Accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids. Two models of heterogeneity in the D' layer are considered: (1) a reservior of dense material above the core-mantle boundary and (2) continuous recharge of the D' layer by subduction of cold, negatively buoyant high viscosity slabs of oceanic lithosphere. Survival of a dense reservoir depends on R&rgr;, the ratio of stabilizing compositional buoyanc to destabilizing thermal buoyance. Rapid mixing occurs for R&rgr;≤1; for R&rgr;≥1, mixing occurs by slow entrainment only. The reservoir model predicts D' layer heterogeneity is concentrated beneath lower mantle upwellings. Differentiation of composite slabs, consisting of dense crustal and light depleted components, occurs in a hot thermal boundary layer. Composite slabs pile up in the basal thermal boundary layer, and upon heating the light component is ejected by Rayleigh-Taylor instabilities. The dense component accoumulates in the boundary layer, arrayed in a pattern of interconnected spokes, and is slowly entrained into thermal plumes. According to these experiments the time require for conversion of slabs to plumes in D' is 1--2 Gyr. The correlation between lateral variations in D' layer thickness and lateral variations in lower mantle seismic velocity suggests that D' may consist of dense material, perhaps former oceanic lithosphere, which has sunk through the lower mantle. ¿American Geophysical Union 1991