Compatibility of geobarometers in the system CaO-FeO-Al₂0₃ SiO₂-TiO₂ (CFAST): implications for garnet mixing models.

In the system CaO-FeO-Al₂O₃-SiO₂-TiO₂ several barometers have been experimentally calibrated. These include: Alm + 3 Ru = 3Jlm + Sil + Qtz (GRAIL), 2Alm + Gr + 6 Ru = 6Ilm + 3 An + 3 Qtz (GRIPS), 2 Alm + Gr = 3 Fa + 3 An(FAG), 3 An = Gr + 2 Ky + Qtz (GASP), 2 Fs = Fa + Qtz(FFQ), and Gr + Qtz = An + 2 Wo (WAGS). Because GRIPS, GRAIL, and GSAP form a linearly dependent set, they provide a stringent test for the accuracy of A/X models for grossular/almandine solid-solutions. The Ganguly and Saxena garnet solution model yields a calculated position for GASP 2 kb above a best-fit to the available experimental reversals. Using these reactions to constrain mixing parameters for grossular/almandine garnets (on a Ca + Fe⁺² = 1 cation basis) yields W_H(CaFe) = 6240J, W_H(FeCa) = 23800J, and W_S(CaFe) = W_S(Fe Ca) = 11.4J/K 'Model 1), or W_H(CaFe) = 628J, W_H(FeCa) = 17080J (Model 2) using the measured pyrope/grossular value for W_S(6.276J/K). GRIPS can be further checked by adding Fa + 2 Ru = 2Ilm + SiO₂ (FRIS) to FAG. Available thermodynamic data do not yield GRIPS in good agreement with available experiments, and a revised value for the Gibbs energy of fayalite has been calculated. These calculations yield the following values of /G^o ₂₉₈ (elm) for almandine, fayalite, ferrosilite, grossular, and anorthie: -4940.8, -1377.1, -1116.2, -6282.3, and -4010.9kJ/mol respectively.-Authors