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Bowen worked out his ideas on the evolution of igneous rocks by doing laboratory experiments with artificial magmas. Bowen proposed (1922) that any one magma can potentially crystallize into rocks of different compositions because of two processes
1. REACTION - whether early formed minerals (higher in the reaction series) remain with the composition they first crystallized at, or react with the remaining magma and change composition dependent on cooling history. Rapid cooling prevents reaction, slow cooling allows reaction.
2. FRACTIONATION - whether early formed minerals remain in the magma, or are removed in some way before crystallization is complete.
Bowen's Reaction PrincipleThe premise behind Bowen's reaction principle is that minerals are stable only at the temperature of formation. Under conditions of rapid cooling the first minerals to form remain frozen in place in the final crystallized rock. But under condition of slow cooling the first formed minerals are unstable at the lower temperatures and so react with the remaining magma to form new minerals.
For example, with slow cooling olivine reacts with magma (or melt) to form pyroxene which reacts with melt to form amphibole. Or, calcium rich plagioclase reacts with the melt to form crystals more and more sodium rich. Thus, mineral composition and rock type will differ based on rate of cooling.
Bowen's Fractionation PrincipleThe fractionation principle is that if equilibrium is not maintained, if the melt cools too quickly for all the reactions to take place, or crystals are removed before they have time to react, then the melt can be divided into two fractions of different compositions. If, for example, we begin with an intermediate melt (e.g. rock diorite) and fractionate it, the original intermediate melt will be divided into one fraction more felsic rich than the original (e.g. granite), and a second, residue, fraction more mafic rich than the original (e.g. basalt).
The mechanisms of fractionation are best understood with phase diagrams, such as the solid solution and binary eutectic, discussed below. First some definitions.
A phase is anything that can be mechanically separated. For example, minerals in a rock are each different phases, and liquid and vapor are different phases. More importantly here, in a partially, or fractionally, melted rock the melt portion is one phase and the unmelted residue is a second phase.
Components are defined as chemical compounds. For example, H2O is a chemical component, although it can exist in 3 phases, ice, water, and vapor.
A variable is an environmental condition, such as temperature or pressure.
The solid solution phase diagram demonstrates how the metallic cations in a mineral can be partitioned into fractions. For example, all the ferromagnesium minerals (olivine, pyroxene, amphibole, biotite) are solid solutions of Mg and Fe. High temperature crystallization species are Mg rich, intermediate temperature species mixed Mg and Fe, and low temperature species Fe rich. Plagioclase is also a solid solution mineral with Ca at the high temperature end (high in BRS) and Na at the low temperature end (low in BRS).
The binary eutectic phase diagram demonstrates how two mixed and unrelated minerals can be fractionated. For example, amphibole and plagioclase are both found in diorite, but can be at least partially separated by fractionation.
The discussion below begins with analysis of the solid solution phase diagram for the plagioclase feldspars, and then proceeds to the binary eutectic phase diagram for two minerals like pyroxene and amphibole.
For each of the diagrams we will first lay out how to read the diagram, then give examples of the reaction behavior of the system under equilbrium conditions, and finally how fractionation can occur.
Reading the solid solution phase diagram (example plagioclase).
Reading the binary eutectic phase diagram (example mafic minerals).
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