Stability of iron oxides in Earth's mantle
2004 (English)In: Proceedings of the 32nd International Geological Congress: Abstracts, 2004Conference paper (Refereed)
Synchrotron x-ray diffraction, Raman spectroscopic experiments, and thermodynamic analysis aiming at the investigation of stability of iron oxides at pressures and temperatures reaching the deep lower mantle conditions were carried out using diamond anvil cell technique combined with laser and resistive heating. In the x-ray experiments, the thermodynamically predicted breakdown of magnetite to hematite and wüstite was not observed, but the trace amounts of hematite detected in the sample assemblage may signify the presence of nucleation centers of breakdown products, the growth of which is kinetically hindered due to the energetic requirements for the reaction. This is corroborated by the occurrence of bands of the locally ordered units of hematite in the Raman spectra. Raman spectra of wüstite compressed to 80 GPa and subjected to an intense laser heating show several new strong bands in the range 200-350 , originating presumably from the transformation to a polymorph with the NiAs structure. The transition is reversible as evidenced by the recovery of wüstite upon decompression of sample to a zero pressure. The 298 K isotherm of magnetite derived from the compression experiments is in a very good agreement with the results of earlier studies. The 1-confidence ellipsoid shows large negative correlations for the fit parameters K0, K0', and V0. The transformation of magnetite to a dense polymorph was observed already at 19 GPa. The Rietveld refinement of the diffraction pattern of is consistent with the -type structure. Thermodynamic assessment shows that on the pressure - temperature phase diagram the stability field for the mixture of oxides + FeO has maximum temperature of only 850 K at 14.6 GPa. At 298 K, the equilibrium pressure for the breakdown of magnetite is 13.3 GPa, while the pressure for the synthesis of FeO + to is located between 35 and 47 GPa, depending on the choice of the equation of state. The calculations also predict that the becomes unstable with respect to h- + FeO at pressures higher than 50 GPa. The values of Gibbs formation energies at standard conditions for the high-pressure polymorphs and h- were estimated to be -962 kJ/mol and -610 kJ/mol, respectively. The standard-state entropy of is 172.4 J/K/mol.
Place, publisher, year, edition, pages
iron oxides, Earth's mantle, thermodynamics, x-ray diffraction, Raman spectroscopy
IdentifiersURN: urn:nbn:se:uu:diva-19617OAI: oai:DiVA.org:uu-19617DiVA: diva2:47389