Editing Serpentinization (section)

== Formation and petrology ==
Serpentinization is a form of low-temperature (0 to ~600 °C) <ref>{{Cite journal |last=Evans |first=Bernard W. |date=2004-06-01 |title=The Serpentinite Multisystem Revisited: Chrysotile Is Metastable |url=https://doi.org/10.2747/0020-6814.46.6.479 |journal=International Geology Review |volume=46 |issue=6 |pages=479–506 |doi=10.2747/0020-6814.46.6.479 |bibcode=2004IGRv...46..479E |s2cid=98271088 |issn=0020-6814|url-access=subscription }}</ref> [[metamorphism]] of ferromagnesian minerals in mafic and [[ultramafic]] rocks, such as [[dunite]], [[harzburgite]], or [[lherzolite]]. These are rocks low in [[silica]] and composed mostly of [[olivine]] ({{chem2|(Mg(2+), Fe(2+))2SiO4}}), [[pyroxene]] ({{chem2|XY(Si,Al)2O6}}), and [[chromite]] (approximately {{chem2|FeCr2O4}}). Serpentinization is driven largely by [[Mineral hydration|hydration]] and [[oxidation]] of olivine and pyroxene to [[serpentine subgroup|serpentine group]] minerals (antigorite, lizardite, and chrysotile), [[brucite]] ({{chem2|Mg(OH)2}}), [[talc]] ({{chem2|Mg3Si4O10(OH)2}}), and [[magnetite]] ({{chem2|Fe3O4}}).<ref name=Moody1976>{{cite journal |last1=Moody |first1=Judith B. |title=Serpentinization: a review |journal=Lithos |date=April 1976 |volume=9 |issue=2 |pages=125–138 |doi=10.1016/0024-4937(76)90030-X|bibcode=1976Litho...9..125M }}</ref> Under the unusual chemical conditions accompanying serpentinization, water is the oxidizing agent, and is itself reduced to hydrogen, {{chem|[[hydrogen|H]]|2}}. This leads to further reactions that produce rare [[iron group]] [[native element mineral]]s, such as [[awaruite]] ({{chem|Ni|3|Fe}}) and [[native iron]]; [[methane]] and other [[hydrocarbon]] compounds; and [[hydrogen sulfide]].<ref name=":0" /><ref name=BerndtEtal1996>{{cite journal |last1=Berndt |first1=Michael E. |last2=Allen |first2=Douglas E. |last3=Seyfried |first3=William E. |title=Reduction of {{CO2}} during serpentinization of olivine at 300&nbsp;°C and 500 bar |journal=Geology |date=1 April 1996 |volume=24 |issue=4 |pages=351–354 |doi=10.1130/0091-7613(1996)024<0351:ROCDSO>2.3.CO;2|bibcode=1996Geo....24..351B }}</ref>

