Dynamics and structure of a stirred partially molten ultralow-velocity zone

The seismic properties of thin (5–40 km thickness) patches exhibiting low seismic velocities—termed ultralow-velocity zones or ULVZ—just above the core–mantle boundary (CMB) might be explained by the presence of partially molten rock, where a liquid phase occupies interstices within a skeletal network of solid grains. However, a key problem with this explanation is that in the absence of improbably strong surface tension effects, partial melt is expected to drain by percolation over geological time scales, to form a dense, melt-rich layer at the CMB with physical properties that are inconsistent with seismic and other geophysical constraints. Here we consider whether stirring within ULVZ, driven by viscous coupling to convective motions in the overlying mantle, can inhibit the production of such stratification and maintain a partially molten region with a structure and constitution comparable to what is inferred seismically. We use two-dimensional numerical simulations of the response of a melt–solid mixture to stirring imposed from above and scaling analysis to identify conditions leading to melt separation, retention and drainage over a broad range of parameters. We find that melt migration at plausible ULVZ conditions is governed predominantly by dynamic pressure gradients arising from the viscous deformation related to mantle stirring, rather than by the buoyancy effects driving melt percolation. In particular, dense melt that would otherwise drain downward and accumulate at the CMB is expected to remain in suspension as a result of the stirring driven within ULVZ. In addition, our model predicts that partially molten ULVZ patches will be characterized by a positive gradient in seismic shear velocity (i.e., increasing with depth), consistent with seismic inferences, and may persist in this state over geological time scales.