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Magmatic differentiation

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Magmatic Differentiation

The different types of magmas produced in the different tectonic regimes, are dependent on three factors: source rock composition, melting conditions and processes, and crystallization conditions and processes.  The composition of the source rock, the degree or extent to which the source rock is melted, the pressure and and temperature conditions under which melting takes place, and the processes by which melting and separation of the melt take place may affect the composition of the melt or magma produced.  Many of these same variables will also effect crystallization.  The conditions and processes of melting and crystallization may change over time.  Due to the large number of variables, a wide range of igneous rocks compositions and textures are possible.   

As a magma crystallizes, the magma becomes depleted in the elements that are entering the crystallizing minerals and so the melt changes composition over time.  Magmatic differentiation is the process whereby one parent magma composition can produce a number of igneous rock compositions through mineral crystallization.  As a cooling melt changes composition, the minerals that are in equilibrium with it (i.e. that are stable in the melt at the temperature and pressure conditions of crystallization) typically either change composition and/or change to structurally-more complex minerals as in Bowen's Reaction Series.  Magmatic differentiation occurs through equilibrium crystallization or fractional crystallization or most commonly some crystallization scheme intermediate between the two end member processes.

Bowen's reaction series

Bronzite, Augite, and Spinel Equilibrium crystallization is the process whereby every crystallizing mineral has an opportunity to react with the residual melt and change composition or mineral structure to remain in equilibrium with the melt.  Fractional crystallization is the process whereby crystals once produced are instantly isolated from the melt and prevented from equilibrating with the liquid from which they crystallized, resulting in a series of residual liquids of more extreme compositions than would have resulted from equilibrium crystallization.  In reality, the crystals in most crystallizing magmas are neither perfectly in equilibrium with the residual melt or perfectly isolated from reaction with the melt immediately after crystallization.  

Magmas also change composition as a result of assimilation, magma mixing, or separating into immiscible liquids.  These processes all occur as a result of diffusion of elements or between the magma and other materials with different compositions.  Assimilation is a process whereby magma composition changes as a result of the incorporation and digestion of wall or country rock in the  magma.  If a high temperature magma intrudes a shale (an aluminous clay mineral-rich sedimentary rock), partial or total melting of the shale in contact with the magma, may occur.  The aluminum content of the magma would increase (the aluminum content in the shale far exceeds the aluminum content in the original magma composition) and aluminum-rich minerals such as garnet, corundum, cordierite, and muscovite may occur.  Magma mixing occurs when two or more magmas of different compositions mix, or partially mix to produce a magma of a different composition.  Magmatic differentiation may produce a melt that separates into two or more immiscible fluids. The most common types of immiscible geologic melts are sulfide-rich and silicate-rich melts.  The first geologic evidence for the existence of immiscible fluids was discovered in lunar rocks!



Frequently used abbreviations: NPL  Non-vertebrate Paleontology Laboratory | TNSC Texas Natural Science Center | UTDGS Department of Geological Sciences | BEG  Bureau of Economic Geology | VPL Vertebrate Paleontology Laboratory | JSG  Jackson School of Geosciences | SUPPORT | VOLUNTEER | GLOSSARY

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