Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain to melt, but not all of them. Partial melting is an important part of the formation of all and some Metamorphic rock rocks (e.g., ), as evidenced by a multitude of Geochemistry, Geophysics and Petrology studies.
The parameters that influence partial melting include the composition of the source rock, the pressure and temperature of the environment, and the availability of water or other fluids.
Partial melting is also linked to the formation of . Magmatic and hydrothermal ore deposits, such as chromite, Nickel-Copper Sulfide mineral, rare-metal , , volcanic-hosted massive sulfide deposits are some examples of valuable natural resources closely related to the conditions of the origin, migration and emplacement of partial melts.
Parameters
Melting in the mantle depends on the following parameters: composition of the rocks,
pressure and
temperature, and the presence of volatiles.
Composition
The chemical composition of rocks affects their melting points and the final product of partial melting. For example, the bulk chemistry of melts obtained experimentally from
, such as
and
Greywacke reflects that of the source rocks.
Additionally, rocks containing minerals with lower melting points will undergo partial melting more easily under the same conditions of pressure and temperature if compared to minerals with higher melting points.
Temperature and Pressure
Temperature and
pressure can have a significant impact on the amount of partial melting that occurs in rocks. When temperature is low, the pressure needs to be low as well for melting to occur, and when temperature is high, the pressure needs to be higher to prevent melting from taking place. Higher pressure can suppress melting, while higher temperature can promote it. The extent to which partial melting occurs depends on the balance between temperature and pressure, with both having a strong influence on the process.
Addition of volatiles
The presence of volatiles has the potential to significantly reduce solidus temperatures of a given system.
This allows for melt to be generated at lower temperatures than otherwise predicted, eliminating the need for a change in pressure or temperature conditions of the system. Furthermore, some consider that volatiles control the stability of minerals and the chemical reactions that happen during partial melting,
while others assign a more subordinate role to these components.
Mechanisms
The main mechanisms responsible for partial melting are decompression melting and
flux melting. The first process happens when bodies of rock move from a higher to a lower pressure setting, causing melting of a part of its components, while the second is caused by the addition of
that lower the
melting point of
, leading to their melting at lower temperatures. Although conduction of heat is a known mechanism capable of transferring heat from one body to another, it plays a subordinate role in causing partial melting. This is due to the ineffective
Heat transfer in large rock bodies in the solid portion of the Earth and a lack of heat sources capable of inciting partial melting.
Decompression melting
Main process responsible for the generation of basaltic melts on certain settings, such as
in continents,
, seafloor
Mid-ocean ridge and intraplate hotspots.
Plate tectonics and mantle convection are responsible for the transportation of hot and less dense rock towards the surface. This causes a reduction in pressure without loss of heat, leading to partial melting.
At seafloor spreading zones (
), hot
peridotite ascending from the mantle undergoes partial melting due to a decrease in pressure, generating a
melt and a solid phase. This melt when extruded on the surface is responsible for the creation of new
oceanic crust. In continental rifts, where the
lithosphere is colder and more rigid, decompression melting occurs when material from the hot and more plastic
asthenosphere is transported to lower pressures.
Flux melting
Decompression melting does not explain how
form above
Subduction, since in this setting there is an increase in pressure when the oceanic plate subducts under a colder
Oceanic crust or a continental plate. The mechanism that explains melting in this setting is
flux melting. In this case, when
water,
Oceanic crust and
metamorphosed mantle rocks are added into the system,
can be melted at lower temperatures.
There are arguments that the most efficient way of carrying material from the subducting slab to the
volcanic arc on the surface is by melting the slab itself,
while other views support that melting occurs between the
lithosphere and the slab.
Heat conduction
Although decompression and flux melting are the main mechanisms causing partial melting, the generation of certain igneous systems, such as large
felsic continental
Magma chamber (for example, Yellowstone
), are not explained by them. In this case, heat conduction is the mechanism responsible for that. When basaltic melt moves through the continental crust, it can accumulate and partially
Crystallization. In this event, if sufficient heat is released, it can cause the melting of the surrounding rocks and the creation of felsic magma.
The relevance of this phenomenon to the modification of the continental crust is a topic of discussion in the scientific community.
Significance
Partial melting is an important process in
geology with respect to the chemical differentiation of crustal rocks. On the
Earth, partial melting of the mantle at
produces
oceanic crust, and partial melting of the mantle and oceanic crust at
subduction zones creates continental crust.
Furthermore, the process of partial melting is also associated with the development of a series of ore deposits such as:
