The importance of grain size to mantle dynamics and seismological observations

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.

Rates of Dynamic Recrystallization in Geologic Materials

“…Thus, DRX has long been considered an essential process for the accumulation of large plastic strains in the Earth's crust and mantle (Etheridge & Wilkie, 1979;Montési & Hirth, 2003;Platt & Behr, 2011;Poirier, 1980;Tullis & Yund, 1985;White et al, 1980). More recently, it has also been argued that DRX and grain size evolution play a key role in other geodynamic phenomena, including mantle convection (Barr & McKinnon, 2007;Dannberg et al, 2017;Hall & Parmentier, 2003;Rozel, 2012), mantle plume dynamics (Korenaga, 2005), intermediate-depth earthquake nucleation (Thielmann et al, 2015), passive margin collapse (Mulyukova & Bercovici, 2018), and plate boundary formation (e.g., Bercovici & Ricard, 2012).…”

Inferences of Mantle Viscosity Based on Ice Age Data Sets: The Bias in Radial Viscosity Profiles Due to the Neglect of Laterally Heterogeneous Viscosity Structure

“…There is strong evidence that both the thickness of the lithosphere (e.g., Artemieva, 2009;Conrad & Lithgow-Bertelloni, 2006;Tesauro et al, 2012;Zhong et al, 2003) and mantle viscosity vary globally, with the latter being characterized by variations of many orders of magnitude (e.g., Ammann et al, 2009;Barnhoorn et al, 2011;Dannberg et al, 2017). For instance, Figure 1a shows the lithospheric thickness model (Conrad & Lithgow-Bertelloni, 2006) that we adopt throughout this study.…”

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

“…Note that there is considerable variability between models. In the upper mantle, mineral grain size is controlled by the balance between static grain growth and bulk strain (e.g., Dannberg et al, 2017). The lower right panel shows the median resistivity profiles from the inverse solution of Murphy and Egbert (2017) and from the refined resistivity solutions that are shown in Figure 6.…”

“…However, regardless of the exact model chosen for the anelastic contribution to seismic observables, grain size is an important parameter. In the upper mantle, mineral grain size is controlled by the balance between static grain growth and bulk strain (e.g., Dannberg et al, 2017). At elevated temperatures, crystal grains tend to grow in order to minimize the Gibbs free energy of their surface interfaces (Evans et al, 2001).…”

“…It is worth noting that, although some geodynamic studies have acknowledged the importance of anelasticity in assimilating seismic velocity observations into numerical models (e.g., Dannberg et al, 2017), the intrinsic ambiguity in mapping seismic velocity anomalies to temperature fields appears to be underappreciated and too often ignored in geodynamic modeling. Our results here demonstrate the need for care in mapping seismic results to temperature.…”