MgO-FeO-SiO2 phase diagrams from 20 to 140 GPa.

Authors: ChEd Boukaré, Yanick Ricard, Guillaume Fiquet

At the end of Earth's accretion and after the core-mantle segregation, the existence of a basal magma ocean at the top of the core-mantle boundary (CMB) depends on the physical properties of mantle materials at relevant pressure and temperature. Present-day deep mantle structures such as ultralow-velocity zones and low-shear velocity provinces might be directly linked to the still ongoing crystallization of a primordial magma ocean. 

We provide the first steps toward a self-consistent thermodynamic model of magma ocean crystallization at high pressure. We build a solid-liquid thermodynamic database for silicates in the MgO-FeO-SiO2 system from 20 GPa to 140 GPa. We use already published chemical potentials for solids, liquid MgO, and SiO2. We derive standard state chemical potential for liquid FeO and mixing relations from various indirect observations. 

Using this database, we compute the ternary phase diagram in the MgO-FeO-SiO2 system as a function of temperature and pressure and self-consistent crystallization sequences. We confirm that the melt is lighter than the solid of same composition for all mantle conditions but at thermodynamic equilibrium, the iron-rich liquid is denser than the solid in the deep mantle. We compute a whole fractional crystallization sequence of the mantle and show that an iron-rich and fusible layer should be left above the CMB at the end of the crystallization.

Do not hesitate to contact me if you are interested by using this thermodynamic database.

Caption: Ternary phase diagram in the MgO-FeO-SiO2 system (left) and corresponding binary joints (right) at 25 GPa. (a) MgO-SiO2 joint. We reproduce the two eutectics points predicted by DeKoker et al. [2013]. Pyrolitic composition (point M) is very close to the periclase-bridgmanite eutectic (point A). (b) Mg0.7Fe0.3-SiO2 binary joint. This binary joint corresponds to the Alkemade line of Mg/Fe ratio of 7∕3. (c) FeO-SiO2 joint. We predict an eutectic and a peritectic point. The point C depicts the melt composition at the end of fractional crystallization. 

Caption: Same as above but at 130 GPa. We predict only one eutectic point (and no perictectic point) on the FeO-SiO2 joint.

Caption: Densities of melt and solid depending on the solid fraction (0% or incipient crystallization, red, and 90%, blue). The initial melt is composed of 53% mol of MgO, 6% mol of FeO, and 41% mol of SiO2. The melt (dashed lines) is in equilibrium with the new formed crystal (solid lines). As crystallization proceeds, the Fe-rich remaining liquid becomes very dense (dashed blue) and much denser than the solid at equilibrium (solid blue). Buoyant crystals in the deep mantle may be responsible for an Earth's basal magma ocean. The black dashed line represents the density of the total solid, at the end of the crystallization. The thin vertical lines indicate the pressures at which the liquid becomes denser than the solid at equilibrium.