Size and Composition of the MORB plus OIB Mantle Reservoir

Most efforts to characterize the size and composition of the mantle that complements the continental crust have assumed that the mid-ocean ridge basalt (MORB) source is the incompatible-element depleted residue of continental crust extraction. The use of Nd isotopes to model this process led to the conclusion that the depleted MORB reservoir is confined to the upper similar to 30% of the mantle, leaving the lower mantle in a more primitive state. Here, we use Nb/U and Ta/U to evaluate mass and composition of the mantle reservoir residual to continent extraction and find that it exceeds 60% of the total mantle. Thus, the (Nb, Ta)/U-based mass balance conflicts with the epsilon(Nd)-based mass balance, and this invalidates the classical 3-reservoir silicate Earth model (continental crust, depleted mantle, and primitive mantle). Including the combined MORB + ocean island basalt (OIB) sources in the epsilon(Nd)-based mass balance does not reconcile the conflict as it would require their average epsilon(Nd) to be <= 3.0, much lower than observed MORB + OIB epsilon(Nd) averages. We resolve this conflict by invoking an additional, early enriched reservoir (EER), formed prior to extraction of significant continental crust, but now hidden or lost. This EER differs from EERs previously invoked by having no Nb-Ta anomaly. We suggest that it originated as an early mafic crust, which had unfractionated (Nb, Ta)/U but fractionated Sm/Nd ratios. The corresponding early depleted reservoir generated the present-day continental crust and the residual mantle MORB-OIB reservoir, which occupies at least 63% of the present-day mantle and is only moderately depleted in incompatible trace elements.

Automated summary

Efforts to characterize the mantle have led to conflicting conclusions about the composition and size of different reservoirs within it. While past models assumed that the depleted MORB reservoir is limited to the upper part of the mantle, new findings suggest it exceeds 60% of the total mantle. This conflict has invalidated the classical 3-reservoir Earth model and led to the proposal of an additional enriched reservoir to reconcile discrepancies in mass balance calculations.