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Differentiation - GLY 421 Igneous Metamorphic Petrology...

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GLY 421: Igneous & Metamorphic Petrology El-Shazly, A. K., 2004 1 Magmatic Differentiation Differentiation is the process by which magmas evolve to give rise to a variety of magmas and rock types (that have different compositions). Therefore, certain physical processes are required to cause the chemical diversification of a magma (i.e. its differentiation). The chemical trends of magmatic differentiation are often determined by studying crystal - liquid relations, but the degree or extent of differentiation is controlled by the efficiency of the differentiation mechanism. In this chapter, we will examine the mechanisms of magmatic differentiation, having to some extent, covered their effects in the last two chapters (crystallization paths: crystal-liquid equilibria, and chemical effects: variation diagrams). We will then briefly discuss the application of trace element, and stable and radiogenic isotope geochemistry in identifying some of the mechanisms of differentiation. Mechanisms of magma diversification (differentiation): 1- Partial melting (to produce different magmas) 2- Crystal fractionation 3- Thermogravitational diffusion 4- Liquid immiscibility 5- Vapor transport 6- Magma mixing 7- Assimilation 1- Partial Melting: (a) Equilibrium partial melting: Partially melting different rock types could be an effective way for producing a variety of magmas of different compositions. A series of liquids produced by successive stages of equilibrium partial melting will upon crystallization produce a sequence of rocks that is the exact opposite of that produced by fractional crystallization. Telling these two mechanisms apart is fairly simple through the use of trace element geochemistry (see later). (b) Fractional melting or incremental batch melting: This is a much more efficient mechanism of differentiation which will depend largely on the frequency at which the liquid is removed from the system. Keep in mind that fractional melting is not a continuous process, and will be arrested for a while as soon as one of the phases is completely used up in the liquid. It will resume once the T is high enough to melt the mixture of the remaining solid phases. The resulting magmas will therefore show sharp differences in composition (as opposed to the gradual compositional changes observed in the case of equilibrium partial melting. (c) Zone melting: Consider a magma chamber undergoing cooling and crystallization from the bottom upward (perhaps because the H 2 O pressure at the bottom of the chamber is lower than at the top, causing the liquidus T to be suppressed at the top, hence delaying crystallization). As the magma crystallizes at the base, the heat of crystallization
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GLY 421: Igneous & Metamorphic Petrology El-Shazly, A. K., 2004 2 released may cause the roof of the magma chamber to partially melt. Accordingly, the melt will appear as if it is migrating upwards. As this happens, elements will be fractionated between the crystals forming at the bottom of the chamber and the melt
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