Water follows carbon: CO₂ incites deep silicate melting and dehydration beneath mid-ocean ridges

Hydrous defects in nominally anhydrous minerals influence the physical properties and reduce the melting point of mantle rocks. Consequently, it is believed that extraction of H₂O beneath mid-ocean ridges by dehydration melting enhances the creep strength of the upper mantle and produces a chemical lithosphere. However, recent studies show that trace water induces melting at shallower depths beneath ridges than previously believed. Here we explore the hypothesis that incipient melting, dehydration, and strengthening of the subridge mantle is promoted by trace quantities of carbon. Experiments at 3 GPa on carbonated peridotite with 1 and 2.5 wt% CO₂ produce partial melts that change from carbonatites with <10 wt% SiO₂ to carbonated silicate melts with >25 wt% SiO₂ between 1325 and 1350°C. Enhanced melting of carbonated peridotite relative to volatile free peridotite is parameterized as a simple function of the concentration of CO₂ in the melt. Application of this model to mantle with 100 ppm bulk CO₂ suggests that the subridge mantle undergoes silicate melting ∼130-140 °C below the CO₂-free solidus, corresponding to a depth of ∼110 km beneath ridges, and that 0.2 wt% partial melting will be attained at a depth of ∼75 km. The combined effects of H₂O and CO₂ further enhance deep silicate melting, as H₂O partitions from nominally anhydrous silicates to carbonated silicate melts. Thus, the deepest silicate melting beneath ridges will be induced by the combination of H₂O and CO₂, rather than simply by dehydration melting of nominally anhydrous peridotite, and this combination is likely responsible for dehydration strengthening of the oceanic lithosphere and for geochemical signatures of deep melting in mid-oceanic ridge basalt.