Bowen’s Reaction Series

Bowen’s reaction series is a complex series of chemical reactions during the process of cooling and solidification of magmatic material. The cooling and crystallization of magma is a complex chemical process in which various silicate minerals crystallize in definite order. The sequence of separation of minerals from a silicate melt is now well established. If a mineral remained in the melt after it crystallized, would react with the remaining melt and produce the next mineral in the sequence. While studying the crystallization of cooling silicate melts, N.L Bowen discovered two reaction series: (1) continuous reaction series, and (2) discontinuous reaction series.

Continuous Reaction Series:

Minerals of a solid solution series generally form a continuous reaction series. The plagioclase felspars with end members anorthite and albite, exhibit such a reaction. In a cooling melt An-rich plagioclases crystallize before the Ab-rich members. With falling temperature there is a continuous reaction between the melt and precipitated crystals and thus the composition of crystals is continuously being changed. Such a solid solution series in which the variation in composition is perfectly continuous, is called ”Continuous Reaction Series”. If chemical equilibrium is not maintained between melt and crystals during cooling , the resulting crystals may show compositional zoning.

Discontinuous Reaction Series:

Discontinuous reactions series are those which occur at fairly definite temperatures. Such a reaction takes place in a cooling magma at a certain lower temperature when an early crystallized mineral reacts with the magma to form a new mineral of different composition. A series of Mg-Fe rich minerals that crystallize from a basaltic magma are found to be related to each other by the discontinuous reaction. Such minerals when arranged in a proper order constitute a “discontinuous reaction series”. For example, early formed olivine may react with the melt at a lower temperature to form the pyroxene, then the pyroxene may transform into the amphibole, and biotite may form as a reaction product of earlier crystallized amphibole.

The two adjacent minerals of a discontinuous reaction series are said to form a “reaction pair”. For example, olivine and pyroxene together form a reaction pair. Minerals of the discontinuous series often exhibit incongruent melting.

A criterion which is common to both the continuous and discontinuous reaction series, is the tendency of one mineral to grow around another. In case of the continuous series this commonly forms reaction rims, coronas, etc.

Bowen reaction series is illustrated in the above diagram. At the upper end where mixtures are more basic, there are two distinct reaction series: (i) continuous series of the plageioclases, and (ii) the discontinuous series, Olivine-Pyroxenes-Amphiboles etc. Towars the lower end they become less distinct and finally merge into a single series.

The reaction series suggests that as a magma of basaltic compostion cools, olivine and An-rich plageioclase may crystallize first. If these minerals remain in contact with magma, they will tend to change into pyroxene and more Ab-rich plageoclase, and the resulting rock will be gabbro or a basalt. If, however, most of the early olivine and An-rich plageoclase is removed, as by gravity settling, the composition of the remaining melt will tend to become enriched in Si, Al, Fe⁺² , K, Na and H₂ O, and CO_2. Such melt would produce a mineral assemblage consisting mainly of amphiboles, some mica, alkali felspar and quartz. It should be noted that these minerals are relatively low temperature, late stage crystallization products.

Related Posts:

1. Introduction to rocks
2. Igneous Rocks and their types
3. Chemical Composition of Igneous Rocks
4. Occurrence of Igneous Rocks
5. The Texture of Igneous Rocks
6. Structures of Igneous Rocks
7. Different Forms of Igneous Bodies/Material
8. Mechanics of Intrusion
9. Formation of Igneous Rocks
10. Origin of Igneous Rocks
11. Magmatic Differentiation
12. Assimilation of crustal rocks in Magma (Crustal Assimilation)

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