Spatial asymmetry in the isotopic composition of volcanic rocks has been identified at several Pacific hotspots, including Hawai'i, the Marquesas, Samoa, the Societies, and the Galápagos. At each hotspot, the volcanoes are arranged in two sub-parallel chains that also define distinct fields in isotopic space. Here we present interpreta-tions of data from two additional ocean island systems that exhibit spatial isotopic asymmetry: the Galápagos and Easter hotspots. Both systems display geochemical asymmetry despite being near-ridge hotspots, suggest-ing that compositional zonation in plumes originates at depths greater than the plumbing systems supplying mid-ocean ridges. Furthermore, the correspondence of the compositional boundaries of the Galápagos and Easter hotspots with those of the Large Low Shear Velocity Province (LLSVP) is consistent with the assertion that spatial patterns of isotopic enrichment at hotspots may reflect the distribution of compositional heteroge-neity within the thermal boundary layer at the base of the mantle that gives rise to the plumes. At the Galápagos hotspot, which is located along the northern side of the LLSVP, the southern side of the chain exhibits geo-chemical enrichment, whereas at the Easter hotspot, located along the southern side of the LLSVP, it is the northern side of the chain that is enriched. Consequently, spatial variations in the geochemistry of hotspot lavas may provide a method for mapping the geochemical structure of the lower mantle.

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