Editing Diamond (section)

=== Carbon sources ===
The mantle has roughly one billion [[tonne|gigatonnes]] of carbon (for comparison, the atmosphere-ocean system has about 44,000 gigatonnes).<ref>{{cite book | vauthors = Lee CA, Jiang H, Dasgupta R, Torres M  |chapter=A Framework for Understanding Whole-Earth Carbon Cycling |pages=313–357 |doi=10.1017/9781108677950.011 | veditors = Orcutt BN, Daniel I, Dasgupta R |title=Deep carbon: past to present |date=2019 |publisher=Cambridge University Press |isbn=978-1-108-67795-0|s2cid=210787128 }}</ref> Carbon has two [[stable nuclide|stable isotopes]], [[carbon-12|<sup>12</sup>C]] and [[carbon-13|<sup>13</sup>C]], in a ratio of approximately 99:1 by mass.<ref name=Shirey2013/> This ratio has a wide range in meteorites, which implies that it also varied a lot in the early Earth. It can also be altered by surface processes like [[photosynthesis]]. The fraction is generally compared to a standard sample using a ratio [[isotopic signature#Carbon isotopes|δ<sup>13</sup>C]] expressed in parts per thousand. Common rocks from the mantle such as basalts, carbonatites, and kimberlites have ratios between −8 and −2. On the surface, organic sediments have an average of −25 while carbonates have an average of 0.<ref name=Cartigny/>

Populations of diamonds from different sources have distributions of δ<sup>13</sup>C that vary markedly. Peridotitic diamonds are mostly within the typical mantle range; eclogitic diamonds have values from −40 to +3, although the peak of the distribution is in the mantle range. This variability implies that they are not formed from carbon that is ''primordial'' (having resided in the mantle since the Earth formed). Instead, they are the result of tectonic processes, although (given the ages of diamonds) not necessarily the same tectonic processes that act in the present.<ref name=Shirey2013/> Diamond-forming carbon originates in the top 700 kilometers (430 mi) or so of the upper mantle closest to the surface, known as the [[asthenosphere]].<ref name=Cartigny/>