The rate of mantle processing controls mantle-crust evolution

The formation and recycling of oceanic crust back into Earth’s mantle is the largest material flux on Earth. It determines the outgoing flux of incompatible elements into the oceanic, and ultimately, the continental crust, and thus controls the rate of incompatible element depletion of Earth’s mantle. We developed a five-reservoir box-model to investigate the Sm-Nd and Lu-Hf isotope evolution of Earth’s mantle and crust. The model includes an average mantle reservoir, an oceanic and continental crust reservoir, and two isolated reservoirs that store the recycled oceanic and/or continental crust before being recycled into and homogenized with the mantle reservoir. We evaluate the elemental mass fluxes needed to fit the Nd-Hf isotope ratios in the mantle and crust, with a particular focus on the effect of the mantle processing rate, which scales with the rate of oceanic crust formation, on mantle-crust evolution. We also assess the variable timescales for homogenization of the recycled crust within the mantle. The results show that basaltic crust generation causes moderate to extreme incompatible element depletion starting within the first 100′s of millions of years of Earth’s evolution, depending on how vigorously the Hadean mantle convected, and how effectively the recycled crust was homogenized with the mantle. According to our results, ca. 2.5–7.5% of the total mass of the mantle today is recycled crust. Intriguingly, this amount of recycled crust is comparable to the estimated mass of Large Low Shear Velocity Provinces (LLSVPs) above the core-mantle boundary. It is unlikely, however, that LLSVPs are entirely formed by recycled crust, because this amount of recycled crust contains up to 70% of the total mass of incompatible elements like Nd and Hf in the bulk silicate Earth. For more incompatible elements (e.g., La, Th, Ba), this mass fraction is even higher and results in near-total depletion of these elements in the peridotitic mantle. Hence, LLSVPs may not only consist of recycled crust and/or they are dynamic reservoirs whose fluctuating size (mass) is determined by the balance between the rate of the in- and outgoing mass fluxes