Low ferric iron content of (Mg,Fe)O at high pressures and temperatures

Abstract

New results from high‐pressure multi‐anvil and diamond anvil cell experiments show that the Fe 3+ content of (Mg,Fe)O is low at high pressures and temperatures, and relatively independent of oxygen fugacity. At 18 GPa and 1000°C, the maximum solubility of Fe 3+ in Mg 0.8 Fe 0.2 O ranges from approximately 2% Fe 3+ /ΣFe at the Fe‐FeO buffer to 3–7% Fe 3+ /ΣFe at the Re‐ReO 2 buffer. These low values are likely due to a high‐pressure phase transition in the system Fe 3 O 4 ‐MgFe 2 O 4 , resulting in preferential partitioning of Fe 3+ into the high‐pressure phase. (Fe,Mg)Fe 2 O 4 would be exsolved in (Mg,Fe)O depending on the oxygen fugacity, which could have significant effects on transport properties. Exsolved (Fe,Mg)Fe 2 O 4 could also be useful as an oxygen barometer.