Solid-liquid density and spin crossovers in (Mg, Fe)O system at deep mantle conditions

Abstract

The low/ultralow-velocity zones in the Earth’s mantle can be explained by the presence of partial melting, critically depending on density contrast between the melt and surrounding solid mantle. Here, first-principles molecular dynamics simulations of (Mg, Fe) O ferropericlase in the solid and liquid states show that their densities increasingly approach each other as pressure increases. The isochemical density difference between them diminishes from 0.78 (±0.7) g/cm3 at zero pressure (3000 K) to 0.16 (±0.04) g/cm3 at 135 GPa (4000 K) for pure and alloyed compositions containing up to 25% iron. The simulations also predict a high-spin to low-spin transition of iron in the liquid ferropericlase gradually occurring over a pressure interval centered at 55 GPa (4000 K) accompanied by a density increase of 0.14 (±0.02) g/cm3. Temperature tends to widen the transition to higher pressure. The estimated iron partition coefficient between the solid and liquid ferropericlase varies from 0.3 to 0.6 over the pressure range of 23 to 135 GPa. Based on these results, an excess of as low as 5% iron dissolved in the liquid could cause the solid-liquid density crossover at conditions of the lowermost mantle.

Density‐Pressure Profiles of Fe‐Bearing MgSiO3 Liquid: Effects of Valence and Spin States, and Implications for the Chemical Evolution of the Lower Mantle

“…First‐principles molecular dynamics method, which was previously used in the simulations of molten MgSiO 3 (Stixrude & Karki, ) and iron‐bearing phases (Bajgain et al, ; Ghosh & Karki, ; Holmstrom & Stixrude, ), was used to generate canonical NVT ensembles within the generalized gradient approximation (GGA) and projector‐augmented wave potentials as implemented in VASP (Kresse & Furthmuller, ). The supercells that sampled pure and Fe‐bearing compositions contained 16MgSiO 3 and its variants.…”

“…The appropriateness of U at elevated pressure and temperature is not well founded so we preferred to use GGA only. The GGA + U test calculations for (Mg,Fe)O system have shown that the effects of the U term on the equation of state and density are small (Ghosh & Karki, ).…”

“…In most cases, the liquid phase of silicates, including the dominant MgSiO 3 component, would be less dense than their solid counterparts. With increasing pressure, the liquid‐solid density gap decreases considerably, but the gap consistently remains finite and negative (Funamori & Sato, ; Ghosh & Karki, ; Petitgirard et al, ). This means that a crossover in solid‐melt density comparison is not possible in any isochemical scenario.…”

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

“…In this scenario, a BMO is separated from the "surficial" MO due to an isochemical density crossover between solids and liquids and/or a crossover in the slopes of the mantle solidus and adiabat (Andrault et al, 2017;Thomas et al, 2012). As any potential crossovers are inferred to occur at very high pressures, if at all (Andrault et al, 2012;de Koker et al, 2013;Ghosh and Karki, 2016;Nomura et al, 2011;Stixrude et al, 2009), they may only be relevant for (near-)complete melting of the early Earth's mantle, e.g. in a giant-impact scenario.…”

Experimental Constraints on Ferropericlase (Mg, Fe)O Melt Viscosity Up to 70 GPa

“…It is expected that Fe 2+ in (Mg, Fe) O melt will go through a spin transition from high spin to low spin (Ghosh & Karki, ; Holmstrom & Stixrude, ). Also, it has been argued that the spin transition of the melt has a dramatic effect on Fe partitioning between solid and melt, causing a sudden Fe enrichment in silicate melt after ~ 70 GPa (Nomura et al, ).…”