Iron–silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle

Abstract

The boundary between the Earth's metallic core and its silicate mantle is characterized by strong lateral heterogeneity and sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition1,2,3,4,5,6,7. To investigate the composition and properties of the lowermost mantle, an understanding of the chemical reactions that take place between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earth's lower mantle is first required8,9,10,11,12,13,14,15. Here we present a study of the interaction between iron and silica (SiO2) in electrically and laser-heated diamond anvil cells. In a multianvil apparatus at pressures up to 140 GPa and temperatures over 3,800 K we simulate conditions down to the core–mantle boundary. At high temperature and pressures below 40 GPa, iron and silica react to form iron oxide and an iron–silicon alloy, with up to 5 wt% silicon. At pressures of 85–140 GPa, however, iron and SiO2 do not react and iron–silicon alloys dissociate into almost pure iron and a CsCl-structured (B2) FeSi compound. Our experiments suggest that a metallic silicon-rich B2 phase, produced at the core–mantle boundary (owing to reactions between iron and silicate2,9,10,13), could accumulate at the boundary between the mantle and core and explain the anomalously high electrical conductivity of this region6.

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Figure 1: Representative spectra obtained in experiments with iron and amorphous silica as starting materials.
Figure 2: Experimental results on the chemical interaction of iron and silica and behaviour of Fe-Si alloys at high pressures and temperatures.
Figure 3: Examples of X-ray diffraction patterns collected in experiments with Fe-5.1 wt% Si.

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Acknowledgements

We thank G. Herrmannsdörfer, H. Schulze, R. Eger and C. Kamella, for technical assistance. Discussion with G. D. Price allowed us to improve the manuscript. Financial support of Deutsche Forschungsgemeinschaft, the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), and the EU ‘Access to Research Infrastructures’ Programme is acknowledged.

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Correspondence to L. Dubrovinsky.

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Dubrovinsky, L., Dubrovinskaia, N., Langenhorst, F. et al. Iron–silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle. Nature 422, 58–61 (2003). https://doi.org/10.1038/nature01422

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