Computational support for a pyrolitic lower mantle containing ferric iron

Abstract

The dominant minerals in Earth's lower mantle are thought to be Fe-and Al-bearing MgSiO 3 bridgmanite and (Mg,Fe)O ferropericlase 1 . However, experimental measurements of the elasticity of these minerals at realistic lower-mantle pressures and temperatures remain impractical. As a result, di erent compositional models for the Earth's lower mantle have been proposed 2-4 . Theoretical simulations, which depend on empirical evaluations of the e ects of Fe incorporation into these minerals, support a pyrolitic lower mantle that contains a significant amount of ferropericlase 5,6 , much like the Earth's upper mantle. Here we present first-principles computations combined with a lattice dynamics approach that include the e ects of Fe 2+ and Fe 3+ incorporation. We calculate the densities and elastic-wave velocities of several possible lower-mantle compositions with varying amounts of ferropericlase along a mantle geotherm. On the basis of our calculations of aggregate elasticities, we conclude that neither a perovskitic composition (about 9:1 bridgmanite to ferropericlase by volume) nor an olivine-like composition (about 7:3) reproduces the seismological reference model of average Earth properties. However, an intermediate volume fraction (about 8:2) consistent with a pyrolitic composition can reproduce the reference velocities and densities. Bridgmanite that is rich in ferric iron produces the best fit. Our findings support a uniform chemical composition throughout the present-day mantle, which we suggest is the result of whole-mantle convection.Determination of the chemical composition of the lower mantle (LM) has long been one of the central research topics in the deep Earth sciences. The LM composition is key to understanding its dynamical properties. For example, if the LM has a different composition from the upper mantle (UM), these regions convect separately. If the UM and LM have the same composition, then whole-mantle convection is expected 7 . However, physical properties of LM minerals determined from high-pressure (P) and high-temperature (T ) experiments have some uncertainties due to technical difficulties, which give substantial discrepancies on this issue. Several different models have been proposed 2-6,8-15 depending on the research approaches used. Generally, experiments based on the multianvil apparatus propose a pyrolitic LM (refs 2,8,9). In the pyrolite model, a Mg/Si ratio of ∼1.3 is proposed, yielding a notable amount of (Mg,Fe)O ferropericlase (Fp), a solid solution of MgO and FeO together with Fe-and Al-bearing MgSiO 3 bridgmanite (MgBr). A particular composition with the olivine-like Mg/Si ratio as high as 2.0 was however suggested by the equations of state (EoS) measurements using multianvil apparatus with sintered diamonds 3 . An alternative view that the LM is MgBr dominant, corresponding to the perovskitic or chondritic model (Mg/Si ∼ 1.0), is proposed based on diamond-anvil cell experiments 4,10,11 . On the other hand, some geochemical analyses 12,13 suggest layering in the LM in conjunc...

Small shear modulus of cubic CaSiO3 perovskite

“…Although previous studies suggested that even a small amount of CaPv can be a seismologically visible component in the LM, a relatively similar [Wang et al, 2014]. Black dots indicate the PREM values [Dziewonski and Anderson, 1981].…”

“…Pluses are values before a pressure correction by Li et al [2006b]. Velocities and densities of 12.5 mol % Fe-bearing MgPv at 2000 and 3000 K are also shown as a reference (dashed lines) [Wang et al, 2014]. (d) Azimuthal anisotropy for longitudinal (A P ) and shear (A S ) waves as a function of pressure at 1000, 2000, 3000, and 4000 K. (colored lines).…”

“…Squares denote B S of the EoS [Kawai and Tsuchiya, 2014]. B S and G of 12.5 mol % Fe-bearing MgPv at 2000 and 3000 K are also shown as a reference (dashed lines) [Wang et al, 2014]. (c) Longitudinal (V P ), bulk sound (V Φ ), and shear (V S ) wave velocities and densities of cubic CaPv as a function of pressure at 1000, 2000, 3000, and 4000 K.…”

“…Next, ρ, V P , V S , and V Φ of CaPv calculated along the adiabat of MgPv, which was derived in Kawai and Tsuchiya [2014] from the EoS with an assumption of a potential temperature of 1630 K, are shown in Figure 3, together with those for Fe free and 12.5 mol % Fe-bearing MgPv [Wang et al, 2014] and the preliminary reference Earth model (PREM) [Dziewonski and Anderson, 1981]. At LM conditions, CaPv has 4.8 and 1.5% larger ρ, À4.4 and À2.6% faster V P , À6.7 and À4.2% faster V S , and À2.6 and À1.2% faster V Φ than Fe free and 12.5 mol % Fe-bearing MgPv, respectively.…”

Pressure–volume–temperature equation of state of ε–FeOOH to 11 GPa and 700 K

“…They argue that such a relict of the banded iron formation may explain the observed features of the ultralow-velocity zones of the seismic wave on the core-mantle boundary. A recent theoretical study showed that the seismic wave velocity in the lower mantle can be explained by the existence of ferric iron (Wang et al, 2015). It is expected that iron oxide hydroxide may be formed when the banded iron formations along with the oceanic crust subducted into the mantle.…”