A new thermodynamic model for sapphirine: calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3

The activity-composition (a-x) relations of sapphirine are re-evaluated in the light of a recent new internally-consistent data set of phase end-members for use in phase equilibria modelling, particularly of ultra-high-temperature (UHT) rocks. This is achieved with the aid of relatively oxidized sapphirine+quartz-bearing granulites from Wilson Lake, Canada. Calculated P-T projections and compatibility diagrams in the K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (KFMASHTO) system are used to illustrate sapphirine+quartz-bearing phase equilibria in the context of UHT metamorphism. These new a-x relations for sapphirine should allow pseudosection thermobarometry in NCKFMASHTO for estimating peak P-T conditions of sapphirine-bearing rocks.