Geochemistry: Ions surprise in Earth's deep fluids.

Abstract

Models indicate that there are strong gradients in element concentrations and in the pH of fluids at the slab–mantle interface — a major discontinuity deep within Earth. This transforms our view of global geochemical transport. See Letter p.420 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.