The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds

Regier, M. E., Pearson, D. G., Stachel, T., Luth, R. W., Stern, R. A. and Harris, J. W. (2020) The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds. Nature, 585(7824), pp. 234-238. (doi: 10.1038/s41586-020-2676-z) (PMID:32908266)

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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.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Harris, Dr Jeff
Authors: Regier, M. E., Pearson, D. G., Stachel, T., Luth, R. W., Stern, R. A., and Harris, J. W.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Nature
Publisher:Nature Research
ISSN (Online):1476-4687
Published Online:09 September 2020
Copyright Holders:Copyright © 2020 Nature Research
First Published:First published in Nature 585(7824):234-238
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher
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