Komatiites and the structure of the Archaean mantle

Abstract

The existence of Archaean komatiites with eruption temperatures greater than 1650°C requires that the mantle be vertically differentiated by the time of komatiite eruption. If in the unlikely event that undifferentiated mantle had survived primordial planetary differentiation and had been hot enough to deliver 1650°C komatiite, it would have been extensively molten to depths of ∼250 km, resulting in rapid, profound, vertical differentiation anyway. During primordial differentiation (or Archaean komatiite petrogenesis) the high density and compressibility of ultrabasic melt allowed storage of a global melt layer beneath a buoyant residue of dunite and/or harzburgite. This refractory cap segregated by extraction of melt both upwards and downwards from the depth at which the density contrast between crystals and liquid vanishes. Eruption of komatiite from the melt layer by corrosion of the cap was the Archaean earth's principal means of dissipating excess heat. This subterranean magma ocean precluded vertical homogenization of the Archaean mantle by convection but effectively absorbed lateral mantle heterogeneities and imposed the relative uniformity of maximum eruption temperature and MgO contents (∼32%) seen in primitive Archaean komatiites on all continents. Verification of the postulated density relations of liquids and crystals to 100 kbar becomes a pressing concern in view of the expected consequences these relations may have had.

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