Supercritical aqueous fluids link subducting plates and the return of carbon to Earth's surface in the deep carbon cycle 1,2 . The amount of carbon in the fluids and the identities of the dissolved carbon species are not known, which leaves the deep carbon budget poorly constrained 3 . Traditional models 4,5 , which assume that carbon exists in deep fluids as dissolved gas molecules, cannot predict the solubility and ionic speciation of carbon in its silicate rock environment. Recent advances enable these limitations to be overcome when evaluating the deep carbon cycle 6-8 . Here we use the Deep Earth Water theoretical model 7 to calculate carbon speciation and solubility in fluids under upper mantle conditions. We find that fluids in equilibrium with mantle peridotite minerals generally contain carbon in a dissolved gas molecule form. However, fluids in equilibrium with diamonds and eclogitic minerals in the subducting slab contain abundant dissolved organic and inorganic ionic carbon species. The high concentrations of dissolved carbon species provide a mechanism to transport large amounts of carbon out of the subduction zone, where the ionic carbon species may influence the oxidation state of the mantle wedge. Our results also identify novel mechanisms that can lead to diamond formation and the variability of carbon isotopic composition via precipitation of the dissolved organic carbon species in the subduction-zone fluids.Supercritical aqueous fluids released from subducting plates carry distinctive suites of elements, including carbon, and have long been invoked to trigger partial melting 9 , oxidation of the mantle wedge overlying subduction zones 10,11 , and mantle metasomatism and diamond formation 11 . However, quantitative theoretical models linking ionic species in the fluids to the rock chemistry are lacking. Instead, aqueous fluids in the deep crust and upper mantle have long been modelled as a mixture of neutral gas molecules-such as CO 2 , CH 4 and H 2 O (that is, COH fluids)-without consideration of aqueous ions 4,5 . In such models, the pH values and bicarbonate or carbonate ion concentrations of subduction-zone fluids are not known. Dehydration and decarbonation reactions alone are invoked for C mobility in fluids, leading to an inability to explain why so much C is degassed over subduction zones 1,3 . As a result, the involvement of aqueous fluids in the deep carbon cycle remains enigmatic.Recent studies of carbonate rock dissolution in subduction zones, fluid inclusions in diamonds, solubility and ionic speciation experiments, and theoretical calculations 1,2,6,12-14 have indicated that aqueous carbonate or bicarbonate ions may play a role in the transport of carbon in the deep Earth carbon cycle. However, none of these studies have quantified C-solubilities or C-speciation in equilibrium with silicate minerals. As the solubility and speciation can be expected to depend on the nature of the silicate environment in the deep crust or upper mantle, it is crucial to include silicate minerals when ...

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Diamond formation has typically been attributed to redox reactions during precipitation from fluids or magmas. Either the oxidation of methane or the reduction of carbon dioxide has been suggested, based on simplistic models of deep fluids consisting of mixtures of dissolved neutral gas molecules without consideration of aqueous ions. The role of pH changes associated with water–silicate rock interactions during diamond formation is unknown. Here we show that diamonds could form due to a drop in pH during water–rock interactions. We use a recent theoretical model of deep fluids that includes ions, to show that fluid can react irreversibly with eclogite at 900 °C and 5.0 GPa, generating diamond and secondary minerals due to a decrease in pH at almost constant oxygen fugacity. Overall, our results constitute a new quantitative theory of diamond formation as a consequence of the reaction of deep fluids with the rock types that they encounter during migration. Diamond can form in the deep Earth during water–rock interactions without changes in oxidation state.

Sulfur belongs among H 2 O, CO 2 , and Cl as one of the key volatiles in Earth's chemical cycles. High oxygen fugacity, sulfur concentration, and δ 34 S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ 34 S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30-230 km depth, and the predominant sulfur loss takes place at 70-100 km with a net δ 34 S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver 34 S-enriched sulfur to produce the positive δ 34 S signature in arc settings. Most sulfur has negative δ 34 S and is subducted into the deep mantle, which could cause a long-term increase in the δ 34 S of Earth surface reservoirs.

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