The eruption of nepheline-normative lavas in the early and late stages of formation of the large Hawaiian tholeiite shields is well established, as is the conclusion that volatile components are involved in the genesis of these alkaline lavas. For magmas to be generated, the source materials must be transported across their solidus curves. Solidus curves for volatile-free periodotite and for periodotite-C-H-O provide the depth-temperature framework for the sites of magma generation. The assumption (controversial) that garnet remains in the source material locates the major melting region in plume material at depths of about 80 km, with isotherms in plume center exceeding 1500 ¿C. The plume carries traces of volatile components from depths greater than 300 km. These dissolve in a trace of interstitial melt as the plume crosses the solidus for peridotite-C-H-O at depths decreasing from 350 km to about 150 km with distance from the plume axis. The volatile-charged melt, enriched in incompatible elements, is swamped by the picrites generated in the major melting region. From the outer portions of the plume, the volatile-rich melt enters the lithosphere at 80--90 km depth, where the change in rheology retards its upward percolation. This magma (remaining in equilibrium with peridotite) is carried toward the solidus for peridotite-C-H-O, changing composition toward nephelinite; evolution of vapor as magma approaches the solidus may facilitate intermittent crack propagation, releasing the nephelinitic magmas for eruption from depths of 75--85 km. Movement of the lithosphere plate over the rising plume establishes asymmetry. Eruption of nephelinitic magmas on the upstream side of the plume (early volcanism) may be suppressed or very close in time and space to eruption of alkaline lavas and tholeiites. On the downstream side (late volcanism), eruption of nephelinites is delayed by lateral transport away from the main melting region. ¿ American Geophysical Union 1988