Link between Archaean continent formation and anomalous sub-continental mantle

The petrology and geochemistry of the subcontrnental mantle from 70 to 160 km depth under Archaean cratons, as sampled by kimberlites, differs from the mantle sources of mid-ocean-ridge basalts (MORB) and hotspot volcanics. Here I propose that both the formation of this anomalous mantle and the origin of large masses of tonalitic-granodioritic crust in the early Archaean can be explained by a single open-system fractional crystallization model. If an extensive, partially molten layer existed at shallow levels in the pre-Archaean due to the production of large amounts of MORB-type magma above sites of mantle upwelling, the magma at loci of fractional crystallization would reach a steady state in major- and trace-element compositions which would be similar to that of Archaean granodiorites and tonalites. While this fractional crystallization process operated, the cumulate would chemically resemble a hydrated tholeiite. Pressure-induced breakdown of amphibole could cause partial re-melting of this cumulate, and this could lead to depletion resulting in a sub-continental mantle keel with chemical characteristics similar to those observed.