It is proposed that parts of the Archean mantle, probably mainly beneath centers of continents, remained volatile-poor from the primordial degassing of Earth. Melting temperatues (Tm) were hundreds of degrees higher in dry parts of the mantle than in wet parts. Effective viscosity in the mantle is related to T/Tm. Thus, in dry parts of the Archean mantle the lithosphere was almost two times thicker than would otherwise have been the case. Genesis of komatiites (ultramafic volcanics) was facilitated because the geotherm remained conductive (“steep”) to higher temperatures before turning convective (approximately adiabatic). Partial melts of dry parts of the mantle left more pyroxene in their residua than would otherwise have been the case, leading to two further advantages for komatiite petrogenesis: the melts had higher MgO contents (due to higher normative olivine/pyroxene ratios), and dT/df was smaller during melting. Gradual homogenization of the mantle with reference to volatiles, even more than diminishment of radiogenic heating, probably caused the dwindling of ultramafic volcanism about 2700 Ma.

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