Mg-Fe partitioning between silicate spinel and magnesiowustite at high pressure:: experimental determination and calculation of phase relations in the system Mg2SiO4-Fe2SiO4

Mg-Fe partitioning experiments between (Mg,Fe)(2)SiO4 spinel and (Mg,Fe)O magnesiowustite were carried out at pressures of 17-21.3 GPa at temperatures of 1400 and 1600 OC, using a multi-anvil apparatus, in order to deter-mine interaction parameters of spinel and magnesiowustite solid solutions and also to constrain the equilibrium boundaries of the postspinel transition ill the Fe-rich side in the system Mg2SiO4-Fe2SiO4. The obtained values of the interaction parameters were 3.4 +/- 1.5 and 13.9 +/- 1.4 kJ mol-l respectively, for spinel and magnesiowustite solid solutions at 19 GPa and 1600 degrees C. The partitioning data in the system Mg2SiO4-Fe2SiO4 at 1400 and 1600 degrees C showed that the transition boundary between spinel and the mixture of magnesiowustite and stishovite has a negative dP/dT slope. Using the above interaction parameters and available thermodynamic data of the Mg2SiO4 and Fe2SiO4 end members, the transition boundaries of spinel to the mixture of magnesiowustite and stishovite were calculated. Within the uncertainties of the data used, the calculated boundaries are in good agreement with the boundaries at 1400 and 1600 degrees C experimentally determined in this study. The dissociation boundary of Fe2SiO4 spinel to wustite and stishovite, calculated from the thermodynamic data, has a negative slope of -1.5 +/- 0.6 MPa K-1.