[SCG51-P09] The conditions of sublithospheric diamond formation constrained from ferric iron-rich exsolution from ferropericlase inclusions.
The ferropericlase- magnesioferrite system has been investigated by several studies at room pressure but the stability field of the two components has been shown to become more complicated at high pressure due to the formation of mixed valence oxides. We have combined experimental measurements with thermodynamic modelling in order to address two main questions: at which P, T, fO2 conditions do Fe2O3-rich phases exsolve from ferropericlase? And what is the maximum Fe3+/Fetot ratio of ferropericlase. To answer these questions, multianvil experiments have been performed between 6 – 25 GPa and 1200-1800°C using a starting composition of (Mg86Fe14)O plus 20 % Fe2O3. Pt powder was added to the experiments to act as a redox sensor and minor amounts of Ni, Cr, Mn and Na were also added. Samples were then analyzed with the scanning electron microscope, electron microprobe, Mössbauer spectroscopy and X-ray diffraction. In the recovered experiments ferropericlase coexists with magnetite-magnesioferrite solid solution up to 10 GPa and Mg2Fe2O5-Fe4O5 solid solution at higher pressures. In the calculation of the oxygen fugacity a ferropericlase model in the FeO-Fe2/3O-MgO system was employed and exchange of Mg and Fe2+ in magnetite was accounted for. Oxygen fugacities at which the phases coexist can be calculated in the magnetite-magnesioferrite field using three different equilibria and a quite simple ferropericlase mixing model results in calculated oxygen fugacities that are within 0.1 log units of each other for all three equilibria. The results show that magnetite-magnesioferrite solid solution should not be in equilibrium with ferropericlase in the diamond stability field. Our results imply that the exsolution of Fe3+ rich phases observed in natural samples likely occurred at pressures corresponding to the transition zone or deeper.