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The core-mantle boundary of Earth is a region where iron-rich liquids interact with oxides and silicates in the mantle. Iron enrichment may occur at the bottom of the mantle, leading to low seismic-wave velocities and high electrical conductivity, but plausible physical processes of iron enrichment have not been suggested. Diffusion-controlled iron enrichment is inefficient because it is too slow, although the diffusion can be fast enough along grain boundaries for some elements. More fundamentally, experimental studies and geophysical observations show that the core is under-saturated with oxygen, implying that the mantle next to the core should be depleted in FeO. Here we show that (Mg,Fe)O in contact with iron-rich liquids leads to a morphological instability, causing blobs of iron-rich liquid to penetrate the oxide. This morphological instability is generated by the chemical potential gradient between two materials when they are not in bulk chemical equilibrium, and should be a common process in Earth's interior. Iron-rich melt could be transported 50 to 100 kilometres away from the core-mantle boundary by this mechanism, providing an explanation for the iron-rich regions in the mantle.

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Shear deformation experiments on polycrystalline wadsleyite (water content, ∼200–2200 H/106 Si) have been conducted at 14.4–17.0 GPa, 1690–2100 K, and strain rates of 2.6–16 × 10−5 s−1 using a rotational Drickamer apparatus (RDA) at a synchrotron facility. The stress was measured from the orientation dependence of lattice spacing for the (013), (211), (141), (240) and (244) planes. On the basis of the mechanical and microstructural observations, we infer that deformation occurs by exponential creep through the Peierls mechanism at relatively low temperatures of 1690–2030 K. However, a sample deformed at the temperature of 2100 K showed significant grain‐size reduction, and most of small grains are dislocation‐free, although sub‐boundaries were observed in some grains in the sample. We interpret these observations as evidence for dynamic recrystallization and that diffusion creep (and grain boundary sliding) plays an important role after dynamic recrystallization caused by power law creep. Consequently, the strength observed in the high‐temperature conditions determined by the present study provides an important constraint on strength of diffusion creep and a lower limit for that of the power law dislocation creep. We conclude that the strength of wadsleyite in the power law dislocation creep is higher than or comparable to that of olivine and the strength of wadsleyite in the Peierls regime is similar to that of olivine.

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are reasonably consistent with values estimated from previous experimental and theoretical studies.Abstract Both ferric iron (Fe 3+ ) and hydrogen (H + ) have important influence on several transport properties of minerals such as diffusion. We determined the influence of Fe 3+ and H + on Fe-Mg interdiffusion in (Mg,Fe) O at 1,673-1,873 K and 5-24 GPa under the anhydrous and hydrous conditions using the diffusion couple technique. The diffusion couples consist of single crystals of ferropericlase ((Mg,Fe)O) and periclase (MgO) with Mg/ (Mg + Fe) ratios ranging from 0.44 to 1.0. The oxygen fugacity was controlled by the following assemblages of metal and oxide: Fe-FeO, Ni-NiO, Mo-MoO 2 , and ReReO 2 . After the diffusion experiments, hydrogen (H + ) concentrations were measured using the FTIR spectroscopy. Fe 3+ concentrations were measured using the flank method. Under the conditions investigated, Fe-Mg interdiffusivity increases strongly with Fe 3+ and modestly with H + and the influence of H + relative to that of Fe 3+ on Fe-Mg interdiffusion decreases with temperature. Our results show that, under both anhydrous and hydrous conditions, the dominant defect responsible for diffusion is the same suggesting that H + enhances Fe-Mg interdiffusivity by enhancing the mobility of vacancies at the M-site. Our results indicate that the influence of Fe 3+ likely dominates at temperatures expected for the normal lower mantle conditions (T > 1,900 K), while the influence of both Fe 3+ and H + is important at lower temperature environments such as near the subduction zone. We also estimated the vacancy diffusivity based on Fe-Mg interdiffusion and vacancy concentration estimated from the charge neutrality condition with Fe 3+ . Both Fe-Mg interdiffusivity and vacancy diffusivities