Size and Composition of the MORB+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 ∼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 ε(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 ε(Nd)‐based mass balance does not reconcile the conflict as it would require their average ε(Nd) to be ≤3.0, much lower than observed MORB + OIB ε(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. Plain Language Summary The Earth's continental crust makes up only about half a percent of Earth's mass, but it contains a large portion of its total budget of uranium and thorium, which produce much of Earth's interior heat. In making the crust, these elements have been extracted via melts and volcanism from Earth's mantle. But what portion of the mantle was involved in making the continents? Previously, geochemists concluded that only its uppermost 30% was involved, leaving the lower two‐thirds of the mantle essentially untouched. The measure used for this estimate has been the difference in the isotope ratios of neodymium, 143Nd/144Nd, between crust and mantle. However, when we use an alternative measure for the same calculation, namely the ratio of niobium to uranium, Nb/U, we find the depleted mantle fracti