Editing Diamond (section)

=== Origin in mantle ===
[[File:Eclogite, détail de la roche.jpg|thumb|[[Eclogite]] with centimeter-size [[garnet]] crystals]]
[[File:Garnet inclusion in diamond.jpg|thumb|Red garnet inclusion in a diamond<ref name=DCOdecadal>{{cite book |last1=Deep Carbon Observatory |title=Deep Carbon Observatory: A Decade of Discovery |doi=10.17863/CAM.44064 |date=2019 |location=Washington, DC |url=https://deepcarbon.net/deep-carbon-observatory-decade-discovery |access-date=December 13, 2019 |archive-date=December 17, 2019 |archive-url=https://web.archive.org/web/20191217174901/https://deepcarbon.net/deep-carbon-observatory-decade-discovery |url-status=dead }}</ref>]]
Most gem-quality diamonds come from depths of 150–250&nbsp;km in the [[lithosphere]]. Such depths occur below cratons in ''mantle keels'', the thickest part of the lithosphere. These regions have high enough pressure and temperature to allow diamonds to form and they are not convecting, so diamonds can be stored for billions of years until a kimberlite eruption samples them.<ref name=Shirey2013/>

Host rocks in a mantle keel include [[harzburgite]] and [[lherzolite]], two type of [[peridotite]]. The most dominant rock type in the [[upper mantle (Earth)|upper mantle]], peridotite is an [[igneous rock]] consisting mostly of the minerals [[olivine]] and [[pyroxene]]; it is low in [[Silicon dioxide|silica]] and high in [[magnesium]]. However, diamonds in peridotite rarely survive the trip to the surface.<ref name=Shirey2013/> Another common source that does keep diamonds intact is [[eclogite]], a [[metamorphic]] rock that typically forms from [[basalt]] as an oceanic plate plunges into the mantle at a [[Subduction|subduction zone]].<ref name=Cartigny/>

A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660&nbsp;km, a region that includes the [[Transition zone (Earth)|transition zone]]. They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of [[majorite]] (a form of [[garnet]] with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800&nbsp;km.<ref name=Cartigny/>

Diamond is thermodynamically stable at high pressures and temperatures, with the phase transition from [[graphite]] occurring at greater temperatures as the pressure increases. Thus, underneath continents it becomes stable at temperatures of 950{{nbsp}}degrees Celsius and pressures of 4.5 gigapascals, corresponding to depths of 150{{nbsp}}kilometers or greater. In subduction zones, which are colder, it becomes stable at temperatures of 800&nbsp;°C and pressures of 3.5{{nbsp}}gigapascals. At depths greater than 240&nbsp;km, iron–nickel metal phases are present and carbon is likely to be either dissolved in them or in the form of [[carbide]]s. Thus, the deeper origin of some diamonds may reflect unusual growth environments.<ref name=Cartigny/><ref name=Shirey2013/>

In 2018 the first known natural samples of a phase of ice called [[Ice VII]] were found as inclusions in diamond samples. The inclusions formed at depths between 400 and 800&nbsp;km, straddling the upper and lower mantle, and provide evidence for water-rich fluid at these depths.<ref>{{cite journal| vauthors = Cartier K |title=Diamond Impurities Reveal Water Deep Within the Mantle|journal=Eos|date=April 2, 2018|volume=99|doi=10.1029/2018EO095949|doi-access=free}}</ref><ref name=Perkins>{{cite journal|vauthors=Perkins S|title=Pockets of water may lie deep below Earth's surface|journal=Science|date=March 8, 2018|url=https://www.science.org/content/article/pockets-water-may-lay-deep-below-earth-s-surface|access-date=June 30, 2022|archive-date=March 8, 2018|archive-url=https://web.archive.org/web/20180308220310/http://www.sciencemag.org/news/2018/03/pockets-water-may-lay-deep-below-earth-s-surface|url-status=live}}</ref>