High-MgO liquids erupted in ocean-island settings and in some continental flood-basalt provinces commonly preserve a “depleted” composition, in terms of both highly incompatible trace elements and isotope ratios. These observations strongly imply that their source is also compositionally depleted. However, in at least one case (Iceland and the North Atlantic volcanic province), it can be shown that this depleted source is not the same as that feeding the present-day North Atlantic mid-ocean ridge. The depleted source must also have been much hotter than the mid-ocean ridge basalt (MORB) source to account for the volume of melt and primitive composition of some magmas that were generated. This depleted character, then, is an intrinsic component of mantle plumes, originating from the deep mantle. We propose that mantle plumes consist of a mixture of enriched or fusible streaks in a depleted, refractory matrix; preferential extraction of the enriched component occurs close to the plume axis. The depleted residue from this melting remains in the upper mantle and may therefore be a major contributor to the source region of MORB.