Sapphirine-bearing assemblages in the system MgO-Al2O3-SiO2: a continuing ambiguity

Based oil experimental data for reactions involving sapphirine in the system MgO-Al2O3-SiO2 (MAS), thermodynamic properties of the sapphirine solid solution end-members have been optimized for both ideal and non-ideal models, and internally consistent P-T grids are proposed. According to the calculated P-T relationships in MAS, the sapphirine + quartz assemblage, which is widely recognized as indicative Of ultrahigh-temperature metamorphism, can be stable down to 840 degrees C and 0.67 GPa, i.e., at temperatures Lip to 150 degrees C lower than estimated by Kelsey el al. (2004). The sapphirine + kyanite assemblage has been found stable at temperatures below 860 degrees C and 1.13 GPa, whereas the sapphirine + forsterite assemblage may be stable below 800 degrees C only under specific conditions of a very low activity of water. Calculations of P-T relationships involving sapphirine using the THERMOCALC dataset revealed its inconsistency with both experimental and natural assemblages.