- What does it consist on?
- Bowen series diagram
- Discontinuous series
- Continuous series
- Magmatic differentiation
- References
The Bowen series are primarily a means of categorizing the most common igneous silicate minerals by the temperature at which they crystallize. In the science of geology there are three main types of rocks, which are classified as igneous rocks, sedimentary and metamorphic.
Mainly, igneous rocks are formed by the cooling and solidification of magma or lava from the mantle and the earth's crust, a process that can be caused by an increase in temperature, a decrease in pressure, or a change in composition.
Norman L. Bowen
Solidification can take place under or below the surface of the earth, forming structures other than rocks. In this sense, throughout history a large number of scientists tried to explain the way in which magma crystallized under varying conditions to form different types of rock.
But it was not until the 20th century that the petrologist Norman L. Bowen carried out a long series of studies of fractional crystallization to be able to observe the type of rocks that were produced according to the conditions in which he worked.
Also, what he observed and concluded in this experiment was quickly accepted by the community, and these Bowen series became the correct description of the magma crystallization process.
What does it consist on?
As mentioned before, the Bowen series are used to classify the igneous silicate minerals that are more in existence by means of the temperature at which they crystallize.
The graphical representation of this series allows to visualize the order in which the minerals will crystallize according to this property, the higher minerals being the first to crystallize in a cooling magma, and the lower ones being the last to form. Bowen concluded that the crystallization process is based on five principles:
1- While the melt cools, the crystallizing minerals will remain in thermodynamic equilibrium with it.
2- With the passing of time and the increase in mineral crystallization, the melt will change its composition.
3- The first crystals formed are no longer in equilibrium with the mass with the new composition, and they dissolve again to form new minerals. This is why there is a series of reactions, which develops with the passing of cooling.
4- The most common minerals in igneous rocks can be categorized into two series: a continuous series of reaction for feldspars, and a discontinuous series for ferromagnesic minerals (olivine, pyroxene, hornblende and biotite).
5- This series of reactions supposes that, from a single magma, all types of igneous rocks can originate as a result of magmatic differentiation.
Bowen series diagram
The Bowen series themselves are represented by a "Y" shaped diagram, with horizontal lines intercepting various points on the Y to indicate temperature ranges.
The first line, viewed from top to bottom, represents a temperature of 1800 ºC, and is manifested in the form of ultramafic rocks.
This is the first section, since minerals cannot form at temperatures higher than this. The second section begins at 1100 ºC, and between this temperature and 1800 ºC is where the mafic rocks are formed.
The third section begins at 900ºC and ends at 600ºC; the latter represents the point where the arms of the diagram meet and a single line descends. Between 600ºC and 900ºC intermediate rocks are formed; lower than this, felsic rocks crystallize.
Discontinuous series
The left arm of the diagram belongs to the discontinuous series. This path represents mineral formations that are rich in iron and magnesium. The first mineral to form this way is olivine, which is the only stable mineral around 1800 ºC.
At this temperature (and from this moment on) minerals formed by iron, magnesium, silicon and oxygen will be evident. With the decrease in temperature, pyroxene will become stable and calcium will begin to appear in the minerals formed when 1100 ºC is reached.
When cooling to 900 ºC is reached, amphiboles appear (CaFeMgSiOOH). Finally, this path ends when the temperature drops to 600ºC, where biotites begin to form in a stable way.
Continuous series
This series is called "continuous" because the mineral feldspar is formed in a continuous and gradual series that begins with a high proportion of calcium (CaAlSiO), but is characterized by a greater formation of sodium-based feldspars (CaNaAlSiO).
At a temperature of 900 ºC, the system equilibrates, the magmas cool and the calcium ions are depleted, so that from this temperature the formation of feldspars is based mainly on sodium feldspars (NaAlSiO). This branch culminates at 600 ºC, where the formation of feldspars is almost 100% NaAlSiO.
For the residual phases -which are the last to form and appear as the straight line that descends from the previous series- the mineral known as K-spar (potassium feldspar) will appear at temperatures below 600 ºC, and the muscovite will will generate at lower temperatures.
The last mineral to form is quartz, and only in systems where there is an excess of silicon in the remnant. This mineral is formed at relatively cold magma temperatures (200 ºC), when it has almost solidified.
Magmatic differentiation
This term refers to the separation of magma in batches or series, in order to separate the crystals from the melt.
This is done in order to obtain certain minerals that would not remain intact in the melt if it were allowed to continue cooling.
As mentioned above, the first minerals that form at 1800ºC and 1100ºC dissolve again to form others, so they can be lost forever if they are not separated from the molten mixture in time.
References
- Britannica, E. (nd). Bowen's Reaction Series. Retrieved from britannica.com
- College, C. (sf). Bowen's Reaction Series. Retrieved from colby.edu
- Lerner, KL (sf). Bowen's Reaction Series. Retrieved from science.jrank.org
- University, I. (sf). Bowen's Reaction Series. Retrieved from indiana.edu
- Wikipedia. (sf). Bowen's Reaction Series. Retrieved from en.wikipedia.org