The minerals that make up igneous rocks crystallize (solidfy, freeze) at a range of different temperatures. This explains why a cooling magma can have some crystals within it and yet remain predominantly liquid. The sequence in which minerals crystallize from a magma is known as the Bowen reaction series (Figure 7.5).
How Did We Get Bowen’s Reaction Series?
Understanding how the reaction series was derived is key to understanding what it means.
Norman Levi Bowen (Figure 7.6) was born in Kingston Ontario. He studied geology at Queen’s University and then at MIT in Boston. In 1912 he joined the Carnegie Institution in Washington, D.C., where he carried out groundbreaking experiments into how magma cools.
Working mostly with mafic magmas (magmas rich in iron and magnesium), he determined the order of crystallization of minerals as the temperature drops. The method, in brief, was to melt the rock to a magma in a specially made kiln, allow it to cool slowly to a specific temperature (allowing some minerals to form), and then quench it (cool it quickly) so that no new minerals form (only glass). The results were studied under the microscope and by chemical analysis. This was done over and over, each time allowing the magma to cool to a lower temperature before quenching.
The result of these experiments was the reaction series which, even a century later, is still an important basis for our understanding of igneous rocks.
Discontinuous and Continuous Series
Different Minerals Form in Sequence on the Discontinuous Side
Bowen’s reaction series has two pathways for minerals to form as magma cools. On the left of Figure 7.5 is the discontinuous series. This refers to the fact that one kind of mineral is being transformed into a different mineral through chemical reactions. For example, olivine begins to form at just below 1300°C, but as the temperature drops, olivine becomes unstable. The early-forming olivine crystals react with silica in the remaining liquid and are converted into pyroxene, something like this:
Mg2SiO4 + SiO2 ——> 2MgSiO3
As long as there is silica remaining and the rate of cooling is slow, this process continues down the discontinuous branch: once all of the olivine has reacted to form pyroxene, the pyroxene will react and form amphibole. Under the right conditions amphibole will form to biotite. Finally, if the magma is quite silica-rich to begin with, there will still be some left at around 750 °C to 800 °C, and from this last magma, potassium feldspar, quartz, and maybe muscovite mica will form.
If the magma cools enough, the first minerals to form will be completely used up in later chemical reactions. This is why we never have an igneous rock made of both olivine (at the top of the series) and quartz (at the bottom).
Notice that the sequence of minerals that form goes from isolated tetrahedra (olivine) toward increasingly complex arrangements of silica tetrahedra. Pyroxene consists of single chains, amphibole has double chains, mica has sheets of tetrahedra, and potassium feldspar and quartz at the bottom of the series have tetrahedra connected to each other in three dimensions.
Plagioclase Feldspar Changes Composition on the Continuous Side
On the right of Figure 7.5 is the continuous series. At about the point where pyroxene begins to crystallize, plagioclase feldspar also begins to crystallize. At that temperature, the plagioclase is calcium-rich (anorthite). As the temperature drops, and providing that there is sodium left in the magma, the plagioclase that forms is a more sodium-rich variety (albite). The series is continuous because the mineral is always plagioclase feldspar, but the series involves a transition from calcium-rich to sodium-rich.
When cooling happens relatively quickly, instead of getting crystals which are of uniform composition, individual plagioclase crystals can be zoned from calcium-rich in the centre to more sodium-rich around the outside. This occurs when calcium-rich early-forming plagioclase crystals become coated with progressively more sodium-rich plagioclase as the magma cools. Figure 7.7 shows a zoned plagioclase crystal as seen under a microscope.
Magma Composition: Mafic, Intermediate, and Felsic
The composition of the original magma determines how far the reaction process can continue before all of the silica is used up. In other words, it determines which minerals will form. The compositions of typical mafic, intermediate, and felsic magmas are shown in Figure 7.8. Notice that these compositions are expressed in terms of oxides (e.g., Al2O3 rather than just Al). There are two reasons for this: one is that in the early analytical procedures, the results were always expressed that way, and the other is that all of these elements combine readily with oxygen to form oxides.
There are three trends to notice in Figure 7.8:
- Silica increases from mafic to felsic magmas. Although there is a range of composition for each magma type, mafic magmas are about half SiO2, and felsic magmas are about 75% SiO2.
- In mafic magmas, FeO + MgO + CaO accounts for 25% of the composition, but this decreases to only about 5% in felsic magmas.
- Mafic magmas have about 5% Na2O + K2O and in felsic magmas this increases to around 10%.
Exercise 7.2 Mafic, Felsic, or Intermediate?
The proportions of the main chemical components of felsic, intermediate, and mafic magmas are listed in the table below. (The values are similar to those shown in Figure 7.8.)
|Oxide||Felsic Magma||Intermediate Magma||Mafic Magma|
Chemical data for four rock samples are shown in the following table. Compare these with those in the table above to determine whether each of these samples is felsic, intermediate, or mafic.
How Do We Get Different Magma Compositions?
Why Is There No Ultramafic Magma?
Partial Melting Makes Magma That Is Richer In Silica
In section 7.1 we discussed partial melting, where some components of a mixture melt before others do. In the case of mafic magma, ultramafic rocks undergo partial melting to produce mafic magma. One place this happens is along ocean spreading centres where there is less pressure from the overlying mass of the lithosphere, and decompression is an important driver of melting.
In general, silicate minerals with more silica will melt before those with less silica. This means the melt will have more silica than the rock as a whole.
Fractional Crystallization Also Makes Magma Richer In Silica
A number of processes that take place within a magma chamber can affect the types of rocks produced in the end. If the magma has a low viscosity (i.e., it’s runny) — which is likely if it is mafic — the crystals that form early, such as olivine (Figure 7.9a), may slowly settle toward the bottom of the magma chamber (Figure 7.9b). This process is called fractional crystallization.
The formation of olivine removes iron- and magnesium-rich components, leaving the overall composition of the magma near the top of the magma chamber more felsic. The crystals that settle might either form an olivine-rich layer near the bottom of the magma chamber, or they might re-melt because the lower part is likely to be hotter than the upper part. If any melting takes place, crystal settling will make the magma at the bottom of the chamber more mafic than it was to begin with (Figure 7.9c).
Magma Composition Also Changes When Other Rocks Are Melted And Mixed In
Magma chambers aren’t isolated from their surroundings. The rock in which the magma chamber is located (called the country rock) can melt, adding to the magma already in the magma chamber (Figure 7.10). Sometimes magma carries fragments of unmelted rock, called xenoliths, with it. Melting of xenoliths can also alter the composition of magma, as can re-melting of crystals that have settled out of the magma.
- "Mafic" combines the words MAgnesium and FerrIC (containing iron). ↵
- "Felsic" combines the words FELdspar and SIliCa. ↵
- The komatiites of the Song Da zone in northwestern Vietnam are 270 million years old, and those on Gorgona Island, Columbia are 89 million years old. Exactly how they formed is still a bit of a mystery. See Table 1 of arXiv:physics/0512118v2 [physics.geo-ph] for a compilation of komatiite ages with references. ↵