Kinetics of Bubble Generation in Mafic Enclaves

Abstract:

Volcanically erupted mafic enclaves are typically vesicular, with the bubbles forming when the mafic magma cools after it is injected and disaggregated into a cooler silicic magma. This study uses hydrothermal experiments to investigate the kinetics of pre-eruptive bubble nucleation and growth within mafic magmas, focused on the efficiency of nucleation on different minerals, and to quantify the growth rate of bubbles with varying cooling rates. Starting materials are natural mafic enclaves from Southwest Trident, Alaska. Experiments were initially equilibrated with H₂O at 85 MPa and 1065 °C for 2 hours, producing a melt with blocky crystals of plagioclase and pyroxene, and spherical bubbles with a mean 30 µm diameter and number density (N_v) of 7.2x10⁴ cm^-3. Upon cooling to 1015 °C at 2 °C/h, the mineralogy and N_v did not change (although total crystallinity increased), while the mean bubble diameter increased to 90 µm. Cooling further to 985 °C at 2 °C/h, resulted in the crystallization of Fe-Ti oxides, along with an abrupt N_v increase (3.0x10⁵ cm^-3) of bubbles with a mean diameter of 60 µm. This abrupt bubble nucleation event, coinciding with the formation of Fe-Ti oxides, suggests that plagioclase and pyroxene are poor bubble nucleation sites in mafic melts, and that Fe-Ti oxides are good bubble nucleation sites, similar to previous results using rhyolite melts. Additionally, the occurrence of this nucleation event suggests that cooling related diffusive growth of bubbles in mafic enclaves, under magma chamber conditions, is too slow to keep up with increasing volatile saturation in the melt, and that the melt may become supersaturated until nucleation sites for new bubbles become available. Rapid cooling (1065-985 °C at 110 °C/h) produced abundant acicular plagioclase and pyroxene crystals (no Fe-Ti oxides), and bubbles with a nearly identical mean diameter and N_v to experiments equilibrated at 1065 °C. It is therefore likely that bubbles will not nucleate or grow significantly during rapid cooling and crystallization of mafic enclaves until Fe-Ti oxide nucleation sites are available. Overall, these experimental results indicate that cooling and crystallization induced pre-eruptive bubble generation in mafic enclaves is strongly controlled by the availability of Fe-Ti oxide nucleation sites.