Cratonic lamproites are diamondiferous ultrapotassic rocks that are emplaced through thick continental lithosphere and thought to derive from melting of metasomatized (i.e., geochemically enriched) regions of the subcontinental lithospheric mantle (SCLM). We explored the alternative hypothesis that melts sourced from sublithospheric (i.e., convective mantle) sources dominate the genesis of cratonic lamproites. Supporting evidence includes a robust linear correlation between the Mg/Fe ratios of xenocrystic and magmatic olivine in lamproites worldwide, overlapping the trend observed for kimberlite olivine. This indicates that, similar to kimberlites, primary lamproite melts originate from broadly similar sublithospheric mantle sources before assimilating SCLM material of variable composition. The lamproites are also characterized by a direct correlation between olivine Mg/Fe ratio and bulk-lamproite K2O/Al2O3, an index of potassium enrichment in the melt that is independent of mantle-xenocryst entrainment and magmatic differentiation. Quantitative modeling indicates that this correlation results from the interaction between carbonate-bearing sublithospheric melt and phlogopite-rich wall rocks in the SCLM. Our data show that cratonic lamproites and kimberlites have similar mantle sources in the convective mantle, with lamproites acquiring their peculiar enrichment in K by interaction with metasomatized SCLM en route to the surface. Modification of sublithospheric melts during transit through the continental lithosphere might represent a common process for the genesis of alkaline mafic/ultramafic magmas.