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 degrees 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 degrees C below the CO2-free solidus, corresponding to a depth of similar to 110 km beneath ridges, and that 0.2 wt% partial melting will be attained at a depth of similar to 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.