Hotspot lavas erupted at ocean islands exhibit tremendous isotopic variability, indicating that there are numerous mantle components(1,2) hosted in upwelling mantle plumes that generate volcanism at hotspots like Hawaii and Samoa(3). However, it is not known how the surface expression of the various geochemical components observed in hotspot volcanoes relates to their spatial distribution within the plume(4-10). Here we present a relationship between He and Pb isotopes in Samoan lavas that places severe constraints on the distribution of geochemical species within the plume. The Pb-isotopic compositions of the Samoan lavas reveal several distinct geochemical groups, each corresponding to a different geographic lineament of volcanoes. Each group has a signature associated with one of four mantle end members with low He-3/He-4:EMII(enriched mantle 2), EMI(enriched mantle 1), HIMU(high mu = U-238/Pb-204) and DM(depleted mantle). Critically, these four geochemical groups trend towards a common region of Pb-isotopic space with high He-3/He-4. This observation is consistent with several low-He-3/He-4 components in the plume mixing with a common high-He-3/He-4 component, but not mixing much with each other. The mixing relationships inferred from the new He and Pb isotopic data provide the clearest picture yet of the geochemical geometry of a mantle plume, and are best explained by a high-He-3/He-4 plume matrix that hosts, and mixes with, several distinct low-He-3/He-4 components.