The ferroelastic phase transition in polycrystalline stishovite and seismic scattering in Earth's lower mantle

Mantle convection may disperse subducted oceanic crust and silica-rich material exsolved from the core in Earth's lower mantle. The scattering of seismic waves in the lower mantle has been explained by the presence of silica-rich rocks and the effects of the ferroelastic phase transition from stishovite to a CaCl₂-type SiO₂ phase. The ferroelastic phase transition perturbs the elastic properties of crystalline silica with the potential to affect the propagation of seismic waves.

We investigated the elastic properties of sintered polycrystalline stishovite and CaCl₂-type SiO₂ at high pressures by X-ray diffraction and Brillouin spectroscopy. Disks of sintered polycrystalline silica were compressed in diamond anvil cells using neon as pressure-transmitting medium. We derived the equations of state of stishovite and CaCl₂-type SiO₂ from the compression curve of sintered polycrystalline silica together with a Landau theory description that predicts the changes in elastic properties across the phase transition. Our results suggest that sintered polycrystalline stishovite is less compressible than stishovite powder. At the phase transition to CaCl₂-type SiO₂, we observed a sharp drop of the bulk modulus. Shear wave velocities determined by Brillouin spectroscopy do not appear to be affected by the phase transition. We explain these discrepancies to earlier studies on silica powder and single crystals with the different elastic response and the grain size of sintered polycrystalline silica.

To assess the seismic scattering potential of oceanic crust dispersed in Earth's lower mantle, we combined the equation of state and Landau theory description for sintered polycrystalline silica with literature data and modeled the contrasts in seismic properties between subducted oceanic crust and pyrolite. Our modeling results suggest that the phase transition from stishovite to CaCl₂-type SiO₂ imposes a strong depth dependence of seismic contrasts for both P waves and S waves. Matching these depth-dependent seismic contrasts with the geophysically observed distribution of seismic scatterers may provide constraints on the thermal and compositional state of the lower mantle.