Recall that ice freezes at a specific temperature, whereas basaltic magma crystallizes over a range of at least 200°C of cooling (about 1200° to 1000°C). In a laboratory setting, Bowen and his coworkers demonstrated that as a basaltic magma cools, minerals tend to crystallize in a systematic fashion, based on their melting temperatures.
As shown in Figure 1, the first mineral to crystallize is the ferromagnesian mineral olivine. Further cooling generates calcium-rich plagioclase feldspar as well as pyroxene, and so forth down the diagram.
During this crystallization process, the composition of the remaining liquid portion of the magma also continually changes. For example, at the stage when about one-third of the magma has solidified, the remaining molten material will be nearly depleted in iron, magnesium, and calcium because these elements are major constituents of the minerals that form earliest in the process.
The absence of these elements causes the melt to become enriched in sodium and potassium. Further, because the original basaltic magma contained about 50 percent silica (SiO2), the crystallization of the earliest formed mineral, olivine, which is only about 40 percent silica, leaves the remaining melt richer in SiO2. Thus, the silica component of the remaining melt becomes enriched as the magma evolves.
Bowen also demonstrated that if the solid components in a magma remain in contact with the remaining melt, they will chemically react and change mineralogy (chemical composition), as shown in Figure 1. For this reason, this order of mineral formation became known as Bowen’s reaction series. However, in nature, the earliest-formed minerals can separate from the melt, thus halting any further chemical reaction.
The diagram of Bowen’s reaction series in Figure 1 depicts the sequence in which minerals crystallize from a magma of basaltic composition under laboratory conditions.
Evidence that this highly idealized crystallization model approximates what can happen in nature comes from analysis of igneous rocks. In particular, scientists know that minerals that form in the same general temperature regime depicted in Bowen’s reaction series are found together in the same igneous rocks. For example, notice in Figure 1 that the minerals quartz, potassium feldspar, and muscovite, which are located in the same region of Bowen’s diagram, are typically found together as major constituents of the intrusive igneous rock granite.