What are the Effects of Assimilation On Magma Composition?


Magma is the hot, semi-solid form of rock found within the earth's interior. There are two types; the most common is basaltic magma, an ultramafic kind that accompanies plate tectonics. When magma comes into contact with the host rock, where the magma meets the earth's crust, the temperature decreases and so does the density of the magma. This results in dilution of the magma which means that a certain proportion of the components are consumed by the cooled magma. The composition of magma may also be changed due to metasomatic replacement when other fluids interact with it. When magma comes into contact with the host rock, where the magma meets the earth's crust, the temperature decreases and so does the density of the magma. This results in dilution of the magma which means that a certain proportion of the components are consumed by the cooled magma. The composition of magma may also be changed due to metasomatic replacement when other fluids interact with it.

 Igneous Rocks by Nut Panpia

Geochemistry of Magma

Magma is a medium in which atoms and molecules move freely. When magma starts to cool, this movement becomes blocked, forming crystals. Cooling also lowers the number of dissolved gases in the magma. These two factors can cause the composition of magma to change with time. Atoms from material in the crust tend to become included in magma as it cools and crystallizes. Some elements will preferentially dissolve when their temperature or pressure decreases, while other molecules bond with gas molecules and float away once there is less gas pressure. Secondary minerals such as quartz are usually much less soluble than primary minerals like olivine and pyroxene. The composition of magmas can change over time due to changes in partial melting rate, assimilation, mixing, and other processes. These processes result in large or small-scale changes to the chemical makeup of the parent magma. The difference between a large and small-scale change is due to whether the process is an open or closed system. For example, if a fractional crystallization event occurs that results in an increase in silica content then this result is restricted by how much silica can be dissolved from neighboring rocks and crystals. However, when an open magma is mixed with an equal volume of another open magma the composition change is not restricted by this closed system of available rock and crystal melt but rather is a function of the chemistry between both separate systems. 

mineral composition of igneous rocks

Process of Assimilation

Assimilation is one process that affects magma composition. Assimilation occurs when an outside type of rock (called country rock) is absorbed into the magma chamber and mixed with existing magma. The rate at which assimilation occurs depends on the amount of time available for interaction between magma and country rock, the viscosity of the magma, and the composition of both. Viscosity is a measure of how readily a fluid flows. The higher the viscosity, the more slowly it flows; therefore, low-viscosity melts have more time to assimilate than high-viscosity melts. Assimilation changes the composition of the magma because it adds new elements to the mixture. It also allows for a change in its physical properties as well. It's caused by interaction between rising magma and surrounding rocks. This forms an igneous intrusion.

Magmatic Assimilation process

Ways By Which Assimilation Changes Magma Composition.

There are many ways that assimilation can affect magma composition, and these effects depend on many factors including 1) the composition of the host/assimilant rock, 2) the composition of the magma, 3) the proportions of assimilant vs. magma, 4) crystal content of each body, 5) temperature differences between each body, 6) water content of each body, 7) depth of intrusion/emplacement of the magma, 8) rate of cooling/crystallization of the magma, etc.

How it Affects Magma Viscosity

Assimilation can also change the physical properties of magma. The magma will be less viscous if it assimilates a high proportion of gas or a volatile-rich mineral-like apatite (phosphorus pentoxide) or pyrrhotite (iron sulfide), reflecting an increase in volatile content. The viscosity may increase if the magma assimilates a high proportion of silica-rich minerals like quartz.

Significance of Assimilation

The effects of assimilation are most commonly observed in primitive magmas, such as basalts, andesites, and rhyolites. These relatively simple compositions allow geologists to identify the processes by which they evolved into more complex compositions such as granites. These compositional changes are usually due to assimilation because fractionation (crystallization or melting) normally produces only small changes in composition.

Other Processes that Affect Magma Composition

1. Fractional crystallization

Fractional crystallization occurs when crystals settle out due to gravity. This means that the densest crystals will settle out first with respect to their specific melting point. For example, if we have an olivine-rich rock and it begins to crystallize, then by definition olivine is denser than the rest of the minerals in the rock. It will be the first to settle out as the crystals form because it is heavier than anything else around it. Fractional crystallization changes the composition of the leftover magma because it removes elements from it. This means that each time a crystal settles out from the melt, it takes away some of its chemical constituents and leaves behind others.

Fractional crystallization

2. Magma Mixing

When two or more magmas mix either before or in their host rocks, mixing can occur as well. In this case, hot liquid mixes with the material of different temperatures and compositions to form a new melt that has characteristics different from those of its parent melts. Unlike assimilation, mixing takes place rapidly when two liquids are brought together rather than over a long period of time as with assimilation. When two magmas mix, crystals may or may not be present in one or both. The two different types of magmas (mafic and felsic) can mix when they come into contact with each other either through direct contact or through parallel fractures within a volcano. The mixture is often explosive because felsic magmas have more dissolved gases than mafic ones.

Igneous Rock showing Magma Mixing Structures Kosterhavet Sweden

3. Magmatic Differentiation

This is a process by which minerals that are less dense than the magma rise to the top of the magma chamber and are ejected from the volcano. This material forms igneous rocks known as cumulates. Cumulate rocks have very coarse-grained textures because they were formed by very slow cooling at the top of a magma chamber. They are composed of minerals that do not like water (hydrophobic minerals) such as olivine and pyroxene, which crystallize out of a cooling magma before most other minerals do.

Magma Differentiation.

4. Partial Melting Magma

Partial melting occurs when rocks melt, but not completely. This is important because partial melting in the lower crust can produce basaltic or rhyolitic magmas depending on which minerals melt first. To get from partial melting to magma, it's necessary for molten material to rise and be separated from the rest of the rock. This can happen in lots of ways, but it often involves plate tectonics. The most commonplace for partial melting to happen is on mid-ocean ridges, where plates are being pushed apart. As rocks are heated by the mantle and undergo decompression because they're being pushed apart, they partially melt (melt until they're between 20 and 90 percent molten). But because there isn't enough pressure or heat to melt them completely, the magma doesn't separate from the rest of the rock.

Rock Melting stages


I hope that this paper has been informative and helpful, allowing the reader to better understand the changes that magma composition can undergo due to assimilation and other processes. The study of magma compositions is an important endeavor because it can help us understand a lot about how the Earth was formed and how it has evolved through time.

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