Size and Composition of the MORB+OIB Mantle Reservoir

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