Editing Carbon cycle (section)

==={{anchor|diamond}}Carbon in the lower mantle ===
[[File:Carbon_Outgassing_(Dasgupta_2011).png|thumb|upright=1.8| {{center|Carbon outgassing through various processes{{hsp}}<ref>{{Cite web|url=http://www.deep-earth.org/postAGU2011/Dasgupta-cider-agu2011.ppt|title=From Magma Ocean to Crustal Recycling: Earth's Deep Carbon Cycle|last=Dasgupta|first=Rajdeep|date=10 December 2011|access-date=9 March 2019|archive-url=https://web.archive.org/web/20160424031155/http://www.deep-earth.org/postAGU2011/Dasgupta-cider-agu2011.ppt|archive-date=24 April 2016|url-status=dead}}</ref>}}]]

Carbon principally enters the mantle in the form of [[carbonate]]-rich sediments on [[Plate tectonics|tectonic plates]] of ocean crust, which pull the carbon into the mantle upon undergoing [[subduction]]. Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the element's movement and forms within the region. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the [[lower mantle (Earth)|lower mantle]]. The study analyzed rare, super-deep [[diamond]]s at a site in [[Juína|Juina, Brazil]], determining that the bulk composition of some of the diamonds' inclusions matched the expected result of basalt melting and [[Crystallization|crystallisation]] under lower mantle temperatures and pressures.<ref>{{cite press release |title=Carbon cycle reaches Earth's lower mantle: Evidence of carbon cycle found in 'superdeep' diamonds From Brazil |url=https://www.sciencedaily.com/releases/2011/09/110915141227.htm |work=ScienceDaily |publisher=American Association for the Advancement of Science |date=15 September 2011 }}</ref> Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principle transport mechanism for carbon to Earth's deep interior. These subducted carbonates can interact with lower mantle [[silicate]]s, eventually forming super-deep diamonds like the one found.<ref>{{cite journal |last1=Stagno |first1=V. |last2=Frost |first2=D. J. |last3=McCammon |first3=C. A. |last4=Mohseni |first4=H. |last5=Fei |first5=Y. |title=The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks |journal=Contributions to Mineralogy and Petrology |date=February 2015 |volume=169 |issue=2 |page=16 |doi=10.1007/s00410-015-1111-1 |bibcode=2015CoMP..169...16S }}</ref>

However, carbonates descending to the lower mantle encounter other fates in addition to forming diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800&nbsp;km deep into the Earth, well within the lower mantle. Doing so resulted in the formations of [[magnesite]], [[siderite]], and numerous varieties of [[graphite]].<ref name="Fiquet 5184–5187">{{cite journal |last1=Boulard |first1=Eglantine |last2=Gloter |first2=Alexandre |last3=Corgne |first3=Alexandre |last4=Antonangeli |first4=Daniele |last5=Auzende |first5=Anne-Line |last6=Perrillat |first6=Jean-Philippe |last7=Guyot |first7=François |last8=Fiquet |first8=Guillaume |title=New host for carbon in the deep Earth |journal=Proceedings of the National Academy of Sciences |date=29 March 2011 |volume=108 |issue=13 |pages=5184–5187 |doi=10.1073/pnas.1016934108 |pmid=21402927 |pmc=3069163 |bibcode=2011PNAS..108.5184B |doi-access=free }}</ref> Other experiments—as well as [[Petrology|petrologic]] observations—support this claim, indicating that magnesite is actually the most stable carbonate phase in most part of the mantle. This is largely a result of its higher melting temperature.<ref>{{cite journal |last1=Dorfman |first1=Susannah M. |last2=Badro |first2=James |last3=Nabiei |first3=Farhang |last4=Prakapenka |first4=Vitali B. |last5=Cantoni |first5=Marco |last6=Gillet |first6=Philippe |title=Carbonate stability in the reduced lower mantle |journal=Earth and Planetary Science Letters |date=May 2018 |volume=489 |pages=84–91 |doi=10.1016/j.epsl.2018.02.035 |bibcode=2018E&PSL.489...84D }}</ref> Consequently, scientists have concluded that carbonates undergo [[Reduction (chemistry)|reduction]] as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments.<ref>{{cite book |doi=10.1007/978-3-642-27833-4_4021-3 |chapter=Oxygen Fugacity |title=Encyclopedia of Astrobiology |date=2014 |last1=Albarede |first1=Francis |pages=1–2 |isbn=978-3-642-27833-4 }}</ref> Magnesium, iron, and other metallic compounds act as buffers throughout the process.<ref>{{cite journal |last1=Cottrell |first1=Elizabeth |last2=Kelley |first2=Katherine A. |title=Redox Heterogeneity in Mid-Ocean Ridge Basalts as a Function of Mantle Source |journal=Science |date=14 June 2013 |volume=340 |issue=6138 |pages=1314–1317 |doi=10.1126/science.1233299 |pmid=23641060 |bibcode=2013Sci...340.1314C }}</ref> The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle.

