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

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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Figure 1: ΔpH of solutions in equilibrium with basalt, pelite and peridotite lithologies.
Figure 2: ΔpH along three representative Precambrian and Phanerozoic P–T paths.
Figure 3: Metasomatism at the subduction interface.

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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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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.

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Correspondence to Matthieu E. Galvez.

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

Extended data figures and tables

Extended Data Figure 1 Properties of water at geological conditions.

The relative permittivity20 (εr) and density87 (ρ) of pure water as functions of pressure and temperature. Adapted from ref. 21.

Extended Data Figure 2 Oxide chemical potential across a peridotite–crust interface.

Shown are profiles of oxide chemical potentials (μ) in the metasomatic model (Fig. 3c, d) relative to that for component at 600 °C and 2 GPa.

Extended Data Figure 3 Relative activity of selected neutral species across a serpentinite–crust interface.

Shown are relative activities of neutral polynuclear clusters , and , as well as Na+ and , in the fluid, at 600 °C and 2 GPa (see Fig. 3c, d).

Extended Data Table 1 Rock compositions used for phase equilibria computations
Extended Data Table 2 Thermodynamic data source for solutes used in this study
Extended Data Table 3 Estimates of global annual C fluxes

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Galvez, M., Connolly, J. & Manning, C. Implications for metal and volatile cycles from the pH of subduction zone fluids. Nature 539, 420–424 (2016). https://doi.org/10.1038/nature20103

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