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The Bowen’s Reaction Series is a concept in geology that explains the order in which minerals crystallize from cooling magma. The series was developed by Canadian geologist Norman L. Bowen in the early 20th century. Bowen began a series of laboratory experiments to establish the sequence of silicate minerals crystallization from a melt. To carry out this experiment he melted powdered mafic igneous rock by raising its temperature to about 1280°C to make artificial magma. Then he let the melt slowly cool to a preset temperature and pressure. The same temperature and pressure were maintained for a month to cause part of it to crystallize. This created a mixture of crystals and melt. Finally, he “quenched” the remaining melt by submerging it quickly in cold mercury. The remaining melt is converted into glass by quenching. The glass trapped the earlier-formed crystals within it. The mineral crystals formed before quenching were identified under a microscope, and the chemical composition of the remaining glass was analyzed.
This experiment was repeated at different temperatures to form crystals before quick cooling. Bowen noticed that as new crystal form, it preferentially extracts certain chemical from the melt. Hence, the chemical composition of the residual melt gradually changes as the melt cools. Bowen identified the specific sequence of reactions crystallizing minerals from a cooling magma that was initially mafic. This sequence is now called Bowen’s reaction series in his honor.
Bowen’s reaction series
The order in which minerals crystallize is called the Bowen reaction series.
A series of minerals in which any early-formed mineral phase tends to react with the residual melt, later in the differentiation, to yield a new mineral further down in the series; e.g. early-formed crystals of olivine react with later liquids to form pyroxene crystals, and these, in turn, may further react with still later liquids to form amphiboles. There are two different series, a continuous reaction series and a discontinuous reaction series.
Discontinuous reaction series
A reaction series in which the reaction of early-formed crystals with later liquid represents an abrupt phase change resulting in one mineral changes to another over specific temperature ranges e.g., the minerals olivine, pyroxene, amphibole, and biotite form a discontinuous reaction series (Figure 1).
In the beginning, as the temperature decreases, a temperature range is reached where a specific mineral starts to crystallize. The earlier formed mineral in the sequence reacts with the residual magma (the melt) to form the next mineral in the sequence. Olivine [(Mg,Fe)2SiO4], for example, is the first ferromagnesian silicate to crystallize from basic magma. As the magma cools further, it reaches the temperature range at which pyroxene is stable; a reaction happens between the olivine and the remaining melt, and pyroxene forms.
Continued cooling causes pyroxene-melt reactions to rearrange the pyroxene structure and form amphibole. Upon further cooling, the amphibole and melt react and reorganize their structure to form a layered structure of biotite mica. The reactions described here tend to transform one mineral to the next, but the reactions are not always complete. For example, olivine may have pyroxene rims, indicating an incomplete reaction. When magma cools rapidly, all discontinuous series of iron-magnesia silicates can occur together in one rock because the early-formed minerals do not have time to react with the melt. Either way, by the time biotite, crystallizes, essentially all of the magnesium and iron present in the original magma has been consumed.
Continuous reaction series
A reaction series in which the reaction of early-formed crystals with later liquids takes place without abrupt phase changes; e.g. the plagioclase feldspars form a continuous reaction series (Figure 1).
In the continuous series, calcium-rich plagioclase crystallizes first. As the magma cools further, the calcium-rich plagioclase reacts with the melt, and proportionately more sodium-bearing plagioclase crystallizes, consuming all the calcium and sodium. In many cases, the cooling is too rapid for the complete conversion of calcium-rich plagioclase to sodium-rich plagioclase. The plagioclase that forms under these conditions is zoned. That is, a calcium-rich core is gradually surrounded by sodium-rich zones.
Since minerals crystallize simultaneously along two series of Bowen reactions, iron and magnesium are consumed for use in ferromagnesian silicates, and calcium and sodium are consumed in plagioclase feldspars. At this point, the residual magma becomes rich in potassium, aluminum, and silicon, which combine to form potassium feldspar, orthoclase (KAlSi3O8), and if water pressure is high, the sheet silicate muscovite forms. The remaining magma is rich in silicon and oxygen (silica) to form quartz mineral (SiO2). The crystallization of orthoclase and quartz is not a true sequence of reactions as it forms independently rather than by the reaction of orthoclase with the melt.
Significance of Bowen’s Reaction Series
- Bowen’s Reaction Series is an important tool in the study of igneous rocks, as it provides a framework for understanding the sequence in which minerals crystallize from cooling magma. This sequence can be used to identify the original composition of the magma and the conditions under which it cooled and solidified.
- Bowen’s Reaction Series can help predict which minerals are most susceptible to weathering based on their chemical composition and stability. The minerals at the top of the Bowen’s Reaction Series, such as olivine, pyroxene, and amphibole, are generally more susceptible to weathering than those at the bottom, such as quartz and feldspar. This is because the minerals at the top of the series are less stable at Earth’s surface conditions and are susceptible to chemical reactions that lead to their breakdown. For example, olivine weathers relatively quickly because it contains iron and magnesium, which are chemically reactive with water and oxygen in the air. In contrast, quartz is one of the most resistant minerals to weathering due to its high chemical stability. It does not react easily with water or oxygen and can withstand the physical forces of weathering, such as abrasion and erosion.
Who is Norman L. Bowen?
Norman L. Bowen (1887-1956) was a Canadian geologist who made important contributions to the field of petrology, the study of rocks and their origins. He is best known for developing the Bowen reaction series, concepts describing how minerals crystallize from cooling magma or lava. Born in Kingston, Ontario, Bowen received his bachelor’s degree and his doctorate from Queen’s University in 1910. After teaching at the University of Chicago, Harvard, and Caltech, he returned to Canada in 1947 and became director of the Geophysical Laboratory at the University of Toronto.
Throughout his career, Bowen made many contributions to the study of igneous rocks. This includes discovering discontinuous sequences of mineral crystallization and developing theoretical models for the formation of igneous rocks. He also studied metamorphic rock formation and mineral chemistry. Bowen was elected to the National Academy of Sciences in 1943 and has received numerous awards and honors for his contributions to the field of geology. Today he is considered one of the most important figures in the history of petrology.
