View source for Mantle oxidation state

{{Short description|Application of oxidation state to the study of the Earth's mantle}}
[[File:Oxygen fugacity range where cation pairs dominate.png|thumb|Oxygen fugacity range where common cation pairs dominate. Data for plotting are from ''Shearer et al.'', (2006).<ref>{{Cite journal |last1=Shearer |first1=C.K. |last2=Papike |first2=J.J. |last3=Karner |first3=J.M. |date=2006-10-01 |title=Pyroxene europium valence oxybarometer: Effects of pyroxene composition, melt composition, and crystallization kinetics |journal=American Mineralogist |volume=91 |issue=10 |pages=1565–1573 |doi=10.2138/am.2006.2098 |issn=0003-004X |bibcode=2006AmMin..91.1565S |s2cid=2080884}}</ref>
IW represents [[iron]]-[[Wüstite]] [[mineral redox buffer|buffer]] and QFM represents [[quartz]]-[[fayalite]]-[[magnetite]] [[mineral redox buffer|buffer]].|upright=1.35]]

'''Mantle oxidation state (redox state)''' applies the concept of [[oxidation state]] in chemistry to the study of the [[Earth's mantle]]. The chemical concept of oxidation state mainly refers to the [[valence (chemistry)|valence state]] of one [[chemical element|element]], while mantle oxidation state provides the degree of decreasing or increasing valence states of all [[wiktionary:polyvalent#Adjective|polyvalent]] elements in mantle materials confined in a closed system. The mantle oxidation state is controlled by [[oxygen]] [[fugacity]] and can be benchmarked by specific groups of [[mineral redox buffer|redox buffers]].

{{expand section|1=Lead needs to summarize ''how'' mantle oxidation state implicates the other mentioned phenomena|date=October 2024}}
Mantle oxidation state changes because of the existence of polyvalent elements (elements with more than one valence state, e.g. [[iron|Fe]], [[chromium|Cr]], [[vanadium|V]], [[titanium|Ti]], [[cerium|Ce]], [[europium|Eu]], [[carbon|C]] and others). Among them, Fe is the most abundant (≈8 wt% of the mantle<ref>{{Cite journal 
|last1=McDonough |first1=W. F. |last2=Sun |first2=S. -s. |date=1995-03-01 |title=The composition of the Earth |journal=Chemical Geology |series=Chemical Evolution of the Mantle |volume=120 |issue=3 |pages=223–253 |doi=10.1016/0009-2541(94)00140-4 |issn=0009-2541 |bibcode=1995ChGeo.120..223M}}</ref>) and its oxidation state largely reflects the oxidation state of mantle. Examining the [[valence (chemistry)|valence state]] of other polyvalent elements could also provide the information of mantle oxidation state.

It is well known{{clarification needed|reason=As established how?|date=October 2024}} that the oxidation state can influence the [[partition coefficient|partitioning]] behavior of elements<ref>{{Cite journal |last1=Fischer |first1=Rebecca A. |last2=Nakajima |first2=Yoichi |last3=Campbell |first3=Andrew J.|last4=Frost |first4=Daniel J.|last5=Harries |first5=Dennis |last6=Langenhorst |first6=Falko |last7=Miyajima |first7=Nobuyoshi |last8=Pollok |first8=Kilian |last9=Rubie |first9=David C. |date=2015-10-15 |title=High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O |journal=Geochimica et Cosmochimica Acta |volume=167 |pages=177–194 |doi=10.1016/j.gca.2015.06.026 |issn=0016-7037 |bibcode=2015GeCoA.167..177F |doi-access=free}}</ref><ref>{{Cite journal |last1=Corgne |first1=Alexandre |last2=Keshav |first2=Shantanu |last3=Wood |first3=Bernard J. |last4=McDonough |first4=William F. |last5=Fei |first5=Yingwei |date=2008 |title=Metal–silicate partitioning and constraints on core composition and oxygen fugacity during Earth accretion |journal=Geochimica et Cosmochimica Acta |language=en |volume=72 |issue=2 |pages=574–589 |doi=10.1016/j.gca.2007.10.006 |bibcode=2008GeCoA..72..574C}}</ref> and liquid water<ref name=":0">{{Cite journal |last1=Frost |first1=Daniel J. |last2=McCammon |first2=Catherine A. |author-link2=Catherine McCammon |date=2008-04-29 |title=The Redox State of Earth's Mantle |journal=Annual Review of Earth and Planetary Sciences |volume=36 |issue=1 |pages=389–420|bibcode=2008AREPS..36..389F |doi=10.1146/annurev.earth.36.031207.124322 |issn=0084-6597}}</ref> between melts and minerals, the [[ion speciation|speciation]] of C-O-H-bearing fluids and melts,<ref>{{Citation |last1=Holloway |first1=John R. |title=Chapter 6. Application of Experimental Results to C-O-H Species in Natural Melts |date=1994-12-31 |work=Volatiles in Magmas |pages=187–230 |publisher=De Gruyter |isbn=9781501509674 |last2=Blank |first2=Jennifer G. |doi=10.1515/9781501509674-012}}</ref> as well as transport properties like electrical conductivity and creep.<ref name=":0" />

