Fractional crystallization (geology): Difference between revisions

{{short description|OneProcess of therock main processes of magmatic differentiationformation}}

'''Fractional crystallization''', or '''crystal fractionation''', is one of the most important geochemical and physical processes operating within [[crust (geology)|crust]] and [[Mantle (geology)|mantle]] of a rocky planetary body, such as the Earth. It is important in the formation of [[igneous rock]]s because it is one of the main processes of [[magmatic differentiation]].<ref>''Petrology The Study of Igneous...Rocks'', Loren A. Raymond, 1995, McGraw-Hill, p. 91</ref> Fractional crystallization is also important in the formation of [[sedimentary]] [[evaporite]] rocks or simply fractional crystallization is the removal of early formed crystals from an Original homogeneous magma so that the crystals are prevented from further reaction with the residual melt.

Fractional crystallization is the removal and [[Segregation (materials science)|segregation]] from a melt of [[mineral]] precipitates; except in special cases, removal of the crystals changes the composition of the magma.<ref name="Wilson">{{cite book | url=https://books.google.com/books?id=FkriS1341gYC&pg=PA93&dqq=fractional+crystallization+igneous&hl=en&sa=X&ei=yJAYVOGdCtCI7Aao04HYBg&ved=0CEgQ6AEwBQ#vpg=onepage&q=fractional%20crystallization%20igneous&f=falsePA93 | title=Igneous Petrogenesis A Global Tectonic Approach | publisher=Springer | author=Wilson B. M. | year=1989 | pages=82 | isbn=9780412533105}}</ref> In essence, fractional crystallization is the removal of early formed crystals from an originally homogeneous magma (for example, by gravity settling) so that these crystals are prevented from further reaction with the residual melt.<ref>{{Cite journal |last1=Emeleus |first1=C. H. |last2=Troll |first2=V. R. |date=August 2014 |title=The Rum Igneous Centre, Scotland |journal=Mineralogical Magazine |language=en |volume=78 |issue=4 |pages=805–839 |doi=10.1180/minmag.2014.078.4.04 |issn=0026-461X|doi-access=free |bibcode=2014MinM...78..805E }}</ref> The composition of the remaining melt becomes relatively depleted in some components and enriched in others, resulting in the precipitation of a sequence of different minerals.<ref>''Petrology The Study of Igneous...Rocks'', Loren A. Raymond, 1995, McGraw-Hill, p. 65</ref>

Fractional crystallization in silicate melts ([[magma]]s) is complex compared to crystallization in chemical systems at constant pressure and composition, because changes in pressure and composition can have dramatic effects on magma evolution. Addition and loss of water, [[carbon dioxide]], hydrogen, and oxygen are among the compositional changes that must be considered.<ref name="lange-carmichael-1990">{{cite journal |last1=Lange |first1=R. L. |last2=Carmichael |first2=Ian S. E. |title=Thermodynamic properties of silicate liquids with emphasis on density, thermal expansion and compressibility |journal=Reviews in Mineralogy and Geochemistry |date=1990 |volume=24 |issue=1 |pages=25–64 |url=https://pubs.geoscienceworld.org/msa/rimg/article-abstract/24/1/25/87284/Thermodynamic-properties-of-silicate-liquids-with?redirectedFrom=fulltext |access-date=8 November 2020}}</ref> For example, the [[partial pressure]] ([[fugacity]]) of water in silicate melts can be of prime importance, as in near-[[solidus (chemistry)|solidus]] crystallization of magmas of [[granite]] composition.<ref>{{cite journal |last1=Huang |first1=W. L. |last2=Wyllie |first2=P. J. |title=Melting relations of muscovite-granite to 35 kbar as a model for fusion of metamorphosed subducted oceanic sediments |journal=Contributions to Mineralogy and Petrology |date=March 1973 |volume=42 |issue=1 |pages=1–14 |doi=10.1007/BF00521643|bibcode=1973CoMP...42....1H |s2cid=129917491 }}</ref><ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |pages=604–612 |edition=2nd}}</ref> The crystallization sequence of [[oxide]] minerals such as [[magnetite]] and [[ulvospinel]] is sensitive to the [[mineral redox buffer|oxygen fugacity]] of melts,<ref>{{cite book |last1=McBirney |first1=Alexander R. |title=Igneous petrology |date=1984 |publisher=Freeman, Cooper |location=San Francisco, Calif. |isbn=0877353239 |pages=124–127}}</ref> and separation of the oxide phases can be an important control of [[silica]] concentration in the evolving magma, and may be important in [[andesite]] genesis.<ref>{{cite journal |last1=Juster |first1=Thomas C. |last2=Grove |first2=Timothy L. |last3=Perfit |first3=Michael R. |title=Experimental constraints on the generation of FeTi basalts, andesites, and rhyodacites at the Galapagos Spreading Center, 85°W and 95°W |journal=Journal of Geophysical Research |date=1989 |volume=94 |issue=B7 |pages=9251 |doi=10.1029/JB094iB07p09251|bibcode=1989JGR....94.9251J }}</ref>{{sfn|Philpotts|Ague|2009|pp=609-611}}

