Mikhail, S.; (2011) Stable isotope fractionation during diamond growth and the Earth’s deep carbon cycle. Doctoral thesis , UCL (University College London).

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Abstract

The flux of carbon between the mantle and crustal reservoirs can have a large impact on the melting of mantle rocks, the long and short term stability of the climate, and the growth of a very precious mineral; diamond. Diamond can be as young as 200 Ma and as old as > 4000 Ma, which is most of Earth’s entire 4500 Ma history and can also contain samples of the Earth’s mantle over a depth range of > 600 km, from the base of the crust and into the lower mantle. This spatial and temporal sampling of the Earth is unrivalled; therefore the study of mantle diamond is the best way to place constraints on the geodynamic carbon cycle over geological time. This study has used the stable isotopes of carbon and nitrogen in natural diamond of three groups and found that monocrystalline diamonds from Dachine, French Guyana have carbon and nitrogen isotopic compositions consistent with a crustal origin, inferring Phanerozoic type subduction > 2 Ga. The same is true for the source for the same isotopic systems in diamondites; however they appear to be evidence of mobilised subducted fluids that metasomatise primary mantle peridotites and induced melting and contemporaneous diamond formation. This study has also explored the potential effects of isotopic fractionation during diamond growth, y quantifying the magnitude and direction of carbon isotope fractionation between diamond and Fe-carbide in natural and experimental samples. This has shown that carbon isotope fractionation in the lower mantle should be larger than in the upper mantle, despite the higher temperatures. The implications for this new data are discussed in light of terrestrial and extraterrestrial geodynamic carbon cycling.

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