Modelling Grain Size Evolution and its Role in Mantle Dynamics: From Small-scale Convection to Passive Margin Collapse

Dynamic models of Earth's lithosphere and convecting mantle often simplify the rheological behavior of mantle rocks, for example by assuming constant grain size or considering limited changes in material properties with mineral assemblage. While these simplifications reduce computational requirements, they neglect key processes such as shear localization and transient rheological behaviour associated with phase transitions, which can profoundly impact mantle flow patterns. As incorporating the effect of an evolving grain size in dynamic models has garnered more interest in the geodynamics community, there is a growing need for accurate, scalable, and computationally efficient approaches to address this complexity.

Here, we present recent advancements in the finite-element code ASPECT that address this challenge. These include a higher-order particle method for tracking grain size evolution and the integration of the ARKode solver library, which offers adaptive time-stepping for solving the ordinary differential equation governing grain size evolution. Our implementation captures the simultaneous and competing effects of different mechanisms affecting grain size, such as dynamic recrystallization driven by dislocation creep, grain growth in multiphase assemblages, Zener pinning, and recrystallisation at phase transitions.

We showcase three applications that highlight the importance of grain size evolution—and its interaction with stress and strain rate—for mantle dynamics: (i) global-scale mantle flow, (ii) small-scale convection beneath lithospheric plates, and (iii) the collapse of passive margins. Our models reveal that grain size evolution induces viscosity variations spanning several orders of magnitude, promoting strain localization in all three settings. It therefore controls the shape of upwellings and downwellings as well as the onset time of instabilities. For instance, beneath oceanic plates, the development of large grain sizes before the onset of convection, when strain rates are low, can delay the initiation of cold downwellings. These initial downwellings, in turn, reduce both grain size and viscosity at the base of the lithosphere, allowing subsequent cold drips to form at younger plate ages. Grain damage can also facilitate the collapse of a passive margin through grain size reduction in the lower parts of the lithosphere—but only within a specific range of grain size evolution parameters. Furthermore, additional weakening mechanisms are required for breaking the upper ≥25 km of the plate for subduction initiation to occur. These applications illustrate the applicability of our method to large-scale 2D and 3D models of the convecting mantle and lithosphere and emphasize the critical role of grain-scale processes in shaping the dynamics of Earth’s interior.