Density heterogeneities in the mantle influence the dynamics of mantle upwellings and therefore modify plume characteristics. Using analog laboratory experiments, we explore the dynamics of “thermo‐chemical” plumes containing both thermal and chemical density anomalies inherited from a stratified boundary layer at the base of the mantle. Because all plumes cool by thermal diffusion as they rise, a chemically composite thermal plume will eventually attain a level of neutral buoyancy, at which it will begin to “fail”. Separation within the plume will occur, whereby the chemically denser material will start to sink back while the heated surrounding mantle keeps rising. It more generally implies that 1) mantle plumes are not necessarily narrow and continuous throughout the mantle but can be fat and patchy such as Iceland, 2) a hot mantle region may not be buoyant and rising, but on contrary may be sinking, and 3) mantle plumes dynamics are strongly time‐dependent.