Deep mantle earthquakes linked to CO₂ degassing at the mid-Atlantic ridge

Volatiles (CO₂, H₂O) play a fundamental role in mantle melting beneath ocean spreading centers, but what role they play during the melt migration remains unknown. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10-20 km depth in the mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary. Syntheses of regional basaltic rock samples and their geochemical analyses indicate the presence of an abnormally high quantity of CO₂ (~0.4-3.0 wt%) in the primary melts. As the degassing of a high concentration of dissolved CO₂ produces volume change, we suggest that deep earthquakes in the mantle result from the degassing of CO₂. The large concentration of CO₂ in the primitive melt will influence the presence of melt beneath the lithosphere-asthenosphere boundary at sub-solidus temperatures.