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

Area of Science:

• Geophysics
• Mineral Physics
• Computational Materials Science

Background:

• Low/ultralow-velocity zones in Earth's mantle are often attributed to partial melting.
• The density contrast between melt and solid mantle is crucial for understanding these zones.
• Ferropericlase ((Mg, Fe)O) is a significant component of the lower mantle.

Purpose of the Study:

• To investigate the density behavior of ferropericlase in solid and liquid states under high pressure and temperature.
• To determine how pressure and iron content affect the density difference between solid and liquid ferropericlase.
• To explore the implications for partial melting and density crossovers in the lowermost mantle.

Main Methods:

• First-principles molecular dynamics simulations were employed.
• Simulations covered pure and iron-alloyed ferropericlase in solid and liquid states.
• Conditions ranged up to 135 GPa and 4000 K.

Main Results:

• The density difference between solid and liquid ferropericlase decreases significantly with increasing pressure.
• A high-spin to low-spin transition of iron in liquid ferropericlase was observed around 55 GPa, causing a density increase.
• An excess of ~5% iron in the liquid phase can lead to a solid-liquid density crossover in the lowermost mantle.

Conclusions:

• The convergence of solid and liquid ferropericlase densities under pressure challenges previous assumptions about melt buoyancy.
• Iron's spin transition plays a key role in modulating liquid ferropericlase density at deep mantle pressures.
• These findings provide a mechanism for explaining seismic observations in low-velocity zones through partial melting dynamics.