The formation of [[diamond]] requires both reaching high pressures and high temperatures and a carbon source. The most common carbon source in the Earth's [[carbon cycle#diamond|lower mantle]] is not elemental carbon, hence [[redox|redox reactions]] need to be involved in diamond formation. Examining the oxidation state aids in  predicting the P-T conditions of diamond formation and can elucidate the origin of deep diamonds.<ref>{{Cite journal |last1=Luth |first1=R. W. |last2=Stachel |first2=T. |date=2015 
|title=Diamond formation — Where, when and how? |url=https://www.infona.pl//resource/bwmeta1.element.elsevier-c4cbc5bf-f8a5-3cb9-a8d8-53058b5d1a49 |journal=Lithos |language=English |volume=Complete |issue=220–223 |pages=200–220 |doi=10.1016/j.lithos.2015.01.028 |issn=0024-4937 |bibcode=2015Litho.220..200S}}</ref>

== Thermodynamic description of oxidation state ==
Mantle oxidation state can be quantified as the oxygen [[fugacity]] (<math>fO_2</math>) of the system within the framework of [[thermodynamics]]. A higher oxygen fugacity implies a more oxygen-rich and more oxidized environment. At each given [[pressure]]-[[temperature]] conditions, for any compound or element M that bears the potential to be oxidized by oxygen<ref>{{Cite journal|last1=Zhang|first1=H.L.|last2=Hirschmann|first2=M.M.|last3=Cottrell|first3=E.|last4=Withers|first4=A.C.|date=2017|title=Effect of pressure on Fe 3+ /ΣFe ratio in a mafic magma and consequences for magma ocean redox gradients|journal=Geochimica et Cosmochimica Acta|volume=204|pages=83–103|doi=10.1016/j.gca.2017.01.023|bibcode=2017GeCoA.204...83Z|issn=0016-7037|doi-access=free}}</ref><ref>{{Cite journal|last1=Campbell|first1=Andrew J.|last2=Danielson|first2=Lisa|last3=Righter|first3=Kevin|last4=Seagle|first4=Christopher T.|last5=Wang|first5=Yanbin|last6=Prakapenka|first6=Vitali B.|date=2009|title=High pressure effects on the iron–iron oxide and nickel–nickel oxide oxygen fugacity buffers|journal=Earth and Planetary Science Letters|volume=286|issue=3–4|pages=556–564|doi=10.1016/j.epsl.2009.07.022|bibcode=2009E&PSL.286..556C|issn=0012-821X}}</ref>

<math>M+\frac{x}{2}O_2\rightleftharpoons MO_x</math>

For example, if M is Fe, the [[Redox|redox equilibrium reaction]] can be Fe+1/2O<sub>2</sub>=FeO; if M is FeO, the [[Redox|redox equilibrium reaction]] can be 2FeO+1/2O<sub>2</sub>=Fe<sub>2</sub>O<sub>3</sub>.

[[Gibbs free energy|Gibbs energy]] change associated with this reaction is therefore

<math>\Delta G= G(MO_x)-G(M)=\frac{x}{2}RT lnfO_2</math>

Along each [[Isothermal process|isotherm]], the partial derivation of ''ΔG'' with respect to ''P'' is ''ΔV'',

<math>\frac{\partial \Delta G}{\partial P_{|T}}={\Delta V}</math>.{{CN|date=May 2024}}

Combining the 2 equations above,

<math>\frac{\partial (lnfO_2))}{\partial P|_T}=\frac{2}{xRT}\Delta V</math>.

