The effect of Fe3+ on the equations of state of Mg-silicate perovskite and post-perov

A significant fraction of iron in perovskite (Pv) and post-perovskite (PPv) has been found to be ferric (Fe³⁺/ΣFe = 10-60%). The amount of Fe³⁺ would be even greater for regions of the mantle with Al enrichment. However, the effect of ferric iron on the physical properties of these dominant lower-mantle phases has not been assessed. Here we investigate the effect of ferric iron on the equations of state of Mg-silicate Pv and PPv. We use a glass starting material with a 0.1Fe2O3•0.9MgSiO3 composition which was synthesized using the laser levitation method. Diffraction measurements were performed in the laser-heated diamond-anvil cell in a quasi-hydrostatic Ar medium with Au pressure standard at the GSECARS sector of the APS. Pv was synthesized at 50 GPa and 2000 K, and volume measurements were made up to 100 GPa on compression. Before each measurement, the sample was annealed by heating to 1800 K. PPv was synthesized at 135 GPa and 2500 K, and volume measurements were performed between 40 and 140 GPa on decompression without heating. The pressure-volume data of Pv and PPv were fit to the Birch-Murnaghan equation. At 125 GPa (2700-km depth), volume and bulk modulus of ferric PPv are 3.5(2)% and 17(4)% smaller than those of ferric Pv, respectively, resulting in a 10(1)% decrease in bulk sound speed at the PPv transition. This magnitude of decrease in bulk sound speed is a factor of four greater than that found in the ferrous system.
At 125 GPa, the volume of ferric Pv (18 mol% Fe³⁺) is 0.9(2)% greater than ferrous Pv (9 mol% Fe²⁺), however, ferric PPv has a 0.4(2)% lower volume than ferrous PPv. This leads to a factor of two larger density jump (Δρ/ρ) at the PPv transition in the ferric system over the ferrous. The smaller volume of ferric PPv compared with ferrous PPv, even though the iron content in the ferric system is larger, perhaps indicates that the ferric ions in PPv are in the low spin state whereas the ferrous ions in PPv are in either a mixed or intermediate spin state. If so, the PPv transition in the ferric system may be accompanied by a spin transition. Our results suggest that an enrichment in Fe³⁺ increases the difference in density and bulk sound speed between Pv and PPv, which has important implications for mantle flow and interpretations of the seismic wave variations at the lowermost mantle, respectively.