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Abstract The geochemical modelling of many small-volume continental magmas shows that their source regions must have been depleted by basalt formation, and later enriched by the addition of a metasomatic melt, formed by melting ∽0·3% of the MORB source. The presence of such magmas throughout western Turkey and the Aegean, where no plume is present, requires such magmas to be formed at temperatures considerably below the dry solidus. Similar magmas elsewhere bring up nodule suites, many of which have the same composition as the source regions of the host magmas. Pressure and temperature estimates from garnetbearing suites, and temperature estimates from those without garnet, show that the nodules last equilibrated at pressures and temperatures close to those of the wet solidus. Magmas from the smaller oceanic islands and from some seamounts closely resemble small-volume continental magmas, and also come from sources that have been metasomaticaUy enriched. However, no data sets from any of the oceanic islands that have yet been modelled require their source regions to have been depleted before being enriched The density of the sources of continental and oceanic basalts can be obtained from their calculated modes. In the garnet peridotite stability field the sources of ocean island basalts have densities that are slightly greater than that of the MORB source, whereas those of most small-volume continental magmas are lighter. Therefore ocean island sources alone are easily entrained into the thermal convection beneath the plates. A numerical experiment shows that material in the hot and cold boundary layers of high Rayleigh number time-dependent convection tends to remain in the boundary layers for several overturns, rather than moving into the interior of the circulation. A simple model that can account for the elemental and isotopic composition of ocean island basalts forms their sources by the addition of metasomatic melt to the undcplcUd MORB source while it forms the lower part of the mechanical boundary layer beneath continents. The isotopic differences between ocean island basalt and MORB are generated before the source becomes entrained in the cold sinking plumes that fall to the base of the convecting layer. At the base the material is heated and rises as part of a hot plume. Because the metasomatic melt contains water and carbonates, the enriched regions start to melt and generate more melt on decompression than does the MORB source. Such regions can therefore generate islands and seamounts. Even when the enriched material moves into the interior of the circulation and acquires the mean potential temperature of the mantle, it will still generate more melt on decompression than will the MORB source, and the isotopic and elemental composition will still be distinctive. The model can therefore account for the observed composition of magmas from seamounts that cannot be produced from either the MORB or the primitive source.