Therefore,

<math>logfO_2(P)=logfO_2(1bar)+(\frac{0.8686}{RT})\int_{1bar}^{P}  \Delta VdP</math> (note that [[Natural logarithm|ln]](e as the base) changed to [[Logarithm|log]](10 as the base) in this formula.

For a closed system, there might exist more than one of these equilibrium oxidation reactions, but since all these reactions share a same <math>fO_2</math>, examining one of them would allow extraction of oxidation state of the system.

== Pressure effect on oxygen fugacity ==
The physics and chemistry of mantle largely depend on [[pressure]]. As mantle minerals are compressed, they are transformed into other minerals at certain depths. Seismic observations of velocity discontinuities and experimental simulations on phase boundaries both verified the structure transformations within the mantle. As such, the mantle can be further divided into three layers with distinct mineral compositions.
{| class="wikitable"
|+Mantle Mineral Composition<ref>{{Cite journal|last=Frost|first=Daniel J.|date=2008-06-01|title=The Upper Mantle and Transition Zone|journal=Elements|language=en|volume=4|issue=3|pages=171–176|doi=10.2113/GSELEMENTS.4.3.171|bibcode=2008Eleme...4..171F |s2cid=129527426 |issn=1811-5209}}</ref>
!Mantle Layer
!Depth
!Pressure
!Major Minerals
|-
|Upper Mantle
|≈10–410&nbsp;km
|≈1-13 GPa
|[[Olivine]], [[Orthopyroxene]], [[Clinopyroxene]], Garnet
|-
|Transition Zone
|410–660&nbsp;km
|13-23 GPa
|[[Wadsleyite]], [[Ringwoodite]], Majoritic [[Garnet]]
|-
|Lower Mantle
|660–2891&nbsp;km
|23-129 GPa
|[[Ferropericlase]], [[Bridgmanite]], Ca-perovskite
|}

Since mantle mineral composition changes, the mineral hosting environment for polyvalent elements also alters. For each layer, the mineral combination governing the redox reactions is unique and will be discussed in detailed below.

=== Upper mantle ===
Between depths of 30 and 60&nbsp;km, oxygen fugacity is mainly controlled by [[Olivine]]-[[Orthopyroxene]]-[[Spinel]] oxidation reaction.

<math>6Fe_2SiO_4+O_2\rightleftharpoons3Fe_2Si_2O_6+2Fe_3O_4</math>

Under deeper upper mantle conditions, [[Olivine]]-[[Orthopyroxene]]-[[Garnet]] oxygen barometer<ref>{{Cite journal|last1=McCammon|first1=C.|last2=Kopylova|first2=M. G.|date=2004-07-17|title=A redox profile of the Slave mantle and oxygen fugacity control in the cratonic mantle|journal=Contributions to Mineralogy and Petrology|volume=148|issue=1|pages=55–68|doi=10.1007/s00410-004-0583-1|issn=0010-7999|bibcode=2004CoMP..148...55M|s2cid=54778255}}</ref> is the redox reaction that is used to calibrate oxygen fugacity. <br /><math>4Fe_2SiO_4+2FeSiO_3+O_2\rightleftharpoons2Fe_3^{2+}Fe_2^{3+}Si_3O_{12}</math>

In this reaction, 4 [[Mole (unit)|mole]] of [[ferrous]] ions were oxidized to ferric ions and the other 2 [[Mole (unit)|mole]] of [[ferrous]] ions remain unchanged.

=== Transition zone ===
Garnet-Garnet<ref name=":1">{{Cite journal|last1=Kiseeva|first1=Ekaterina S.|last2=Vasiukov|first2=Denis M.|last3=Wood|first3=Bernard J.|last4=McCammon|first4=Catherine|last5=Stachel|first5=Thomas|last6=Bykov|first6=Maxim|last7=Bykova|first7=Elena|last8=Chumakov|first8=Aleksandr|last9=Cerantola|first9=Valerio|date=2018-01-22|title=Oxidized iron in garnets from the mantle transition zone|journal=Nature Geoscience|volume=11|issue=2|pages=144–147|doi=10.1038/s41561-017-0055-7|issn=1752-0894|url=http://bib-pubdb1.desy.de/record/418155|bibcode=2018NatGe..11..144K|s2cid=23720021}}</ref> reaction can be used to estimate the redox state of [[Transition zone (Earth)|transition zone]].

