First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer

Area of Science:

• Geophysics
• Mineral Physics
• Computational Materials Science

Background:

• Post-perovskite magnesium silicate (MgSiO3) is a key mineral phase hypothesized to exist in the Earth's D'' layer.
• Its properties are crucial for understanding seismic observations like the D'' reflector and seismic anisotropy.
• Ionic diffusion in post-perovskite influences mantle viscosity and thermal/chemical coupling with the core.

Purpose of the Study:

• To calculate absolute ionic diffusion rates in post-perovskite under lower mantle conditions.
• To investigate the impact of diffusion anisotropy on post-perovskite strength and seismic properties.
• To reconcile experimental and observational data regarding the D'' layer.

Main Methods:

• Utilizing first-principles computational methods.
• Simulating ionic diffusion (Mg2+ and Si4+) in post-perovskite.
• Analyzing diffusion rates under high-pressure and high-temperature conditions.

Main Results:

• MgSiO3 diffusion is extremely anisotropic, with an eight-order-of-magnitude difference between fast and slow directions.
• Anisotropic diffusion can render post-perovskite up to four orders of magnitude weaker than perovskite if lattice-preferred orientation is present.
• Weak post-perovskite significantly increases heat flux across the core-mantle boundary and alters the geotherm.

Conclusions:

• The presence of weak, anisotropic post-perovskite explains laterally varying viscosity in the lowermost mantle.
• A rapid change in seismic anisotropy, due to the onset of rapid deformation in post-perovskite, likely causes the observed sharp D'' reflector.
• The findings reconcile seismic observations with experimental constraints on the perovskite-to-post-perovskite phase transition width.