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, 101 (45), 15867-9

Ferromagnesian Postperovskite Silicates in the D'' Layer of the Earth

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Ferromagnesian Postperovskite Silicates in the D'' Layer of the Earth

Wendy L Mao et al. Proc Natl Acad Sci U S A.

Abstract

Natural olivine with 12 mol % Fe(2)SiO(4) and synthetic orthopyroxenes with 20% and 40% FeSiO(3) were studied beyond the pressure-temperature conditions of the core-mantle boundary. All samples were found to convert entirely or partially into the CaIrO(3) postperovskite structure, which was recently reported for pure MgSiO(3). The incorporation of Fe greatly reduces the pressure needed for the transition and establishes the new phase as the major component of the D'' layer. With the liquid core as an unlimited reservoir of iron, core-mantle reactions could further enrich the iron content in this phase and explain the intriguing seismic signatures observed in the D'' layer.

Figures

Fig. 1.
Fig. 1.
Two-dimensional x-ray diffraction image of En80 at 108 GPa and 300 K after laser-heating to 2,000 K showing azimuthal angle versus 2θ. The large diamond spots are a result of single-crystal diffraction from the diamond anvils and were masked for the azimuthal integration of the spectra.
Fig. 2.
Fig. 2.
X-ray diffraction patterns of En80 at 108 GPa and San Carlos olivine at 144 GPa. Both were taken at 300 K after laser-heating to 2,000 K. ppv peaks are marked with *. The San Carlos olivine spectra shows that the sample had converted to only ppv + mw phases. Splitting of the Re 100 and 101 peaks may be a result of repeated temperature cycling that transformed the edge of the gasket material into a double hexagonal close-packed (hcp) structure [similar to behavior observed in Fe and Co (21, 22)] or a hcp phase reaction product. The En80 sample shows conversion of the amorphized sample into ppv + pv. The ppv 132 peak is on the short-wavelength shoulder of a pv peak. The wavelength of the monochromatic beam was 0.3344 Å.
Fig. 3.
Fig. 3.
Schematic phase diagram of the MgSiO3–FeSiO3 binary system at 2,000–2,500 K. Depending on the temperature at the core–mantle boundary, the phase boundaries could shift upward by 10–20 GPa based on a positive Clapeyron slope (5, 6).
Fig. 4.
Fig. 4.
Schematic cross section of the D″ zone. The shading in the squares (pv) and striped rectangles (ppv) reflects the changing Fe/Mg ratio, with an increasing red color indicating higher Fe content in that phase. The Fe/Mg and pv/ppv ratio follows the two phase loops in the phase diagram.

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