Low-viscosity mantle blobs are sampled preferentially at regions of surface divergence and stirred rapidly into the mantle

The persistence of mantle heterogeneity depends on its density and viscosity and as well as the characteristics of the convecting mantle, such as the vigour and possible stratification of convection. Low-viscosity regions stretch and deform more rapidly than their high-viscosity counterparts. Low-viscosity regions are also drawn into, and hence preferentially sampled at, regions of surface divergence such as mid-ocean ridges and plume-fed hotspots. Low-viscosity heterogeneities in the form of blobs are thus more rapidly stirred into a convecting mantle. Discrete blobs of primitive or enriched mantle are unlikely to persist for billions of years.