Lab 4: Igneous Rocks - Evolution of Magmas


We've discussed the origin of magma. Now let's take a look at what happens when magma moves, primarily upward, through the mantle and crust. First, there are two fairly obvious things that can happen to a magma to change it, as a result of movement:


Assimilation of Existing Rocks


You've heard that word assimilation, perhaps in the context of culture. You could say that the habits and lifestyles of Irish settlers or any other cultural group in the United States, have gradually, at least a little bit, for most families, been slowly combined with the cultures of all sorts of people. We can say that cultures are assimilated into the whole, which is whatever you say is the set of "North American" habits and lifestyles. Magma moving upward, or sideways, within the Earth, will take in rock along the edges of magma chambers, even in huge pieces, and the aped material will be melted into the magma, changing its composition somewhat. If an intermediate composition magma moves upward into continental crust, which is a hodgepodge of all types of rocks, assimilation of country rock will result in a diluting of iron and magnesium content (the aped material doesn't have as much iron and magnesium, usually). So, the magma may become felsic in composition after the assimilation.


Mixing of Magma


Two magmas, perhaps of differing composition, can mix together if they come into contact, and the resulting magma is a kind of combination, most likely different from either of the original magmas.


But there is a less obvious way that magmas can evolve. What happens to the remaining magma as crystals grow, taking out the raw materials for their crystals from the magma. In the case of olivine, which starts crystallizing first, at high temperatures, iron and magnesium, along with silica, are taken out of the magma, depleting it of these substances. As other minerals grow, they will likewise remove certain chemical elements from the magma. So, the overall composition of the magma changes, as crystallization happens. Norman Bowen did laboratory experiments to determine the order of crystallization of minerals growing in a magma as it cools, and made a chart showing the order. This is called Bowen's Reaction Series, and is important to understanding how magmas evolve. Why is it important? Because early formed igneous rock might contain one set of minerals, but a later forming igneous rock, forming from the same magma, but changed, will have a different mineralogy. For such things as ore deposits like gold and silver, understanding how minerals crystallize helps you map out where you think deposits are located within mountain ranges. Here is Bowen's Reaction Series:



Bowens Reaction Series


First, observe the layout of the chart. High temperature is on top, so you can imagine that time progresses down the page; minerals crystallize in a sequence, from top to bottom (This chart is just an abstraction, used to explain the process). On the right, you see the compositions you've learned: ultramafic, mafic, intermediate, down to felsic. In the middle you have familiar silicate minerals. On the left side you see olivine, augite (pyroxene), hornblende (amphibole), and biotite. On the right side you see plagioclase, ranging in composition from all calcium, to 50:50 calcium and sodium, to all sodium toward the bottom (Sodium and calcium substitute for one another, in a proportion dependent on temperature). And under the left and right sides, you see orthoclase (potassium feldspar), muscovite, and then quartz at the bottom. The two sides have names: the left side is called the discontinuous reaction series and the right side is called the continuous reaction series:


Discontinuous Reaction Series


Why discontinuous, as in start-stop? Because there are different minerals crystallizing in an order, first olivine, then pyroxene, then amphibole, then biotite, with a little bit of overlap in time for each transition.


Continuous Reaction Series


You can see why it is called continuous: there is only one mineral involved, plagioclase, and the only thing that happens as the temperature drops (going down the chart), is that sodium takes the place of calcium more and more.


And, finally, as you trace down from the two sides, you see orthoclase, then muscovite, and finally, emphasis on finally, quartz crystallizes. Now you might be seeing some things:


There is a range of temperature across which a given mineral will grow. Crystals of the mineral start to grow, they continue to form and grow as the temperature drops, but then they stop forming and growing, and other minerals grow instead.


Earlier formed minerals, such as olivine, will "hang around" in the magma, as other minerals grow. The period of "hanging around" is one wherein the rims of early formed crystals will react a bit with the magma, which is changing in chemistry; it is evolving over time. When you look at olivine and pyroxene crystals up close with a microscope, you can sometimes see reaction rims.


Not so for plagioclase. Instead of a start-stop affair, it just keeps growing, simply taking in more sodium at the expense of calcium as the temperature drops. A single plagioclase crystal will have a different composition on the inside (formed when the crystal was a "baby"), and a different composition in the middle, and on the outside. Such zoned plagioclase crystals are common.


Late forming minerals, especially quartz, have to grown in the space remaining between the earlier formed mineral crystals. Usually, quartz is crammed in between the other minerals, because it is the last to form.


Now, and most importantly, take a look at the compositions along the right hand side of the chart:


Ultramafic igneous rocks form at high temperatures from minerals that begin to grow at those higher temperatures (olivine, Ca-rich plagioclase).


Mafic igneous rocks also form at higher temperatures, as minerals such as pyroxene (augite), amphibole (hornblende) and Ca-Na plagioclase grow.


Intermediate igneous rocks form in a mid-level temperature range, and contain perhaps a little bit of pyroxene, a good bit of amphibole, biotite, and Na-rich plagioclase.


Felsic igneous rocks form at the lowest temperatures, from the suite of minerals toward the bottom of the chart.


So there you have it. Norman Bowen's chart helps you to make sense of magma crystallization.


Finally, we've been discussing what happens as the temperature drops and minerals grow. This same chart is useful to help thinking about what happens in the reverse process, when existing solid rock gets heated up, to the point when minerals start to melt. Which minerals are the first to melt? The last ones to crystallize, those on the bottom, such as quartz and muscovite and orthoclase. If heating doesn't take it all the way to total melting, these more siliceous minerals get melted off, and magma generated never gets any of the high iron and magnesium content of minerals like olivine, pyroxene, and amphibole. Recall partial melting? Now you might be getting the importance of this process to the generation of intermediate composition magmas from a mafic source rock.