Carbon-dioxide-rich silicate melt in the Earth's upper mantle

The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exo-sphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges(1,2), owing to the small fraction (similar to 0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at similar to 200km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO2 beneath ridges commences at similar to 180 km. Accounting for the effect of 50-200 p.p.m. H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with similar to 100 p.p.m. CO2 becomes as deep as similar to 220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front similar to 250-200 km deep(6), with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite(7) and that of the oceanic upper mantle(5), suggest that mantle at depth is CO2-rich but H2O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H2O and other highly incompatible elements at ridges becomes controlled by the flux of the former.