Igneous differentiation

Igneous differentiation, often called magmatic differentiation, is a group of geological processes. These processes change the chemical makeup of magma over time. This change can happen during partial melting, cooling, emplacement, or eruption. As the magma changes, it often becomes more silicic, which means it has more silica. This sequence of changing magmas is known as a magma series. Understanding these changes helps geologists reconstruct the history of the Earth's interior.

To understand these changes, scientists look at different types of melts. A primary melt is the very first liquid produced when a rock melts. It represents the original starting composition of the magma. When these melts come directly from the Earth's mantle, they are called primitive melts. Because we cannot observe the mantle directly, primitive melts are vital tools. They allow scientists to model the composition of the rocks that formed the mantle. If a primary melt cannot be found, scientists look for a parental melt. A parental melt is the specific composition that produced a group of observed magmas through differentiation.

One of the most important mechanisms is fractional crystallization. This occurs when minerals crystallize out of the liquid melt and separate from it. This separation changes the chemical composition of the remaining liquid. This process is highly complex and depends on temperature, pressure, and the magma's initial composition. For example, in mafic or ultramafic melts, the levels of magnesium oxide (MgO) and silica (SiO2) determine which minerals form. These levels decide if forsterite olivine or enstatite pyroxene will precipitate. Pressure also plays a role. High-pressure conditions can produce different granites than low-pressure conditions.

Another major process is assimilation. This happens when hot magma melts and incorporates the surrounding wall rocks. This process can change the chemistry of the magma significantly. For instance, a hot primitive melt rising through a cooler, felsic crust will melt that crust. The resulting mix alters the original composition of the magma. Scientists have found that assimilation is fundamental to changing the isotopic composition of magmas. It also plays a role in creating important ore deposits and influencing volcanic eruptions.

Magma chambers are rarely static, single environments. They are often subject to replenishment, which is the injection of fresh, hot magma. This new magma provides extra heat and can trigger vigorous convection. It can also cause existing minerals to dissolve back into the melt. Replenishment changes the chemistry of the liquid, which changes which minerals crystallize next. For example, injecting fresh magma can change the type of plagioclase feldspar that forms. This can even reverse previous trends in mineral layers.

Magma mixing is another common occurrence in volcanic systems. This happens when two different magmas meet and blend together. The resulting magma has a composition somewhere between the two original types. This is a primary way that intermediate rocks, such as monzonite and andesite, are formed. In some areas, granitic melts called underplates form from the melting of the crust. When basaltic melts from the mantle rise and meet these underplates, they create an intermediate composition. This mixing is a key part of how open-system magma chambers operate.

Geologists use the acronym FARM to remember the four main drivers of differentiation. FARM stands for fractional crystallization, assimilation, replenishment, and magma mixing. Other factors like interface entrapment and partial melt extraction also contribute. Interface entrapment happens when friction and viscosity trap crystals near the walls of a magma chamber. This can create flow banding. Partial melt extraction occurs when the liquid portion of a cooling chamber is removed. This leaves behind a remnant that no longer matches the original parental magma.

Finally, the physical properties of the magma influence how it behaves. Granite magmas have very high viscosity, meaning they are thick and not very fluid. Because they move in large, concerted masses, they often form large structures called plutons. Mafic magmas are much more fluid and flow more easily. This allows them to form thinner shapes like dikes and sills. Mafic magmas are also more likely to undergo rapid crystallization and frequent replenishment. Even dissolved gases, especially water, are integral parts of these complex magmatic systems.