Fe-Mg partitioning between ringwoodite and magnesiowustite and the effect of pressure, temperature and oxygen fugacity

The partitioning of Mg and Fe between magnesiowustite and ringwoodite solid solutions has been measured between 15 and 23 GPa and 1200-1600 degreesC u sing both Fe and Re capsule materials to vary the oxidation conditions. The partitioning results show a clear dependence on the capsule material used due to the variation in Fe3+ concentrations as a consequence of the differe nt oxidation environments. Using results from experiments performed in Fe c apsules, where metallic Fe was also added to the starting materials, the di fference in the interaction parameters for the two solid solutions (W-FeMg( mw) - W-FeMg(ring)) is calculated to be 8.5 +/- 1 kJ mol(-1). Similar exper iments performed in Re metal capsules result in a value for W-FeMg(mw) - W- FeMg(ring) that is apparently 4 kJ higher, if all Fe is assumed to be FeO. Electron energy-loss near-edge structure (ELNES) spectroscopic analyses, ho wever, show Fe3+ concentrations to be approximately three times higher in m agnesiowustite produced in Re capsules than in Fe capsules and that Fe3+ pa rtitions preferentially into magnesiowustite, with K-DFe3+(ring/mw) estimat ed between 0.1 and 0.6. Using an existing activity composition model for ma gnesiowustite, a least-squares fit to the partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W-F eMg(ring)) of 3.5 +/- 1 kJ mol(-1). The equivalent regular interaction para meter for magnesiowustite (W-FeMg(mw)) is 12.1 +/- 1.8 kJ mol. These determ inations take into account the Fe3+ concentrations that occur in both phase s in the presence of metallic Fe. The free energy change in J mol(-1) for t he Fe exchange reaction can be described, over the range of experimental co nditions, by 912 + 4.15 (T - 298) + 18.9P with T in K, P in kbar. The estim ated volume change for this reaction is smaller than that predicted using c urrent compilations of equation of state data and is much closer to the vol ume change at ambient conditions. These results are therefore a useful test of high pressure and temperature equation of state data. Using thermodynam ic data consistent with this study the reaction of ringwoodite to form magn esiowustite and stishovite is calculated from the data collected using Fe c apsules. Comparison of these results with previous studies shows that the p resence of Fe3+ in phases produced in multianvil experiments using Re capsu les can have a marked effect on apparent phase relations and determined the rmodynamic properties.