Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

Journal name:
Nature Geoscience
Volume:
9,
Pages:
481–489
Year published:
DOI:
doi:10.1038/ngeo2733
Received
Accepted
Published online

Abstract

Seismic images of Earth's interior reveal two massive anomalous zones at the base of the mantle, above the core, where seismic waves travel slowly. The mantle materials that surround these anomalous regions are thought to be composed of cooler rocks associated with downward advection of former oceanic tectonic plates. However, the origin and composition of the anomalous provinces is uncertain. These zones have long been depicted as warmer-than-average mantle materials related to convective upwelling. Yet, they may also be chemically distinct from the surrounding mantle, and potentially partly composed of subducted or primordial material, and have therefore been termed thermochemical piles. From seismic, geochemical and mineral physics data, the emerging view is that these thermochemical piles appear denser than the surrounding mantle materials, are dynamically stable and long-lived, and are shaped by larger-scale mantle flow. Whether remnants of a primordial layer or later accumulations of more-dense materials, the composition of the piles is modified over time by stirring and by chemical reactions with material from the surrounding mantle, underlying core and potentially from volatile elements transported into the deep Earth by subducted plates. Upwelling mantle plumes may originate from the thermochemical piles, so the unusual chemical composition of the piles could be the source of distinct trace-element signatures observed in hotspot lavas.

At a glance

Figures

  1. Large low shear velocity provinces (LLSVPs) with other phenomena.
    Figure 1: Large low shear velocity provinces (LLSVPs) with other phenomena.

    a Lowermost mantle shear velocity perturbations (δVS) are shown for model S40RTS (ref. 1) at depth (z) of 2,800 km. Blue and red shading corresponds to positive and negative δVS, respectively. The thick orange line at δVS = −0.27% highlights LLSVP structure corresponding to 30% of the core–mantle boundary (CMB) area. Convergent margins (blue lines) overlay high δVS. Surface hotspots (red dots), large igneous province origination locations (purple circles6) and some kimberlites (green dots34) overlay low δVS, especially LLSVP margins. b, Strong lateral gradients () in δVS are shown (darker colours are strongest), which match well with the LLSVP contour of a. c, The 30% area LLSVPs of eight models, represented by eight coloured lines as detailed in the key (Supplementary Fig. S1 displays original models). d, The 30% area LLSVP of S40RTS is shaded orange, and shown with red lines corresponding to forward modelling studies that map sharp LLSVP edges, which to first order are near LLSVP margins seen in tomography (more detail is given in Supplementary Fig. S3).

  2. LLSVP observations and interpretations.
    Figure 2: LLSVP observations and interpretations.

    a Surface features (upper panel) and seismically determined lower-mantle phenomena (lower panel). See text for details. b–e, Idealized possibilities proposed to explain LLSVPs. In all cases, subducted material (possibly including post-perovskite, pPv) surrounds the structure of interest that maps as the LLSVP. b, Plume cluster. c, Thermochemical superplume. d, Stable thermochemical pile. e, Metastable thermochemical pile. LIPs, large igneous provinces; CMB, core–mantle boundary; ULVZs, ultralow velocity zones.

  3. Thermochemical pile evolution.
    Figure 3: Thermochemical pile evolution.

    Two end-member evolutionary pathways for a present-day thermochemical pile are illustrated. a, Primordial layer: in this scenario, early Earth processes establish a global layer, which develops into separate piles over time. b, Growth of the thermochemical layer over time: here, material with elevated density collects at the core–mantle boundary over time, which grows into a thicker layer and eventually into distinct thermochemical piles.

  4. Chemical composition and mineralogy of pyrolite versus mid-ocean ridge basalt (MORB).
    Figure 4: Chemical composition and mineralogy of pyrolite versus mid-ocean ridge basalt (MORB).

    a, Bulk chemical compositions of pyrolite97 and MORB98. b, Mineralogy of pyrolite99 and MORB22, 100 in the lower mantle. c, Chemical compositions of bridgmanite in pyrolite99 and MORB100 compositions. Bm, bridgmanite; Fp, ferropericlase; Ca-Pv, CaSiO3 perovskite; CF, Ca-ferrite-type phase. All proportions are given in weight%.

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  1. School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6004, USA

    • Edward J. Garnero,
    • Allen K. McNamara &
    • Sang-Heon Shim

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E.J.G., A.K.M., and S.-H.S. equally contributed to the text. E.J.G. constructed the figures with active involvement from A.K.M. and S.-H.S.

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