Figure 1.

The “Standard Model.” Hotspots are fed by narrow plumes from primordial undegassed lower mantle. The recycled component, treated as a package, goes deep into the mantle before it returns as a plume. The upper mantle is completely degassed but is injected with ³He leaking from the lower mantle (5, 6, 9). High ³He/⁴He ratios are interpreted as excess ³He. Temporal variations in magmas at hotspots reflect a compositionally zoned plume and motion of the plate across the plume. In this model, magmatism is initiated by impingement of a hot plume on the base of the plate. In contrast, Pb-isotopes have been used to argue that U, and other lithophile elements, was transported to the outer layers in early Earth history (3, 40, 41).