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

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 degrees C, the maximum solubility of Fe(3+) in Mg(0.8)Fe(0.2)O ranges from approximately 2% Fe(3+)/Sigma Fe at the Fe-FeO buffer to 3-7% Fe(3+)/Sigma Fe 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.