Fig. 4

The radial relative viscosity profiles of ferropericlase inferred from homologous temperature scaling and the solidus melting curves. Profiles based on the ideal (red, open symbols) and regular solution models (black, solid symbols) for (Mg_0.75Fe_0.25)O, (Mg_0.8Fe_0.2)O, (Mg_0.85Fe_0.15)O and (Mg_0.9Fe_0.1)O assuming a a “cool” geotherm³⁸ and a relatively b “hot” geotherm³⁹. Symbol identification is the same as in Fig. 3. Viscosity values are scaled relative to the viscosity at 27 GPa. For these curves, the non-dimensional constant g in the homologous temperature scaling is given a value of 12 (Methods). Uncertainties on the viscosity are approximately enveloped by the values given by the solidus and liquidus (Supplementary Fig. 5) and two different solid solution models under two different geotherms. The blue shaded regions include spin transition pressure range for Mg#75-90 at 300 K (~35–70 GPa)³³. The spin transition pressure ranges at corresponding high temperatures are likely broader but begin at pressures similar to those at 300 K^65,66. The curves are drawn as a guide for the eye