[[File:Carbon tetrahedral oxygen.png|thumb|upright=0.8|left|Carbon is tetrahedrally bonded to oxygen]]
[[Polymorphism (materials science)|Polymorphism]] alters carbonate compounds' stability at different depths within the Earth. To illustrate, laboratory simulations and [[density functional theory]] calculations suggest that [[Tetrahedral molecular geometry|tetrahedrally coordinated]] carbonates are most stable at depths approaching the [[core–mantle boundary]].<ref>{{cite book |doi=10.1016/C2016-0-01520-6 |title=Magmas Under Pressure |date=2018 |isbn=978-0-12-811301-1 |editor1-first=Yoshio |editor1-last=Kono |editor2-first=Chrystèle |editor2-last=Sanloup }}{{pn|date=July 2024}}</ref><ref name="Fiquet 5184–5187"/> A 2015 study indicates that the lower mantle's high pressure causes carbon bonds to transition from sp<sub>2</sub> to sp<sub>3</sub> [[Orbital hybridisation|hybridised orbitals]], resulting in carbon tetrahedrally bonding to oxygen.<ref>{{cite journal |last1=Boulard |first1=Eglantine |last2=Pan |first2=Ding |last3=Galli |first3=Giulia |last4=Liu |first4=Zhenxian |last5=Mao |first5=Wendy L. |title=Tetrahedrally coordinated carbonates in Earth's lower mantle |journal=Nature Communications |date=18 February 2015 |volume=6 |issue=1 |page=6311 |doi=10.1038/ncomms7311 |pmid=25692448 |arxiv=1503.03538 |bibcode=2015NatCo...6.6311B }}</ref> CO<sub>3</sub> trigonal groups cannot form polymerisable networks, while tetrahedral CO<sub>4</sub> can, signifying an increase in carbon's [[coordination number]], and therefore drastic changes in carbonate compounds' properties in the lower mantle. As an example, preliminary theoretical studies suggest that high pressure causes carbonate melt viscosity to increase; the melts' lower mobility as a result of its increased viscosity causes large deposits of carbon deep into the mantle.<ref>{{cite journal |last1=Jones |first1=A. P. |last2=Genge |first2=M. |last3=Carmody |first3=L. |title=Carbonate Melts and Carbonatites |journal=Reviews in Mineralogy and Geochemistry |date=January 2013 |volume=75 |issue=1 |pages=289–322 |doi=10.2138/rmg.2013.75.10 |bibcode=2013RvMG...75..289J }}</ref>

Accordingly, carbon can remain in the lower mantle for long periods of time, but large concentrations of carbon frequently find their way back to the lithosphere. This process, called carbon outgassing, is the result of carbonated mantle undergoing decompression melting, as well as [[mantle plume]]s carrying carbon compounds up towards the crust.<ref>{{cite journal |last1=Dasgupta |first1=Rajdeep |last2=Hirschmann |first2=Marc M. |title=The deep carbon cycle and melting in Earth's interior |journal=Earth and Planetary Science Letters |date=September 2010 |volume=298 |issue=1–2 |pages=1–13 |doi=10.1016/j.epsl.2010.06.039 |bibcode=2010E&PSL.298....1D }}</ref> Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO<sub>2</sub>. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.<ref>{{cite journal |last1=Frost |first1=Daniel J. |last2=McCammon |first2=Catherine A. |title=The Redox State of Earth's Mantle |journal=Annual Review of Earth and Planetary Sciences |date=May 2008 |volume=36 |issue=1 |pages=389–420 |doi=10.1146/annurev.earth.36.031207.124322 |bibcode=2008AREPS..36..389F }}</ref>

[[File:Speeds_of_seismic_waves.svg|thumb|Knowledge about carbon in the core can be gained by analysing shear wave velocities]]