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Knowing the elasticity of ferropericlase across the spin transition can help explain seismic and mineralogical models of the lower-mantle including the origin of seismic heterogeneities in the middle to lowermost parts of the lower mantle 1  2  3  4 . However, the effects of spin transition on full elastic constants of ferropericlase remain experimentally controversial due to technical challenges in directly measuring sound velocities under lower-mantle conditions 1  2  3  4  5 . Here we have reliably measured both  V   P   and  V   S   of a single-crystal ferropericlase ((Mg 0.92 ,Fe 0.08 )O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa. The derived elastic constants show drastically softened  C   11   and  C   12   within the spin transition at 40–60 GPa while  C   44   is not affected. The spin transition is associated with a significant reduction of the aggregate  V   P   /V   S   via the aggregate  V   P   softening because V  S   softening does not visibly occur within the transition. Based on thermoelastic modelling along an expected geotherm, the spin crossover in ferropericlase can contribute to 2% reduction in  V   P   /V   S   in a pyrolite mineralogical model in mid lower-mantle. Our results imply that the middle to lowermost parts of the lower-mantle would exhibit enhanced seismic heterogeneities due to the occurrence of the mixed-spin and low-spin ferropericlase.

Elasticity of Ferropericlase across the Spin Crossover in the Earth’s Lower Mantle