THERMOBAROMETRY USING MULTI-EOUILIBRIUM CALCULATIONS: A NEW TECHNIOUE, WITH PETROLOGICAL APPLICATIONS-

An "internally consistent method" (Powell 1985) for performing thermobarometric calculations is described in which all possible equilibria implied by a given mineral assemblage are computed using an internally consistent set of thermodynamic data for end members and mixing propeilies. Results are presented graphically and as best estimates determined by weighted averaging of the graphical results, Several examples illustrate that convergence of all equilibria in a single P-T region correlates well with textural and chemical indications of equilibrium, suggesting that this technique makes it possible to reasonably assess the state of equilibration of samples for systems in which the thermodynamic properties appear to be well described, One implication is that portions of P-T paths may be reconstructed from rocks that display disequilibrium textures but that pass this test for local equilibrium. The general applicability of this technique is presently limited by the accuracy of thermodynamic data, particularly for solid solutions, but should expand as these properties are refined in the future. This method facilitates the refinement process by (1) highlighting which minerals are most incompatible in each calculation and thus may be in need of refinement, (2) illustrating the sensitivity of results to different solution-models and offering the chance to correlate these differences with petrographic observations, and (3) providing a means to select "well-equilibrated" samples that can be used to refine thermodynamic properties in the absence of appropriate experimental data.