Grain size plays a key role in controlling the mechanical properties of the Earth’s mantle, affecting both long-term flow patterns and anelasticity on the timescales of seismic wave propagation. In turn, the deformation in the Earth’s mantle also affects grain size evolution. 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.
In “The importance of grain size to mantle dynamics and seismological observations”, Dannberg et al. (2017) use the community mantle convection code ASPECT (Advanced Solver for Problems in Earth’s Convection) to study grain size evolution in the Earth's mantle. The presented geodynamic models include the simultaneous and competing effects of grain growth, dynamic recrystallization resulting from dislocation creep (decreasing the grain size), and recrystallization at phase transitions. They show that grain size evolution drastically affects both rheology and the dynamics of mantle convection. Changes in grain size alone can lead to lateral viscosity variations of six orders of magnitude in the upper mantle, and control the shape of upwellings and downwellings.
Positive feedback between grain size reduction and viscosity reduction results in shear localization, for example at the edges of mantle plumes and in a low-viscosity layer at the base of the lithosphere. Hence, viscosity at the edges of thermal plumes is lower than within, despite lower temperatures (bottom panel in Figure 1). Low temperatures and high stresses in and near to slabs result in small grains, and make slabs weaker than predicted in conventional mantle convection models. Slab material can have the same viscosity as the surrounding mantle despite lower temperatures, and mixing is faster than in models without grain size evolution (top panel in Figure 1).
In addition to the interplay of a dynamically evolving grain size with stress and strain rate in the convecting mantle, the study also investigates the influence of grain size on seismic velocities and attenuation. Using laboratory-derived scaling relationships, the authors convert the output of their geodynamic models to these seismically-observable quantities, allowing a comparison to Earth's observed 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. The authors also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth’s true temperature variations.
The CIG-supported ASPECT code, together with an extensive documentation, is available online under an Open Source license (https://geodynamics.org/cig/software/aspect/) and the modifications used in the study are available at: https://github.com/gassmoeller/aspect/tree/grain_size_dependent_rheology.
Dannberg, J., Eilon, Z., Faul, U., Gassmöller, R., Moulik, P., & Myhill, R. (2017). The importance of grain size to mantle dynamics and seismological observations. Geochemistry, Geophysics, Geosystems 18, 3034–3061, doi:10.1002/2017GC006944.
Shape and dynamics of subducting slabs (top) and mantle plumes (bottom) in models with dynamically evolving grain size. Snapshots show viscosity (left), grain size (center) and temperature (right).
(Top) White arrows mark three points in the evolution of the slab where its temperature, grain size and viscosity difference compared to the surrounding mantle are shown. While the temperature difference becomes smaller over time, the grain size difference increases. This means that the initially strong viscosity contrast between slab and mantle becomes smaller, until slab and mantle have the same viscosity even though the slab is still approximately 200 K colder.
(Bottom) Due to shear localization, plumes are narrow features and the viscosity at the edges of plumes is lower than within.