Partially molten materials resist shearing and compaction. This resistance
is described by a fourth-rank effective viscosity tensor. When the tensor
is isotropic, two scalars determine the resistance: an effective shear and
an effective bulk viscosity. In this seminar, calculations are presented of
the effective viscosity tensor during diffusion creep for a 3D tessellation of
tetrakaidecahedrons (truncated octahedrons). The geometry of the melt is
determined by assuming textural equilibrium. Two parameters
control the effect of melt on the viscosity tensor: the porosity and the
dihedral angle. Calculations for both Nabarro-Herring (volume diffusion)
and Coble (surface diffusion) creep are presented. For Nabarro-Herring
creep the bulk viscosity becomes singular as the porosity vanishes. This
singularity is logarithmic, a weaker singularity than typically assumed in
geodynamic models. The presence of a small amount of melt (0.1% porosity)
causes the effective shear viscosity to approximately halve. For Coble creep,
previous modelling work has argued that a very small amount of melt may
lead to a substantial, factor of 5, drop in the shear viscosity. Here, a
much smaller, factor of 1.4, drop is obtained.