The importance of grain size to mantle dynamics and seismological observations

Juliane Dannberg, Zach Eilon, Ulrich Faul, Rene Gassmoeller, Pritwiraj Moulik, Robert Myhill

Research output: Contribution to journalArticle (Academic Journal)peer-review

26 Citations (Scopus)
453 Downloads (Pure)

Abstract

Grain size plays a key role in controlling the mechanical properties of the Earth's mantle, affecting both long-time-scale flow patterns and anelasticity on the time scales of seismic wave propagation. However, dynamic models of Earth's convecting mantle usually implement flow laws with constant grain size, stress-independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of 6 orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory-derived scaling relationships, we convert model output to seismologically observable parameters (velocity and attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earth's attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our reexamination of high-pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth's true temperature variations.
Original languageEnglish
Pages (from-to)3034-3061
Number of pages28
JournalGeochemistry, Geophysics, Geosystems
Volume18
Issue number8
Early online date4 Aug 2017
DOIs
Publication statusPublished - 13 Aug 2017

Keywords

  • Grain size evolution
  • Mantle Convection
  • geodynamic model
  • seismic attenuation and anelasticity
  • translationg dynamic models to seismic structure

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