The effect of viscosity variations associated with large scale chemical heterogeneities on mixing in the mantle is studied for a model two-dimensional problem. Low viscosity regions are rapidly deformed, develop long tendrils, and are entrained towards regions of surface divergence. Very viscous regions are only slowly stretched; thus, geochemical reservoirs can persist relatively undisturbed for long periods of time if they are >O(10--100) times more viscous than the surrounding mantle. High viscosity ratio blobs have a tendency to aggregate, leading to the formation of large scale heterogeneities from smaller ones. ¿ American Geophysical Union 1996