Editing Magma (section)

=== The melting process ===
[[File:Diopside-anorthite phase diagram.png|thumb|Phase diagram for the diopside-anorthite system]]When rocks melt, they do so over a range of temperature, because most rocks are made of several [[minerals]], which all have different melting points. The temperature at which the first melt appears (the solidus) is lower than the melting temperature of any one of the pure minerals. This is  similar to the lowering of the melting point of ice when it is mixed with salt. The first melt is called the ''[[eutectic]]'' and has a composition that depends on the combination of minerals present.{{sfn|Philpotts|Ague|2009|pp=195-197}}

For example, a mixture of [[anorthite]] and [[diopside]], which are two of the predominant minerals in [[basalt]], begins to melt at about 1274&nbsp;°C. This is well below the melting temperatures of 1392&nbsp;°C for pure diopside and 1553&nbsp;°C for pure anorthite. The resulting melt is composed of about 43 wt% anorthite.<ref>{{cite journal |last1=Osborn |first1=E.F. |last2=Tait |first2=D.B. |year=1952 |title=The system diopside-forsterite-anorthite |journal=Am. J. Sci. |volume=250 |pages=413–433 |url=http://earth.geology.yale.edu/~ajs/1952A/413.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://earth.geology.yale.edu/~ajs/1952A/413.pdf |archive-date=2022-10-09 |url-status=live |access-date=9 February 2021}}</ref> As additional heat is added to the rock, the temperature remains at 1274&nbsp;°C until either the anorthite or diopside is fully melted. The temperature then rises as the remaining mineral continues to melt, which shifts the melt composition away from the eutectic. For example, if the content of anorthite is greater than 43%, the entire supply of diopside will melt at 1274&nbsp;°C., along with enough of the anorthite to keep the melt at the eutectic composition. Further heating causes the temperature to slowly rise as the remaining anorthite gradually melts and the melt becomes increasingly rich in anorthite liquid. If the mixture has only a slight excess of anorthite, this will melt before the temperature rises much above 1274&nbsp;°C. If the mixture is almost all anorthite, the temperature will reach nearly the melting point of pure anorthite before all the anorthite is melted. If the anorthite content of the mixture is less than 43%, then all the anorthite will melt at the eutectic temperature, along with part of the diopside, and the remaining diopside will then gradually melt as the temperature continues to rise.{{sfn|Philpotts|Ague|2009|pp=195-197}}

Because of eutectic melting, the composition of the melt can be quite different from the source rock. For example, a mixture of 10% anorthite with diopside could experience about 23% partial melting before the melt deviated from the eutectic, which has the composition of about 43% anorthite.  This effect of partial melting is reflected in the compositions of different magmas. A low degree of partial melting of the upper mantle (2% to 4%) can produce highly alkaline magmas such as [[melilitite]]s, while a greater degree of partial melting (8% to 11%) can produce alkali olivine basalt.<ref>{{cite journal |last1=Zou |first1=Haibo |last2=Zindler |first2=Alan |title=Constraints on the degree of dynamic partial melting and source composition using concentration ratios in magmas |journal=Geochimica et Cosmochimica Acta |date=February 1996 |volume=60 |issue=4 |pages=711–717 |doi=10.1016/0016-7037(95)00434-3|bibcode=1996GeCoA..60..711Z }}</ref> Oceanic magmas likely result from partial melting of 3% to 15% of the source rock.<ref>{{cite journal |last1=Haase |first1=Karsten M. |title=The relationship between the age of the lithosphere and the composition of oceanic magmas: Constraints on partial melting, mantle sources and the thermal structure of the plates |journal=Earth and Planetary Science Letters |date=October 1996 |volume=144 |issue=1–2 |pages=75–92 |doi=10.1016/0012-821X(96)00145-8|bibcode=1996E&PSL.144...75H }}</ref> Some [[Calc-alkaline magma series|calk-alkaline]] [[granitoid]]s may be produced by a high degree of partial melting, as much as 15% to 30%.<ref>{{cite journal |last1=Farahat |first1=Esam S. |last2=Zaki |first2=Rafat |last3=Hauzenberger |first3=Christoph |last4=Sami |first4=Mabrouk |title=Neoproterozoic calc-alkaline peraluminous granitoids of the Deleihimmi pluton, Central Eastern Desert, Egypt: implications for transition from late- to post-collisional tectonomagmatic evolution in the northern Arabian-Nubian Shield |journal=Geological Journal |date=November 2011 |volume=46 |issue=6 |pages=544–560 |doi=10.1002/gj.1289|bibcode=2011GeolJ..46..544F |s2cid=128896568 }}</ref>
High-magnesium magmas, such as [[komatiite]] and [[picrite]], may also be the products of a high degree of partial melting of mantle rock.{{sfn|Philpotts|Ague|2009|p=400}}

