97

Holdaway_p582-595_97 - American Mineralogist, Volume 82,...

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582 0003–004X/97/0506–0582$05.00 American Mineralogist, Volume 82, pages 582–595, 1997 Garnet-biotite geothermometry revised: New Margules parameters and a natural specimen data set from Maine M.J. H OLDAWAY , 1 B ISWAJIT M UKHOPADHYAY , 1 M.D. D YAR , 2 C.V. G UIDOTTI , 3 AND B.L. D UTROW 4 1 Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, U.S.A. 2 Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, U.S.A. 3 Department of Geological Sciences, University of Maine, Orono, Maine 04469, U.S.A. 4 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. A BSTRACT The garnet-biotite geothermometer has been recalibrated using recently obtained Mar- gules 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 D W Al 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 Bt W 5 40719 2 30 T J/mole, MgFe Bt Bt Bt D W 5 W 2 W 5 210190 2 245.40 T J/mole, Al FeAl MgAl Bt Bt Bt D W 5 W 2 W 5 310990 2 370.39 T J/mole. Ti FeTi MgTi Based on this model, the exchange reaction D H is 41952 J/mol and D S is 10.35 J/(K mol). Estimated uncertainty for this geothermometer is 25 8 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 O 2 with graphite, a limited range of P ( ; 3 to 4.5 kbar), and a moderate range in T ( ; 550– 650 8 C), and which were all analyzed on a single microprobe using the same standards. Results indicate that the Maine staurolite zone averages 574 8 C compared with 530 8 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 8 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 8 C and discriminates the assemblages in the Maine data set better than other models. I NTRODUCTION The garnet-biotite Fe-Mg exchange geothermometer is the most widely used thermometer for estimating T of equilibration of medium-grade pelitic metamorphic rocks. The landmark experimental calibrations of Ferry and Spear (1978) and Perchuk and Lavrent’eva (1983) form the basis for this geothermometer, together with recent modifications that account for nonideality in the garnet and biotite (e.g., Ganguly and Saxena 1984, Indares and Martignole 1985, Williams and Grambling 1990, Berman 1990, McMullin et al. 1991, Bhattacharya et al. 1992, Kleemann and Reinhardt 1994). The combination of Ber- man’s (1990) garnet model and Kleemann and Rein- hardt’s (1994) biotite model appears to be the best overall calibration currently available. However, when applied to
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Holdaway_p582-595_97 - American Mineralogist, Volume 82,...

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