The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks

Abstract

Recent phase equilibria studies, and the concepts of basic magma evolution to which they have given rise, are reviewed. The variations with pressure of liquid compositions in equilibrium with peridotite are summarised, and their bearing on the origins of peridotite nodules in basalt and upon upper mantle mineralogy are discussed.The origin of observed concentrations of the elements K, Ti, P from source rocks containing only very low concentrations of these elements is readily explained by ca. 15% partial melting combined with variable eclogite fractionation and up to 50% further extraction of olivine, yielding optional concentration factors from 8 to 70. Orthopyroxene fractionation cannot explain these effects. There is no to 50% further extraction of olivine, yielding optional concentration factors of from 8–70. Orthopyroxene fractionation cannot explain these effects. There is no need for wall-rock reaction hypotheses which are difficult to envisage operating throughout the long history of a particular volcano.A revised closed system model of basalt magma evolution is presented, based upon these controlling factors; depth of partial melting, extent of partial melting, speed of movement towards the surface. The source rock of basic magmas at depths greater than ca. 60 km is probably a garnet-two pyroxenes-peridotite (garnet-lherzolite), which on partial melting yields and garnet-enstatite-peridotite (garnet-harzburgite), not two pyroxenes-peridotite (lherzolite) residua. The pyroxenite stage of basalt evolution, like the eclogite stage, is located at temperatures below the garnet- or spinel-lherzolite solidus and only affects slow moving magma batches.Six hypotheses for the origin of the calc-alkali series are reviewed. The fractionation of water-enriched high-alumina basalt at relatively high partial pressures of oxygen is concluded to be the mechanism in best accord with available facts.Fig.4–11 and their captions contain material and arguments not duplicated in the text. The figures, together with the conclusions section summarise the main themes of this paper.