Quantifying the Influence of an Evolving Mineral Grain Size on the Characteristics of Mantle Flow

Abstract:

The evolution of the mineral grain size is a strong control on the deformation behavior of the Earth's interior. In particular, it provides a mechanism for localization of strain through the formation of shear zones with small grain sizes that facilitate rapid deformation. In addition, the Earth’s variable grain size influences seismic velocities of finite-frequency waves, which has important implications for inferring mantle temperatures from seismic velocities. However, the feedback between deformation and the micro-structure of the rock poses substantial challenges for computational models, and for that reason generally is not accounted for in convection simulations.

We have developed geodynamic models that combine insights from geodynamic modeling, mineral physics and seismic observations to better understand the spatially variable mineral grain size in the Earth’s mantle. Specifically, we will present two types of models, created using the community code ASPECT. (1) Time-dependent models of mantle flow below oceanic plates show how an evolving mineral grain size controls mantle rheology, the balance between diffusion and dislocation creep, and the distribution of deformation in dependence of plate age. (2) Instantaneous models based on a conversion of seismic velocities from tomography models to material properties relevant for geodynamic simulations can provide better constrains on both temperature and grain size if a variable, deformation-dependent grain size is taken into account.

Our models show that considering an evolving mineral grain size leads to deformation that is more localized and occurs at a shallower depth within the asthenosphere, just below the base of plates. These results are important for our understanding of how oceanic plates interact with the underlying mantle, and have implications for where fabric development is expected to occur. Comparing these model predictions to seismic observations can help provide better constraints on the rheology and characteristics of mantle flow.