The influences of depth-dependent mantle properties on the interaction of thermal-chemical plumes with an internal compositional boundary have been studied with a 2D model, using the extended Boussinesq approximation. In contrast to previous results with constant thermal expansivity, the decrease of thermal expansivity with depth produces focused plumes, which promote the breakthrough of plumes through the compositional boundary separating the upper and lower mantles. In terms of an averaged buoyancy ratio (compositionally induced density/thermally induced density), plumes can penetrate intermittently the chemical barrier for a buoyancy ratio of around two. Those plumes which can reach the surface are sprayed out and may explain the strong episodicity in the heat-flow and the generation of flood basalts. There is very little entrainment of the upper-mantle material into these mega-plumes. By these singular injection events the upper mantle becomes more and more heterogeneous, whereas the lower mantle remains barely mixed.