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Compositions of basalts erupted between the main zone of Galapagos plume upwelling and adjacent Galapagos Spreading Centre (GSC) provide important constraints on dynamic processes involved in transfer of deep-mantle-sourced material to mid-ocean ridges. We examine recent basalts from central and northeast Galapagos including some that have less radiogenic Sr, Nd and Pb isotopic compositions than plume influenced basalts (E-MORB) from the nearby ridge. We show that the location of E-MORB, greatest crustal thickness and elevated topography on the GSC correlates with a confined zone of low-velocity, high-temperature mantle connecting the plume stem and ridge at depths of ~100 km. At this site on the ridge, plume-driven upwelling involving deep melting of partially-dehydrated, recycled ancient oceanic crust, plus plate-limited shallow melting of anhydrous peridotite, generate E-MORB and larger amounts of melt than elsewhere on the GSC. The first order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (> 5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained “anchored” to a contact point on the GSC. This deep, confined NE plume stem-to-ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid-state outflow of heterogeneous plume material to the ridge. This article is protected by copyright. All rights reserved.

[1] Analyses of basalts from the Galapagos archipelago and adjacent spreading center have shown that the underlying mantle plume is isotopically heterogeneous over length scales of 10s to 100s of km. We show that the convecting mantle is also compositionally heterogeneous on smaller length scales (kms). Our evidence comes from recent small-volume mildly alkaline and tholeiitic basalts on Isla Santiago, central Galapagos. Flows from volcanic vents

Funder: Royal Commission for the Exhibition of 1851; Id: http://dx.doi.org/10.13039/501100000700, Funder: University of Cambridge; Id: http://dx.doi.org/10.13039/501100000735, Funder: Geological Society of London; Id: http://dx.doi.org/10.13039/100008066, Geophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.