Silicate dissolution boosts the CO2 concentrations in subduction fluids

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 ΔFMQ ⠈ -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.