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Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s or a mineral that displays
solid solution A solid solution describes a family of materials which have a range of compositions (e.g. AxB1−x) and a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" is used to desc ...

solid solution
, this melt can be different from the bulk composition of the solid. Partial melting occurs where the
solidus Solidus (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republi ...
and
liquidus Image:SiO2 Li2O.GIF, thumbnail, 250px, Liquidus temperature curve in the binary glass system SiO2-Li2O, based on 91 published data collected iSciGlass model fit froGlassproperties.com The liquidus temperature, TL or Tliq, specifies the temperature ...
temperatures are different. For single minerals this can happen when they exhibit solid solution, for example in
olivine The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

olivine
s between
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
and
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
. In rocks made up of several different minerals, some will melt at lower temperatures than others.


Partial melting of the mantle

Melting in the
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
requires one of three possible events to occur: an increase in temperature, a decrease in pressure, or the addition of
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily Volatility (chemistry), vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as Refractory (planetary scien ...
to the system (a change in composition).


Temperature

In the case of raising the temperature, mantle melting will only occur if the mantle is heated past the normal geotherm. It is believed that
heat flux Heat flux of \mathbf(\mathbf) with the unit normal vector \mathbf(\mathbf) ''(blue arrows)'' at the point \mathbf multiplied by the area dS. The sum of \mathbf\cdot\mathbf dS for each patch on the surface is the flux through the surface Flux ...
from the
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
and lower mantle is responsible for increasing the temperature of the upper mantle. Local perturbations of the geothermal gradient, such as hotspots, are not well understood but are considered to be a likely heat source for the mantle. The decay of
radioactive element A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by ...
s, though considered to be one of the simplest ways of generating heat in the mantle, is not realistically responsible for mantle melting, as it would take over 10 million years for the radioactive decay of K, U and Th to increase the temperature of
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spok ...
by 1 degree Celsius. Furthermore, even if this process did generate a small fraction of melt, the radioactive elements would concentrate in the melt and escape the system, ultimately halting the process of melt generation.


Pressure

Melting in the mantle can also occur if there is a sufficient drop in pressure in the system at a given temperature. In order to decrease pressure, mantle rocks must rise to shallower levels, while experiencing a minimal loss of heat to the surroundings. This process can be referred to as if the heat loss is zero. As the mass of mantle rock continues to rise through the Earth's layers, it follows a P-T path that may eventually cross the solidus, initiating melting. This melting process is known as decompression melting.


Addition of volatiles

The presence of volatiles (particularly H2O and CO2) 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. However, the mantle typically has a very low volatile content and this can limit the amount of melt generated. Partial melting is an important process in
geology Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...

geology
with respect to the chemical
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
of crustal rocks. On the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, partial melting of the mantle at
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate ...
s produces
oceanic crust The oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates This is a list of tectonic plates on Earth's surface. Tectonic plates are pieces of Earth's crust 350px, Plates in the crust of Earth Earth's crust i ...
, and partial melting of the mantle and oceanic crust at
subduction zones Subduction is a geological process in which the oceanic lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On ...
creates continental crust. In all these places partial melting is often associated with
volcanism Volcanism (or volcanicity) is the phenomenon of eruption of molten rock (magma) onto the Earth#Surface, surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called ...
, although some melts do not make it to the surface. Partial melts are thought to play an important role in enriching old parts of the continental
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
in
incompatible elementIn petrology Image:LvMS-Lvm.jpg, A volcanic lithic fragment (geology), sand grain seen under the microscope, with plane-polarized light in the upper picture, and cross polarized light in the lower picture. Scale box is 0.25 mm. Petrology (from the ...
s. Partial melts produced at depth move upwards due to the compaction of the surrounding
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
.


References

{{Magmatic processes Volcanology Igneous petrology