Chemical interaction of Fe and Al2O3 as a source of heterogeneity at the Earth's core-mantle boundary

Seismological studies have revealed that a complex texture or heterogeneity exists in the Earth's inner core and at the boundary between core and mantle(1-4). These studies highlight the importance of understanding the properties of iron when modelling the composition and dynamics of the core and the interaction of the core with the lowermost mantle(5-7). One of the main problems in inferring the composition of the lowermost mantle is our lack of knowledge of the high-pressure and high-temperature chemical reactions that occur between iron and the complex Mg-Fe-Si-Al-oxides which are thought to form the bulk of the Earth's lower mantle. A number of studies(6,8-12) have demonstrated that iron can react with MgSiO3-perovskite at high pressures and high temperatures, and it was proposed(6,8) that the chemical nature of this process involves the reduction of silicon by the more electropositive iron. Here we present a study of the interaction between iron and corundum (Al2O3) in electrically- and laser-heated diamond anvil cells at 2,000-2,200 K and pressures up to 70 GPa, simulating conditions in the Earth's deep interior. We found that at pressures above 60 GPa and temperatures of 2,200 K, iron and corundum react to form iron oxide and an iron-aluminium alloy. Our results demonstrate that iron is able to reduce aluminium out of oxides at core-mantle boundary conditions, which could provide an additional source of light elements in the Earth's core and produce significant heterogeneity at the core-mantle boundary.