Mantle flow and multistage melting beneath the Galápagos hotspot revealed by seismic imaging

Journal name:
Nature Geoscience
Volume:
7,
Pages:
151–156
Year published:
DOI:
doi:10.1038/ngeo2062
Received
Accepted
Published online

Abstract

Some of Earth’s largest magmatic provinces result from the interaction between mid-ocean ridges and near-ridge hotspots, which are hypothesized to overlie plumes of upwelling mantle. Geodynamic models predict that upwelling plumes are sheared by the motion of the overlying tectonic plates and can connect to a nearby mid-ocean ridge by shallow flow beneath thin, young lithosphere. Here we present seismic tomographic images of the upper 300km of the mantle beneath the Galápagos Archipelago in the eastern Pacific Ocean. We observe a low-velocity anomaly, indicative of an upwelling plume, that is not deflected in the direction of plate motion. Instead, the anomaly tilts towards the mid-ocean ridge at depths well below the lithosphere. These characteristics of the plume–ridge connection beneath the Galápagos Archipelago are consistent with the presence of multiple stages of partial melting, melt extraction, and melt remixing within the plume and surrounding mantle. These processes affect the viscosity of the asthenosphere, alter the upwelling plume and influence the compositions of surface lavas. Our results imply that the coupling between the oceanic plate and plume upwelling beneath the Galápagos is weak. Multistage melting may similarly affect the geophysical and geochemical characteristics of other hotspots.

At a glance

Figures

  1. Map of the Galapagos Islands and seismic network.
    Figure 1: Map of the Galápagos Islands and seismic network.

    Triangles indicate seismic stations; the network consisted of ten portable broadband stations and the Global Seismographic Network (GSN) station PAYG. Selected islands and volcanoes are labelled in italics and red, respectively (SN, Sierra Negra; CA, Cerro Azul; W, Wolf). The black arrow indicates the direction of motion of the Nazca plate in a hotspot reference frame12. Bathymetry and topography in metres above sea level (masl). Inset shows the location in a broader context.

  2. Delay times of teleseismic S waves.
    Figure 2: Delay times of teleseismic S waves.

    Each delay time (circle) is plotted at the piercing point where the seismic wave path to a given station (triangle) intersects a horizontal plane at 300km depth. Delay times have been adjusted by station corrections (see Supplementary Information). Red and blue circles indicate positive (late) and negative (early) delays, respectively. The size of the symbol is proportional to the observed delay, as indicated by the scale at the top right in a. a, Station G06, located on Fernandina. b, GSN station PAYG, located on Santa Cruz. c, Station G03, located on Floreana. d, Station G09, located on Genovesa.

  3. Results of tomographic inversion for S-wave velocity structure.
    Figure 3: Results of tomographic inversion for S-wave velocity structure.

    Colour scale denotes the percentage deviation from the starting model (see Fig. 11 of ref. 22). ac, Map-view horizontal section at 40km depth (a), 80km depth (b), and 200km depth (c). d, East–west cross-section at 0°45′S. e, North–south cross-section at 91°W. Dashed lines in ac indicate locations of cross-sections d,e.

  4. Schematic illustration of mantle flow and melting and their relation to geochemical observations.
    Figure 4: Schematic illustration of mantle flow and melting and their relation to geochemical observations.

    a, North–south cross-section at 91°W, perpendicular to the plate motion. Also indicated are the mantle flow direction (green); ascent of carbonated and hydrous melts (dashed red arrows); ascent of silicate melts (solid red arrows); zone of melt mixing (thick red line); enriched plume (orange); dehydrated volume (grey); and zone of elevated temperature and water content within the plume (black curve). The latitudinal extent of the He anomaly is indicated by the box above the map. b, Map of 3He/4He ratios and anomalous seismic velocities at 200km depth (coloured region denotes shear velocities at least 0.75% lower than average). Helium data compiled from the GEOROC database. Numbers are average values for individual volcanoes; 3He/4He contoured at values of 11 and 15.

  5. Predicted and observed trace element compositions of magmas produced by melting of the Galapagos plume.
    Figure 5: Predicted and observed trace element compositions of magmas produced by melting of the Galápagos plume.

    A 0.1% carbonatite melt (thin line) and a calculated hybrid of that melt and a 5% silicate melt of the residue (dashed line) are compared with a basalt sample from Fernandina (thick line). The source is assumed to have 20% clinopyroxene and 10% garnet. Carbonatite–liquid distribution coefficients are from ref. 30. The Fernandina basalt composition48 has been corrected for 50% fractional crystallization. The source composition is depleted MORB (mid-ocean ridge basalt) mantle49. Concentrations are normalized to the primitive mantle composition50.

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Author information

Affiliations

  1. Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA

    • Darwin R. Villagómez,
    • Douglas R. Toomey &
    • Emilie E. E. Hooft
  2. Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA

    • Dennis J. Geist
  3. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

    • Sean C. Solomon
  4. Present address: ID Analytics, San Diego, California 92128, USA

    • Darwin R. Villagómez

Contributions

D.R.V., D.R.T. and D.J.G. wrote the initial manuscript. D.R.V., D.R.T., E.E.E.H. and S.C.S. contributed to experiment design and collection of seismic data. D.R.V., D.R.T. and E.E.E.H. contributed to the analysis of seismic data and methods development. D.J.G. contributed to the petrologic calculations and to the development of geochemical models. All authors discussed the results and their implications and assisted in the final revisions to the manuscript.

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The authors declare no competing financial interests.

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