Partial melting

Partial melting is a geological phenomenon where only specific minerals within a rock turn into liquid. This happens when a rock reaches temperatures high enough to melt some parts but not all of its components. It is a fundamental process in the formation of igneous rocks. It also helps create certain metamorphic rocks, such as migmatites. Scientists use geochemical, geophysical, and petrological studies to understand this process. By studying how rocks melt, we can learn how the Earth's crust is built and changed over time.

The process of melting is governed by the relationship between a rock's solidus and its liquidus. The solidus is the temperature below which a substance remains entirely solid. When a rock reaches its solidus temperature, it begins to produce a liquid phase. This initial liquid has a different chemical composition than the original solid rock. As the temperature continues to rise toward the liquidus, more of the solid phase melts. The liquidus is the temperature at which the entire substance has become liquid. During this transition, the chemical composition of the melt changes as it grows.

Several critical parameters influence the extent of partial melting. The chemical composition of the source rock determines its melting points. Rocks containing minerals with low melting points will melt more easily than those with high melting point minerals. Temperature and pressure also play a constant tug-of-war during this process. Higher temperatures promote melting, while higher pressures tend to suppress it. To melt a rock at a lower temperature, the pressure must also be lower. Additionally, the presence of volatiles, such as water, can significantly reduce the solidus temperature. This allows melting to occur without needing to change the temperature or pressure of the system.

There are two primary mechanisms that drive partial melting in the Earth. The first is decompression melting, which occurs when rocks move from high-pressure zones to lower-pressure zones. This is common in rift zones, seafloor spreading zones, and intraplate hotspots. In these areas, plate tectonics and mantle convection move hot, less dense rock toward the surface. As the rock rises, the pressure drops, which lowers the melting point of its minerals. At mid-ocean ridges, this process turns hot peridotite into a basaltic melt. This melt eventually rises to create new oceanic crust.

The second main mechanism is flux melting. This process is responsible for volcanic activity in subduction zones. In these settings, an oceanic plate slides under another plate, which actually increases the pressure. Decompression melting cannot explain volcanism here, so flux melting takes over. When water, oceanic crust, or metamorphosed mantle rocks are added to the hot system, the melting point of the minerals drops. This allows the rock to melt at much lower temperatures. The Klyuchevskoi volcano in Kamchatka, Russia, is a famous example of a volcano formed by flux melting.

A third, less common mechanism is heat conduction. Conduction is the transfer of heat from one body to another. Most large rock bodies in the Earth's solid portion do not melt through conduction because it is too slow and inefficient. However, it is important for certain systems like Yellowstone. In these cases, basaltic melt moves through the continental crust and accumulates. If the melt releases enough heat, it can melt the surrounding rocks to create felsic magma. This process is a key way the continental crust is modified over time.

Partial melting is vital for the chemical differentiation of the Earth's crust. Melting in the mantle at mid-ocean ridges produces the oceanic crust. Meanwhile, melting at subduction zones helps create the continental crust. This process is also closely linked to the formation of many valuable ore deposits. For example, it is related to chromite deposits and Ni-Cu sulfide deposits. It also helps form rare-metal pegmatites and diamond deposits in kimberlites and lamproites. Even light rare-earth elements in carbonatites are connected to these melting conditions. These natural resources are essential products of the complex melting cycles within our planet.