Isograds in internally buffered systems without solid solutions: principles and examples

Abstract

Internal buffering is recognised as an important end-member process when considering the factors governing the composition of a fluid phase present in a rock during metamorphism. In such processes, the major isograd-forming reactions take place at intersections in T-XCO2 space. In P-T space, the trace of such intersections with changing pressure forms a univariant reaction along which the composition of the coexisting fluid phase changes. In this paper, it is demonstrated that the stoichiometry of these univariant reactions changes at singular points. These singularities are controlled by divariant reactions which liberate CO2 and/or H2O, and thus form degenerate univariant reactions in the complete chemical system. A series of phase diagrams illustrating these points is presented for ultrabasic, margarite-kyanite-clinozoisite and siliceous dolomite assemblages. These diagrams provide a complete description of potential isograd-forming reactions within each chemical system. P-T pseudosections are constructed for particular bulk compositions to illustrate the continuum between ‘externally controlled’ and internally buffered conditions and the effects of changes in fluid and solid phases.