Fe²⁺-Mg partitioning experiments between orthopyroxene and spinel using ultrahigh-temperature granulite from the Napier Complex, East Antarctica

Temperature dependence of the Fe²⁺-Mg exchange between orthopyroxene (Opx) and spinel (Spl), was experimentally determined at 9-13 kbar and 900-1200 °C using an ultrahigh-temperature (UHT) granulite collected from the Napier Complex in Enderby Land, East Antarctica. The Fe²⁺-Mg distribution coefficient, is empirically obtained as where X is the cationic mole fraction, and pressure P and temperature T are in kbar and Kelvin, respectively. The new geothermometer was applied to various natural UHT and associated high-grade metamorphic rocks from East Antarctica and other regions of the world. The results indicate temperatures between 735 and 902 °C at pressures in the range of 5-14 kbar. This geothermometer utilizing spinel does not give peak metamorphic condition, because it is relatively easy for spinel to re-equilibrate during the cooling stage of metamorphism. Hence, we conclude that the geothermometer is suitable for evaluating the closure temperature for the K_D between aluminous orthopyroxene and spinel during retrograde metamorphism rather than the thermal peak.