The transport of carbon into Earth’s mantle is a critical pathway in Earth’s carbon cycle, affecting both the climate and the redox conditions of the surface and mantle. The largest unconstrained variables in this cycle are the depths to which carbon in sediments and altered oceanic crust can be subducted and the relative contributions of these reservoirs to the sequestration of carbon in the deep mantle1. Mineral inclusions in sublithospheric, or ‘superdeep’, diamonds (derived from depths greater than 250 kilometres) can be used to constrain these variables. Here we present oxygen isotope measurements of mineral inclusions within diamonds from Kankan, Guinea that are derived from depths extending from the lithosphere to the lower mantle (greater than 660 kilometres). These data, combined with the carbon and nitrogen isotope contents of the diamonds, indicate that carbonated igneous oceanic crust, not sediment, is the primary carbon-bearing reservoir in slabs subducted to deep-lithospheric and transition-zone depths (less than 660 kilometres). Within this depth regime, sublithospheric inclusions are distinctly enriched in 18O relative to eclogitic lithospheric inclusions derived from crustal protoliths. The increased 18O content of these sublithospheric inclusions results from their crystallization from melts of carbonate-rich subducted oceanic crust. In contrast, lower-mantle mineral inclusions and their host diamonds (deeper than 660 kilometres) have a narrow range of isotopic values that are typical of mantle that has experienced little or no crustal interaction. Because carbon is hosted in metals, rather than in diamond, in the reduced, volatile-poor lower mantle2, carbon must be mobilized and concentrated to form lower-mantle diamonds. Our data support a model in which the hydration of the uppermost lower mantle by subducted oceanic lithosphere destabilizes carbon-bearing metals to form diamond, without disturbing the ambient-mantle stable-isotope signatures. This transition from carbonate slab melting in the transition zone to slab dehydration in the lower mantle supports a lower-mantle barrier for carbon subduction.
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Nature Communications Open Access 14 October 2022
Communications Earth & Environment Open Access 09 September 2021
Oceanic and super-deep continental diamonds share a transition zone origin and mantle plume transportation
Scientific Reports Open Access 20 August 2021
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We acknowledge Canada Excellence Research Chairs and the Deep Carbon Observatory for funding this study. We thank Diamond Trading Company (a member of the DeBeers Group of Companies) for the donation to J.W.H. of the diamonds used in this study.
The authors declare no competing interests.
Peer review information Nature thanks John Eiler and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
The oxygen isotopic composition of coexisting garnets and olivines from peridotitic mantle xenoliths were analysed using the ion probe to determine the instrumental fractionation associated with the Cr2O3 content of garnets. The plot defines the olivine δ18O and the deviation of the measured garnet δ18O from equilibrium after Ca# matrix correction46, versus the Cr2O3 contents of the garnets. Because all the olivines have δ18O within the error of the mantle, we assume isotopic equilibrium between garnet and olivine51 and contend that the trend of SIMS-determined garnet δ18O with Cr2O3 content is a matrix effect. The trendline indicates the correction of the δ18O values to a hypothetical Cr-free garnet. Errors are 2σ.
The instrumental mass fractionation with enstatite Mg# was assessed using reference material S0170 (Mg# of 91.2; laser fluorination δ18O of +5.64‰) and S0444 (Mg# of 94.1; laser fluorination δ18O of +5.76)52. Error bars incorporate 0.10‰ analytical uncertainty in the laser fluorination measurements.
Elemental and isotopic measurements of Kankan inclusions in diamond. This Excel file contains the elemental and oxygen isotope analyses of the garnet, majoritic garnet, and enstatite inclusions in Kankan diamonds that were produced in this study.
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Regier, M.E., Pearson, D.G., Stachel, T. et al. The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds. Nature 585, 234–238 (2020). https://doi.org/10.1038/s41586-020-2676-z
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