Fractional crystallization (geology) 5782810 212224881 2008-05-13T22:54:44Z PipepBot 4984067 robot Adding: [[it:Cristallizzazione frazionata]] '''Fractional crystallization''' is one of the most important geochemical and physical processes operating within the Earth's [[crust (geology)|crust]] and [[Earth's mantle|mantle]]. Fractional crystallization is the removal and segregation from a melt of [[mineral]] precipitates; except in special cases, removal of the crystals changes the major element composition of the magma. Fractional crystallization in silicate melts ([[magma]]s) is complex compared to crystallization in chemical systems at constant pressure and composition, because changes in pressure and composition can have dramatic effects on magma evolution. Addition and loss of water, carbon dioxide, hydrogen, and oxygen are among the compositional changes that must be considered. For example, the [[partial pressure]] ([[fugacity]]) of water in silicate melts can be of prime importance, as in near-[[solidus (chemistry)|solidus]] crystallization of magmas of [[granite]] composition. The crystallization sequence of [[oxide]] minerals such as [[magnetite]] and [[ulvospinel]] is sensitive to the [[mineral redox buffer|oxygen fugacity]] of melts, and separation of the oxide phases can be an important control of [[silica]] concentration in the evolving magma, and may be important in [[andesite]] genesis. Experiments have provided many examples of the complexities that control which mineral is crystallized first as the melt cools down past the [[liquidus]]. One example concerns crystallization of melts that crystallize to [[mafic]] and [[ultramafic]] rocks. MgO and SiO<sub>2</sub> concentrations in melts are among the variables that determine whether [[forsterite]] [[olivine]] or [[enstatite]] [[pyroxene]] is precipitated, but the water content and pressure are also important. In some compositions, at high pressures without water crystallization of enstatite is favored, but in the presence of water at high pressures, olivine is favored. Granitic magmas provide additional examples of how melts of generally similar composition and temperature but at different pressure may crystallize different minerals. Pressure determines the maximum water content of a magma of granite composition. High-temperature fractional crystallization of relatively water-poor [[granite]] magmas may produce single-[[feldspar|alkali-feldspar]] granite, and lower-temperature crystallization of relatively water-rich magma may produce two-feldspar granite. During the process of fractional crystallization, melts become enriched in [[incompatible element]]s. Hence, knowledge of the crystallization sequence is critical in understanding how melt compositions evolve. Textures of rocks provide insights, as documented in the early 1900's by [[Bowen's reaction series]]. Experimentally-determined [[phase diagram]]s for simple mixtures provide insights into general principles. Numerical calculations with software like that referenced below have become increasing able to simulate natural processes accurately. * Main article: [[Igneous differentiation]] ==See also== * [[Fractional crystallization (chemistry)]] * [[Flow banding]] * [[Ultramafic to mafic layered intrusions]] * [[Cumulate rocks]] * Software for modeling of phase equilibria in magmatic systems: [http://geo.web.ru/~ariskin/soft.html-id=comagmat.htm COMAGMAT], [http://geo.web.ru/~kbs/index.html COMAGMAT online-version], [http://melts.ofm-research.org/ MELTS] [[Category:Igneous rocks]] [[Category:Petrology]] [[it:Cristallizzazione frazionata]] [[nl:Kristalfractionatie]]