Partial melting is the phenomenon that occurs when arock is subjected to temperatures high enough to cause certainminerals to melt, but not all of them. Partial melting is an important part of the formation of alligneous rocks and somemetamorphic rocks (e.g.,migmatites), as evidenced by a multitude ofgeochemical,geophysical andpetrological studies.[1]
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.[2][1] As for the mechanisms that govern partial melting, the main aredecompression melting andflux melting. Decompression melting occurs when rocks are brought from higher to lower pressure zones in theEarth's crust, lowering the melting point of its mineral components, thus generating a partial melt. Flux melting, on the other hand, occurs when water and other volatiles get in contact with hot rock, reducing the melting point of minerals, leading to partial melting.[2] With a few exceptions (e.g.,Yellowstone[3]),conduction of heat is considered a mechanism too slow and inefficient to partially melt large bodies of rock.[2]
Partial melting is also linked to the formation ofores. Magmatic and hydrothermal ore deposits, such aschromite,Ni-Cusulfides, rare-metalpegmatites,kimberlites,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.[4]
Melting in themantle depends on the following parameters:composition of therocks,pressure andtemperature, and the presence ofvolatiles.
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 fromsedimentary rocks, such asshales andgraywacke reflects that of the source rocks.[7] 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.[4]
Temperature andpressure 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.[5]
The presence ofvolatiles has the potential to significantly reduce solidus temperatures of a given system.[8][9] 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,[10] while others assign a more subordinate role to these components.[11]
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The main mechanisms responsible for partial melting aredecompression melting andflux 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 offluids that lower themelting point ofminerals, leading to their melting at lower temperatures. Althoughconduction 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 ineffectiveheat flow in large rock bodies in the solid portion of the Earth and a lack of heat sources capable of inciting partial melting.[2]
Main process responsible for the generation of basaltic melts on certain settings, such asrift zones in continents,back-arc basins, seafloorspreading zones and intraplatehotspots.Plate tectonics andmantle convection are responsible for the transportation of hot and less dense rock towards the surface. This causes areduction in pressure without loss of heat, leading to partial melting.[13] At seafloor spreading zones (mid-ocean ridges), hotperidotite ascending from the mantle undergoes partial melting due to a decrease in pressure, generating abasaltic melt and a solid phase. This melt when extruded on the surface is responsible for the creation of newoceanic crust. In continental rifts, where thelithosphere is colder and more rigid, decompression melting occurs when material from the hot and more plasticasthenosphere is transported to lower pressures.[2]
Decompression melting does not explain howvolcanoes form abovesubduction zones, since in this setting there is an increase in pressure when the oceanic plate subducts under a colderoceanic plate or acontinental plate. The mechanism that explains melting in this setting isflux melting. In this case, whenwater,oceanic crustal material andmetamorphosed mantle rocks are added into the system,minerals can be melted at lower temperatures.[15] There are arguments that the most efficient way of carrying material from thesubducting slab to thevolcanic arc on the surface is by melting the slab itself,[16] while other views support that melting occurs between thelithosphere and theslab.[17][18]
Although decompression and flux melting are the main mechanisms causing partial melting, the generation of certain igneous systems, such as largefelsic continentalmagma reservoirs (for example,Yellowstone[3]), 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 partiallycrystallize. In this event, if sufficient heat is released, it can cause the melting of the surrounding rocks and the creation of felsic magma.[19] The relevance of this phenomenon to the modification of thecontinental crust is a topic of discussion in the scientific community.[20]
Partial melting is an important process ingeology with respect to the chemicaldifferentiation ofcrustal rocks. On theEarth, partial melting of the mantle atmid-ocean ridges producesoceanic crust, and partial melting of the mantle and oceanic crust atsubduction zones createscontinental crust.[5]
Furthermore, the process of partial melting is also associated with the development of a series of ore deposits such as:[4]
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