Persistence of strong silica-enriched domains in the Earth’s lower mantle

Journal name:
Nature Geoscience
Volume:
10,
Pages:
236–240
Year published:
DOI:
doi:10.1038/ngeo2898
Received
Accepted
Published online

Abstract

The composition of the lower mantle—comprising 56% of Earth’s volume—remains poorly constrained. Among the major elements, Mg/Si ratios ranging from ~0.9–1.1, such as in rocky Solar-System building blocks (or chondrites), to ~1.2–1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing, and thus homogenous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity associated with a 20-fold change in viscosity, such as due to the dominance of intrinsically strong (Mg, Fe)SiO3–bridgmanite in low-Mg/Si domains, is sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize mantle convection patterns, and coherently persist at depths of about 1,000–2,200km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such bridgmanite-enriched ancient mantle structures (BEAMS) may reconcile the geographical fixity of deep-rooted mantle upwelling centres, and geophysical changes in seismic-tomography patterns, radial viscosity, rising plumes and sinking slabs near 1,000km depth. Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.

At a glance

Figures

  1. Predicted evolution of the mantle for two regimes of mixing.
    Figure 1: Predicted evolution of the mantle for two regimes of mixing.

    af, Numerical-model results for the reference case I (a,b) and the example case (cf) show efficient mixing and persistence of large-scale heterogeneity, respectively (model time as annotated). b,c,f, Snapshots of composition with isotherms (spaced 450K). a,d, Snapshots of potential temperature with compositional contours that mark small-scale heterogeneity in a and large-scale BEAMS in d. This difference in mantle-mixing efficiency between cases highlights the role of compositional rheology, given that both cases have similar Nusselt numbers Nu (Supplementary Table 3)—that is, a criterion for convective vigour50. e, Snapshot of viscosity shows that BEAMS are more viscous than upwelling and downwelling conduits. Also see Supplementary Movies 1–4.

  2. Summary of numerical-model results.
    Figure 2: Summary of numerical-model results.

    Regime map of all cases (Supplementary Table 3) shows that compositional viscosity contrasts of ~1.5 orders of magnitude and small-to-moderate compositional density contrasts are required for long-term persistence of SiO2-enriched material (blue squares). This conclusion is independent of whether all cases, or the subset of cases with 10 ≤ Nu ≤ 11 (highlighted by black frames) are considered. In reference cases I/II and III (circles), a global viscosity jump at 660km depth of factor λ = 8 and λ = 2.5, respectively, is imposed to ensure that Nu is comparable to Nu of the example case (Fig. 1c–f), which is marked by a white cross.

  3. Map with possible distributions of BEAMS in the Earth/'s lower mantle.
    Figure 3: Map with possible distributions of BEAMS in the Earth’s lower mantle.

    Colours show mid-mantle shear-velocity anomalies51, radially averaged as annotated. As LLSVPs are primarily confined to 2,300–2,891km depth22, 24, they do not dominate the radial average shown here. Note that the blue fast anomalies (downwelling conduits: ‘1’,‘3’), are ~2× weaker than the red slow anomalies (upwelling conduits: ‘2’,‘4’) (Supplementary Fig. 7). BEAMS probably occupy the volume between conduits (dashed outlines); arrows mark the sense of associated upper-mantle flow. Stagnant slabs1 (‘S’) should overlie BEAMS, guiding our assessment of BEAMS distributions, which agree well with cluster analysis of seismic-tomography models25.

  4. Illustration of the BEAMS hypothesis.
    Figure 4: Illustration of the BEAMS hypothesis.

    BEAMS (light grey) are stable high-viscosity structures that reside in Earth’s lower mantle, while streaks of pyrolitic–harzburgitic rocks (light blue/green) and basalt (dark blue/green) circulate between the shallow and deep mantle through rheologically weak channels. BEAMS can coexist with, and stabilize the LLSVPs in the lowermost ~500km of the mantle (yellow), which are interpreted as intrinsically dense (Fe-rich) piles32, 33, 34, 35 and plume-generation zones31.

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Affiliations

  1. Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan

    • Maxim D. Ballmer,
    • Christine Houser,
    • John W. Hernlund,
    • Renata M. Wentzcovitch &
    • Kei Hirose
  2. Institute of Geophysics, ETH Zurich, 8092 Zurich, Switzerland

    • Maxim D. Ballmer
  3. Department of Applied Physics and Applied Mathematics, Columbia University, New York City, New York 10027, USA

    • Renata M. Wentzcovitch
  4. Department of Earth and Environmental Sciences, Columbia University, Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA

    • Renata M. Wentzcovitch

Contributions

M.D.B., C.H. and J.W.H. wrote the manuscript and composed the figures. M.D.B. performed and analysed geodynamic models. C.H. and R.M.W. computed seismic velocities in the lower mantle. J.W.H. and K.H. analysed the influence of composition on density and viscosity. All authors contributed to the BEAMS hypothesis, and the design of the study.

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