Effect of Fe-enrichment on seismic properties of perovskite and post-perovskite in the deep lower mantle

Abstract

Recent experimental measurements of the equation of state of perovskites and post-perovskites in the (Mg,Fe)SiO3 and (Mg,Fe,Al)(Fe,Al,Si)O3 systems over a wide range of iron contents are used to constrain the effects of Fe and Al on density and bulk modulus of these phases at deep mantle pressures. The density of Fe-bearing perovskite follows a linear relationship with Fe-content at a representative mid-mantle depth of 1850 km (80 GPa): ρ80 (g cm− 3) = 5.054(1) + 1.270(3)XFe. The bulk modulus of silicate perovskite is not sensitive to Fe-content and follows the relationship, K80 (GPa) = 546(2) + 12(25)XFe. The velocity heterogeneity parameter, ∂ln VB/∂XFe, determined by experimental values for the bulk sound speed is 0.10(1), in agreement with theory and the behaviour of other Fe-bearing silicates. Near the core–mantle boundary, Fe-rich post-perovskite is observed to be more compressible than the Mg-end-member, in contrast to theoretical predictions. From experimental data, the densities of perovskite and post-perovskite at 125 GPa (2700 km depth) are ρ125,Pv (g cm−3) = 5.426(11) + 1.38(4)XFe and ρ125,pPv (g cm−3) = 5.548(1) + 1.41(3)XFe. The density contrast across the post-perovskite transition is ∼2 per cent, irrespective of Fe-content, but the contrast in bulk sound speed increases with Fe-content. Al-rich silicates exhibit no significant differences in density or compressibility relative to Al-free silicates, but may be responsible for seismic heterogeneities due to differences in the depth and width of the post-perovskite transition. Observations of increased densities in large low shear velocity provinces and ultra-low-velocity zones may be consistent with local iron enrichment from Mg#90 to Mg# 78–88 and Mg# <50, respectively.