Editing Basalt (section)

=== Geochemistry ===
Relative to most common igneous rocks, basalt compositions are rich in [[magnesium oxide|MgO]] and [[calcium oxide|CaO]] and low in [[silicon dioxide|SiO<sub>2</sub>]] and the alkali oxides, i.e., [[sodium oxide|Na<sub>2</sub>O]] + [[potassium oxide|K<sub>2</sub>O]], consistent with their [[TAS classification]]. Basalt contains more silica than [[picrobasalt]] and most [[basanite]]s and [[tephrite]]s but less than [[basaltic andesite]]. Basalt has a lower total content of alkali oxides than [[trachybasalt]] and most basanites and tephrites.{{sfn|Philpotts|Ague|2009|pp=139–143}}

Basalt generally has a composition of 45–52 [[wt%]] SiO<sub>2</sub>, 2–5 wt% total alkalis,{{sfn|Philpotts|Ague|2009|pp=139–143}} 0.5–2.0 wt% [[titanium dioxide|TiO<sub>2</sub>]], 5–14 wt% [[iron(II) oxide|FeO]] and 14 wt% or more [[alumina|Al<sub>2</sub>O<sub>3</sub>]]. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.<ref name="irvine-baragar-1971">{{cite journal |last1=Irvine |first1=T. N. |last2=Baragar |first2=W. R. A. |title=A Guide to the Chemical Classification of the Common Volcanic Rocks |journal=Canadian Journal of Earth Sciences |date=1 May 1971 |volume=8 |issue=5 |pages=523–548 |doi=10.1139/e71-055|bibcode=1971CaJES...8..523I }}</ref>

High-alumina basalts have aluminium contents of 17–19 wt% Al<sub>2</sub>O<sub>3</sub>; [[boninite]]s have [[magnesium]] (MgO) contents of up to 15 percent. Rare [[feldspathoid]]-rich [[mafic]] rocks, akin to alkali basalts, may have Na<sub>2</sub>O + K<sub>2</sub>O contents of 12% or more.{{sfn|Irvine|Baragar|1971}}

The abundances of the [[lanthanide]] or [[rare-earth element]]s (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the [[europium anomaly]]. It arises because Eu<sup>2+</sup> can substitute for Ca<sup>2+</sup> in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form <sup>3+</sup> [[cation]]s.{{sfn|Philpotts|Ague|2009|p=359}}

Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in [[Compatibility (geochemistry)|incompatible]] trace elements, and they have relatively flat REE patterns normalized to mantle or [[chondrite]] values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt is considered a key to understanding [[plate tectonics]], its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the [[Mid-Atlantic Ridge]], and the compositions also define different ranges in different ocean basins.<ref>{{cite book |doi=10.1016/B978-0-08-095975-7.00203-5 |chapter=Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements |title=Treatise on Geochemistry |year=2014 |last1=Hofmann |first1=A.W. |pages=67–101 |isbn=978-0-08-098300-4 }}</ref> Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).{{sfn|Philpotts|Ague|2009|p=312}}

[[Isotope]] ratios of [[chemical element|elements]] such as [[strontium]], [[neodymium]], [[lead]], [[hafnium]], and [[osmium]] in basalts have been much studied to learn about the evolution of the [[Earth's mantle]].{{sfn|Philpotts|Ague|2009|loc=Chapter 13}} Isotopic ratios of [[noble gas]]es, such as <sup>3</sup>[[Helium|He]]/<sup>4</sup>He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from [[mantle plume]]s.<ref name="class-goldstein-2005">{{cite journal |last1=Class |first1=Cornelia |last2=Goldstein |first2=Steven L. |title=Evolution of helium isotopes in the Earth's mantle |journal=Nature |date=August 2005 |volume=436 |issue=7054 |pages=1107–1112 |doi=10.1038/nature03930|pmid=16121171 |bibcode=2005Natur.436.1107C |s2cid=4396462 }}</ref>

Source rocks for the partial melts that produce basaltic magma probably include both [[peridotite]] and [[pyroxenite]].<ref name="sobolev-etal-2007">{{cite journal|author=Alexander V. Sobolev|author2=Albrecht W. Hofmann|author3=Dmitry V. Kuzmin|author4=Gregory M. Yaxley|author5=Nicholas T. Arndt|author6-link=Sun-Lin Chung|author6=Sun-Lin Chung|author7=Leonid V. Danyushevsky|author8=Tim Elliott|author9=Frederick A. Frey|author10=Michael O. Garcia|author11=Andrey A. Gurenko|author12=Vadim S. Kamenetsky|author13=Andrew C. Kerr|author14=Nadezhda A. Krivolutskaya|author15=Vladimir V. Matvienkov|author16=Igor K. Nikogosian|author17=Alexander Rocholl|author18=Ingvar A. Sigurdsson|author19=Nadezhda M. Sushchevskaya|author20=Mengist Teklay|name-list-style=amp |title=The Amount of Recycled Crust in Sources of Mantle-Derived Melts|journal=Science|date=20 April 2007|volume=316|issue=5823|pages=412–417|bibcode=2007Sci...316..412S|doi=10.1126/science.x|pmid=17395795|url=http://eprints.utas.edu.au/2614/1/Science2007.pdf}}
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