Basalts from intraplate or hotspot ocean islands (e.g., the Hawaiian, Galápagos, and Canary Islands) are believed to be formed by mantle plumes, which emanate from mantle boundary layers such as the core-mantle boundary. The long-term chemical structure of mantle plumes, however, remains poorly constrained. Spatial variation in the chemical composition has long been recognized in lavas from the Galápagos Islands: Enriched plume material forms a horseshoe-shaped region with depleted mantle, similar in composition to mid-ocean ridge basalt, in its inner part. The enriched horseshoe-shaped region can be subdivided into three distinct geochemical domains. We show that these same domains occur in the same relative positions with respect to morphology in a geochemical profile across the Galápagos hotspot track off the coast of Costa Rica, indicating that the asymmetrical spatial zonation of the Galápagos hotspot has existed for at least 14 m.y. Combined with published He isotope data, the results of this study imply that plume material can ascend from the lower mantle, possibly from the core-mantle boundary, with little stirring occurring during ascent, and that zonation in hotspot lavas may in some cases reflect spatial heterogeneity within the lower mantle source.

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