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

- We model the seismic velocity of FexMg1 − xO at lower mantle conditions.
- For x>50% there is a large decrease in K due to a mixed spin state.
- Ultra-Low Velocity Zones (ULVZ) cannot be explained by FexMg1 − xO alone.
- A mixture of FexMg1 − xO and FeyMg1 − ySiO3 can fit a wide range of ULVZ properties.
- Average ULVZ properties are best fit with x∼1 and y<0.3.

The elasticity of FexMg1 − xO 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 FexMg1 − xO 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 Vs/dln Vp)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 Vs decreases of up to 45%, which allows the range of ULVZ Vp, Vs and densities to be matched. We also find that (dln Vs/dln Vp)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.