How Magmas Evolve

Describe how magmatic differentiation can generate a body that has mineralogy (chemical composition) that is different from its parent magma.

Geologists have observed that, over time, a volcano may extrude lavas that vary in composition.
Data of this type led them to examine the possibility that magma might change (evolve), and thus one body could become the parent to a variety of igneous rocks. To explore this idea, N. L. Bowen carried out a pioneering investigation into the crystallization of magma early in the twentieth century.

Bowen’s Reaction Series and the Composition of Igneous Rocks

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 it 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.

Figure 1 – Bowen’s reaction series
This diagram shows the sequence in which minerals crystallize from a mafic-magma.

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 magma of basaltic composition under laboratory conditions.

Evidence that this highly idealized crystallization model approximates what can happen in nature comes from the 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.

Magmatic Differentiation and Crystal Settling

Bowen demonstrated that minerals crystallize from magma in a systematic fashion. But how do Bowen’s findings account for the great diversity of igneous rocks? It has been shown that at one or more stages during the crystallization, separation of various components can occur. One mechanism that causes this to happen is called crystal settling.

This process occurs when the earlier-formed minerals are denser (heavier) than the liquid portion and sink toward the bottom of the chamber, as shown in Figure 2. When the remaining melt solidifies—either in place or in another location, if it migrates into fractures in the surrounding rock —it will form a rock with a mineral composition that is different than the parent magma. The formation of a body having mineralogy or chemical composition that is different from the parent magma is called differentiation.

Figure 2 – Crystal settling results in a change in the composition of the remaining melt A magma evolves as the earliest-formed minerals (those richer in iron, magnesium, and calcium) crystallize and settle to the bottom of the chamber, leaving the remaining melt richer in sodium, potassium, and silica (SiO2).

A classic example of magmatic differentiation is found in the Palisades Sill, which is a 300-meter-thick (1000-foot-thick) slab of dark igneous rock exposed along the west bank of the lower Hudson River across from New York City (Figure 3). Because of its great thickness and subsequent slow rate of solidification, crystals of olivine (the first mineral to form) sank and makeup about 25 percent of the lower portion of the Palisades Sill.

By contrast, near the top of this igneous body, where the last melt crystallized, olivine represents only 1 percent of the rock mass.

Figure 3 – The Palisades Sill, as seen from New York City The Palisades form impressive cliffs along the west side of the Hudson River for more than 80 kilometers (50 miles). This structure, which is visible from Manhattan, formed when magma was injected between layers of sandstone and shale. (Terese Loeb Kreuzerl/Alamy)

Assimilation and Mixing

Bowen successfully demonstrated that through magmatic differentiation, a single parent magma can generate several mineralogically different igneous rocks. However, more recent work indicates that differentiation involving crystal settling cannot, by itself, account for the entire compositional spectrum of igneous rocks.

Once a body forms, the incorporation of foreign material can also change its composition. For example, in near-surface environments where rocks are brittle, the magma pushing upward can cause numerous fractures in the overlying rock. The force of the injected magma is often sufficient to dislodge and incorporate the surrounding host rock (Figure 4).

Figure 4 – Assimilation of the host rock by a body The composition changes when the molten mass incorporates pieces of surrounding host rock, in a process called assimilation.

Melting of these blocks, a process called assimilation changes the overall chemical composition of the body.
Another means by which the composition can be altered is called magma mixing.
I may occur during the ascent of two chemically distinct bodies as the more buoyant mass overtakes the more slowly rising body (Figure 5). Once they are joined, convective flow stirs the two magmas, generating a single mass that has an intermediate composition.

Figure 5 – Mixing. This is one way the composition of a magma body can change.

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