Elasticity of Single-Crystal Ferropericlase across the Spin Transition in the Lower Mantle

Abstract:

Recent experimental and theoretical studies on the lower-mantle ferropericlase have demonstrated that its physical and chemical properties can be affected by the spin transition, which in turn can affect our understanding of deep-Earth seismic structures, geochemistry, and geodynamics. The sound velocities of ferropericlase at lower-mantle pressures have been reported using various techniques including Brillouin Light Scattering (BLS), Impulsive Stimulate Light Scattering (ISS), Inelastic X-ray Scattering (IXS), and Nuclear Resonant Inelastic X-ray Scattering [1,2,3]. However, the compressional wave and shear wave velocities have never been measured simultaneously up to lower mantle conditions to solve for full elastic constants of ferropericlase, C₁₁, C₁₂ and C₄₄ via Christoffel’s equations. Thus far, the effects of the spin transition on elasticity of ferropericlase remains highly debated. Using the combinations of experimental results from BLS and ISS measurements in the Mineral Physics Lab at the University of Texas at Austin, we have directly measured Vp and Vs of ferropericlase (Mg_0.92Fe_0.08)O simultaneously along [100] and [110] crystallographic axes up to megabar pressures. These results permit the derivation of reliable full elastic constants and the modeling of the elastic and seismic properties in the high-spin, low-spin and mixed-spin states. Single-crystal X-ray diffraction experiments were also performed to provide the equation of state parameters of ferropericlase for the modelling. The compressional wave velocities from ISS measurements show significant softening, while shear wave velocities from BLS experiments were only slightly affected by the spin transition. Using thermoelastic modelling [4], we will discuss the effects of the spin transition on elastic constants, sound velocities, elastic anisotropies, and seismic parameters of ferropericlase at lower-mantle pressure-temperature conditions. These results are compared with seismic observations of the deep lower mantle in order to better understand seismic signatures and mineralogical models of the lower mantle.

References:

1. Antonangeli, D., et al., 2011, Science 331, 64

2.Marquardt, H., et al., 2009, Science 324, 224.

3. Crowhurst, J., et al., 2008, Science 319, 451.

4.Wu, Z.Q., et al., 2013, PRL 110, 228501.