This widely accepted model implies that mantle (or at least the upper mantle) is continually differentiating to form continental crust by a two-stage process. The crust formed is permanent and is not recycled back into the mantle.
(1) Primitive pyrolite mantle rises at mid-ocean ridges, melts to form basaltic ocean crust overlying refractory harzburgite plate.
(2) Plate sinks back into the mantle at subduction zones. Hydrated altered ocean crust dehydrates and causes melting of the basaltic ocean crust and of the overlying mantle wedge to yield andesitic magmas.
(3) Andesitic magmas fractionate en route to the surface to produce more siliceous magmas. Hence sialic crust accretes laterally at continental margins, is of low density and is indestructible.
|Fig. 19. Simple "box model" of mantle evolution, showing how melting at spreading ridges produces ocean crust, which is then altered by hydrothermal activity and then subducted. Part of this subducted crust is then melted to form continental crust, and the residues then subducted to become part of the reservoir of the depleted (DMM) mantle. Small degree melts migrate upwards to enrich the sub-continental mantle and provide the source for alkali basalts. Sediment subduction may modify the sub-continental lithosphere. (after Tarney et al. 1980)|
A consequence of course is that if the continental crust has been extracted from the convecting mantle, the convecting mantle must have become progressively depleted in lithophile elements. This is now known as the 'DM' mantle reservoir. This is the reservoir that supplies depleted mid-ocean ridge basalt ("MORB"). The real story is a little more complicated, as may be deduced form Fig. 19. Sediments may be subducted and contaminate the lithosphere under continental margins as well as the material stored at the 650 km discontinuity. Small degree melts permeate upwards and vein both the sub-continental and sub-oceanic lithosphere, but because the former is older, we generally observe more complex effects under the continents.