Water and Magma

Abstract

Water has been continuously degassed from the Earth's interior by magmatism throughout evolution, and can significantly affect dynamic processes of its carrier, i.e., magmas, during their transport from the mantle to the Earth's surface. This paper summarizes the effects of water on the physical and thermodynamic properties of magmas, and their roles in magmatic processes. Magmas commonly contain at least 0.2 wt.% of water, and some magmas can have up to 6 wt. %. Despite the fact that water is a minor component in silicate liquids, the effects of dis-solved water on the properties of silicate melt are significant because it has a much lower molecular weight at 18.0 than those of the other components (SiO₂. 60.1, for example). Dissolved water greatly affects the density and the viscosity of silicate melts, thereby controlling rates of dynamic processes of magmas, such as segregation of primary melts in the mantle, transport of magmas from the mantle to the crust, convections and crystal-melt separation in crustal magma chambers, and ascent of magmas in volcanic conduits. Water also influences solid-melt thermodynamic equilibrium relationships, and this affects the chemical differentiation paths of magmas, in addition to the amount of melt production in the mantle by changing solidus temperatures. The eruptive behavior of volcanoes is driven by the exsolution of magmatic water, and as such depends on the water solubility of magmas mainly as a function of pressure. Water has also played important roles in the evolution of the Earth. Magma generation has been induced by water in the Earth's interior, and magmas have carried materials and energy from the interior to the surface of the Earth. In particular, water transport beneath an island arc is important in the global water cycle, and has greatly affected the environment of the Earth's surface.