Archean mantle fractionation

Nd isotopic data require that most Archean igneous rocks are derived from depleted or undepleted mantle sources while geochemical model stuides necessitate enriched sources for most Archean tonalites. This apparent contradiction can be reconciled if the enriched sources are produced by short‐lived (≤150 m.y.) enrichment of depleted sources. Extensive depleted mantle reservoirs were probably produced during early degassing of the upper mantle (≥4 4 b.y. ago) and complementary enriched reservoirs collected at the base of the lithosphere. Sources for tonalites may be produced from short‐lived enrichment of basalt above convective downcurrents by metasomatic and magmatic components derived chiefly from depleted mantle and by degassing of sinking, short‐lived komatiitic lithosphere.