Ferric iron content of ferropericlase as a function of composition, oxygen fugacity, temperature and pressure: Implications for redox conditions during diamond formation in the lower mantle

We investigated the ferric iron (Fe3+) concentration in (Mg,Fe)O ferropericlase using the flank method applied to Mg–Fe interdiffusion couples of ferropericlase. Diffusion couples with Mg/(Mg+Fe) in the range 0.44 to 1 were annealed at temperatures of 1673–1873 K and pressures of 5–24 GPa over a wide range of oxygen fugacities. Oxygen fugacity was controlled by Fe, Ni, Mo, and Re metal capsules and their corresponding oxide phases. Based on our results and available experimental data, we derived an equation for the Fe3+ solubility in ferropericlase applicable to depths at the top of the lower mantle: [Fe3+]=C (XFe4 fO2)m exp{−((1−XFe)E⁎Mg+XFeE*Fe+PV*)/RT}, where C=2.6(1)×10−3, m=0.114(3), E*Mg=−35(3) [kJ/mol], E*Fe=−98(2) [kJ/mol], and V*=2.09(3) [cm3/mol]. The value of the oxygen fugacity exponent m implies that Fe3+ mostly occupies tetrahedral sites under these conditions, which is consistent with the results of previously reported Mössbauer spectroscopy studies. Based on this relationship, we calculated the redox conditions of ferropericlase inclusions in diamonds believed to have come from the lower mantle. The estimated oxygen fugacities are close to the upper stability limit of diamond in mantle peridotite at the top of the lower mantle at adiabatic or slightly superadiabatic temperatures, which suggests that ferropericlase inclusions recorded and preserved the conditions at which diamond was precipitated from carbonates or carbonatite melts near the top of the lower mantle.