Ultra-Low Velocity Zones and the Effect of Iron on the Elasticity of the Lower-Mantle

Ultra-low velocity zones (ULVZ) are small (<100 km long) regions at the core-mantle boundary (CMB) with anomalously low velocities and increased densities (Δρ +5-15%, ΔV_p 8-15% ΔV_s 24-45%) (Thorne, M. et al. Earth Planet. Sci. Lett (2013) 364, 59). A possible source of these regions is the enrichment of lower mantle minerals with iron. As of yet the effect of iron on the elasticity and density of lower mantle minerals at lower mantle conditions is unknown.

In this study we used molecular dynamic simulations to study iron (both ferric and ferrous) substitution into the magnesium (silicate) oxide structures that are most commonly found in the lower mantle- perovskite, post-perovskite and ferropericlase- at the pressures (136 GPa) and temperatures of the CMB (3000-4000 K). The effect of temperature in the simulation is profound with very different temperature derivatives between iron-rich and iron-poor substances leading to different results statically and thermally. All iron species have a roughly linear relationship between properties and iron concentration allowing comparisons to be drawn with ULVZs and expected parameters to be calculated. The substitution of iron causes a large decrease in the oxide velocities with a particularly pronounced shear wave decreases. At a density increase of 10% from a lower mantle background we find that V_p decreases are around 5-12% and V_s decreases are around 10-20% for all iron structures. Thus V_p decreases in ULVZs are easily accounted for with iron substitution but V_s decreases in ULVZs are larger than would be expected from iron substitution. This can be seen in the dLnV_s:dLnV_p ratios which reach a maximum of 3.5 with iron substituted structures but remain around 2 at appropriate densities which is smaller than the expected 3.