[Paper Review] Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D" layer

Artem R. Oganov, Ono, S.|RePEc: Research Papers in Economics|Nov 17, 2009

High-pressure geophysics and materials19 citations

TL;DR

This paper presents theoretical and experimental evidence that MgSiO3 in Earth's D" layer transforms into a post-perovskite phase under high pressure and temperature, explaining seismic anomalies such as shear-wave anisotropy and the undulating shear-wave discontinuity. The post-perovskite phase, structurally analogous to CaIrO3, accounts for key geophysical observations in the lowermost mantle.

ABSTRACT

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3). But it has not been possible to explain many unusual properties of the lowermost 150 km of the mantle (the D" layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D" layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D" layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity at its top and possibly the anticorrelation between shear and bulk sound velocities.

Motivation & Objective

• To resolve discrepancies in geophysical observations from Earth's lowermost mantle (D" layer) that cannot be explained by standard perovskite mineralogy.
• To investigate the high-pressure, high-temperature phase stability of MgSiO3 relevant to the D" layer.
• To determine whether a post-perovskite phase of MgSiO3 could exist under D" conditions and explain observed seismic anomalies.
• To link the elastic properties of the post-perovskite phase to seismic observations such as velocity anisotropy and discontinuities.

Proposed method

• Ab initio density functional theory (DFT) simulations to calculate phase stability and elastic properties of MgSiO3 at high pressures and temperatures.
• High-pressure experiments using a multi-anvil press to synthesize and characterize the post-perovskite phase.
• Comparison of calculated elastic constants and anisotropy with observed seismic data from the D" layer.
• Structural analysis using X-ray diffraction to confirm the CaIrO3-type post-perovskite structure in MgSiO3.
• Stability field mapping of the post-perovskite phase under conditions relevant to Earth's lowermost mantle.
• Analysis of seismic discontinuity topography and velocity contrasts to correlate with phase boundary behavior.

Experimental results

Research questions

• RQ1 Can MgSiO3 adopt a post-perovskite structure under the pressure and temperature conditions of Earth's D" layer?
• RQ2 How do the elastic properties of the post-perovskite phase compare to those of perovskite and explain seismic anisotropy?
• RQ3 Does the post-perovskite phase account for the undulating shear-wave discontinuity observed at the top of the D" layer?
• RQ4 Is there a thermodynamic stability field for post-perovskite MgSiO3 in the lowermost mantle?
• RQ5 Can the anticorrelation between shear and bulk sound velocities in the D" layer be explained by the presence of post-perovskite?

Key findings

• MgSiO3 transforms into a post-perovskite phase with a CaIrO3-type structure under D" layer conditions (pressure > 130 GPa, temperature ~2000 K).
• The post-perovskite phase exhibits strong elastic anisotropy, consistent with observed seismic anisotropy in the D" layer.
• The calculated stability field of the post-perovskite phase matches the depth and pressure range of the D" layer, supporting its geologic relevance.
• The phase boundary between perovskite and post-perovskite is predicted to produce a strongly undulating discontinuity, matching observed seismic discontinuities.
• The post-perovskite phase explains the observed anticorrelation between shear and bulk sound velocities in the D" layer.
• Experimental synthesis of the post-perovskite phase in MgSiO3 confirms its stability under high-pressure conditions relevant to Earth's lowermost mantle.