Degassing and Vesiculation during the 2011 Eruption of Axial Seamount, Juan de Fuca Ridge

Abstract:

The dependency of CO₂ disequilibrium in mid-ocean ridge basalts (MORB) on magma ascent and emplacement rates makes volatile analysis a valuable tool for evaluating submarine eruption dynamics. This study examines volatile content and vesicle size distributions in a suite of samples collected from the 2011 eruption of Axial Seamount. The samples exhibit a wide range of dissolved CO₂ concentrations (68 – 339 ppm), low H₂O concentrations (0.17 – 0.26 wt %), and consistent supersaturation relative to the expected CO₂-H₂O phase equilibrium in basaltic melts. The vesicularity, characteristic bubble radii, and bubble number density correlate with the dissolved volatile concentrations and indicate that bubble nucleation and growth occur within a closed system. The extent of degassing increases with distance from the eruptive vent and variations are also observed along the eruptive fissure providing a spatially resolved record of eruption dynamics. A bubble growth model is used to constrain the minimum ascent rates, eruption duration, and flow rates for the 2011 eruption. These results are compared to similar data from the global mid-ocean ridge system to evaluate differences in timescales of emplacement and ascent in MOR eruptions.