Magnesium silicate perovskite and effect of iron oxidation state on its bulk sound velocity at the conditions of the lower mantle

- We refine structure of single crystal (Mg, Fe) (Si, Al)O3 at high P-T conditions.
- We approach the problem of spin state transition of iron in (Mg, Fe) (Si, Al)O3.
- We develop a general model for bulk modulus for high P-T conditions.
- We examine influence P, T and composition on bulk sound velocity of material.
- In particular, we feature effect of iron oxidation state on bulk sound velocity.

A comprehensive understanding of the iron- and aluminum-bearing magnesium silicate perovskite (Pv) crystal structure and its evolution under pressure is vital for our understanding of the deep Earth's lower mantle physics and chemistry as well as for correct interpretation of seismic data. In this study we investigate material with composition relevant to subducted oceanic crust, by means of single crystal X-ray diffraction at high pressure and high temperature conditions. We found that (a) there is no spin state crossover in ferric iron occupying the bicapped trigonal prism (“A”) crystallographic site, and (b) ferric iron does not enter the octahedral (“B”) site at any conditions of our experiments. We constrain the thermal equation of state for Pv with variable content of iron and aluminum. For compositions of Pv relevant for Earth's lower mantle, variation of Fe3+/ΣFe can lead to significant changes of Pv bulk sound velocity (over 1%), demonstrating the importance of iron oxidation state to the interpretation of seismic tomography data.