Effects of Fe spin transition on the elasticity of (Mg, Fe)O magnesiowüstites and implications for the seismological properties of the Earth's lower mantle

S. Speziale, V. E. Lee, S. M. Clark, J. F. Lin, M. P. Pasternak, R. JeanLoz

Research output: Contribution to journalArticlepeer-review

Abstract

High-pressure x-ray diffraction of (Mg0.80 Fe0.20)O at room temperature reveals a discontinuity in the bulk modulus at 40 (±5) GPa, similar to the pressure at which an electronic spin-pairing transition of Fe2+ is observed by Mössbauer spectroscopy. We determine the zero-pressure bulk modulus of low-spin magnesiowüstite to be between K70=136 and 246 GPa, with a pressure derivative (∂&∂P)70 between 5.2 and 3.9. The best fit unit-cell volume at zero pressure, V0=71 (±5) Å, is consistent with past estimates of the ionic radius of octahedrally-coordinated low-spin Fe2+ in oxides. A spin transition at lower-mantle depths between 1100 and 1900 km (40-80 GPa) would cause a unit-cell volume decrease (ΔV) of 3.7 (±0.5) to 2.0 (±0.1) percent and bulk sound velocity increase (Avø) of 7.6 (±4) percent at 40 GPa and 7.6 (±1.2) percent at 80 GPa. Even in the absence of a visible seismic discontinuity, we expect the spin transition of iron to imply correction to current compositional models of the lower mantle, with up to 10 mol percent increase of magnesiowüstite being required to match the seismological data.

Original languageEnglish
Article numberB10212
JournalJournal of Geophysical Research: Solid Earth
Volume112
Issue number10
DOIs
StatePublished - 4 Oct 2007

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