Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amount of melting initiates at about 150-300 km depths in the presence of volatiles (CO2, H2O)1–3, but the extensive dry melting commences at 60-70 km depths due to the upwelling of the mantle as two diverging plates move apart4,5. However, how these melts migrate to the surface and what happens to these melts in the upper part of the mantle are still not understood. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10-20 km depth in the hot mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary, suggesting that these earthquakes are caused by a volume change associated with the CO2 degassing from the ascending melt. The geochemical analyses of basalts from the ridge axis show an abnormally high quantity of CO2 (>1.9 wt%) in the primitive melt, confirming the CO2 degassing hypothesis. The large concentration of CO2 in the primitive melt will influence the presence of melt beneath the lithosphere-asthenosphere boundary at sub-solidus temperatures.