Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas

The abundance of volatile compounds, and particularly CO2, in the upper oceanic mantle affects the style of volcanic eruptions. At mid-ocean ridges, eruptions are generally dominated by the gentle effusion of basaltic lavas with a low volatile content. However explosive volcanism has been documented at some ocean spreading centres(1-3), indicative of abundant volatile compounds. Estimates of the initial CO2 concentration of primary magmas can be used to constrain the CO2 content of the upper oceanic mantle, but these estimates vary greatly(4,5). Here we present ion microprobe measurements of the CO2 content of basaltic melt trapped in plagioclase crystals. The crystals are derived from volcanic ash deposits erupted explosively at Axial Seamount, Juan de Fuca Ridge, in the northeast Pacific Ocean. We report unusually high CO2 concentrations of up to 9,160 ppm, which indicate that the upper oceanic mantle is more enriched in carbon than previously thought. We furthermore suggest that CO2 fluxes along mid-ocean ridges(4,5) vary significantly. Our results demonstrate that elevated fluxes of CO2 from the upper oceanic mantle can drive explosive eruptions at mid-ocean ridges.