マグマの起源

抄録

Great advances in studies on the genesis of magma have come during the last ten years from progress in the fields of experimental petrology, geochemistry, and plate tectonics. In experimental petrology, the effects of pressure and of volatile components such as H₂O and CO₂ on the composition of magmas formed in the upper mantle have been determined : with increase of pressure magmas formed by partial melting of upper mantle peridotite become more silica-undersaturated and more enriched in olivine component ; in the presence of H₂O magmas become more silica-rich, whereas in the presence of CO₂ they become more silicaundersaturated. Primary magmas thus formed may change their compositions by fractional crystallization or by partial zone melting during their ascent. Recent research has been concentrated on the origin of several possible primary magmas. Beneath the mid-ocean ridges abyssal tholeiite magmas may be formed by a relatively high degree of partial melting of diapirs of upper mantle materials, with or without subsequent fractional crystallization. In orogenic belts calc-alkaline andesite magmas may be formed by melting of hydrous upper mantle, with or without subsequent crystallization. Kimberlite magma and some very low-silica magmas erupted in the continental areas may be formed in the upper mantle in the presence of CO₂. Major problems, however, remain unsolved, such as a mechanism for generation of plateau basalts and mechanisms to explain compositional variations in time and space in both orogenic belts and mid-ocean ridges. Other research areas indispensable to further understanding of magma genesis are knowledge of the physical properties, such as density and viscosity, of magmas at high pressures and knowledge of the partitioning of trace elements between crystals and melts under known conditions.