Stability of sapphirine-bearing mineral assemblages in the system FeO-MgO-Al2O3-SiO2 and metamorphic P-T parameters of aluminous granulites

Abstract

Consistent thermodynamic data on the properties of pure mineral end members and the mixing properties of solid solutions in the system FeO-MgO-Al2 O 3 -SiO 2 were employed to simulate phase rela� tions of sapphirine, garnet, spinel, orthopyroxene, cordierite, quartz, Al silicates, and corundum. Composi� tional variations of the solid solutions with temperature notably modify the topology of the P-T diagrams, which differ from the petrogenetic grids widely used in the literature. It is worth noting that the evaluation of PT metamorphic conditions based on reaction relations in sapphirinebearing assemblages cannot be so far considered reliable enough. The lower stability limit of the sapphirine + quartz assemblage in the system in question is possibly located at much lower P-T parameters: at least 835°C and ~6 kbar. The sapphirine + kya� nite assemblage can be stable at temperatures below 860°C and a pressure of ~11 kbar, and the stability field of the sapphirine + olivine assemblage is narrow and constrained to temperatures no higher than ~800°C.