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

Matthew Lloyd Morgan Gleeson , Mark Richards, Cinzia G Farnetani, Kaj Hoernle, Sally Gibson

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 Centre above the plume since ~20 Ma, this relationship suggests that geochemical enrichment in the Galápagos basalts is always greatest above the region of the plume furthest from the nearby mid-ocean ridge. Here we demonstrate that the temporal variations in geochemical enrichment associated with Galápagos plume stripes likely reflect ''shallow mantle control'' associated with differences in the mean depth of melting. We conduct forward melting models of a mixed peridotite-pyroxenite mantle to calculate the isotopic composition of the resulting melts formed under two 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 strongly confirm the paradigm of persistent plume striping, with variations in the degree of geochemical enrichment modulated by shallow mantle processes.