Making and unmaking continental mantle: Geochemical and geophysical perspectives

Continents survive through time only if they have a thick, stiff, buoyant underlay (the “life raft”) of depleted SCLM; without this support, the weak continental crust will ultimately be “stirred” back into the convecting mantle. It is important to recognize that a buoyant lithospheric mantle requires not only a high Mg# (the result of melt extraction) but a low content of FeO (<8 wt%). Such FeO-depleted mantle is not produced by the melting processes that function on Earth today, e.g. at spreading ridges or beneath Large Igneous Provinces. Regardless of the degree of depletion, the FeO content of the residues of these “modern” processes stays constant at around 8%, and they are gravitationally unstable relative to the convecting mantle. Experimental studies suggest that such Fe-depleted mantle residues were produced by a specifically Archean process, involving high-temperature, high-degree melting at depths of >150 km. These residues range from dunite to harzburgite (olivine + orthopyroxene); the extracted magmas may have been broadly komatiitic in composition.