UNIVARIANT MIXED-VOLATILE REACTIONS - PRESSURE TEMPERATURE PHASE-DIAGRAMS AND REACTION ISOGRADS

For any isobaric invariant point on a fully expanded temperature versus mol fraction (T-X) phase diagram, there is a pair of phase assemblages whose stability fields span the largest divariant sectors on the low-T and high-T sides of the point and do not overlap along the T axis. The two assemblages have no solid phases in common, but together they include all the phases of the isobarically invariant assemblage. The closed-system reaction that relates these two assemblages produces a fluid phase of isobarically invariant composition, and it can be mass-balanced algebraically without prior knowledge of the stable T-X topology, thus identifying the low-T and high-T phase assemblages in a simple and direct manner. These considerations justify and facilitate labeling of reactant and product assemblages on the polybaric P-T trace of any isobaric invariant point. In turn, such labeling facilitates direct chemographic expansion of H2O-CO2 or other mixed-volatile equilibria in P-T projection, bypassing the usual first step of constructing a complete set of stable T-X diagrams in the P range of interest. The resulting P-T phase diagrams are convenient in the determination and display of the P-T stability fields of silicate-carbonate mineral assemblages. Such diagrams also serve to identify critical subassemblages that are necessary and sufficient for rigorous mapping of isograds based on univariant mixed-volatile reactions and bathograds based on invariant mixed-volatile reactions.