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

• Published in: Nature (London) 2016-11, Vol.539 (7629), p.420-424
• Main Authors: Galvez, Matthieu E., Connolly, James A. D., Manning, Craig E.
• Format: Article
• Language: English
• Subjects: 639/638/161; 704/2151/209; 704/2151/598; Acids; Alkali metals; Chemical properties; Chlorine; Earth; Fluids; Geochemistry; Halogens; Humanities and Social Sciences; letter; Lithosphere; Metals; Mineralogy; multidisciplinary; Observations; Ocean-atmosphere interaction; Rocks; Science; Seawater; Slabs; Speciation; Subduction zones; Volatile organic compounds
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature20103

A thermodynamic model of fluid pH and its variability in Earth’s mantle and subducting crust highlights chemical feedbacks that connect deep Earth to surface processes. Subduction zone fluid pH and rock chemistry Matthieu Galvez and co-authors present thermodynamic predictions of fluid–rock equilibria that tie together models of subduction-zone thermal structure, mineralogy and fluid speciation. They find that the pH of fluids in subducted crustal lithologies is uniform and confined to a mildly alkaline range, controlled by rock volatile and chlorine contents, but that the pH of mantle wedge fluids exhibits marked sensitivity to minor variations in rock chemistry. They conclude that this sensitivity of fluid chemistry to carbon, alkali metals and halogens illustrates a feedback between Earth's atmosphere–ocean chemistry and the speciation of subduction-zone fluids via the hydrothermally altered oceanic lithosphere. The chemistry of aqueous fluids controls the transport and exchange—the cycles—of metals 1 , 2 , 3 , 4 , 5 and volatile elements 3 , 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 exchange 2 , 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 science 11 , 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 eon 13 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 system 14 , 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