Partial melting

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Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorphic rocks (e.g., migmatites), as evidenced by a multitude of geochemical, geophysical and petrological studies. [1]

Contents

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 are decompression melting and flux melting. Decompression melting occurs when rocks are brought from higher to lower pressure zones in the Earth'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 of ores. Magmatic and hydrothermal ore deposits, such as chromite, Ni-Cu sulfides, rare-metal pegmatites, 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]

Parameters

A rock with composition CB starts to melt when its temperature is TA and reaches the solidus curve, the temperature below which all the substance is solid. The newly formed liquid phase has an initial composition of CL at TA. As the temperature increases towards TB, the partial melting of the solid phase leads to changes in composition from CB to CS (blue line). As the liquid phase increases, its composition gets closer to the rock's original composition CB (red line). When the temperature reaches TB, the whole solid phase has melted, characterizing the substance being above the liquidus curve. Partial Melting Phase Diagram.svg
A rock with composition CB starts to melt when its temperature is TA and reaches the solidus curve, the temperature below which all the substance is solid. The newly formed liquid phase has an initial composition of CL at TA. As the temperature increases towards TB, the partial melting of the solid phase leads to changes in composition from CB to CS (blue line). As the liquid phase increases, its composition gets closer to the rock’s original composition CB (red line). When the temperature reaches TB, the whole solid phase has melted, characterizing the substance being above the liquidus curve.

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 sedimentary rocks, such as shales and graywacke 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 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. [5]

Addition of volatiles

The presence of volatiles 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]

Mechanisms

Diagram showing the physical processes inside the Earth that lead to the generation of magma. The plots above show the rate at which the temperature (red line) and the solidus (green line) change based on depth and tectonic setting (A to D). Partial melting asthenosphere EN.svg
Diagram showing the physical processes inside the Earth that lead to the generation of magma. The plots above show the rate at which the temperature (red line) and the solidus (green line) change based on depth and tectonic setting (A to D).
A close-up showing a mid-ocean ridge with a magma reservoir below. Hot and less dense mantle rocks rise to lower pressure zones leading to decompression melting. Mid-ocean ridge topography.gif
A close-up showing a mid-ocean ridge with a magma reservoir below. Hot and less dense mantle rocks rise to lower pressure zones leading to decompression melting.
At 4,800 m above sea level, Klyuchevskoi is located in Kamchatka, Russia and is a product of flux melting on a subduction zone. Klyuchevskaya Sopka (Klyuchevskoi) with some activity, Kamchatka, Russia - March 8th, 2020 (49637900967).jpg
At 4,800 m above sea level, Klyuchevskoi is located in Kamchatka, Russia and is a product of flux melting on a subduction zone.

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 fluids that lower the melting point of minerals, 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 flow in large rock bodies in the solid portion of the Earth and a lack of heat sources capable of inciting partial melting. [2]

Decompression melting

Main process responsible for the generation of basaltic melts on certain settings, such as rift zones in continents, back-arc basins, seafloor spreading zones 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. [13] At seafloor spreading zones (mid-ocean ridges), hot peridotite ascending from the mantle undergoes partial melting due to a decrease in pressure, generating a basaltic 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. [2]

Flux melting

Decompression melting does not explain how volcanoes form above subduction zones, since in this setting there is an increase in pressure when the oceanic plate subducts under a colder oceanic plate or a continental plate. The mechanism that explains melting in this setting is flux melting. In this case, when water, oceanic crustal material and metamorphosed 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 the subducting slab to the volcanic arc on the surface is by melting the slab itself, [16] while other views support that melting occurs between the lithosphere and the slab. [17] [18]

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 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 partially crystallize. 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 the continental crust is a topic of discussion in the scientific community. [20]

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 mid-ocean ridges produces oceanic crust, and partial melting of the mantle and oceanic crust at subduction zones creates continental crust. [5]

Furthermore, the process of partial melting is also associated with the development of a series of ore deposits such as: [4]

Related Research Articles

<span class="mw-page-title-main">Granite</span> Type of igneous rock

Granite is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

<span class="mw-page-title-main">Magma</span> Hot semifluid material found beneath the surface of Earth

Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles.

<span class="mw-page-title-main">Asthenosphere</span> Highly viscous, mechanically weak, and ductile region of Earths mantle

The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

<span class="mw-page-title-main">Andesite</span> Type of volcanic rock

Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

<span class="mw-page-title-main">Migmatite</span> Mixture of metamorphic rock and igneous rock

Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock that was reconstituted subsequently by partial melting ("neosome"), while the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance ("paleosome"). Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains.

<span class="mw-page-title-main">Island arc</span> Arc-shaped archipelago formed by intense seismic activity of long chains of active volcanoes

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

<span class="mw-page-title-main">Komatiite</span> Magnesium-rich igneous rock

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% magnesium oxide (MgO). It is classified as a 'picritic rock'. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

Anatexis is the partial melting of rocks. Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term "partial melting" refers to the partial melting of all rocks, in both the crust and mantle.

In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of magmas produced by igneous differentiation is known as a magma series.

