Degassing and Vesiculation during the 2011 eruption of Axial Volcano, Juan de Fuca Ridge

Abstract:

Our knowledge of mid-ocean ridge eruption dynamics remains speculative due to a lack of direct observations. However, the dependency of CO₂ disequilibrium in MORB on depressurization rates and timescales makes volatile analyses, which can be assessed long after an eruption has concluded, a promising method for evaluating submarine eruption dynamics. In this study, we quantitatively assess magma ascent and emplacement rates and timescales during the 2011 eruption of Axial Volcano based on dissolved volatile contents, noble gases, vesicularities, and vesicle size distributions combined with numerical modeling of diffusion-controlled bubble growth. Systematic sampling along a series of eruptive fissures and individual lava flow units, combined with previously published constraints on seafloor deformation and temperature anomalies, allows a uniquely well-constrained evaluation of submarine eruption dynamics at Axial Volcano. In this eruption, lava erupted from the vent supersaturated in CO₂ and experienced CO₂ degassing during lava emplacement, leading to systematically decreasing dissolved CO₂ concentrations and increasing vesicularity with distance from the eruptive vents. The relative contributions of bubble nucleation, growth, and coalescence during emplacement are evaluated from the vesicle size distributions and dissolved CO₂ concentrations in these samples. In order to reproduce observed dissolved volatile concentrations, our model requires lava emplacement rates from ~0.1-0.7 m/s. In addition, magma ascent dynamics are assessed through the variable dissolved CO₂ content and vesicularity observed in samples collected along the 10km eruptive fissure. The observed variability may be explained by magma ascent paths of varying lengths with ascent rates exceeding 0.025 MPa/s. Longer ascent paths occur at the northern and southern ends of a narrow 5km region of the eruptive fissured indicating the site of dike nucleation at depth.