Persistent Geochemical Zonation (“Striping”) within the Galápagos Mantle Plume

Some hotspot tracks, such as those formed by the Hawai’i and Galápagos mantle plumes, exhibit long-lived cross-track isotopic zonation, thought to reflect the streaking out of heterogeneous material in the plume conduit during upwelling. In lavas associated with the Galápagos mantle plume, three geochemical domains, present for at least 15 Myr, have been identified: northern, southern and central. The most extreme isotopic enrichments are observed in the northern domain of the Cocos Ridge at ~15 Ma, and in the southern domain of the Galápagos Archipelago at the present day. Owing to the northward migration of the Galápagos Spreading Center above the plume at ~5-10 Ma, this relationship suggests that geochemical enrichment in the Galápagos basalts is greatest above the region of the plume furthest from the nearby mid-ocean ridge. We examine the hypothesis that these temporal and spatial variations in geochemical enrichment reflect a ''shallow mantle control'', associated with differences in the mean depth of melting. We conducted forward melting models of a mixed peridotite-pyroxenite mantle to calculate the isotopic composition of the resulting melts formed under two different mantle flow regimes. Our results demonstrate that variations in the average pressure of melt generation, due to the influence of the nearby ridge axis, may explain the range of isotopic compositions across ~15 Ma of Galápagos plume-related volcanism. The patterns of isotopic zonation observed along the hotspot track confirm the paradigm of persistent plume striping, with variations in the degree of geochemical enrichment modulated by shallow mantle processes.