Geophysical and geochemical models help constrain the relative importance of oceanic and continental crust recycling

The formation of enriched oceanic crust and depleted mantle at mid-oceanic ridge forms is a long-lived mechanism of differentiation affecting the structure and composition of the Earth's mantle. The recycling of dense oceanic crust is predicted to pool at the base of the mantle where it can contribute significantly to the formation of LLSVPs. Formation and recycling of oceanic crust is a primary source of and complements the role of continental crust extraction in the formation of the DMM, HIMU, and EMI endmembers in multiple lithophile isotope systems.

We will use new finite element models that build on earlier work (Brandenburg et al., EPSL, 2008) to quantify the role of oceanic crust formation and recycling on the structure of the Earth's mantle and the relative differences between upper mantle, lower mantle, and the core-mantle-boundary region. The new models are based on both the force-balanced plate formulation of Brandenburg et al. (2008) in cylindrical geometry that allow for modeling at Earth-like convective vigor, as measured by surface velocities and heat flow, and models using a yield-stress rheology in a spherical annulus that are close to Earth-like convective vigor.

Tomographic filtering shows that the structure of LLSVPs could be satisfactorily explained by oceanic crust recycling alone, but this does not rule out a role of dense heterogeneous material created during the formation of the Earth.

We record melting in the new models and use these for a new generation of geochemical models that track the lithophile and noble gas trace isotope evolution of the Earth's crust and mantle. New Hf and Nd isotopic studies suggest a significant role for oceanic crust recycling in addition to continental crust extraction and recycling. Trace element concentrations in the Earth's upper mantle appear to require on average more layering and recycling of slabs at shallower levels than was modeled in Brandenburg et al. (2008). Our models do no match the helium isotopic evolution of the mantle without taking into account extant sources such as interplanetary dust particles or core outgassing.