[S15-1] Structure and dynamics of the mid mantle

[S15-1-02] Large-scale compositional heterogeneity in the Earth's mantle

^○Maxim Ballmer (ETH Zurich, Zurich, Switzerland)

invited

Seismic imaging of subducted Farallon and Tethys lithosphere in the lower mantle has been taken as evidence for whole-mantle convection, and efficient mantle mixing. However, cosmochemical constraints point to a lower-mantle composition that has a lower Mg/Si compared to upper-mantle pyrolite. Moreover, geochemical signatures of magmatic rocks indicate the long-term persistence of primordial reservoirs somewhere in the mantle. In this presentation, I use numerical models to establish geodynamic mechanisms for sustaining large-scale heterogeneity in the Earth's mantle. Mantle flow is controlled by rock density and viscosity. Variations in intrinsic rock density, such as due to variations in basalt or iron content, can induce layering or partial layering in the mantle. Layering can be sustained in the presence of persistent whole mantle convection due to active “unmixing" of heterogeneity in low-viscosity domains, e.g. in the transition zone or near the core-mantle boundary [1]. On the other hand, lateral variations in intrinsic rock viscosity, such as due to variations in Mg/Si, can strongly affect the mixing timescales of the mantle. In the extreme case, intrinsically strong lateral heterogeneity may remain unmixed through the age of the Earth, and persist as large-scale domains in the mid-mantle due to focusing of deformation along weak conveyor belts [2]. That large-scale lateral heterogeneity and/or layering can persist in the presence of whole-mantle convection can explain the stagnation of some slabs, as well as the deflection of some plumes, in the mid-mantle [1,2]. These findings indeed motivate new seismic studies for rigorous testing of model predictions.
[1] Ballmer, M. D., N. C. Schmerr, T. Nakagawa, and J. Ritsema (2015), Science Advances, doi:10.1126/sciadv.1500815.
[2] Ballmer, M. D., C. Houser, J. W. Hernlund, R. Wentzcovitch, and K. Hirose (2017), Nature Geoscience, in press.