Diapiric ascent is often assumed to occur in a mantle with a viscosity of order 1021 P, the value estimated from glacial rebound studies. Segregation of melt from residual crystals en route is then a necessary consequence of the slow ascent velocity. The temperatures and stresses associated with diapirs, however, are much greater than those involved in glacier unloading, and the controlling viscosity is therefore relatively low. High stresses may also result from the volume change associated with the partial-melting process itself. Stress and temperature feedback considerably increase the velocity of ascent of a deep diapir, and this may allow the melt and the residual crystals to rise together without separating. The material delivered to shallow-mantle magma chambers may therefore be similar in composition to the deeper source region. This reopens the possibility that a deep eclogite layer, ≥200 km depth, may be the source region for mid-ocean ridge basalts.