Certain chemical elements, called [[incompatible element]]s, have a combination of [[ionic radius]] and [[ionic charge]] that is unlike that of the more abundant elements in the source rock. The ions of these elements fit rather poorly in the structure of the minerals making up the source rock, and readily leave the solid minerals to become highly concentrated in melts produced by a low degree of partial melting.  Incompatible elements commonly include [[potassium]], [[barium]], [[caesium]], and [[rubidium]], which are large and weakly charged (the large-ion lithophile elements, or LILEs), as well as elements whose ions carry a high charge (the high-field-strength elements, or HSFEs), which include such elements as [[zirconium]], [[niobium]], [[hafnium]], [[tantalum]], the [[rare-earth elements]], and the [[actinide]]s. Potassium can become so enriched in melt produced by a very low degree of partial melting that, when the magma subsequently cools and solidifies, it forms unusual potassic rock such as [[lamprophyre]], [[lamproite]], or [[kimberlite]].<ref name="albarede">{{cite book | title = Geochemistry: an introduction | url = https://books.google.com/books?id=doVGzreGq14C&pg=PA17 | publisher = Cambridge University Press | year = 2003 | isbn = 978-0-521-89148-6 | first =Francis | last = Albarède }}</ref>

When enough rock is melted, the small globules of melt (generally occurring between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent of partial melting may be sufficient to cause melt to be squeezed from its source.<ref>{{Cite journal|last=Faul|first=Ulrich H.|date=2001|title=Melt retention and segregation beneath mid-ocean ridges|journal=Nature|volume=410|issue=6831|pages=920–923|doi=10.1038/35073556|pmid=11309614|bibcode=2001Natur.410..920F|s2cid=4403804|issn=0028-0836}}</ref> Melt rapidly separates from its source rock once the degree of partial melting exceeds 30%. However, usually much less than 30% of a magma source rock is melted before the heat supply is exhausted.{{sfn|Philpotts|Ague|2009|p=400, 599}}

[[Pegmatite]] may be produced by low degrees of partial melting of the crust.<ref>{{cite journal |last1=Barros |first1=Renata |last2=Menuge |first2=Julian F. |title=The Origin of Spodumene Pegmatites Associated With the Leinster Granite In Southeast Ireland |journal=The Canadian Mineralogist |date=July 2016 |volume=54 |issue=4 |pages=847–862 |doi=10.3749/canmin.1600027|bibcode=2016CaMin..54..847B |hdl=10197/11562 |s2cid=134105127 |hdl-access=free }}</ref> Some [[granite]]-composition magmas are [[eutectic]] (or cotectic) melts, and they may be produced by low to high degrees of partial melting of the crust, as well as by [[fractional crystallization (geology)|fractional crystallization]].<ref>{{cite journal |last1=Harris |first1=N. B. W. |last2=Inger |first2=S. |title=Trace element modelling of pelite-derived granites |journal=Contributions to Mineralogy and Petrology |date=March 1992 |volume=110 |issue=1 |pages=46–56 |doi=10.1007/BF00310881|bibcode=1992CoMP..110...46H |s2cid=129798034 }}</ref>