A global geochemical model for the evolution of the mantle

Basalt, eclogite, and harzburgite, differentiation products of the Earth, appear to be trapped in the upper mantle above the 670 km seismic discontinuity. It is proposed that the upper mantle transition region, 220 to 670 km, is composed of eclogite, or olivine eclogite, which has been derived from primitive mantle by about 20% partial melting and that this is the source and sink of oceanic lithosphere. The remainder of the upper mantle is garnet peridotite, or pyrolite, the source of continental basalts and hotspot magmas. This region is enriched in incompatible elements by partial melts or hydrous and CO_2 rich metasomatic fluids which have depleted the underlying layers in the L.I.L. elements and L.R.E.E. The eclogite layer is internally heated. It may control the convection pattern in the upper mantle. Material can only escape from this layer by melting. The insulating effect of thick continental lithosphere may lead to partial melting in both the peridotite and eclogite layers. Hotspots and ridges would then mark the former locations of continents. Most of the basaltic or pyroxenitic fraction of the oceanic lithosphere returns to the eclogite layer