Gessmann et al., 1997 GARB

Gessmann et al., 1997 GARB - American Mineralogist Volume...

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1225 0003–004X/97/1112–1225$05.00 American Mineralogist, Volume 82, pages 1225–1240, 1997 Experimental study of the Fe-Mg exchange between garnet and biotite: Constraints on the mixing behavior and analysis of the cation-exchange mechanisms C.K. G ESSMANN ,* B. S PIERING , AND M. R AITH Mineralogisch-Petrologisches Institut, Universita ¨t Bonn, 53115, Bonn, Germany A BSTRACT New experimental data are presented for the Fe-Mg exchange between garnet and biotite in the temperature range 600–800 8 C at 0.2 GPa. The Fe-Mg-Al mixing properties of biotite were evaluated and the garnet-biotite geothermometer was recalibrated. SEM ob- servations and comparative laser granulometry show that solution-precipitation largely controls the cation exchange mechanism, involving about 50% of the mineral volume. Mass balance calculations emphasize the effectiveness of the experimental design: A high Gt/Bio ratio ensures that the garnet composition remains approximately constant and close to equilibrium, even if the entire garnet volume is not involved in the cation exchange. Progressively decreasing partition coefficients with decreasing Fe content of garnet indicate nonideal thermodynamic mixing behavior. The application of various garnet activity mod- els support nearly ideal Fe-Mg mixing in garnet. The remaining nonidealities were attrib- uted to nonideal Fe, Mg, and Al mixing in biotite as the initially binary biotite samples changed toward more aluminous compositions during the experiments. Adopting the stan- dard state properties and the garnet-mixing model of Berman (1988, 1990), least square regressions reveal nearly ideal mixing of Fe and Mg in biotite with W FeMg 52 2.3 6 1.6 kJ/mol, while the difference between Fe-Al and Mg-Al interactions yield D W Al 17.6 6 2.4 kJ/mol (1 cation). This interaction parameter is strictly valid only for Tschermak- substituted [6] Al in biotite according to the operational substitution. Application of the suggested garnet-biotite geothermometer reproduces well the reference temperatures of experimental and natural garnet biotite assemblages. I NTRODUCTION The Fe-Mg exchange between garnet and biotite is strongly temperature dependent and therefore is widely used as a geothermometer. The cation exchange reaction can be expressed as Mg Al Si O 1 KFe AlSi O (OH) 3 2 31 2 3 31 0 2 pyrope annite Fe Al Si O 1 KMg AlSi O (OH) . (1) 2 3 0 2 almandine phlogopite Early calibrations of this exchange reaction were based either on natural assemblages for which the P-T condi- tions were constrained by independent methods (e.g., Thompson 1976; Goldman and Albee 1977) or on exper- imental data (Ferry and Spear 1978; Perchuk and La- vrent’eva 1983 and Perchuk et al. 1985). The two exper- imental studies differ in starting materials and experimental conditions employed: Ferry and Spear (1978) carried out exchange experiments with synthetic binary (Fe-Mg) phases at temperatures from 550 to 800 8 C at 0.2 GPa; Perchuk and Lavrent’eva (1983) used pre- dominantly natural minerals as reactants for their exper- * Current address: Bayerisches Geoinstitut, Universita ¨t Bay- reuth, 95440 Bayreuth, Germany
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Gessmann et al., 1997 GARB - American Mineralogist Volume...

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