Deep penetration of molten iron into the mantle caused by a morphological instability

A morphological instability causing blobs of iron-rich liquid to penetrate iron oxides is described, providing a mechanism for the iron-rich regions in the mantle. How molten iron moves to the mantle Several observations suggest that iron enrichment might occur at the bottom of the Earth's mantle, but a plausible physical mechanism to produce such enrichment has remained elusive. Kazuhiko Otsuka and Shun-ichiro Karato now show that contact between solid (Mg,Fe)O and iron-rich liquids leads to a morphological instability—caused by the chemical potential gradient—that allows the efficient penetration of iron-rich liquid blobs into the oxide. The authors calculate that such iron-rich melts could be transported up to 100 kilometres from the core–mantle boundary, thereby explaining the presence of iron-rich regions in the lowermost mantle. The core–mantle boundary of Earth is a region where iron-rich liquids interact with oxides and silicates in the mantle 1 . Iron enrichment may occur at the bottom of the mantle, leading to low seismic-wave velocities and high electrical conductivity 2 , 3 , 4 , 5 , but plausible physical processes of iron enrichment have not been suggested. Diffusion-controlled iron enrichment is inefficient because it is too slow 6 , although the diffusion can be fast enough along grain boundaries for some elements 7 . 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.