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 mant le(1-4). These studies highlight the importance of understanding the proper ties of iron when modelling the composition and dynamics of the core and th e interaction of the core with the lowermost mantle(5-7). One of the main p roblems in inferring the composition of the lowermost mantle is our lack of knowledge of the high-pressure and high-temperature chemical reactions tha t occur between iron and the complex Mg-Fe-Si-Al-oxides which are thought t o form the bulk of the Earth's lower mantle. A number of studies(6,8-12) ha ve demonstrated that iron can react with MgSiO3-perovskite at high pressure s and high temperatures, and it was proposed(6,8) that the chemical nature of this process involves the reduction of silicon by the more electropositi ve iron. Here we present a study of the interaction between iron and corund um (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 dee p interior. We found that at pressures above 60 GPa and temperatures of 2,2 00 K, iron and corundum react to form iron oxide and an iron-aluminium allo y. Our results demonstrate that iron is able to reduce aluminium out of oxi des at core-mantle boundary conditions, which could provide an additional s ource of light elements in the Earth's core and produce significant heterog eneity at the core-mantle boundary.