The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle

Abstract

MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200-300 km above the core-mantle boundary (the D" discontinuity) or the polarization anisotropy observed in the lowermost mantle. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase. However, recent in situ X-ray diffraction measurements have revealed a transition to a 'post-perovskite' phase above 125 GPa and 2,500 K--conditions close to those at the D" discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D" layer.

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO 3 orthopyroxene

“…Combining the results for volume and bulk modulus indicates that at 120 GPa the bulk sound velocity changes by Ϫ2.3(2.1)% across the transition at deep mantle pressures. Theoretical studies have predicted that the decrease in bulk sound velocity will be Ͻ1% across the transition (5,8).…”

Iron-rich silicates in the Earth's D″ layer

“…Density of the ppv phase as a function of x. For comparison, results are corrected to the same pressure at 130 GPa using the theoretical bulk modulus and elastic constants (11,29). Density is shown with comparison to Fs12 silicate perovskite (ϫ symbol shown with 1% error bar) (21) and the ␣-PbO2-type (dotted line) and CaCl 2-type (dashed line) SiO2 phases (30).…”

“…Without a stable iron-rich silicate phase, previous explanations of the complex geochemical and geophysical signatures of the DЉ layer have been limited to heterogeneous, solid͞melt mixtures of iron-poor silicates and iron-rich metals and oxides (6, 7). Recently, MgSiO 3 has been found to transform from perovskite to CaIrO 3 structure under the P-T conditions of the DЉ layer (8)(9)(10)(11)(12). This postperovskite (ppv) phase also was observed to coexist with silicate perovskite and magnesiowüstite in experiments with orthopyroxene and olivine starting materials with x up to 0.4, but the iron content in this phase is undefined because of the unknown Fe͞Mg distributions among multiple coexisting ferromagnesian phases (13).…”

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

“…(Mg,Fe)O ferropericlase, although it makes up perhaps 20-25% of the lower mantle by volume, has a very large intrinsic shear wave anisotropy (*50% or more) at depth; perovskite is considerably less anisotropic (e.g., Karki et al 1999;Wentzcovitch et al 2006;Mainprice 2007;Marquardt et al 2009). The single-crystal elasticity of postperovskite is not yet well understood; several different sets of elastic constants obtained from first-principles calculations have been published (Iitaka et al 2004;Stackhouse et al 2005;Wentzcovitch et al 2006) but discrepancies among studies exist.…”