<math>2Ca_3Al_2Si_3O_{12}+\frac{4}{3}Fe_3Al_2Si_3O_{12}+2.5Mg_4Si_4O_{12}+O_2
\rightleftharpoons2Ca_3Fe_2Si_3O_{12}+\frac{10}{3}Mg_3Al_2Si_3O_{12}+SiO_2</math>
[[File:Garnet-Group-215473.jpg|thumb|Garnet, a major mineral in the transition zone, controls the oxidation state there.]]
A recent study<ref name=":1" /> showed that the oxygen fugacity of transition referred from Garnet-Garnet reaction is -0.26 <math>logfO_2</math> to +3 <math>logfO_2</math> relative to the Fe-FeO (IW, [[iron]]- [[Wüstite|wütstite]]) [[Mineral redox buffer|oxygen buffer]].

=== Lower mantle ===
Disproportionation of ferrous iron at lower mantle conditions also affect the mantle oxidation state. This reaction is different from the reactions mentioned above as it does not incorporate the participation of free oxygen.

<math>3Fe^{2+}(Fp)\rightleftharpoons Fe+2Fe^{3+}(Bdg)</math>,<ref name=":0" /><ref>{{Cite journal|last1=Rubie|first1=David C.|last2=Trønnes|first2=Reidar G.|last3=Catherine A. McCammon|last4=Langenhorst|first4=Falko|last5=Liebske|first5=Christian|last6=Frost|first6=Daniel J.|date=2004|title=Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle|journal=Nature|language=en|volume=428|issue=6981|pages=409–412|doi=10.1038/nature02413|pmid=15042086|issn=1476-4687|bibcode=2004Natur.428..409F|s2cid=32948214}}</ref>

FeO resides in the form of [[ferropericlase]] (''Fp'') and Fe<sub>2</sub>O<sub>3</sub> resides in the form of [[bridgmanite]] (''Bdg''). There is no oxygen fugacity change associated with the reaction. However, as the reaction products differ in density significantly, the metallic iron phase could descend downwards to the Earth's core and get separated from the mantle. In this case, the mantle loses metallic iron and becomes more oxidized.

== Implications for diamond formation ==
[[File:Diamond-diamond macle1.jpg|thumb|Diamond formed in the Earth's interior]]The equilibrium reaction involving diamond is[[File:Flux of crustal material in the mantle.jpg|thumb|Carbon Cycle involving deep Earth|upright=1.8]]
<math>Mg_2Si_2O_6+2MgCO_3\rightleftharpoons2Mg_2SiO_4+2C(Diamond)+2O_2</math>.

Examining the oxygen fugacity of the upper mantle and transition enables us to compare it with the conditions ([[Chemical equilibrium|equilibrium reaction]] shown above) required for diamond formation. The results show that the <math>logfO_2</math> is usually 2 units lower than the carbonate-carbon reaction<ref name=":1" /> which means favoring the formation of diamond at transition zone conditions.

It has also been reported that [[pH]] decrease would also facilitate the formation of diamond in Mantle conditions.<ref>{{Cite journal|last1=Sverjensky|first1=Dimitri A.|last2=Huang|first2=Fang|date=2015-11-03|title=Diamond formation due to a pH drop during fluid–rock interactions|journal=Nature Communications|volume=6|issue=1|pages=8702|doi=10.1038/ncomms9702|pmid=26529259|issn=2041-1723|bibcode=2015NatCo...6.8702S|pmc=4667645}}</ref>

<math>HCOO^{-}+H^++H_{2,aq} \rightleftharpoons C_{diamond}+2H_2O
</math>

<math>CH_3CH_2COO^-+H^+\rightleftharpoons3C_{diamond}+H_{2,aq}+2H_2O</math>

where the subscript ''aq'' means 'aqueous', implying H<sub>2</sub> is dissolved in the solution.

