Geochemical variations within the young Hawaiian Islands occur in two particularly prominent forms: differences between volcanic stages and differences between the “Loa” and “Kea” subchains. These observations have been interpreted to reveal spatial patterns of compositional variation in the mantle, such as concentric zoning about the hot spot or elongate streaks along the hot spot track. Our numerical models of a hot plume of upwelling mantle that is interacting with, and melting beneath, a moving lithospheric plate suggest some of the above interpretations should be reevaluated. The mantle plume is assumed to be uniformly isotopically heterogeneous, thus without any compositional zoning. Nonetheless, our models predict geographic zoning in lava isotope composition, an outcome that is caused by differences in melting depths of distinct source components and plume-lithosphere interaction. Isotope compositions of model volcanoes that grow as they pass over the melting zone can explain some of the gross aspects of isotope variation at Hawaii. The results illustrate that chemical zoning at the surface is not necessarily a map of zoning in the mantle, and this affects further inferences about the chemical structure of the mantle.