Research ArticleGEOLOGY

Pure climb creep mechanism drives flow in Earth’s lower mantle

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Science Advances  10 Mar 2017:
Vol. 3, no. 3, e1601958
DOI: 10.1126/sciadv.1601958
  • Fig. 1 Creep models.

    (A) Diffusion creep where grain boundaries act as sources (C+) and sinks (C) for vacancies. The schematic represented here corresponds to NH creep where vacancies diffuse through the lattice. In Coble, creep vacancies would diffuse along grain boundaries. (B) Dislocation creep. The Weertman model where gliding dislocations are emitted by sources (S). Interactions are then released by some recovery mechanisms (red arrows) such as climb. (C) Pure climb creep. Strain is produced by climb motion of two orthogonal slip systems, which exchange vacancies.

  • Fig. 2 Comparison between the glide velocity (vg) and the climb velocity (vc) of dislocations.

    The ratio of vg/vc is mapped as a function of temperature and resolved stress for (A) olivine at ambient pressure, (B) ringwoodite at 20 GPa, (C) and bridgmanite at 30 GPa. The red line where vg = vc indicates the transition between two regimes. At high stress (green to blue), glide is the strain-producing mechanism. At low stress (yellow to purple), climb dominates.

  • Fig. 3 Pure climb creep simulation in bridgmanite.

    (A) Sketch of the 2D simulation box and of the loading condition. The chosen loading conditions were set in analogy with the conditions usually used for describing NH creep. They correspond to a pure shear loading. Two slip systems are considered. Here, dislocations characterized by [010] Burgers vector (labeled as “1” and shown in blue) move in response to the tensile stress along the [100] climb direction by emitting vacancies. The excess of vacancies created by these dislocations is absorbed by the dislocations with [100] Burgers vector (slip system as “2” and shown in red), which move in response to the compressive stress along the [100] direction by absorbing vacancies. (B) DD stress-strain curve obtained by applying a creep stress σ of 40 MPa at T = 1900 K and P = 24 GPa. In this particular case, the initial dislocation density is 1012 m−2, and an equilibrium vacancy concentration of Xv = 10−5 is assumed. After an initial transient stage where dislocation multiplication occurs, the steady state is attained and the steady-state strain rate value is extracted (dashed line slope). (C) Dislocation microstructure and σxx stress field extracted from the creep simulation at εxx = 0.045%. (D) Detail of the dislocation microstructure shown in (C). Black (purple) symbols indicate the dislocations with [100] ([010]) Burgers vector, whereas plus (cross) symbols are used to denote the positive (negative) sign of their Burgers vector.

  • Fig. 4 Comparison between the strain rates obtained by pure climb creep and by diffusion creep.

    Strain rate values resulting from diffusion creep are calculated (see Supplementary Materials) for the Coble (blue dotted lines) and the NH (“N-H,” black and gray dashed lines) for two grain sizes: 0.10 and 10 mm. They are compared to strain rates resulting from pure climb creep (red symbols) calculated as shown in Fig. 3 for dislocation densities (ρo) ranging from 108 to 1012 m−2. For grain sizes larger than ca. 0.1 mm, pure climb creep is a more efficient strain-producing mechanism than diffusion creep.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1601958/DC1

    fig. S1. Sketch of the simulation box.

    table S1. Parameters used to compute the glide mobility laws for dislocation in olivine, ringwoodite, and bridgmanite.

    table S2. Parameters used to compute the climb mobility laws for dislocation in olivine, ringwoodite, and bridgmanite.

    References (33, 34)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Sketch of the simulation box.
    • table S1. Parameters used to compute the glide mobility laws for dislocation in olivine, ringwoodite, and bridgmanite.
    • table S2. Parameters used to compute the climb mobility laws for dislocation in olivine, ringwoodite, and bridgmanite.
    • References (33, 34)

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