Fe 2+ –Mg partitioning experiments between orthopyroxene and spinel using ultrahigh-temperature granulite from the Napier Complex, East Antarctica

Abstract

Abstract Temperature dependence of the Fe 2+ –Mg exchange between orthopyroxene (Opx) and spinel (Spl), 1 / 2 Fe 2 Si 2 O 6 + MgAl 2 O 4 = 1 / 2 Mg 2 Si 2 O 6 + FeAl 2 O 4 Opx Spl Opx 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 Fe 2+ –Mg distribution coefficient, K D = X Fe Spl X Mg Opx X Mg Spl X Fe Opx is empirically obtained as ln K D = − 1.134 + [ 2341 + 7.5 ( P − 11 ) ] / T 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.