<span class="mw-page-title-main">Adakite</span> Volcanic rock type

Adakites are volcanic rocks of intermediate to felsic composition that have geochemical characteristics of magma originally thought to have formed by partial melting of altered basalt that is subducted below volcanic arcs. Most magmas derived in subduction zones come from the mantle above the subducting plate when hydrous fluids are released from minerals that break down in the metamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when young oceanic crust is subducted, adakites are typically produced in the arc. They postulated that when young oceanic crust is subducted it is "warmer" than crust that is typically subducted. The warmer crust enables melting of the metamorphosed subducted basalt rather than the mantle above. Experimental work by several researchers has verified the geochemical characteristics of "slab melts" and the contention that melts can form from young and therefore warmer crust in subduction zones.

<span class="mw-page-title-main">Igneous rock</span> Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.

<span class="mw-page-title-main">Subduction zone metamorphism</span> Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the Earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets produced by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types formed by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process generates and alters water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

<span class="mw-page-title-main">Tonalite–trondhjemite–granodiorite</span> Intrusive rocks with typical granitic composition

Tonalite–trondhjemite–granodiorite (TTG) rocks are intrusive rocks with typical granitic composition but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean TTG rocks are present in arc-related batholiths, as well as in ophiolites, while Archean TTG rocks are major components of Archean cratons.

<span class="mw-page-title-main">Earth's crustal evolution</span>

Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.

<span class="mw-page-title-main">Crystal mush</span>

A crystal mush is magma that contains a significant amount of crystals suspended in the liquid phase (melt). As the crystal fraction makes up less than half of the volume, there is no rigid large-scale three-dimensional network as in solids. As such, their rheological behavior mirrors that of absolute liquids.

<span class="mw-page-title-main">Dharwar Craton</span> Part of the Indian Shield in south India

The Dharwar Craton is an Archean continental crust craton formed between 3.6-2.5 billion years ago (Ga), which is located in southern India and considered as the oldest part of the Indian peninsula.

<span class="mw-page-title-main">Volcanic and igneous plumbing systems</span> Magma chambers

Volcanic and igneous plumbing systems (VIPS) consist of interconnected magma channels and chambers through which magma flows and is stored within Earth's crust. Volcanic plumbing systems can be found in all active tectonic settings, such as mid-oceanic ridges, subduction zones, and mantle plumes, when magmas generated in continental lithosphere, oceanic lithosphere, and in the sub-lithospheric mantle are transported. Magma is first generated by partial melting, followed by segregation and extraction from the source rock to separate the melt from the solid. As magma propagates upwards, a self-organised network of magma channels develops, transporting the melt from lower crust to upper regions. Channelled ascent mechanisms include the formation of dykes and ductile fractures that transport the melt in conduits. For bulk transportation, diapirs carry a large volume of melt and ascent through the crust. When magma stops ascending, or when magma supply stops, magma emplacement occurs. Different mechanisms of emplacement result in different structures, including plutons, sills, laccoliths and lopoliths.

Magmatism along strike-slip faults is the process of rock melting, magma ascent and emplacement, associated with the tectonics and geometry of various strike-slip settings, most commonly occurring along transform boundaries at mid-ocean ridge spreading centres and at strike-slip systems parallel to oblique subduction zones. Strike-slip faults have a direct effect on magmatism. They can either induce magmatism, act as a conduit to magmatism and magmatic flow, or block magmatic flow. In contrast, magmatism can also directly impact on strike-slip faults by determining fault formation, propagation and slip. Both magma and strike-slip faults coexist and affect one another.

