We experimentally investigated phlogopite- and C–O–H-bearing lherzolite to model the mantle wedge fluxed by volatiles released from a subducting crustal slab. Experiments have been carried out at 900–1050°C and 1·6–3·2 GPa, at fluid- and carbon-saturated conditions. We used an end-loaded piston cylinder apparatus and a conventional double-capsule technique to constrain the redox state of the experiments, using the nickel–nickel oxide oxygen buffer (NNO). Following thermodynamic calculations, we expect inner fO₂ values to be systematically below NNO, with fluids that are mixtures of CO₂ and H₂O. Estimated fO₂ in the runs are between ΔFMQ ≈ –0·7 at 3 GPa and ΔFMQ ≈ –1·1 at 1·8 GPa, values that have been reported for natural mantle-wedge xenoliths. At the conditions investigated, the hydrous phases are phlogopite and pargasitic amphibole. Whereas phlogopite is ubiquitous, amphibole disappears at 3·1 GPa at 900°C and 2·7 GPa at 1050°C, where the solidus is encountered. The amphibole-out reaction also consumes orthopyroxene and liberates water. From low to high P, we observed first carbonate-free, amphibole-bearing assemblages, then carbonate + amphibole-bearing assemblages, and finally amphibole-free, carbonate-bearing assemblages. Carbonate-free assemblages melt to produce trachyandesite at T > 1050°C, whereas dolomitic carbonatites have been found beyond the solidus of carbonate-bearing assemblages. Carbonates occur as dolomite at <1·9 GPa, 900°C and at 2·4 GPa, 900°C and >2·7, 1050°C; between these limits, a magnesite + dolomite-bearing assemblage constitutes a two-carbonate field. P–T pseudosections fail to reproduce the experimental results concerning amphibole breakdown and reaction positions involving carbonates. The amount of COH fluid is thought to have a major role, even in fluid-saturated peridotites. Clinopyroxene and olivine are not expected at fluid-oversaturated conditions, for which dolomite or magnesite are stable respectively. The presented results are useful for unravelling the exhumation history of orogenic lherzolites bearing COH phases and to suggest a way to transfer carbon species to the mantle wedge. We suggest that once carbon-bearing fluids react with mantle-wedge peridotites, a sort of buoyant ‘cold plume’ will form containing low-density phases such as amphibole, carbonates and carbonatitic melt. This plume could represent an important source of CO₂ and H₂O, and it is one of a series of processes that ultimately lead to arc magmatism.

Estimates of dissolved CO₂ in subduction-zone fluids are based on thermodynamic models, relying on a very sparse experimental data base. Here, we present experimental data at 1–3 GPa, 800 °C, and ∆FMQ ≈ −0.5 for the volatiles and solute contents of graphite-saturated fluids in the systems COH, SiO₂–COH ( + quartz/coesite) and MgO–SiO₂–COH ( + forsterite and enstatite). The CO₂ content of fluids interacting with silicates exceeds the amounts measured in the pure COH system by up to 30 mol%, as a consequence of a decrease in water activity probably associated with the formation of organic complexes containing Si–O–C and Si–O–Mg bonds. The interaction of deep aqueous fluids with silicates is a novel mechanism for controlling the composition of subduction COH fluids, promoting the deep CO₂ transfer from the slab–mantle interface to the overlying mantle wedge, in particular where fluids are stable over melts.

Ocean sediments consist mainly of calcium carbonate and organic matter (phytoplankton debris). Once subducted, some carbon is removed from the slab and returns to the atmosphere as CO₂ in arc magmas. Its isotopic signature is thought to reflect the bulk fraction of inorganic (carbonate) and organic (graphitic) carbon in the sedimentary source. Here we challenge this assumption by experimentally investigating model sediments composed of ¹³C-CaCO₃ + ¹²C-graphite interacting with water at pressure, temperature and redox conditions of an average slab–mantle interface beneath arcs. We show that oxidative dissolution of graphite is the main process controlling the production of CO₂, and its isotopic composition reflects the CO₂/CaCO₃ rather than the bulk graphite/CaCO₃ (i.e., organic/inorganic carbon) fraction. We provide a mathematical model to relate the arc CO₂ isotopic signature with the fluid–rock ratios and the redox state in force in its subarc source.

High-pressure COH fluids have a fundamental role in a variety of geological processes. Their composition in terms of volatile species can control the solidus temperature and carbonation/decarbonation reactions, as well as influence the amount of solutes generated during fluid–rock interaction at depth. Over the last decades, several systems have been experimentally investigated to unravel the effect of COH fluids at upper-mantle conditions. However, fluid composition is rarely tackled as a quantitative issue, and rather infrequently fluids are analyzed in the same way as the associated solid phases in the experimental assemblage. A comprehensive characterization of carbon-bearing aqueous fluids in terms of composition is hampered by experimental difficulties in synthetizing and analyzing high-pressure fluids without altering their composition upon quenching. Recently, improved techniques have been proposed for the analyses of experimental carbon-saturated COH fluids, leading to a significant advancement in synthetic fluid characterization. Here, we present a review of carbon-bearing aqueous fluid experiments conducted at lower-crust and upper-mantle P–T (pressure and temperature) conditions, in which fluids have been characterized quantitatively through ex situ techniques. We review the experimental background of the most commonly employed thermodynamic models for COH fluids, together with the techniques to synthetize them and analyze their composition when the fluid coexists with solid phases. We highlight how a quantitative approach to COH fluid analyses is a fundamental step to understand the effect of these fluids at upper-mantle conditions and to provide a strong experimental foundation to thermodynamic models to ultimately unravel the deep cycling of elements.