During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure.{{sfn|Moody|1976|p=128-129}} The density changes from {{convert|3.3|to|2.5|g/cm3|abbr=on}} with a concurrent volume increase on the order of 30-40%.<ref name=Mevel2003>{{cite journal |last1=Mével |first1=Catherine |title=Serpentinization of abyssal peridotites at mid-ocean ridges |journal=Comptes Rendus Geoscience |date=September 2003 |volume=335 |issue=10–11 |pages=825–852 |doi=10.1016/j.crte.2003.08.006|bibcode=2003CRGeo.335..825M }}</ref> The reaction is highly [[exothermic]], releasing up to {{convert|40|kJ|kcal|sp=us}} per mole of water reacting with the rock, and rock temperatures can be raised by about {{convert|260|Celsius}},<ref name="LC">[http://www.lostcity.washington.edu/science/chemistry/serpentinization.html Serpentinization: The heat engine at Lost City and sponge of the oceanic crust]</ref><ref name="FruhGreenEtal20042">{{cite journal |last1=Früh-Green |first1=Gretchen L. |last2=Connolly |first2=James A.D. |last3=Plas |first3=Alessio |last4=Kelley |first4=Deborah S. |last5=Grobéty |first5=Bernard |date=2004 |title=Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological activity |journal=Geophysical Monograph Series |volume=144 |pages=119–136 |bibcode=2004GMS...144..119F |doi=10.1029/144GM08 |isbn=0-87590-409-2}}</ref> providing an energy source for formation of non-volcanic [[hydrothermal vent]]s.<ref name=Lowell2002>{{cite journal |last1=Lowell |first1=R. P. |title=Seafloor hydrothermal systems driven by the serpentinization of peridotite |journal=Geophysical Research Letters |date=2002 |volume=29 |issue=11 |pages=1531 |doi=10.1029/2001GL014411|bibcode=2002GeoRL..29.1531L |doi-access=free }}</ref> The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea [[chemotroph]] [[microorganism]]s.<ref name=FruhGreenEtal2004>{{cite journal |last1=Früh-Green |first1=Gretchen L. |last2=Connolly |first2=James A.D. |last3=Plas |first3=Alessio |last4=Kelley |first4=Deborah S. |last5=Grobéty |first5=Bernard |title=Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological activity |journal=Geophysical Monograph Series |date=2004 |volume=144 |pages=119–136 |doi=10.1029/144GM08|bibcode=2004GMS...144..119F |isbn=0-87590-409-2 }}</ref><ref name=LC/>

===Formation of serpentine minerals ===
Olivine is a [[solid solution]] of [[forsterite]], the [[magnesium]] endmember of {{chem2|(Mg(2+), Fe(2+))2SiO4}}, and [[fayalite]], the [[iron]] endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks.<ref name=SnowDick1995>{{cite journal |last1=Snow |first1=Jonathan E. |last2=Dick |first2=Henry J.B. |title=Pervasive magnesium loss by marine weathering of peridotite |journal=Geochimica et Cosmochimica Acta |date=October 1995 |volume=59 |issue=20 |pages=4219–4235 |doi=10.1016/0016-7037(95)00239-V|bibcode=1995GeCoA..59.4219S }}</ref> [[Serpentine subgroup|Serpentine]] can form from [[olivine]] via several reactions:

{{NumBlk|:
|{{overset|[[Forsterite]]|3 {{chem|Mg|2|SiO|4}}}} + {{overset|silicon dioxide|{{chem|SiO|2}}}} + 4 {{chem|H|2|O}} → {{overset|serpentine|2 {{chem|Mg|3|Si|2|O|5|(OH)|4}}}}
|{{EquationRef|Reaction 1a}}}}

{{NumBlk|:
|{{overset|[[Forsterite]]|2 {{chem|Mg|2|SiO|4}}}} + {{overset|water|3 {{chem|H|2|O}}}} → {{overset|serpentine|{{chem|Mg|3|Si|2|O|5|(OH)|4}}}} + {{overset|[[brucite]]|{{chem|Mg|(OH)|2}}}}
|{{EquationRef|Reaction 1b}}}}

Reaction 1a tightly binds silica, lowering its [[chemical activity]] to the lowest values seen in common rocks of the [[Earth's crust]].<ref name=FrostBeard2007>{{cite journal |last1=Frost |first1=B. R. |last2=Beard |first2=J. S. |title=On Silica Activity and Serpentinization |journal=Journal of Petrology |date=3 April 2007 |volume=48 |issue=7 |pages=1351–1368 |doi=10.1093/petrology/egm021|url=http://petrology.oxfordjournals.org/content/48/7/1351.full.pdf }}</ref> Serpentinization then continues through the hydration of olivine to yield serpentine and brucite (Reaction 1b).<ref name="Coleman77">{{cite book|last=Coleman|first=Robert G.|title=Ophiolites|date=1977|publisher=Springer-Verlag|isbn=978-3540082767|pages=100–101}}</ref> The mixture of brucite and serpentine formed by Reaction 1b has the lowest silica activity in the [[serpentinite]], so that the brucite phase is very important in understanding serpentinization.<ref name=FrostBeard2007/> However, the brucite is often blended in with the serpentine such that it is difficult to identify except with [[X-ray diffraction]], and it is easily altered under surface weathering conditions.{{sfn|Moody|1976|p=127}}

