Editing Geochemistry (section)

==Differentiation and mixing==
The chemical composition of the Earth and other bodies is determined by two opposing processes: differentiation and mixing. In the Earth's [[mantle (geology)|mantle]], differentiation occurs at [[mid-ocean ridge]]s through [[partial melting]], with more refractory materials remaining at the base of the [[lithosphere]] while the remainder rises to form [[basalt]]. After an oceanic plate descends into the mantle, [[convection]] eventually mixes the two parts together. [[Erosion]] differentiates [[granite]], separating it into [[clay]] on the ocean floor, [[sandstone]] on the edge of the continent, and dissolved minerals in ocean waters. [[Metamorphism]] and [[anatexis]] (partial melting of crustal rocks) can mix these elements together again. In the ocean, biological organisms can cause chemical differentiation, while dissolution of the organisms and their wastes can mix the materials again.<ref name=Albarede/>{{rp|23&ndash;24}}

===Fractionation===
{{See also|magmatic differentiation}}
A major source of differentiation is [[fractionation]], an unequal distribution of elements and isotopes. This can be the result of chemical reactions, [[Phase transition|phase changes]], kinetic effects, or [[radioactivity]].<ref name=Albarede/>{{rp|2&ndash;3}}
On the largest scale, ''[[planetary differentiation]]'' is a physical and chemical separation of a planet into chemically distinct regions. For example, the terrestrial planets formed iron-rich cores and silicate-rich mantles and crusts.<ref name="cosmo"/>{{rp|218}} In the Earth's mantle, the primary source of chemical differentiation is [[partial melting]], particularly near mid-ocean ridges.<ref>{{cite book|last1=Olson|first1=Gerald Schubert; Donald L. Turcotte; Peter|title=Mantle convection in the earth and planets|date=2001|publisher=Cambridge Univ. Press|location=Cambridge|isbn=9780521798365}}</ref>{{rp|68,153}} This can occur when the solid is heterogeneous or a [[solid solution]], and part of the melt is separated from the solid. The process is known as ''equilibrium'' or ''batch'' melting if the solid and melt remain in equilibrium until the moment that the melt is removed, and ''fractional'' or ''Rayleigh'' melting if it is removed continuously.<ref>{{cite book|last1=Wilson|first1=Marjorie|title=Igneous petrogenesis|date=2007|publisher=Springer|location=Dordrecht|isbn=9789401093880}}</ref>

[[Isotopic fractionation]] can have mass-dependent and mass-independent forms. Molecules with heavier isotopes have lower [[Zero-point energy|ground state energies]] and are therefore more stable. As a result, chemical reactions show a small isotope dependence, with heavier isotopes preferring species or compounds with a higher oxidation state; and in phase changes, heavier isotopes tend to concentrate in the heavier phases.<ref name=Kendall>{{cite book|last1=Kendall|first1=Carol|author-link1=Carol Kendall (scientist)|last2=Caldwell|first2=Eric A.|chapter=Chapter 2: Fundamentals of Isotope Geochemistry|editor-last1=Kendall|editor-first1=Carol|editor-last2=McDonnell|editor-first2=J. J.|title=Isotope tracers in catchment hydrology|date=2000|publisher=Elsevier|location=Amsterdam|isbn=9780444501554|pages=51&ndash;86|chapter-url=http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html|access-date=24 October 2017|archive-date=14 March 2008|archive-url=https://web.archive.org/web/20080314192517/http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html|url-status=live}}</ref> Mass-dependent fractionation is largest in light elements because the difference in masses is a larger fraction of the total mass.<ref name=Hoefs>{{cite book|last1=Hoefs|first1=Jochen|title=Stable Isotope Geochemistry |chapter=Isotope Fractionation Processes of Selected Elements |date=2015|pages=47&ndash;134|doi=10.1007/978-3-319-19716-6_2|isbn=978-3-319-19715-9|s2cid=100690717 }}</ref>{{rp|47}}

Ratios between isotopes are generally compared to a standard. For example, sulfur has four stable isotopes, of which the two most common are <sup>32</sup>S and <sup>34</sup>S.<ref name=Hoefs/>{{rp|98}} The ratio of their concentrations, {{math|''R''{{=}}<sup>34</sup>S/<sup>32</sup>S}}, is reported as
:<math>\delta{}^{34}\mathrm{S} = 1000\left(\frac{R}{R_\mathrm{s}}-1\right),</math>
where {{math|''R''<sub>s</sub>}} is the same ratio for a standard. Because the differences are small, the ratio is multiplied by 1000 to make it parts per thousand (referred to as parts per mil). This is represented by the symbol {{math|‰}}.<ref name=Kendall/>{{rp|55}}

====Equilibrium====
''[[Equilibrium fractionation]]'' occurs between chemicals or phases that are in equilibrium with each other. In equilibrium fractionation between phases, heavier phases prefer the heavier isotopes. For two phases A and B, the effect can be represented by the factor
:<math> a_\mathrm{A-B} = \frac{R_\mathrm{A}}{R_\mathrm{B}}. </math>
In the liquid-vapor phase transition for water, {{math|''a''<sub>l-v</sub>}} at 20 degrees [[Celsius]] is 1.0098 for <sup>18</sup>O and 1.084 for <sup>2</sup>H. In general, fractionation is greater at lower temperatures. At 0&nbsp;°C, the factors are 1.0117 and 1.111.<ref name=Kendall/>{{rp|59}}

====Kinetic====
When there is no equilibrium between phases or chemical compounds, ''[[kinetic fractionation]]'' can occur. For example, at interfaces between liquid water and air, the forward reaction is enhanced if the humidity of the air is less than 100% or the water vapor is moved by a wind. Kinetic fractionation generally is enhanced compared to equilibrium fractionation and depends on factors such as reaction rate, reaction pathway and bond energy. Since lighter isotopes generally have weaker bonds, they tend to react faster and enrich the reaction products.<ref name=Kendall/>{{rp|60}}

Biological fractionation is a form of kinetic fractionation since reactions tend to be in one direction. Biological organisms prefer lighter isotopes because there is a lower energy cost in breaking energy bonds. In addition to the previously mentioned factors, the environment and species of the organism can have a large effect on the fractionation.<ref name=Kendall/>{{rp|70}}