Fe–Mg partitioning between ringwoodite and magnesiowüstite and the effect of pressure, temperature and oxygen fugacity |
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Authors: | D J Frost F Langenhorst P A van Aken |
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Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth 95440 Bayreuth, Germany Tel.:+49-921-553737; Fax:+49-921-553769 e-mail: Dan.Frost@uni-bayreuth.de, DE Institut für Mineralogie, Technische Universit?t Darmstadt, Schnittspahnstr. 9, 64287 Darmstadt, Germany, DE
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Abstract: | The partitioning of Mg and Fe between magnesiowüstite and ringwoodite solid solutions has been measured between 15 and 23 GPa
and 1200–1600 ∘C 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 Fe3+ 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 Fe3+ concentrations to be approximately three times higher in magnesiowüstite produced in Re capsules than in Fe capsules and
that Fe3+ partitions preferentially into magnesiowüstite, with K
D
Fe3+
ring/mw estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowüstite, 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 magnesiowüstite (W
FeMg
mw) is 12.1±1.8 kJ mol. These determinations take into account the Fe3+ 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, P in 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 magnesiowüstite and stishovite is calculated from the data collected using Fe capsules. Comparison of
these results with previous studies shows that the presence of Fe3+ in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined
thermodynamic properties.
Received: 13 September 2000 / Accepted: 25 March 2001 |
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Keywords: | Ringwoodite Magnesiowüstite Oxygen fugacity Element partitioning Transition zone High pressure |
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