A similar suite of reactions involves [[pyroxene]]-group minerals:

{{NumBlk|:
|{{overset|[[Enstatite]]|3 {{chem|Mg|SiO|3}}}} + {{overset|silicon dioxide|{{chem|SiO|2}}}} + {{chem|H|2|O}} → {{overset|[[talc]]|{{chem|Mg|3|Si|4|O|10|(OH)|2}}}}
|{{EquationRef|Reaction 2a}}}}

{{NumBlk|:
|{{overset|[[Enstatite]]|6 {{chem|Mg|SiO|3}}}} + 3 {{chem|H|2|O}} → {{overset|serpentine|{{chem|Mg|3|Si|2|O|5|(OH)|4}}}} + {{overset|[[talc]]|{{chem|Mg|3|Si|4|O|10|(OH)|2}}}}
|{{EquationRef|Reaction 2b}}}}

Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over.{{sfn|Frost|Beard|2007|p=1355}} When olivine is abundant, silica activity drops low enough that talc begins to react with olivine:

{{NumBlk|:
|{{overset|[[Forsterite]]|6 {{chem|Mg|2|SiO|4}}}} + {{overset|[[talc]]|{{chem|Mg|3|Si|4|O|10|(OH)|2}}}} + {{overset|water|9 {{chem|H|2|O}}}} → {{overset|serpentine|5 {{chem|Mg|3|Si|2|O|5|(OH)|4}}}}
|{{EquationRef|Reaction 3}}}}

This reaction requires higher temperatures than those at which brucite forms.{{sfn|Moody|1976|p=127}}

The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed {{convert|600|C|F|abbr=on}} during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface.{{sfn|Moody|1976|p=125, 127, 131}}

===Breakdown of diopside and formation of rodingites===
Ultramafic rocks often contain calcium-rich pyroxene ([[diopside]]), which breaks down according to the reaction:

{{NumBlk|:
|{{overset|[[Diopside]]|3 {{chem|Ca|Mg|Si|2|O|6}}}} + 6 {{chem|H|+}} → {{overset|serpentine|{{chem|Mg|3|Si|2|O|5|(OH)|4}}}} + 3 {{chem|Ca|2+}} + {{chem|H|2|O}} + {{overset|silicon dioxide|4 {{chem|SiO|2}}}}
|{{EquationRef|Reaction 4}}}}

This raises both the [[pH]], often to very high values, and the calcium content of the fluids involved in serpentinization. These fluids are highly reactive and may transport [[calcium]] and other elements into surrounding [[mafic]] rocks. Fluid reaction with these rocks may create [[metasomatism|metasomatic]] reaction zones enriched in calcium and depleted in silica, called [[rodingite]]s.{{sfn|Frost|Beard|2007|pp=1360-1362}}

===Formation of magnetite and hydrogen===
{{See also|Schikorr reaction}}
In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the [[Mineral redox buffer#Common redox buffers and mineralogy|fayalite-magnetite-quartz (FMQ) buffer]].{{sfn|Moody|1976|p=129}} The very low chemical activity of silica during serpentinization eliminates this buffer, creating highly [[Reduction (chemistry)|reducing]] conditions<ref name=FrostBeard2007/> that allow water to oxidize ferrous ({{chem|Fe|2+}}) ions in fayalite. This reaction modifies minerals and [[Hydrogen_cycle#Sources|liberates hydrogen gas]]:<ref name=":0" /><ref>{{cite web| title = Methane and hydrogen formation from rocks – Energy sources for life| url = http://www.lostcity.washington.edu/science/chemistry/methane.html| access-date = 2011-11-06}}</ref><ref>{{Cite journal| last = Sleep| first = N.H.| author2 = A. Meibom, Th. Fridriksson, R.G. Coleman, D.K. Bird| year = 2004| title = H<sub>2</sub>-rich fluids from serpentinization: Geochemical and biotic implications| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 101| issue = 35| pages = 12818–12823| doi = 10.1073/pnas.0405289101|bibcode = 2004PNAS..10112818S| pmid=15326313| pmc=516479| doi-access = free}}</ref>

