Silicate dissolution boosts the CO(2) concentrations in subduction fluids

Estimates of dissolved CO(2) 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, SiO(2)–COH ( + quartz/coesite) and MgO–SiO(2)–COH ( + forsterite and enstatite). The CO(2) 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 CO(2) transfer from the slab–mantle interface to the overlying mantle wedge, in particular where fluids are stable over melts.