Effects of the Fe³⁺ spin transition on the properties of aluminous perovskite-New insights for lower-mantle seismic heterogeneities

We have measured the effects of the coupled substitution of Fe³⁺ and Al on the density and compressibility of mantle silicate perovskite (Pv) up to 95GPa. X-ray emission spectroscopy and synchrotron Mössbauer spectroscopy reveal a rapid increase in the population of low-spin Fe³⁺ in Fe³⁺, Al-bearing Pv over a narrow pressure range near 70GPa, which is in sharp contrast with Al-free Fe³⁺-bearing Pv, where Fe³⁺ undergoes a gradual spin transition, and with Al-free Fe²⁺-bearing Pv, where Fe²⁺ does not become low spin. At low pressure, Fe³⁺ and Al expand the perovskite lattice. However, near the pressure range of the abrupt increase in the low-spin population, the unit-cell volume of Fe³⁺, Al-bearing Pv becomes similar to that of Mg-endmember Pv, while those of Al-free Fe³⁺-bearing Pv and Al-free Fe²⁺-bearing Pv remain larger throughout the lower mantle. Consequently, Pv in Al-rich systems should have lower density in the shallow lower mantle but similar or greater density than Pv in pyrolite in the deep lower mantle, affecting the buoyancy and mechanical stability of heterogeneities. Although the Fe³⁺ spin transition in Pv is unlikely to cause a seismic discontinuity at mantle temperatures, it may result in a large change in bulk sound speed at 1200-1800km depth, such that a vertically extending structure with an elevated amount of Fe³⁺ would generate slower and faster anomalies above and below the depth of the spin transition, respectively, relative to the surrounding mantle. This may have important implications for bulk sound speed anomalies observed at similar depths in seismic tomography studies.