Carbon-driven Chemical Interactions between Alumina and Iron: A possible reaction pathway in Earth's interior

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AbstractSeismological and geochemical observations have revealed a complex structure for the earth's core-mantle boundary (CMB) region, with lateral and chemical heterogeneities. The presence of higher than expected concentrations of siderophile elements (Ni, Co, Pt etc) in the earth's mantle, iron enrichment of the lower mantle relative to the upper mantle, and a possible carbon flux from the core suggest the possibility of continual long-term exchange of materials between the core and the mantle. The chemical interactions of molten iron with complex mantle oxides and diffusion have been postulated as key mechanisms. A number of studies have been carried out on the reduction reactions taking into account the extreme conditions of high-temperature and high-pressure in earth's interior. These studies have, however, neglected to consider the influence of carbon on these reactions. The earth's metallic core is rich in carbon (~ 5 wt% C), and there is a growing evidence for the presence of carbon in the earth's mantle as well. Carbon can affect redox conditions through chemical interactions with oxygen, and is a critical element in determining the oxidation state of siderophile elements. Here we present a study of the interactions between liquid iron and alumina-carbon substrates at 1,823K in argon atmosphere, and report on the formation of a Fe-Al~0.25-0.5~ alloy at ambient pressure. Iron induced reduction of alumina in the absence of carbon, has been previously reported to occur only at pressures above 60 GPa and temperatures of 2,200K. Our results demonstrate that carbon enriched iron is capable of reducing alumina in regions of much lower pressures. These chemical reactions could provide an important mechanism for the reduction reactions occurring in earth's interior, and be responsible for far higher levels of heterogeneities than currently believed.