{{NumBlk|:
|{{overset|[[Fayalite]]|3 {{chem|Fe|2|SiO|4}}}} + {{overset|water|2 {{chem|H|2|O}}}} → {{overset|[[magnetite]]|2 {{chem|Fe|3|O|4}}}} + {{overset|silicon dioxide|3 {{chem|SiO|2}}}} + {{overset|hydrogen|2 {{chem|H|2}}}}
|{{EquationRef|Reaction 5}}}}

Studies of serpentinites suggest that in nature iron minerals are first converted to [[wikt:ferroan|ferroan]] brucite, that is, brucite containing {{chem2|Fe(OH)2}},<ref>{{cite journal |last1=Bach |first1=Wolfgang |last2=Paulick |first2=Holger |last3=Garrido |first3=Carlos J. |last4=Ildefonse |first4=Benoit |last5=Meurer |first5=William P. |last6=Humphris |first6=Susan E. |title=Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274) |journal=Geophysical Research Letters |date=2006 |volume=33 |issue=13 |pages=L13306 |doi=10.1029/2006GL025681|bibcode=2006GeoRL..3313306B |hdl=1912/3324 |s2cid=55802656 |hdl-access=free }}</ref> which then undergoes the [[Schikorr reaction]] in the anaerobic conditions of serpentinization:<ref name=Esource>{{cite journal |doi=10.1111/j.1472-4669.2010.00249.x|title=Serpentinization as a source of energy at the origin of life|year=2010|last1=Russell|first1=M. J.|last2=Hall|first2=A. J.|last3=Martin|first3=W.|journal=Geobiology|volume=8|issue=5|pages=355–371|pmid=20572872|s2cid=41118603 }}</ref><ref>{{cite journal |doi=10.2138/rmg.2013.75.18|title=Serpentinization, Carbon, and Deep Life|year=2013|last1=Schrenk|first1=M. O.|last2=Brazelton|first2=W. J.|last3=Lang|first3=S. Q.|journal=Reviews in Mineralogy and Geochemistry|volume=75|issue=1|pages=575–606|bibcode=2013RvMG...75..575S}}</ref>

{{NumBlk|:
|{{underset|ferrous hydroxide|6 {{chem|Fe|(OH)|2}}}} → {{underset|magnetite|2 {{chem|Fe|3|O|4}}}} + {{underset|water|4 {{chem|H|2|O}}}}  + {{underset|hydrogen|2 {{chem|H|2}}}}
|{{EquationRef|Reaction 6}}}}

Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between {{convert|200 and 315|C||sp=us}}<ref>{{cite journal |last1=McCollom |first1=Thomas M. |last2=Bach |first2=Wolfgang |title=Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks |journal=Geochimica et Cosmochimica Acta |date=February 2009 |volume=73 |issue=3 |pages=856–875 |doi=10.1016/j.gca.2008.10.032|bibcode=2009GeCoA..73..856M }}</ref> and when fluids are carbonate undersaturated.<ref name=":0" /> If the original ultramafic rock (the ''[[protolith]]'') is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen.<ref>{{cite journal |last1=Klein |first1=Frieder |last2=Bach |first2=Wolfgang |last3=McCollom |first3=Thomas M. |title=Compositional controls on hydrogen generation during serpentinization of ultramafic rocks |journal=Lithos |date=September 2013 |volume=178 |pages=55–69 |doi=10.1016/j.lithos.2013.03.008|bibcode=2013Litho.178...55K }}</ref>

Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite).{{sfn|Moody|1976|p=128}} During serpentinization, the rock is enriched in [[chlorine]], [[boron]], [[fluorine]], and sulfur. Sulfur will be reduced to hydrogen sulfide and sulfide minerals, though significant quantities are incorporated into serpentine minerals, and some may later be reoxidized to sulfate minerals such as [[anhydrite]].<ref>{{Cite journal |last1=Debret |first1=Baptiste |last2=Andreani |first2=Muriel |last3=Delacour |first3=Adélie |last4=Rouméjon |first4=Stéphane |last5=Trcera |first5=Nicolas |last6=Williams |first6=Helen |date=2017-05-15 |title=Assessing sulfur redox state and distribution in abyssal serpentinites using XANES spectroscopy |url=https://www.sciencedirect.com/science/article/pii/S0012821X17300973 |journal=Earth and Planetary Science Letters |language=en |volume=466 |pages=1–11 |doi=10.1016/j.epsl.2017.02.029 |bibcode=2017E&PSL.466....1D |issn=0012-821X|doi-access=free |hdl=20.500.11850/207239 |hdl-access=free }}</ref> The sulfides produced include nickel-rich sulfides, such as [[mackinawite]].<ref>{{cite journal |last1=Delacour |first1=Adélie |last2=Früh-Green |first2=Gretchen L. |last3=Bernasconi |first3=Stefano M. |title=Sulfur mineralogy and geochemistry of serpentinites and gabbros of the Atlantis Massif (IODP Site U1309) |journal=Geochimica et Cosmochimica Acta |date=October 2008 |volume=72 |issue=20 |pages=5111–5127 |doi=10.1016/j.gca.2008.07.018|bibcode=2008GeCoA..72.5111D }}</ref>

===Methane and other hydrocarbons===
Laboratory experiments have confirmed that at a temperature of {{convert|300|C||sp=us}} and pressure of 500 bars, olivine serpentinizes with release of hydrogen gas. In addition, methane and complex hydrocarbons are formed through reduction of carbon dioxide. The process may be catalyzed by magnetite formed during serpentinization.<ref name=BerndtEtal1996/> One reaction pathway is:<ref name=Esource/>

{{NumBlk|:
|{{overset|forsterite|18 {{chem|Mg|2|SiO|4}}}} + {{overset|fayalite|6 {{chem|Fe|2|SiO|4}}}} + 26 {{chem|H|2|O}} + {{chem|CO|2}} → {{overset|serpentine|12 {{chem|Mg|3|Si|2|O|5|(OH)|4}}}} + {{overset|magnetite|4 {{chem|Fe|3|O|4}}}} + {{overset|methane|{{chem|CH|4}}}}
|{{EquationRef|Reaction 7}}}}

===Metamorphism at higher pressure and temperature===
Lizardite and chrysotile are stable at low temperatures and pressures, while antigorite is stable at higher temperatures and pressure.<ref>{{Cite journal |last=Evans |first=Bernard W. |date=2004-06-01 |title=The Serpentinite Multisystem Revisited: Chrysotile Is Metastable |journal=International Geology Review |volume=46 |issue=6 |pages=479–506 |doi=10.2747/0020-6814.46.6.479 |bibcode=2004IGRv...46..479E |s2cid=98271088 |issn=0020-6814}}</ref> Its presence in a serpentinite indicates either that serpentinization took place at unusually high pressure and temperature or that the rock experienced higher grade metamorphism after serpentinization was complete.<ref name="Moody1976" />

Infiltration of {{CO2}}-bearing fluids into serpentinite causes distinctive ''[[Talc carbonate|talc-carbonate alteration]]''.<ref>{{cite journal |last1=Naldrett |first1=A. J. |title=Tale-Carbonate Alteration of some Serpentinized Ultramafic Rocks south of Timmins, Ontario |journal=Journal of Petrology |date=1 October 1966 |volume=7 |issue=3 |pages=489–499 |doi=10.1093/petrology/7.3.489}}</ref> Brucite rapidly converts to [[magnesite]] and serpentine minerals (other than antigorite) are converted to talc. The presence of [[pseudomorph]]s of the original serpentinite minerals shows that this alteration takes place after serpentinization.<ref name="Moody1976" />

