Geophysics of the continental mantle: an historical perspective

Abstract

The continental lithosphere is a small fraction of the Earth and cannot be understood in isolation from the rest of the planet. It is at the top of the mantle and, therefore, is probably buoyant with respect to the bulk of the upper mantle in spite of the fact that it is also the coldest part of the mantle. The shield or cratonic lithosphere is ancient. Because of its long-term stability it is probably deficient in garnet, the densest abundant mantle mineral, and/or FeO relative to the rest of the upper mantle. It has high seismic velocities; this in addition to its anisotropy favours a high olivine, possibly forsterite-rich, content. High temperatures, high pyroxene contents, and high FeO contents all serve to decrease the seismic velocities. Since melts are enriched in CaO, Al_2O_3, and FeO compared to their source rocks the immediate implication of long-term buoyancy is that the cratonic lithosphere represents, at least in part, the refractory, buoyant residue, i.e. olivine and orthopyroxene, of mantle differentiation processes. In so far as it is related to the overlying continental crust, at least in age, it may be a product of the differentiation of the whole mantle rather than just the upper mantle. The argument here is that the continental crust is so enriched in the most incompatible elements that it would require efficient processing of the whole mantle in order to obtain the observed concentrations.

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