Bubble growth and resorption in magma: insights from dissolved water distributions in volcanic glass

Abstract

Volcanic eruptions are driven by the growth of gas bubbles in magma, which grow and shrink as volatile species exsolve from and dissolve back into the melt in response to changes in the local environment, particularly in pressure and temperature. This movement of volatiles, particularly water, is recorded in the glass around vesicles and recent studies have used this record to interpret natural samples. This thesis investigates the processes that control bubble growth and resorption in magma, by measuring the distribution of dissolved H2O in experimentally-vesiculated volcanic glasses. H2O concentration profiles obtained using SIMS-calibrated BSEM imaging and H2O speciation data obtained using FTIR spectroscopy, are interpreted in the context of the known pressure and temperature history of the samples.
Samples are found to have undergone partial bubble resorption during the quench to glass at the end of experiments, as a result of increasing H2O solubility with decreasing temperature. Analysis of the lengthscale and timing of the resulting H2O concentration profiles demonstrates that the majority of resorption occurs above the glass transition. This quench resorption is associated with a reduction in bubble volumes which creates characteristic textures, such as buckled melt films between adjacent vesicles and reoriented cracks around resorption halos. Highly disequilibrium H2O speciation ratios within resorption halos are found to be diagnostic of quench resorption and can preserve evidence of pre-quench bubble growth
Quench resorption can increase sample H2O concentrations and H2Om:OH ratios and reduce bubble volumes and sample porosities. Studies based on these parameters must therefore consider the potential impact of quench resorption, which is expected to be greatest for samples with high H2O concentrations, slow quench and low initial sample porosities. H2O speciation data offer a way to investigate these impacts in unconstrained natural samples and could provide a tool for forensic interrogation of their eruptive history.