Solubilities of mantle oxides in molten iron at high pressures and temperatures: implications for the composition and formation of Earth's core

Abstract

The solubilities of several oxide components of the mantle in molten iron have been measured at 16 GPa and 1700–2000°C. Their relative solubilities are as follows: FeO > Cr 2O 3 > MnO, V 2O 3, TiO 2 > SiO 2 > MgO > Al 2O 3. Oxide contents reach significant levels, varying from ∼ 10 wt% FeO to ∼ 1 wt% SiO 2 at the respective metal-oxide eutectics, but are very small for MgO and Al 2O 3. The solubilities of these oxides are expected to increase substantially at temperatures above 2000°C and at pressures above 16 GPa and are relevant to the core-formation process in the Earth. Individual oxide species dissolve quasi-congruently under conditions which maintain oxygen fugacities near the iron-wüstite buffer. However, the dissolution of mantle mineral phases is highly incongruent. If a mixture of metallic iron and mantle silicates were subjected to increasing pressures and temperatures (above 16 GPa and 2000°C) the most soluble species, FeO, would first be extracted into the metallic liquid. When the FeO activity had been lowered sufficiently, significant amounts of SiO 2 would then enter this melt. At extremely high temperatures and pressures, appreciable amounts of MgO could even dissolve. These results provide a background for interpreting recent diamond-anvil experiments in which molten iron was observed to react with mantle silicates at temperatures of 2700–3700°C and pressures of 20–70 GPa. They also elucidate the nature of chemical reactions between the core and mantle which may occur in the “D” layer of the lower mantle. The differing solubilities of mantle oxides in molten iron place constraints on models of accretion of the Earth and of accompanying core-formation processes. The observation that TiO 2 is undepleted in the Earth's mantle (relative to CI chondrites) makes it improbable that the core contains a substantial amount of dissolved Si or SiO 2. It therefore seems likely that oxygen (as dissolved FeO) is the principal light element component of the core. This conclusion is reconciled more readily with a modified version of homogeneous accretion of the Earth than with some current models of heterogeneous accretion. Accretion of the Earth from a population of giant planetesimals would have led to complete melting of the mantle and core at very high temperatures. Under these conditions, nearly all FeO and other transition metal oxides would have been extracted from the mantle into the core, together with a significant amount of SiO 2. It is difficult to reconcile the geochemical consequences of this accretion scenario with the present composition of the mantle.