Serpentinite may contain [[chlorite group|chlorite]] (a [[phyllosilicate]] mineral), [[tremolite]] (Ca<sub>2</sub>(Mg<sub>5.0-4.5</sub>Fe<sup>2+</sup><sub>0.0-0.5</sub>)Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>), and metamorphic olivine and [[diopside]] (calcium-rich pyroxene). This indicates that the serpentinite has been subject to more intense metamorphism, reaching the upper [[greenschist]] or [[amphibolite]] [[metamorphic facies]].<ref name="Moody1976" />

Above about {{convert|450|C||sp=us}}, antigorite begins to break down. Thus serpentinite does not exist at higher metamorphic facies.<ref name=FruhGreenEtal2004/>

===Extraterrestrial production of methane by serpentinization===
The presence of traces of [[Methane on Mars|methane in the atmosphere of Mars]] has been hypothesized to be a possible evidence for [[life on Mars (planet)|life on Mars]] if methane was produced by [[bacteria]]l activity. Serpentinization has been proposed as an alternative non-biological source for the observed methane traces.<ref>{{cite journal|jstor=27858733|title=Life on Mars?|date=March–April 2006|journal=American Scientist|volume=94|issue=2|pages=119–120|last1=Baucom|first1=Martin|doi=10.1511/2006.58.119}}</ref><ref>{{Cite web|url=https://www.esa.int/Our_Activities/Human_and_Robotic_Exploration/Exploration/ExoMars/The_methane_mystery|title=The methane mystery|last=esa|website=European Space Agency|language=en-GB|access-date=2019-04-22}}</ref> In 2022 it was reported that microscopic examination of the [[ALH 84001]] meteorite, which came from Mars, shows that indeed the organic matter it contains was formed by serpentinization, not by life processes.<ref>{{cite journal |display-authors=etal|last1=Andrew Steele |title=Organic synthesis associated with serpentinization and carbonation on early Mars |journal=Science |date=13 January 2022 |volume=375 |issue=6577 |pages=172–177 |doi=10.1126/science.abg7905|pmid=35025630 |bibcode=2022Sci...375..172S |s2cid=245933224 }}</ref><ref>{{cite journal |last1=Leah Crane |title=Mars: Organic compounds were made by water interacting with rocks |journal=New Scientist |date=22 January 2022 |url=https://www.newscientist.com/article/2304270-organic-compounds-on-mars-were-produced-by-water-and-rocks-not-life/}}</ref>

Using data from the [[Cassini–Huygens|Cassini]] probe flybys obtained in 2010–12, scientists were able to confirm that Saturn's moon [[Enceladus]] likely has a liquid water ocean beneath its frozen surface. A model suggests that the ocean on Enceladus has an alkaline [[pH]] of 11–12.<ref name="pH 2015">{{cite journal |title=The pH of Enceladus' ocean |journal=Geochimica et Cosmochimica Acta |date=16 April 2015 |last1=R. Glein |first1=Christopher |last2= Baross |first2= John A. |last3=Waite |first3=Hunter |doi=10.1016/j.gca.2015.04.017 |bibcode=2015GeCoA.162..202G |volume=162 |pages=202–219|arxiv=1502.01946 |s2cid=119262254 }}</ref> The high pH is interpreted to be a key consequence of serpentinization of [[chondrite|chondritic rock]], that leads to the generation of {{chem|H|2}}, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules.<ref name="pH 2015"/><ref>{{cite news |last=Wall |first=Mike |url=http://www.space.com/29334-enceladus-ocean-energy-source-life.html |title=Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life |work=Space.com |date=7 May 2015 |access-date=2015-05-08 }}</ref>