Experimental calibration of sapphirine–spinel Fe2+–Mg exchange thermometer: Implication for constraints on P–T condition of Howard Hills, Napier Complex, East Antarctica

The Fe2+–Mg distribution coefficients between sapphirine and spinel:KD=XFeSplXMgSprXMgSplXFeSpr,were experimentally determined at pressures of 9–13 kbar and temperatures of 950–1150 °C using a natural ultrahigh-temperature (UHT) granulite with paragenesis of these minerals from the Napier Complex in East Antarctica [XMg = Mg / (Fe + Mg); XFe = Fe / (Fe + Mg)]. A new sapphirine–spinel geothermometer has been obtained as:lnKD=−1.257+[2940+9.5(Pkbar−11)]/T(K).We applied the exchange thermometer to UHT or high-grade metamorphic rocks that were reported from various complexes in the world. If the KD values of 2.63–4.34 obtained from low-Cr mineral pairs such as XCrSpr < 0.016 and XCrSpl < 0.047 were substituted into the equation, their temperature conditions would be estimated as 806–1050 °C at 11 kbar. The XCr means Cr / (Al + Cr(+ Fe3+)). These temperatures are reasonable retrograde or near peak metamorphic condition.