Thermodynamics of the system Fe--FeO--MgO at high pressure and temperature and a model for formation of the Earth's core

Abstract

Summary Recent shock wave data of Jeanloz & Ahrens have established the presence of a phase transformation in Fe0.94 O, occurring at approximately 70 GPa, which has profound implications for the phase behaviour of the geophysically important system FeO—MgO at high pressure. We calculate the phase diagram based on the available shock wave data for FeO and MgO, and suggest that an increase of pressure in the system FeO—MgO will result in a gradual exsolution of an almost pure FeO high-pressure phase (hpp), leaving an iron-depleted (Fe, Mg)O rocksalt (Bl) phase. Using current geophysical data, exsolution of FeO (hpp) from the rocksalt phase in the present-day lower mantle is calculated to occur only in the region immediately overlying the core. Before core segregation took place, however, the increased iron content of the rocksalt phase could place the exsolution region at lesser depths, perhaps 2000 km. Extrapolation of the solubility data of oxygen in liquid iron indicates that the liquid miscibility gap in the system Fe—FeO at atmospheric pressure should close at approximately 4250K. We construct schematic phase diagrams for the system Fe—FeO at high pressure by considering the effect of pressure upon end-member melting points, upon the extent of liquid immiscibility and upon eutectic temperatures and compositions. We suggest that at high pressure (P˜ 90 GPa) the phase diagram of Fe—FeO becomes qualitatively similar to the system Fe—FeS, displaying complete liquid miscibility and a eutectic temperature well below the melting point of pure iron. On the basis of our inferences regarding behaviour in the system Fe—FeO—MgO at high pressure, we have constructed a model for core segregation. Assuming the Earth to have accreted from the primordial solar nebula as a relatively homogeneous mixture of metallic iron and silicate phases, core segregation involving oxygen as the principal light alloying element in the core would commence at a depth where pressure is sufficiently high to cause exsolution of FeO (hpp) from the rocksalt phase, and temperature is sufficiently high to allow formation of an Fe—FeO (hpp) melt without extensively melting the silicate phases. A gravitational instability arises, leading to vertical differentiation of the Earth as molten blobs of the metal phase sink downwards to form the core and the residual depleted silicate material coalesces to form large bodies which rise diapirically upwards to form the mantle.