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Carbonate dissolution during subduction revealed by diamond-bearing rocks from the Alps

Nature Geoscience volume 4, pages 703–706 (2011)Cite this article

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Abstract

Subduction zones modulate the global carbon cycle. Carbon is transported into the mantle by the subducting slab and returned to the surface by degassing at arc volcanoes above the subduction zone1,2. However, the mechanisms for the transfer of carbon from the subducting slab and sediments into the overlying mantle wedge are poorly understood. Decarbonation—a metamorphic reaction between silicate and carbonate minerals that releases CO2—was thought to be the primary mechanism. Yet, thermodynamic models show that decarbonation occurs at much greater pressures and temperatures than those found in typical subduction zones3,4,5,6. Carbon should therefore be retained in the slab and transported to great depths in the mantle, rather than supply the arc volcanoes. Here we identify diamonds in ultrahigh-pressure rocks from the Italian western Alps that have an oceanic origin. We assess the geochemistry of diamond-bearing fluid inclusions and find that they contain bicarbonate, carbonate and sulphate ions, silica monomers, and crystals of carbonate and silicate. This fluid geochemistry indicates that carbon was released from the slab at relatively shallow depths through dissolution, not decarbonation. We conclude that dissolution, driven by fluids released from the subducted slab, is an important mechanism for the transfer of carbon into the mantle and ultimately back into the atmosphere, helping to balance the carbon flux.

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Figure 1: Examples of diamond and fluid inclusions in garnet.
Figure 2: Examples of Raman spectra from aqueous fluid inclusions associated with diamonds.
Figure 3: Iron X-ray map of garnet showing Fe-rich haloes surrounding open fluid inclusions (for example, arrows).

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Acknowledgements

Funding provided by the Italian PRIN 2008BYTF98 (M.L.F. and R.C.) and US NSF grant EAR-0809218 (J.S. and Z.D.S.). Comments from M. Kopylova, C. Manning, and S. Poli greatly improved the manuscript.

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Authors and Affiliations

  1. Dipartimento Scienze della Terra, Università di Siena, Via Laterina 8, Siena 53100, Italy

    M. L. Frezzotti

  2. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-0001, USA

    J. Selverstone & Z. D. Sharp

  3. Dipartimento Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, Torino 10125, Italy

    R. Compagnoni

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  1. M. L. Frezzotti
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Contributions

Raman analyses were carried out by M.L.F., petrographic analyses by J.S. and R.C., microprobe analyses by J.S. and stable isotope analyses by Z.D.S. Samples were collected by J.S. and R.C. All authors participated in extensive discussions and the preparation of the manuscript.

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Correspondence to M. L. Frezzotti.

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Frezzotti, M., Selverstone, J., Sharp, Z. et al. Carbonate dissolution during subduction revealed by diamond-bearing rocks from the Alps. Nature Geosci 4, 703–706 (2011). https://doi.org/10.1038/ngeo1246

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