Elasticity of Single-Crystal Ferropericlase across the Spin Transition of Iron Investigated by Brillouin and Impulsive Spectroscopies

Understanding the effect of the electronic spin transition of the iron on the elasticity of the lower-mantle ferropericlase (Mg,Fe)O is critical in our understanding of the seismic velocity structures of the Earth's lower mantle. Previous studies have reported inconsistent results about the elastic behavior of ferropericlase across the spin transition--- ISLS results showed that the spin transition is associated with softening of all elastic moduli by as much as 25% between 40 and 50 GPa (Crowhurst et al., 2008), whereas Brillouin spectroscopic studies revealed an increase in shear wave anisotropy across the transition. Inelastic X-ray scattering results, on the other hand, did not show any visible effects of the spin transition. To decipher the elasticity of the lower-mantle ferropericlase within the spin transition and in the low-spin state, we have used both Brillouin Light Scattering (BLS; sensitive to Vs) and Impulsive Stimulated Light Scattering (ISLS; sensitive to Vp) techniques to simultaneously measure the shear wave (BLS) and compressional wave (ISLS) velocities of a single-crystal ferropericlase in a high-pressure diamond anvil cell. Together with X-ray diffraction analyses, our results reveal various magnitudes of the effects of the spin transition on elastic moduli as well as Vp/Vs anisotropies of ferropericlase. These experimental results are used in thermal elastic modeling to address potential geophysical consequences of the spin transition in the lower mantle.