Chemical exchange across the core-mantle boundary J. VAN ORMAN Case Western Reserve University , USA

The core-mantle boundary separates the two largest geochemical reservoirs on Earth. Relatively little is known about the direction in which chemical species travel across the core-mantle boundary, owing to our lack of knowledge of the initial state of the core-mantle system and the thermodynamic properties of the materials. Despite this lack of knowledge, we can place constraints on the magnitude of chemical exchange between the core and mantle over Earth’s history by considering the kinetics of the chemical mass transport reactions involved. During core formation, it is likely that the liquid metal that formed the core equilibrated with silicate materials at pressures significantly lower than that at the core-mantle boundary. Because silicates and oxides become more soluble in liquid iron alloys as pressure increases, the core initially would have been undersaturated in mantle materials. Dissolution of mantle materials into the core is limited by the solubility and rate of mass transport in liquid iron. Rates of mass transport in the vigorously convecting core are extremely high, and dissolution reactions would have proceeded very rapidly until a layer of residual insoluble materials (probably dominated by ferropericlase) developed at the base of the mantle. Further chemical exchange between core and mantle would be limited by the rate of diffusive and advective mass transport through the mantle. I will review experimental and theoretical constraints on diffusional mass transport rates in lower mantle minerals and grain boundaries and discuss their implications for the magnitude of core-mantle chemical exchange.