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Important role for organic carbon in subduction-zone fluids in the deep carbon cycle

Nature Geoscience volume 7, pages 909–913 (2014)Cite this article

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Abstract

Supercritical aqueous fluids link subducting plates and the return of carbon to Earth’s surface in the deep carbon cycle1,2. The amount of carbon in the fluids and the identities of the dissolved carbon species are not known, which leaves the deep carbon budget poorly constrained3. Traditional models4,5, which assume that carbon exists in deep fluids as dissolved gas molecules, cannot predict the solubility and ionic speciation of carbon in its silicate rock environment. Recent advances enable these limitations to be overcome when evaluating the deep carbon cycle6,7,8. Here we use the Deep Earth Water theoretical model7 to calculate carbon speciation and solubility in fluids under upper mantle conditions. We find that fluids in equilibrium with mantle peridotite minerals generally contain carbon in a dissolved gas molecule form. However, fluids in equilibrium with diamonds and eclogitic minerals in the subducting slab contain abundant dissolved organic and inorganic ionic carbon species. The high concentrations of dissolved carbon species provide a mechanism to transport large amounts of carbon out of the subduction zone, where the ionic carbon species may influence the oxidation state of the mantle wedge. Our results also identify novel mechanisms that can lead to diamond formation and the variability of carbon isotopic composition via precipitation of the dissolved organic carbon species in the subduction-zone fluids.

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Figure 1: Predicted log f O 2 versus pH diagrams for aqueous C-species constructed using equilibrium constants calculated with the DEW model.
Figure 2: Pressure-dependent stability of an organic acid anion versus aqueous phase pH.
Figure 3: Aqueous C-speciation in equilibrium with a model metasedimentary eclogite containing diamond, jadeite, pyrope, kyanite, and coesite.
Figure 4: Aqueous C-speciation in peridotitic fluid containing 1.0 m C and Cl.

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Acknowledgements

This research was supported by the WDC Research Fund (V.S.), a Johns Hopkins Graduate Fellowship (F.H.), grants from the Sloan Foundation through the Deep Carbon Observatory (Reservoirs and Fluxes and Extreme Physics and Chemistry programmes to D.A.S.) and grant DOE DE-FG-02-96ER-14616 (D.A.S.). We are also grateful for the help and support of the Johns Hopkins University and the Geophysical Laboratory of the Carnegie Institution of Washington. We wish to acknowledge reviews of the manuscript by R. E. Cohen, R. M. Hazen and C. M. Schiffries, as well as helpful discussions with I. Daniel, Y. Fei, M. S. Ghiorso, R. J. Hemley, S. Lobanov, C. E. Manning, S. Mikhail, B. O. Mysen and E. L. Shock.

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA

    Dimitri A. Sverjensky & Fang Huang

  2. Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA

    Vincenzo Stagno

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  1. Dimitri A. Sverjensky
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Contributions

All authors contributed to the calculations and the writing of the manuscript. The activity diagrams were calculated by F.H., the aqueous speciation and solubility modelling was carried out by D.A.S. The selection of systems of interest and oxidation states in the mantle were guided and calculated by V.S.

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Correspondence to Dimitri A. Sverjensky.

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Sverjensky, D., Stagno, V. & Huang, F. Important role for organic carbon in subduction-zone fluids in the deep carbon cycle. Nature Geosci 7, 909–913 (2014). https://doi.org/10.1038/ngeo2291

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