UNDERSTANDING THE KINETICS OF HETEROGENEOUS BUBBLE NUCLEATION IN MID-OCEAN RIDGE LAVAS FROM THE 2011 AXIAL SEAMOUNT ERUPTION

 Heterogeneous bubble nucleation in CO₂ supersaturated Mid-Ocean Ridge Basalts (MORBs) is poorly understood. We examine a subset of samples from the 2011 Axial Seamount eruption to better characterize degassing processes of MORB lavas. This study utilizes both Fourier transform infrared spectroscopy (FTIR) and optical analyses of Axial Seamount glasses to investigate how CO₂ supersaturated lavas degas through bubble nucleation, growth, and coalescence. FTIR transects of MORB glasses around bubbles reveal concentrations ranging from 0.13 to .34 wt. % H₂O and 87 to 299 ppm CO₂. There seems to be a diffusion profile for H₂O, but not CO₂. Porosities measured from this sample subset range from .04% to .76%. Using porosity as a proxy for the extent of degassing experienced by each sample, it appears the bubble number density decreases from 6.2x10⁵ to 5.9x10⁴ bubbles/cm³, while average bubble diameter increases from 11 to 63 µm. Crystal number densities stay relatively constant (1.3x10⁶ to 2.5x10⁶ crystals/cm³), and 29% to 84% of bubbles are visibly attached to microlites of olivine, plagioclase, and sulfide globules. Using this information, it appears the lava initially degassed through bubble nucleation on crystal surfaces before or during the eruption. Once the lava erupted onto the surface, crystal number densities indicate that crystallization essentially stopped, limiting the amount of crystal nucleation sites for new bubbles to nucleate on. This appears to have encouraged the lava to degas through diffusive growth and coalescence of smaller bubbles into larger bubbles, which is supported by the decreasing bubble number density and increasing bubble diameter. These results indicate that heterogeneous bubble nucleation can influence the style of degassing in CO₂ saturated MORB lavas, which may be pertinent to understanding degassing processes in other MORB settings.