  The thermoelastic properties of ferropericlase Mg1-xFexO (x = 0.1875)
throughout the iron high-to-low spin crossover have been investigated by first
principles at Earth's lower mantle conditions. This crossover has important
consequences for elasticity such as an anomalous bulk modulus (KS) reduction.
At room temperature the anomaly is somewhat sharp in pressure but broadens with
increasing temperature. Along a typical geotherm it occurs across most of the
lower mantle with a more significant KS reduction around 1400-1600 km depth.
This anomaly might also cause a reduction in the effective activation energy
for viscous creep and lead to a viscosity minimum in the mid-lower mantle, in
apparent agreement with results from inversion of data related with mantle
convection and postglacial rebound.
