Viscosity-depth profiles in the Earth's mantle are estimated on the basis of microrheological equations for ultrabasic silicates, the preliminary reference Earth model (PREM), and two assumed geotherms (one with, and one without, a lower mantle upper thermal boundary layer). The estimation includes assessment of variations of creep parameters with depth. Both constant-strain rate non-linear viscosity and Newtonian viscosity are in the range 1020-1021 Pa s in the upper mantle. In the lower mantle, there is a moderate increase in viscosity with depth, and upper limit estimates are of the order of 1022 and 1023 Pa s, respectively. As uncertainties are ±one order of magnitude, the Newtonian-viscosity mantle inferred from geophysical observations cannot be rejected on the strength of rheological considerations. By the same token, convection should be mantle-wide, as the viscosity layering is not sufficient to prevent it. On the other hand, if the mantle is chemically stratified, and separate convective circulations exist in the upper and lower mantle, the thermal boundary layer below the 670-km discontinuity should have a discernible viscosity signature.

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Journal Physics of the Earth and Planetary Interiors
Ranalli, G, & Fischer, B. (B.). (1984). Diffusion creep, dislocation creep, and mantle rheology. Physics of the Earth and Planetary Interiors, 34(1-2), 77–84. doi:10.1016/0031-9201(84)90086-4