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Abstract

Temperature dependence of the Fe2+–Mg exchange between orthopyroxene (Opx) and spinel (Spl), 
formula
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, 
formula
is empirically obtained as 
formula
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 KD between aluminous orthopyroxene and spinel during retrograde metamorphism rather than the thermal peak.
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