title: Constraints on Magma Ocean Crystallization in the Early Earth: Experiments, Thermodynamics and Ab initio simulations
Creators: name: Yao, Jie
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Magma Ocean
subject: Early Earth
subject: melting phase relations
subject: density
subject: structure
subject: 550

description: Giant impact events in the early Earth have led to large-scale melting, potentially up to the entire planet becoming liquid. Some highly fractioned residual melt may have persisted at the base of the mantle contributing to the existence of ultra-low velocity zones and large low shear velocity provinces. The crystallization of silicate melt from the magma ocean, or residual reservoirs, therefore provides an important constraint on chemical differentiation and possible initial stratification in the Earth. Characterizing and modelling melting relations in the system MgO-SiO2 at lower mantle pressures (P) relies on the location of the eutectic points for MgO-MgSiO3 and MgSiO3-SiO2. While at an uppermost lower mantle pressure (P~25 GPa) there is general consensus on the eutectic composition in the MgO-MgSiO3 system, large discrepancies exist for MgSiO3-SiO2 from experiments in the diamond anvil cell and ab-initio simulations, as well as models built on them. In order to address this discrepancy, we have performed multi-anvil press experiments at 24 GPa for Mg0.4Si0.6O1.6 and Mg0.3Si0.7O1.7 at temperatures (T) of 2650 ± 100 K and 2750 ± 100 K. At 2750 ± 100 K, we observe the presence of partial melt, and the recovered sample from the experiment with Mg0.4Si0.6O1.6 starting composition shows SiO2 stishovite as the liquidus phase, and electron microprobe analysis determines XSiO2 = 0.53 ± 0.03 as the eutectic composition. Based on the experimental results, we fit a thermodynamic model for the melting relations in the MgO-SiO2 system and extrapolate to core-mantle boundary pressure. At 136 GPa, we predict that the eutectic points have moved further away from MgSiO3, and solidus temperatures are similar for MgO-MgSiO3 and MgSiO3-SiO2. We have further extended the melting experiments in the multi-anvil press at 24 GPa to compositions along the MgSiO3-FeSiO3 join, with starting compositions Mg0.9Fe0.1SiO3, Mg0.8Fe0.2SiO3, Mg0.7Fe0.3SiO3 and Mg0.5Fe0.5SiO3, heated to 2650 ± 100, 2615 ± 100, 2580 ± 100 and 2510 ± 100 K, respectively. Scanning electron microscope images of the recovered samples show coexisting bridgmanite solid with melt. From their chemical analysis by electron microprobe, we determine KD = (Fesolid/Mgsolid)/(Feliquid/Mgliquid), the iron equilibrium constant. With increasing FeSiO3 content of the starting composition, it decreases quasi-linearly from KD ~ 0.31 ± 0.07 to ~ 0.22 ± 0.02. We construct a thermodynamic model for the FeO-MgO-SiO2 system based on these partitioning data and the melting curve of the endmembers, and binary melting phase diagrams along the MgO-SiO2, MgSiO3-FeSiO3, MgO-FeO, FeO-SiO2 systems are calculated at 24 and 136 GPa. At 24 GPa they compare favorably with available experimental phase relations that were not used as constraints. At core mantle boundary conditions, we find that iron partition strongly into the melt. The lower solidus T of FeO-rich melts provides a possible explanation for low velocity features in the lowermost mantle. Melt density is a key parameter affecting the dynamic process of magma ocean crystallization. We have performed molecular dynamic simulation of liquid Mg28Fe4Si16O64 and Mg32Si16O64 over a wide compression range (reaching 370 GPa) at 3000 K, based on density functional theory, using the generalized gradient approximation plus local Coulomb repulsion (GGA + U) approach. We find iron in Mg28Fe4Si16O64 in a high-spin state with an average magnetic moment of 4.0 ± 0.1 μB up to 35 GPa, followed by a quasi-linear decrease with P, reaching 0.5 ± 0.4 μB at 370 GPa. We explore melt structure and fit the isothermal Pvolume results using the Tait equation of state. Partial molar volumes of FeO and MgO are virtually identical across the whole P range, which is also reflected in very similar radial distribution functions. Based on the molar weight difference alone, olivine melt with 25%mol of Fe2SiO4 shows higher density than seismic models for the Earth mantle above ~85 GPa, suggesting that a basal magma ocean in the early earth may have been gravitationally stable given the high iron equilibrium constant predicted from our thermodynamic model. Experimental measurements of the structure and physical properties of silicate liquids are challenging due to their high melting point. Many experiments therefore rely on silicate glass, quenched from the melt. The extent to which they represent liquid structure, however, cannot be unambiguously evaluated. We have therefore performed molecular dynamics simulations based on density functional theory for Mg2SiO4 and MgSiO3 melt at 2000 K and glass at 300 K, cooled from the melt with different rates: immediate quench and 0.1 K per femtosecond (slow cooling). Analyzing the resulting glass and melt structure by means of total and partial radial distribution functions and structure factors, we find that slowly cooled glass shows more structural features than the quench or melt. Differences in total X-ray and neutron structure factors are too small between them, and too large between different measurements, to understand whether the cooling effects are reflected in experiments.