Deep diamonds have become important windows to look into the [[mineralogy]] of the [[Structure of the Earth|Earth's interior]]. Minerals not stable at the surface could possibly be found within inclusions of superdeep diamonds<ref>{{Cite journal|last1=Zhu|first1=Feng|last2=Li|first2=Jie|last3=Liu|first3=Jiachao|last4=Lai|first4=Xiaojing|last5=Chen|first5=Bin|last6=Meng|first6=Yue|date=2019-02-18|title=Kinetic Control on the Depth Distribution of Superdeep Diamonds|journal=Geophysical Research Letters|volume=46|issue=4|pages=1984–1992|language=en|doi=10.1029/2018GL080740|bibcode=2019GeoRL..46.1984Z|doi-access=free|hdl=2027.42/148362|hdl-access=free}}</ref>—implying they were stable where these diamond crystallized. Because of the hardness of diamonds, the high pressure environment is retained even after transporting to the surface. So far, these superdeep minerals brought by diamonds include [[ringwoodite]],<ref>{{Cite journal|last1=Pearson|first1=D. G.|last2=Brenker|first2=F. E.|last3=Nestola|first3=F.|last4=McNeill|first4=J.|last5=Nasdala|first5=L.|last6=Hutchison|first6=M. T.|last7=Matveev|first7=S.|last8=Mather|first8=K.|last9=Silversmit|first9=G.|date=2014-03-12|title=Hydrous mantle transition zone indicated by ringwoodite included within diamond|journal=Nature|volume=507|issue=7491|pages=221–224|doi=10.1038/nature13080|issn=0028-0836|url=http://bib-pubdb1.desy.de/record/168146/files/10.1038_nature13080.pdf|bibcode=2014Natur.507..221P|pmid=24622201|s2cid=205237822}}</ref> [[Ice VII|ice-VII]],<ref>{{Cite journal|last1=Tschauner|first1=O.|last2=Huang|first2=S.|last3=Greenberg|first3=E.|last4=Prakapenka|first4=V. B.|last5=Ma|first5=C.|last6=Rossman|first6=G. R.|last7=Shen|first7=A. H.|last8=Zhang|first8=D.|last9=Newville|first9=M.|date=2018-03-09|title=Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth's deep mantle|journal=Science|language=en|volume=359|issue=6380|pages=1136–1139|doi=10.1126/science.aao3030|issn=0036-8075|bibcode=2018Sci...359.1136T|pmid=29590042|doi-access=free}}</ref>  cubic δ-[[Nitrogen|N<sub>2</sub>]]<ref>{{Cite journal|last1=Navon|first1=Oded|last2=Wirth|first2=Richard|last3=Schmidt|first3=Christian|last4=Jablon|first4=Brooke Matat|last5=Schreiber|first5=Anja|last6=Emmanuel|first6=Simon|date=2017|title=Solid molecular nitrogen ( δ -N 2 ) inclusions in Juina diamonds: Exsolution at the base of the transition zone|journal=Earth and Planetary Science Letters|language=en|volume=464|pages=237–247|doi=10.1016/j.epsl.2017.01.035|bibcode=2017E&PSL.464..237N}}</ref> and Ca-[[perovskite]].<ref>{{Cite journal|last1=Nestola|first1=F.|last2=Korolev|first2=N.|last3=Kopylova|first3=M.|last4=Rotiroti|first4=N.|last5=Pearson|first5=D. G.|last6=Pamato|first6=M. G.|last7=Alvaro|first7=M.|last8=Peruzzo|first8=L.|last9=Gurney|first9=J. J.|date=2018-03-07|title=CaSiO3 perovskite in diamond indicates the recycling of oceanic crust into the lower mantle|journal=Nature|volume=555|issue=7695|pages=237–241|doi=10.1038/nature25972|pmid=29516998|bibcode=2018Natur.555..237N|s2cid=3763653|issn=0028-0836|url=http://discovery.ucl.ac.uk/10049984/}}</ref>

== See also ==

* [[Ultra-high-pressure metamorphism]]
* [[Polymorphism (materials science)]]
* [[Table of thermodynamic equations]]
* [[List of oxidation states of the elements]]

==References==
{{reflist}}

{{Chemical equilibria}}
{{Earthsinterior}}

[[Category:Earth]]
[[Category:Geochemistry]]
[[Category:Geophysics]]
[[Category:High pressure science]]