Evolution of the lowermost mantle and development of a mantle plume. Colors represent the Delta57Fe anomalies. Thermodiffusion occurs in a thin layer above the core-mantle boundary where the temperature exceeds the iron solidus and isotopic anomalies start to develop. When the thermal boundary layer becomes unstable, a plume starts to rise, entraining the layer enriched in 57Fe. This structure is preserved as the plume rises and approaches the surface, where the Delta57Fe anomalies will be sampled in the melting process.

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