Supercritical pE-pH diagrams, with applications to the stability of biotite

Abstract

A pE-pH diagram for supercritical aqueous fluids at 500° C and 2 kb total pressure is calculated from thermodynamic data assuming ideal mixing of gas species. The experimental data on the stability of annite-phlogopite solid-solutions (Wones and Eugster 1965) and on the stability of aluminous biotite (Rutherford 1968) have been used to calculate the stability of these biotite solid solutions in sanidine, muscovite and Al₂SiO₅ bearing assemblages. The resulting pE-pH diagrams show that Al increases the stability field of annite more per atom than does Mg. However, the addition of Al to biotite does not increase the stability of annite in very acid and alkaline solutions. Environments sufficiently acid to render aluminous biotite metastable are probably not found in nature, but the mildly alkaline environments are attainable. At constant f _{O 2}, f _{H 2} and $f_{H_2 O}$ the Al content of biotite is at a minimum in muscovite + sanidine bearing assemblages, and increases relatively rapidly with increases in pH in the sanidine field, and increases more slowly with decreases in pH through the muscovite and Al₂SiO₅ fields.

These diagrams show that the composition of biotite solid-solutions containing more aluminum than ideal annite-phlogopite and coexisting with sanidine and magnetite cannot be used to infer intensive parameters (T f _{O 2}, $f_{H_2 O}$) prevailing during the crystallization of the biotite, and that in no case can any biotite composition in muscovite or Al₂SiO₅ bearing assemblages be used to extract this information.