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Continental lithosphere is a sandwich of two layers, each composed of materials that are rare in the upper parts of the Earth. Continental crust consists of low-temperature distillates produced during a succession of melting events; the underlying lithosphere is a remarkably pure concentrate of high-temperature, highly refractory minerals. Material with intermediate compositions, which should have been far more abundant, is missing. The two dominant components of Archaean lithospheric mantle, olivine with 92–94% forsterite and similarly magnesian orthopyroxene, form only a small proportion of the residue of large-scale mantle melting. Their accumulation to form the lithosphere requires efficient sorting, to separate them from the products of lower-degree melting. This sorting, which is driven by the buoyancy of these low-density phases and their high viscosity, takes place during: (1) plume ascent through the segregation of residues of high- and low-degree melting; (2) recycling of the residues through the mantle; (3) crystallization in the crust of a Hadean magma ocean. The higher-than-normal orthopyroxene content in the Kaapvaal and other cratons is due in part to density sorting and in part to exsolution of majorite, a residual phase during komatiite melting. Secular variation in lithosphere composition (a decrease in the proportion of magnesian olivine and orthopyroxene and an increase in clinopyroxene and spinel) reflects a progressive decline in high-degree melting, a consequence of falling mantle temperatures.

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