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

摘要

The partitioning of Mg and Fe between magnesiowustite and ringwoodite solid solutions has been measured between 15 and 23 GPa and 1200-1600 ℃ using 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 Fe~(3+) concentrations as a consequence of the different oxidation environments. Using results from experiments performed in Fe capsules, where metallic Fe was also added to the starting materials, the difference 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 experiments 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, however, show Fe~(3+) concentrations to be approximately three times higher in magnesiowustite produced in Re capsules than in Fe capsules and that Fe~(3+) partitions preferentially into magnesiowustite, with K_(D_(Fe~(3+)))~(ring/mw) estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowustite, a least-squares fit to the partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W_(FeMg)~(ring)) of 3.5 ± 1 kJ mol~(-1). The equivalent regular interaction parameter for magnesiowustite (W_(FeMg)~(mw)) is 12.1 ± 1.8 kJ mol. These determinations take into account the Fe~(3+) concentrations that occur in both phases in the presence of metallic Fe. The free energy change in J mol~(-1) for the Fe exchange reaction can be described, over the range of experimental conditions, by 912 + 4.15 (T - 298) + 18.9P with T in K, Pin kbar. The estimated volume change for this reaction is smaller than that predicted using current compilations of equation of state data and is much closer to the volume change at ambient conditions. These results are therefore a useful test of high pressure and temperature equation of state data. Using thermodynamic data consistent with this study the reaction of ringwoodite to form magnesiowustite and stishovite is calculated from the data collected using Fe capsules. Comparison of these results with previous studies shows that the presence of Fe~(3+) in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined thermodynamic properties.