Constraints on Primordial Heterogeneity Preservation in the Lower Mantle and Implications for Global-Scale Mantle Dynamics

The composition and structure of the mantle are shaped by the evolution of the Earth's interior, but also control its dynamics through time. One question that receives particular attention is whether the lower mantle is heterogeneous in composition on a large scale, and what such heterogeneity would imply for the evolution of mantle convection and plate tectonics. Recent geodynamic models suggest that intrinsically viscous domains (e.g., enriched in the strong mineral bridgmanite) in the lower mantle can remain unmixed by whole-mantle convection for longer than the age of the Earth, potentially reconciling recent seismic observations of mantle heterogeneity and geochemical evidence for primordial-material preservation^[1]. While this regime has been successfully reproduced in 2D spherical annulus convection models with composition-dependent rheology^[2], the effects of rheological and tectonic parameters upon primordial heterogeneity preservation, as well as those of internal heating, remain to be tested.

In this study, we use state-of-the-art 2D numerical models in a spherical annulus geometry to provide a more rigorous assessment of primordial-material preservation in a convective mantle. We explore the effects of rheological parameters such as lower-mantle viscosity and its temperature/pressure dependency. We also investigate the effects of lithospheric yield strength, focusing on models that display plate-like tectonic behavior, and of the distribution of radioactive internal heat sources on mantle mixing. In this parameter space, we define the physical requirements for primordial-material preservation and put the model predictions in context with recent discoveries from seismology and geochemistry. For example, we compare the predicted wavelengths of thermal and compositional anomalies with those inferred from seismic tomography. In the future, we aim at constraining our models by rigorous comparison with geophysical constraints, including the geoid and dynamic topography.

^[1] Ballmer et al. (2017): doi:10.1038/ngeo2898 ^[2] Gebhardt, D., Gülcher, A. J. P., Ballmer, M.D. and Tackley, P.J.: Variable dynamic styles of primordial heterogeneity preservation in Earth's lower mantle. EPSL (in review)