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Size and composition of the MORB+OIB mantle reservoir
  • Albrecht W. Hofmann,
  • Cornelia Class,
  • Steven L. Goldstein
Albrecht W. Hofmann
Max Planck Institute for Chemistry

Corresponding Author:[email protected]

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Cornelia Class
Lamont-Doherty Earth Observatory
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Steven L. Goldstein
Columbia University
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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 67% 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, 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 (EDR) generated the present-day continental crust and the “residual mantle” MORB-OIB reservoir, which occupies at least 70% of the present-day mantle and is only moderately depleted in incompatible trace elements.