6.3.2: Bowen’s Reaction Series

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N. L. Bowen pioneered in the study of magma crystallization, for which he received the Roebling Medal from the Mineralogical Society of America in 1950. By studying naturally occurring igneous rocks and conducting laboratory experiments, he derived an idealized model for equilibrium crystallization in a magmatic system. We call the model Bowen’s reaction series; it is depicted below in Figure 6.17. Although Bowen’s series is a generalization that does not apply in detail to all magma types, it is an excellent model to describe the process of crystallization. Some petrologists have developed more precise models for magmas of specific compositions.

6.17.png — Figure 6.17: Bowen’s Reaction Series

Bowen’s reaction series shows the (hypothetical) order in which minerals crystallize from cooling magma. It contains nine mineral names arranged in a Y-shape. We call the left-hand side of the Y the discontinuous side because abrupt changes occur as different minerals crystallize in sequence. We call the right-hand side the continuous side because plagioclase is continually present during crystallization, starting as Ca-rich plagioclase at high temperature and changing to Na-rich plagioclase as cooling progresses. The minerals at the bottom of Bowen’s series crystallize at the lowest temperatures. Although not evident from this diagram, just like plagioclase, minerals on the discontinuous side of the series change composition as cooling proceeds. At higher temperatures, for example, olivine usually has a greater magnesium to iron ratio (Mg:Fe) than at lower temperatures.

Mineral crystallization temperatures depend on mineral composition. So, Bowen’s series reflects general trends in mineral chemistry. Minerals at the top of the series (olivine, pyroxene, and calcium-rich plagioclase) are mafic (relatively silica poor). Those at the bottom are silicic (relatively silica rich). Silica content is the most significant factor controlling melting and crystallization temperatures. The mafic minerals at the top of the discontinuous series also are deficient in aluminum and alkalis and rich in iron and magnesium, compared with minerals at the bottom.

Bowen’s Reaction Series is not meant to imply that all magmas start out crystallizing olivine and end up crystallizing quartz. Consider, for example, a hypothetical magma that is 100% SiO₂. It cannot crystallize any minerals in Bowen’s reaction series except quartz because it does not contain the necessary elements. It will skip all the other minerals and just form quartz.

On the other hand, if a melt were 100% Mg₂SiO₄, it would crystallize forsterite (olivine of composition Mg₂SiO₄) and be completely solidified at high temperature. These two extreme examples do not exist in nature, but compositions of natural magmas do control the extent to which crystallization follows Bowen’s reaction series and which minerals crystallize first (and last). And, although all magmas crystallize different minerals at different temperatures, none follow the complete series.

Bowen’s Reaction Series is a convenient way to remember the minerals that are common in different kinds of rocks. Mafic magmas, which crystallize at high temperature, produce rocks containing minerals at the top of the series. Silicic magmas, which crystallize at lower temperatures, produce rocks that contain minerals at the bottom of the series. So mafic rocks such as basalt or gabbro commonly contain olivine, pyroxene and Ca-rich plagioclase. Felsic rocks such as rhyolite or granite are generally rich in K-feldspar and quartz. And, intermediate composition rocks may contain pyroxene, amphibole, or biotite with plagioclase.