Low-pressure-temperature stability of pyrope plus quartz relative to orthopyroxene plus kyanite: a new model for aluminous orthopyroxene with vacancies

Determination of the stability of pyrope with quartz could provide theoretical pressure-temperature (P-T) constraints on silica-saturated eclogites as well as near end-member pyrope-bearing terrains. The lower P-T stability of pyrope in the presence of quartz in the system MgO-Al2O3-SiO2 can be modeled by the reaction: 3 enstatite+2 kyanite=2 pyrope+2 quartz. Reaction reversals were experimentally obtained using a 1/2-in.-diameter piston-cylinder press and a 1000-t multi-anvil press. Besides natural quartz, all other starting materials, including orthopyroxene, pyrope, and kyanite were synthesized. The reaction has been bracketed at 1.50 GPa at 1040-1080 degrees C; 1.69-1.79 GPa at 1000 degrees C; 2.26 GPa at 920-940 degrees C; 2.90 GPa at 840-860 degrees C. The bracketed boundary is higher in temperature and pressure than that calculated using the newest dataset of Holland et al. (J Petrol 59:881-900, 10.1093/petrology/egy048, 2018), particularly at higher pressures. Aluminum is usually thought to be taken into orthopyroxene in the form of Mg-Tschermak's pyroxene (MgTs; MgAl2SiO6) under these experimental P-T conditions. In contrast, the observed orthopyroxene composition indicated the involvement of a non-stoichiometric component Mg-Eskola pyroxene (MgEs; Mg-0.5?0.5AlSi2O6), an aluminous orthopyroxene with octahedral vacancies (?). End-member thermodynamic properties for MgEs as well as a mixing model for the ternary enstatite-MgTs-MgEs solid solution have been derived. The thermodynamic calculations, employing the mixing model, are in good agreement with the experimental brackets. The new model has potential applications to constrain P-T conditions of tectonic settings such as ultrahigh-pressure and ultrahigh-temperature metamorphisms.