The abundance of volatile compounds, and particularly 18 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. But, explosive volcanism has been documented at some ocean
spreading centres1-3, 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 greatly4,5. 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.
And we furthermore suggest that CO2 fluxes along mid-ocean ridges4,5 vary
significantly. Our results demonstrate that elevated fluxes of CO2 from the upper
oceanic mantle can drive explosive eruptions at mid-ocean ridges.