Bowen's Reaction Series
As hot magma cools, it undergoes specific reactions. Bowen's reaction series describes the temperature dependent formation of minerals as magma cools. Rocks formed from magma are igneous rocks, and minerals crystallize as magma cools. The temperature of the magma and the rate of cooling determine which minerals are stable (i.e., which minerals can form) and the size of the mineral crystals formed (i.e., texture). The slower a magma cools, the larger crystals can grow.
Named after geologist Norman L. Bowen (1887–1956), Bowen's reaction series allows geologists to predict chemical composition and texture based upon the temperature of a cooling magma.
Bowen's reaction series is usually diagramed as a "Y" with horizontal lines drawn across the "Y." The first horizontal line—usually placed just above the top of the "Y"—represents a temperature of 3272°F (1800°C). The next horizontal line, represents a temperature of 2012°F (1100°C) and is located one-third of the way between the top of the "Y" and the point where the two arms join the base. A third line representing a temperature of 1652°F (900°C) is located two-thirds of the way from the top of the "Y" to juncture of the upper arms. A fourth horizontal line—representing a temperature of 1112°F (600°C) intersects the triple point junction where the upper arms of the "Y" meet the base portion.
The horizontal temperature lines divide the "Y" into four compositional sections. Mineral formation is not possible above 3272°F (1800°C). Between 2012°F (1100°C) and 3272°F (1800°C), rocks are ultramafic in composition. Between 1652°F (900°C) and 2012°F (1100°C), rocks are mafic in composition. Between 1112°F (600°C) and 1652°F (900°C), rocks are intermediate in composition. Below 1112°F (600°C), felsic rocks form.
The upper arms of the "Y" represent two different formation pathways. By convention, the left upper arm represents the discontinuous arm or pathway. The upper right arm represents the continuous arm or continuous path of formation. The discontinuous arm represents mineral formations rich in iron and magnesium. The first mineral to form is olivine—it is the only mineral stable at or just below 3272°F (1800°C). As the temperature decreases, pyroxene becomes stable. The general chemical compositional formula—used throughout this article and not to be confused with a balanced molecular or empirical chemical formula—at the highest temperatures includes iron, magnesium, silicon and oxygen (FeMgSiO, but no quartz). At approximately 2012°F (1100°C), calcium containing minerals (CaFeMgSiO) become stable. As the temperature lowers to 1652°F (900°C), amphibole (CaFeMgSiOOH) forms. As the magmas cools to 1112°F (600°C), biotite (KFeMg-SiOOH) formation is stable.
The continuous arm of Bowen's reaction series represents the formation of feldspar (plagioclase) in a continuous and gradual series that starts with calcium rich feldspar (Ca-feldspar, CaAlSiO) and continues with a gradual increase in the formation of sodium containing feldspar (Ca-Na-feldspar, CaNaAlSiO) until an equilibrium is established at approximately 1652°F (900°C). As the magmas cool and the calcium ions are depleted, the feldspar formation becomes predominantly sodium feldspar (Na-feldspar, NaAlSiO). At 1112°F (600°C), the feldspar formation is nearly 100% sodium feldspar (Na-feldspar, NaAlSiO).
At or just below 1112°F (600°C), the upper arms of the "Y" join the base. At this point in the magma cooling, K-feldspar or orthoclase (KAlSiO) forms and as the temperature begins to cool further, muscovite (KAlSiOOH) becomes stable. Just above the base of the "Y," the temperature is just above the point where the magma completely solidifies. At these coolest depicted temperatures (just above 392° F [200°C]), quartz (SiO) forms.
The time that the magma is allowed to cool will then determine whether the rock will be pegmatite (produced by extremely slow cooling producing very large crystals), phaneritic (produced by slow cooling that produces visible crystals), aphanitic (intermediate cooling times that produce microscopic crystals), or glassy in texture (a product of rapid cooling without crystal formation). When magmas experience differential cooling conditions, they produce porphyritic rock, a mixture of crystal sizes and exhibit either a phaneritic or aphanitic groundmass.
Although the above temperature and percentage composition data are approximate, simplified (e.g., the formation of hornblende has been omitted), and idealized, Bowen's reaction series allows the prediction of mineral content in rock and—by examination of rock—allows the reverse determination of the conditions under which the magma cooled and igneous rock formed.
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K. Lee Lerner