Elastic properties of ferropericlase at lower mantle conditions and its relevance to ULVZs

Abstract

Editor: P. Shearer Keywords: elasticity and anelasticity composition of the mantle equations of stateThe elasticity of Fe x Mg 1−x O was examined under lowermost mantle temperature and pressure conditions using density functional theory (DFT). The addition of iron decreases the shear modulus of MgO but has varying effects on the bulk modulus depending on the spin state of the iron. The spin state of iron in Fe x Mg 1−x O is dependent on pressure and temperature but also on the concentration of iron. At 136 GPa, Fe in low concentrations (<25%) is nearly entirely low spin, while at very high concentrations (>75%) it is nearly entirely in the high spin state. There is, as expected, a large decrease in seismic velocities with iron substitution. However, the effect of Fe is greater at high-temperatures than at low-temperatures, meaning it is difficult to extrapolate low-temperature experimental results. We cannot simultaneously match the density and seismic velocities of ULVZs with Fe-enriched ferropericlase. This is reflected in (dln V s /dln V p ) T,P , which in ULVZs is generally observed to be about 3, but does not exceed about 1.5 for Fe-enriched periclase. A mixture of ferropericlase and ferrous perovskite can cause V s decreases of up to 45%, which allows the range of ULVZ V p , V s and densities to be matched. We also find that (dln V s /dln V p ) T,P increases up to as much as 3 but this value is strongly dependent on the bounds of the mixing geometry. We conclude, therefore, that the properties of ULVZs can be readily explained by a lower mantle with a single phase that is heavily enriched in Fe.

Origins of ultralow velocity zones through slab-derived metallic melt

“…For instance, Knittle and Jeanloz (20) attributed ULVZs to FeSi and FeO as core−mantle reaction products although a subsequent study did not produce FeSi from reaction between bridgmanite and molten iron (21). Some of these models showed that simultaneous match of density (ρ), compressional wave velocity (Vp), and Vs can be achieved for certain compositions at 300 K (7), but recent theoretical studies concluded that, at high temperatures, iron-rich wüstite or bridgmanite could not reproduce both Vp and Vs (11,12).…”

“…Given uncertainties in the melting behavior of mantle rocks (10), elastic properties of relevant phases (11,12), and iron partitioning between them (13,14), the origin of ULVZs remains enigmatic. Partial melt of silicate composition has been widely considered as the origin of ULVZs because the presence of partial melt reduces shear wave velocity (Vs) effectively, and partial melt was found to be denser than coexisting solids at deep mantle conditions (e.g., refs.…”

Fe-periclase reactivity at Earth’s lower mantle conditions: Ab-initio geochemical modelling

“…Deschamps and Trampert, 2004;Kobayashi et al, 2005;Auzende et al, 2008;Sakai et al, 2009;Nakajima et al, 2012;Prescher et al, 2014;Muir and Brodholt, 2015). In particular, HP-HT experiments carried out between 30-130 GPa and 1760-2500 K (Prescher et al 2014;Sinmyo et al, 2008;Sinmyo and Hirose, 2013;Narygina et al, 2011) yield Kd-estimates showing a trend flattening at LLM pressures like our predictions, although in a restricted range of values (0.35-0.16 versus 0.69-0.16).…”

“…Spin state and excess Gibbs energy of mixing LS-HS mixed states, which affect the physical properties of the (Mg,Fe)O-phases, are often accounted for in a solid mixing by a linear combination of the pure LS and HS systems (Lyubutin et al, 2009;Wentzcovitch et al 2009;Muir and Brodholt, 2015;Vilella et al, 2015). It is straightforward to prove that in a HS-LS-(FeO-MgO) system (x LS-HS -x LS -x HS being x-FeO in mixed HS-LS state, pure LS or HS state, respectively) the exact mixing energy is expressible as…”

Implications for the Melting Phase Relations in the MgO‐FeO System at Core‐Mantle Boundary Conditions

“…Similarly, we adopt a constant value of α (i.e., 3 × 10 −5 K −1 ) for ferropericlase from (Wu et al, ). The thermoelastic properties of ferropericlase at CMB conditions are directly taken from Muir and Brodholt (). The elastic properties of the mixture with various fractions of ferropericlase are calculated using both Voigt and Reuss averages (Hill, ).…”