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Implications for metal and volatile cycles from the pH of subduction zone fluids

Nature volume 539, pages 420424 (17 November 2016) | Download Citation

Abstract

The chemistry of aqueous fluids controls the transport and exchange—the cycles—of metals1,2,3,4,5 and volatile elements3,6,7 on Earth. Subduction zones, where oceanic plates sink into the Earth’s interior, are the most important geodynamic setting for this fluid-mediated chemical exchange2,6,7,8,9,10. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has long been a major challenge in Earth science11,12. Here we report thermodynamic predictions of fluid–rock equilibria that tie together models of the thermal structure, mineralogy and fluid speciation of subduction zones. We find that the pH of fluids in subducted crustal lithologies is confined to a mildly alkaline range, modulated by rock volatile and chlorine contents. Cold subduction typical of the Phanerozoic eon13 favours the preservation of oxidized carbon in subducting slabs. In contrast, the pH of mantle wedge fluids is very sensitive to minor variations in rock composition. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to soluble elements in low abundance in the host rocks, such as carbon, alkali metals and halogens, illustrates a feedback between the chemistry of the Earth’s atmosphere–ocean system14,15 and the speciation of subduction zone fluids via the composition of the seawater-altered oceanic lithosphere. Our findings provide a perspective on the controlling reactions that have coupled metal and volatile cycles in subduction zones for more than 3 billion years77.

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Acknowledgements

The presentation of this work benefited from informal reviews by O. Bachmann and D. Rumble. Discussions with P. Ulmer, X. Zhong, J.A. Padron Navarta, D. Miron, D. Sverjensky, J. Eiler and J. Cohen were helpful. This research was supported by an ETH fellowship ETH/CoFUND Fel-06 13-2 (M.E.G). Partial support from Carnegie and Society in Science/Branco-Weiss fellowships (M.E.G.), Swiss National Science Foundation Grant 200021_146872 (J.A.D.C), Deep Carbon Observatory and National Science Fundation grant EAR 1347987 (C.E.M) are also acknowledged.

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Affiliations

  1. Earth Sciences Department, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland

    • Matthieu E. Galvez
    •  & James A. D. Connolly
  2. Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California 90095-1567, USA

    • Craig E. Manning

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  2. Search for James A. D. Connolly in:

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Contributions

M.E.G. conceived the project, developed the computational tools used and performed the calculations. J.A.D.C. developed the PerpleX software used for phase equilibria computations. M.E.G., J.A.D.C. and C.E.M. analysed the data and wrote the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Matthieu E. Galvez.

Reviewer Information Nature thanks D. Dolejs, K. Evans and H. Keppler for their contribution to the peer review of this work.

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https://doi.org/10.1038/nature20103

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