X-ray diffraction study of iron partitioning between α and γ phases of the (Mg,Fe)2SiO4 system

We have studied the time dependence of the Fe partitioning between olivine and ringwoodite at pressures and temperatures where the phases coexist. Using SAM85, a cubic anvil high-pressure apparatus with synchrotron radiation, we collected energy dispersive X-ray diffraction spectra as a function of time at 900°C/11.5 GPa, 1117°C/11.7 GPa and 900°C/11.1 GPa. These data were analyzed with Rietveld refinement techniques to determine very accurate lattice parameters and phase fractions. The time evolution of the sample volume indicates that iron-magnesium exchange occurs at 1117°C on the scale of minutes, but does not evolve at 900°C. Using unit cell volume as a measure of iron content, the Fe partitioning in olivine and ringwoodite increases by 4% and 5%, respectively within 1 h at 1117°C while the phase fraction increases 10% for olivine and decreases 10% for ringwoodite.