References

  1. 1 2 Kilinc, Attila (1989-12-01). "Partial melting of crustal rocks". Engineering Geology. 27 (1): 279–299. Bibcode:1989EngGe..27..279K. doi:10.1016/0013-7952(89)90036-7. ISSN   0013-7952.
  2. 1 2 3 4 5 Asimow, Paul D. (2016), "Partial Melting", in White, William M. (ed.), Encyclopedia of Geochemistry: A Comprehensive Reference Source on the Chemistry of the Earth, Encyclopedia of Earth Sciences Series, Cham: Springer International Publishing, pp. 1–6, doi:10.1007/978-3-319-39193-9_218-1, ISBN   978-3-319-39193-9 , retrieved 2023-02-13
  3. 1 2 Huang, H.-H.; Lin, F.-C.; Schmandt, B.; Farrell, J.; Smith, R. B.; Tsai, V. C. (2015-05-15). "The Yellowstone magmatic system from the mantle plume to the upper crust". Science. 348 (6236): 773–776. Bibcode:2015Sci...348..773H. doi: 10.1126/science.aaa5648 . ISSN   0036-8075. PMID   25908659. S2CID   3070257.
  4. 1 2 3 Ridley, John (2013). Ore Deposit Geology. Cambridge: Cambridge University Press. doi:10.1017/cbo9781139135528. ISBN   978-1-107-02222-5.
  5. 1 2 3 Winter, John D. (2010). Principles of igneous and metamorphic petrology. John D. Winter (2nd ed.). New York: Prentice Hall. ISBN   978-0-321-59257-6. OCLC   262694332.
  6. ↑ Morse, Stearns A. (1980). Basalts and phase diagrams : an introduction to the quantitative use of phase diagrams in igneous petrology. New York: Springer-Verlag. ISBN   0-387-90477-8. OCLC   6143116.
  7. ↑ Kilinc, I. A.; Burnham, C. W. (1972-04-01). "Partitioning of Chloride Between a Silicate Melt and Coexisting Aqueous Phase from 2 to 8 Kilobars". Economic Geology. 67 (2): 231–235. Bibcode:1972EcGeo..67..231K. doi:10.2113/gsecongeo.67.2.231. ISSN   1554-0774.
  8. ↑ Li, Jiahao; Ding, Xing; Liu, Junfeng (2022). "The Role of Fluids in Melting the Continental Crust and Generating Granitoids: An Overview". Geosciences. 12 (8): 285. Bibcode:2022Geosc..12..285L. doi: 10.3390/geosciences12080285 . ISSN   2076-3263.
  9. ↑ Collins, William J.; Huang, Hui-Qing; Jiang, Xiaoyan (2016-01-04). "Water-fluxed crustal melting produces Cordilleran batholiths". Geology. 44 (2): 143–146. Bibcode:2016Geo....44..143C. doi:10.1130/g37398.1. ISSN   0091-7613.
  10. ↑ Safonov, O. G.; Kosova, S. A. (2017-09-01). "Fluid–mineral reactions and melting of orthopyroxene–cordierite–biotite gneiss in the presence of H2O-CO2-NaCl and H2O-CO2-KCl fluids under parameters of granulite-facies metamorphism". Petrology. 25 (5): 458–485. Bibcode:2017Petro..25..458S. doi:10.1134/S086959111705006X. ISSN   1556-2085. S2CID   135331685.
  11. ↑ Newton, Robert C.; Touret, Jacques L.R.; Aranovich, Leonid Y. (2014). "Fluids and H2O activity at the onset of granulite facies metamorphism". Precambrian Research. 253: 17–25. Bibcode:2014PreR..253...17N. doi:10.1016/j.precamres.2014.06.009. ISSN   0301-9268.
  12. ↑ Grotzinger, John P. (2020). Understanding earth. Thomas H. Jordan (8th ed.). New York, NY. ISBN   978-1-319-05532-5. OCLC   1120096743.{{cite book}}: CS1 maint: location missing publisher (link)
  13. 1 2 McKenzie, D.; Bickle, M.J. (1988). "The Volume and Composition of Melt Generated by Extension of the Lithosphere". Journal of Petrology. 29 (3): 625–679. doi:10.1093/petrology/29.3.625 . Retrieved 2023-03-30.
  14. ↑ Lockwood, John P. (2010). Volcanoes : global perspectives. Richard W. Hazlett. Hoboken, N.J.: Wiley-Blackwell. ISBN   978-1-4051-6249-4. OCLC   452272618.
  15. ↑ Grove, Timothy L.; Chatterjee, Nilanjan; Parman, Stephen W.; Médard, Etienne (2006-09-15). "The influence of H2O on mantle wedge melting". Earth and Planetary Science Letters. 249 (1): 74–89. Bibcode:2006E&PSL.249...74G. doi:10.1016/j.epsl.2006.06.043. ISSN   0012-821X.
  16. ↑ Drummond, M. S.; Defant, M. J.; Kepezhinskas, P. K. (1996). "Petrogenesis of slab-derived trondhjemite–tonalite–dacite/adakite magmas". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 87 (1–2): 205–215. Bibcode:1996EESTR..87..205D. doi: 10.1017/S0263593300006611 . ISSN   1755-6929. S2CID   131616869.
  17. ↑ Hacker, Bradley R.; Abers, Geoffrey A.; Peacock, Simon M. (2003). "Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents". Journal of Geophysical Research: Solid Earth. 108 (B1): 2029. Bibcode:2003JGRB..108.2029H. doi: 10.1029/2001jb001127 . ISSN   0148-0227.
  18. ↑ Kogiso, Tetsu; Omori, Soichi; Maruyama, Shigenori (2009-12-01). "Magma genesis beneath Northeast Japan arc: A new perspective on subduction zone magmatism". Gondwana Research. 16 (3): 446–457. Bibcode:2009GondR..16..446K. doi:10.1016/j.gr.2009.05.006. ISSN   1342-937X.
  19. ↑ Annen, C.; Blundy, J.D.; Sparks, R.S.J. (2006). "The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones". Journal of Petrology. 47 (3) (published March 2006): 505–539. doi: 10.1093/petrology/egi084 .
  20. ↑ Bonin, Bernard (2004-10-01). "Do coeval mafic and felsic magmas in post-collisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review". Lithos. Selected Papers presented at the Symposium: 'Interaction between Mafic and Felsic Magmas in Orogenic Suites: Dynamics of Processes, Nature of End-Members, Effects'. 78 (1): 1–24. Bibcode:2004Litho..78....1B. doi:10.1016/j.lithos.2004.04.042. ISSN   0024-4937.
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