HP-HT (Mg,Fe)O sub-solidus reaction modelling implications for lower mantle geochemical heterogeneities

Equilibrium reactions over the sub-solidus MgO-FeO binary, i.e., periclase-like structure (Mg,Fe)O phases, were modelled in the frame of pyrolitic Earth lower mantle via (Mg,Fe)O mixing Gibbs energy as a function of composition, P and T, using quantum mechanical calculations (Hartree-Fock/DFT hybrid scheme), cluster expansion techniques, statistical thermodynamics and chemical equilibrium formalism. We aim to explore how the equilibrium reaction (Mg,Fe)O-products are a source for lower mantle geochemical heterogeneities (e.g., variability of the phases' compositions, phase proportions and density). Iron in Fe-periclase was modelled in both low-spin (LS) and high-spin (HS) state, separately, and the sub-solidus (Mg,Fe)O-system was treated as an open system that exchanges Mg and Fe with the lower mantle, considered as a reservoir. On the basis of our results, LS is the main booster of equilibrium phase changes in the sub-solidus (Mg,Fe)O-system over the lower mantle. The model indicates that the sub-solidus (Mg,Fe)O-system is able to continuously incorporate iron from the reservoir forming equilibrium phases with compositions and proportions that vary over the P-T range 24/1920-80/2530 GPa/K. At higher pressure and temperature, up to 128/3150 GPa/K, the sub-solidus (Mg,Fe)O-system stabilizes a quasi-invariant iron-rich composition [(Mg0.50Fe0.50)O], predominant in terms of phase proportions, and two iron-poor compositions given by (Mg0.10Fe0.90)O and (Mg0.175Fe0.825)O. These theoretical results are in agreement with the compositional variance observed in the lower mantle Fe-periclase from diamond inclusions and are consistent with the frequency of occurrence revealed for the compositions of the natural specimens. The density differences among the equilibrium (Mg,Fe)O-phases increase up to ~1 0%, between 24 and 128 GPa. The calculated bulk Fe/Mg partitioning coefficient between perovskite (here used as a chemical approximation of the reservoir) and Fe-periclase is 0.64 at 24 GPa, then drops to 0.19 at 80 GPa, and becomes quasi-invariant (0.18-0.16) in the lowermost portion of the Earth mantle (~ 80-128 GPa). These values are consistent with those experimentally determined. Altogether our results suggest that Fe-periclase does cause geochemical heterogeneities over the range 24-80 GPa, but it does not give rise to any sharp discontinuity, so that the deepest lower mantle seismic anomalies (i.e., D''-region) do not seem primarily ascribable to the mixing reactivity of the sub-solidus (Mg,Fe)O-system.