Degassing of Mid-Ocean Ridge Basalts during Magma Ascent and Lava Emplacement

Abstract:

Mid-ocean ridge basalt (MORB) glasses are often supersaturated in CO₂. Indeed, volatile saturation pressures, estimated from dissolved CO₂ and H₂O concentrations, exceed eruption pressures (seafloor collection depths) by up to 45–50 MPa. It is thought that rapid ascent to the surface promotes disequilibrium degassing by kinetically limiting bubble nucleation and growth. This study uses supersaturated MORB glasses to experimentally investigate degassing kinetics in order to constrain magma ascent and flow rates. Glasses used were recovered at the fissure of the 2005–06 eruption of the East Pacific Rise, and are supersaturated by about 38 MPa and contain ~10⁵ bubbles/cm³, the largest of which are ~35 µm in size. Away from the fissure, CO₂ concentrations decrease as bubble sizes increase dramatically along the flow length, with bubbles reaching 94 µm in size. Nucleation is being studied by heating samples at 1225 °C and 200 MPa, and then rapidly decompressing them to low pressures, holding them at low pressure for various amounts of time, and then rapidly quenching them. Samples quenched at 200 MPa are bubble free and contain about 550 ppm CO₂, indicating that all original bubbles resorb. Preliminary results show that samples decompressed to 12–21 MPa nucleate ~10⁵ bubbles/cm³ after two minutes, but nucleate many orders of magnitude more after ten minutes, number densities that far exceed those in the natural samples. Samples decompressed to 52 MPa nucleate ~10⁵ bubbles/cm³ after 10 minutes. These preliminary results suggest that bubble nucleation in MORB occurs at relatively high pressures, and then bubble growth suppresses further nucleation at lower pressures. Such a scenario suggests that MORB magmas may not ascend as rapidly as predicted. Bubble growth at low pressure is being studied by rapidly heating glassy vent samples to 1225 °C at 70 MPa, then decompressing and holding them at ~25 MPa for various amounts of time. Mean bubble size increases steadily by ~3 µm/hr during the first 7 hours, but number density decreases significantly between 3 and 5 hours, possibly because of coalescence. Continued experiments will be run to mimic bubble sizes in the natural samples to constrain the rate of lava emplacement.