Temperature estimates obtained with various biotite-garnet thermometers are inconsistent in granulite facies rocks. Although the inconsistency lies partly in the lack of definition of equilibrium criteria, it still remains after rigorous selection of analyzed mineral pairs (isolated biotite and garnet cores for peak equilibrium conditions) and originate from minor element effects in garnet and biotite solid solutions. The departure from ideality due to Ca in garnet can be calculated either with the available thermodynamic data (model A), or with a combination of thermodynamic and empirical data (model B), whereas that due to Mn is probably negligible due to the low Mn content of high-grade garnets. The high Ti content of biotites seems responsible for the abnormally high temperatures obtained with an ideal lnKD/T relation (e.g., the experimental calibration of Ferry and Spear). This deviation and that introduced by Alvi substitution in biotite can be evaluated using the classical equations of regular solutions provided independent T estimates are available. Ti and Al interaction parameters have thus been evaluated as:
(in cal per mole) depending on the model adopted for the non-ideality in garnet. Starting from the experimental ideal model of Ferry and Spear, a new calibration of the biotite–garnet geothermometer based upon the above correction for Al and Ti and the interaction of Ca in Fe–Mg garnets is proposed here:
Application of this calibration to granulite facies rocks gives results compatible with those obtained using Thompson's calibration.
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