Two voluminous magma types generated in the mantle underlying the Pacific plate are mid‐ocean ridge tholeiite (MORB) erupted at the East Pacific Rise spreading center and Hawaiian tholeiite (HT) erupted above the Hawaiian hot spot or melting anomaly. MORB has low initial ⁸⁷Sr/⁸⁶Sr ratios and low amounts of all incompatible trace elements including rare earths; chondrite‐normalized patterns are depleted in light rare earths. HT, by contrast, has higher initial ⁸⁷Sr/⁸⁶Sr and higher amounts of incompatible trace elements; chondrite‐nor‐malized patterns are enriched in the middle and light rare earths. HT is generally poorer in CaO and Al₂O₃ and much richer in total iron and TiO₂ compared with MORB having the same MgO content. Primary magma compositions for the two volcanic systems are calculated in Fe‐Mg equilibrium with residual olivine (Fo₉₂). MORB is generated by partial melting of a trace element depleted Iherzolite source leaving a residual assemblage dominated by olivine and orthopyroxene. The percentage of partial melting for a primary magma containing 15% MgO is calculated to be 35–42% in a source mantle having a heavy rare earth content of 3×chondrite and 33–35% MgO. HT, represented by Kilauea tholeiite, is generated by partial melting of a mixture of unmelted and residual mantle for MORB which has been modified by metasomatic addition of a nephelinitic fluid, amphibole, and minor amounts of apatite and Fe‐bearing phases such as sulfide and magnetite/ilmenite. This model yields a picritic magma in equilbrium with magnesian dunite at high (>40%) degrees of partial melting. The source also has 35% MgO before partial melting. Melting in both systems in polyvariant and not controlled by lower‐temperature invariant equilibria. The low‐velocity zone is considered to be the source of metasomatic fluids that are driven upward into the lowermost lithosphere in response to a thermal plume. Picritic primary magmas are produced by shear melting, localized in the zone of thinned and metasomatized lithosphere beneath Hawaii. Melt extraction is rapid and episodic at intervals of months to decades; magma is not stored in the mantle but passes upward to a plexus of storage reservoirs located 2–6 km beneath the surface of Kilauea. Kilauea primary magmas fractionate olivine during upward transport to reach bulk compositions of 13–14% MgO in storage. Different magma batches erupted to the surface, distinguished by different major and minor element compositons compared at similar MgO content, represent combinations of differing degrees of metasomatic enrichment, differing degrees of partial melting, and some effects of premelting mantle heterogeneity.