One example concerns crystallization of melts that form [[mafic]] and [[ultramafic]] rocks. MgO and SiO₂ concentrations in melts are among the variables that determine whether [[forsterite]] [[olivine]] or [[enstatite]] [[pyroxene]] is precipitated,{{sfn|Philpotts|Ague|2009|pp=201-205}} but the water content and pressure are also important. In some compositions, at high pressures without water crystallization of enstatite is favored, but in the presence of water at high pressures, olivine is favored.<ref>{{cite journal |last1=Kushiro |first1=Ikuo |title=The system forsterite-diopside-silica with and without water at high pressures |journal=American Journal of Science |volume=267.A |year=1969 |pages=269–294 |url=http://earth.geology.yale.edu/~ajs/1969/ajs_267A_11.pdf/269.pdf |access-date=8 November 2020}}</ref>

Granitic magmas provide additional examples of how melts of generally similar composition and temperature, but at different pressure, may crystallize different minerals. Pressure determines the maximum water content of a magma of granite composition. High-temperature fractional crystallization of relatively water-poor [[granite]] magmas may produce single-[[feldspar|alkali-feldspar]] granite, and lower-temperature crystallization of relatively water-rich magma may produce two-[[feldspar]] granite.{{sfn|McBirney|1984|pp=347-348}}

During the process of fractional crystallization, melts become enriched in [[incompatible element]]s.<ref name="Klein2005">{{cite book|first=E. M.|last=Klein|authorlink1author-link1=Emily Klein|chapter =Geochemistry of the Igneous Oceanic Crust|editor=Rudnick, R.|title=The Crust — Treatise on Geochemistry Volume 3|page =442|publisher = Elsevier|location=Amsterdam|publication-date=2005|isbn=0-08-044847-X}}</ref> Hence, knowledge of the crystallization sequence is critical in understanding how melt compositions evolve. Textures of rocks provide insights, as documented in the early 1900s by [[Bowen's reaction series]].<ref>{{Cite book|title=The Evolution of the Igneous Rocks|last=Bowen|first=N. L.|publisher=Dover|year=1956|location=Canada|pages=60–62}}</ref> {{cn span|date=November 2020|An example of such [[Texture (crystalline)|texture]], related to fractioned crystallization, is intergranular (also known as intercumulus) textures that develop wherever a mineral crystallizes later than the surrounding matrix, hence filling the left-over interstitial space. Various oxides of chromium, iron and titanium show such textures, such as intergranular chromite in a siliceous matrix.}} Experimentally-determined [[phase diagram]]s for simple mixtures provide insights into general principles.{{sfn|McBirney|1984|pp=68-102}}{{sfn|Philpotts|Ague|2009|pp=194-240}} Numerical calculations with special software have become increasingly able to simulate natural processes accurately.{{sfn|Philpotts|Ague|2009|pp=239-240}}<ref>{{cite journal |last1=Ghiorso |first1=Mark S. |last2=Hirschmann |first2=Marc M. |last3=Reiners |first3=Peter W. |last4=Kress |first4=Victor C. |title=The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa: pMELTS, A REVISION OF MELTS |journal=Geochemistry, Geophysics, Geosystems |date=May 2002 |volume=3 |issue=5 |pages=1–35 |doi=10.1029/2001GC000217|doi-access=free }}</ref>

Fractional crystallization is important in the formation of sedimentary evaporite rocks.<ref>{{cite journal |last1=Raab |first1=M. |last2=Spiro |first2=B. |title=Sulfur isotopic variations during seawater evaporation with fractional crystallization |journal=Chemical Geology: Isotope Geoscience Section |date=April 1991 |volume=86 |issue=4 |pages=323–333 |doi=10.1016/0168-9622(91)90014-N|bibcode=1991CGIGS..86..323R }}</ref>