Elastic properties of ferrous bearing MgSiO3 and their relevance to ULVZs

We used ab initio molecular dynamics to calculate the elastic constants of MgSiO3, FeSiO3 and (Fe0.5Mg0.5)SiO3 perovskite under lower-most mantle conditions (136 GPa, 2000-4000 K). At 0 K, V-p and V-s for FeSiO3 perovskite are 12 and 18 per cent lower, respectively than those for MgSiO3, in agreement with previous work. The difference in velocity, however, increases with temperature for high spin (but not low spin) states and at 4000 K, V-p and V-s for the end-member FeSiO3 are, respectively, 19 and 34 per cent lower than those of MgSiO3. We find that (dlnV(s)/dlnV(p))(T,P) of FexMg1-xO3 remains below 2 for all relevant iron concentrations and lower than the value of 3 seen in most ultralow velocity zones (ULVZ). Moreover, we are unable to match simultaneously the observed density increase and shear wave velocity reduction shown by ULVZs by increasing the ferrous iron content in perovskite except for the largest density increases and the smallest shear wave decreases observed in ULVZs. Assuming that current seismic estimates of ULVZ properties are accurate, our results suggest that ferrous iron enrichment alone cannot explain ULVZs.