The stability of sapphirine plus quartz: calculated phase equilibria in FeO-MgO-Al2O3-SiO2-TiO2-O

The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra-high-temperature (UHT) metamorphism (> 900 degrees C), an inference based on the restriction of this assemblage to temperatures > 980 degrees C in the conventionally considered FeO-MgO-Al2O3-SiO2 (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe2O3, phase equilibra modelling using thermocalc software has been undertaken in the FeO-MgO-Al2O3-SiO2-O (FMASO) and FeO-MgO-Al2O3-SiO2- TiO2-O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz-saturated systems, the effects of Fe2O3 and TiO2 on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 degrees C in oxidized systems and thus out of the UHT range.