(Fe,Al)-bearing post-perovskite in the Earth's lower mantle

•We have studied the spin and valence states and EoS of PPv at high pressures.•Both of Fe2+ and Fe3+ in PPv samples are in the HS state at ∼165GPa and 300 K.•The volume of PPv is strongly affected by the spin and valence state of Fe.•The density and VΦ of (Fe,Al)-enriched PPv is anti-correlated at high pressures.•Effect of Fe-Al on the density and VΦ across the Pv to PPv transition are studied.

The combined effects of Fe and Al on the electronic spin and valence states as well as the equation of state (EoS) of post-perovskite have been investigated using synchrotron X-ray diffraction and Mössbauer spectroscopy in high-pressure diamond anvil cells. Two post-perovskite samples (Mg0.6Fe0.15Al0.5Si0.75O3 and Mg0.66Fe0.13Al0.28Si0.86O3) were synthesized at approximately 165 GPa and 2200-2500 K, and were subsequently investigated for these properties at 114-170 GPa and 300 K. Analyses of the high-pressure Mössbauer spectra show that Fe2+ and Fe3+ occupy the large bipolar prismatic sites in both of our samples and remain in the high-spin state at ∼165-168GPa and 300 K. Combining the Mössbauer results with the obtained pressure-volume relationship from X-ray diffraction, we have found that the unit cell volume of post-perovskite can be significantly affected by the spin and valence states of Fe and the Al substitution. Mg0.6Fe0.15Al0.5Si0.75O3-PPv with the predominantly high-spin Fe2+ (∼95%) and a greater amount of Al has a unit cell volume similar to that of Mg0.66Fe0.13Al0.28Si0.86O3-PPv in which ∼65% of Fe is in the high-spin Fe3+ state. Our results are used together with previous results regarding the EoS parameters in Fe-bearing perovskite and post-perovskite to model the density and bulk sound velocity variation between perovskite and post-perovskite in the D″ layer, in which the enrichment of Fe and Al can produce an increase in density but substantially reduce the bulk sound velocity across the phase transition. That is, the combined effect of Fe and Al leads to an anti-correlation between the enhanced density and the reduced bulk sound velocity at the pressure condition of the lowermost mantle. Our results indicate that (Fe,Al)-rich silicate post-perovskite existing in the D″ region would be shown as a relatively high-density and low-velocity region in deep-mantle seismic observations.