The study of pelitic rocks for the purpose of deciphering pressure-temperature (P-T) information is an important area for collaboration between mineralogists and petrologists. Accurate geothermobarometry is especially important for studies of pressure-temperature-time paths of terranes during tectonism and for studies of the movement of metamorphic fluids. During the past decade, new developments have included thermodynamic data bases, sophisticated crystal structure determinations and associated site assignments, and analyses for Fe3+ and light elements (especially H and Li) for many minerals within a petrogenetic context. It is now necessary to apply these new findings to geothermobarometry in several important ways: (1) The stoichiometric basis for hydrous minerals should be revised in light of highly variable H and variable Fe3+, which can now be estimated with considerable accuracy as a function of grade or mineral assemblage. (2) Mole fraction (activity) models should be based on the best crystal chemical considerations. For some minerals, H may be omitted from the model if all the substitutions involving H are coupled substitutions. (3) Thermodynamic data should be based on careful analysis of all available experiments and secondary comparison with natural assemblages. (4) The possibility of nonideal solid solution should be considered, as ideality is merely a special case of nonideality. It is better to estimate binary interaction parameters than to assume that they are zero. However, it is difficult to determine ternary interaction parameters. In such cases, little error is likely to result from assuming that strictly ternary interaction parameters are zero, while evaluating all the binary terms. Our understanding of garnet and biotite solutions has improved, and the garnet-biotite geothermometer has undergone numerous refinements, but additional information on these minerals and improvements in the garnet biotite thermometer are still necessary.
As the P and T determinations become more precise, we can better evaluate XH2O, XCO2, and XCH4 in the fluid phase. In all probability, patterns will be discovered in the compositions of pelitic metamorphic fluids, especially when graphite is present. The hematiteilmenite solid solution system is potentially useful for determining fO2, once the solvus is better understood. In graphitic rocks at known P and T, knowledge of fO2 from coexisting hemoilmenite and rutile or hemoilmenite and magnetite enables one to determine the mole fraction of all fluid species.
Careful application of the present state of knowledge, combined with modest improvements, should allow accuracy approaching ±250 bars and ±25 °C in rocks formed at low to moderate pressures. It is very important to continue to revise thermochemical data, activity models, Margules parameters, and stoichiometric information, and it is equally important for petrologists and tectonicists to make use of these revisions as they become available.