Experimental Study of Bubble Size Distribution in Rhyolitic and Phonolitic Melts: Implications for Vesicle Size Distribution in Pumice

University Of Alaska Fairbanks Campus, Fairbanks AK

Investigators

Abstract

Faust Larsen EAR-0074127 It is critical to understand the evolution of bubble size distributions (BSD) during magmatic ascent in order to determine what the textural information from quenched pumice clasts tells us about the processes of magmatic degassing during explosive eruptions. Processes such as coalescence, Ostwald ripening, the effects of concentration gradients in the melt, and multiple nucleation events are all possible ways by which an initially uniform bubble size distribution derived from a single nucleation event may evolve into polymodal distributions more like that observed in pumice clasts. We propose here to experimentally quantify the rates and conditions under which these processes affect BSD's in rhyolitic and phonolitic melts at decompression rates, pressures, temperatures, and water contents applicable to magmatic ascent prior to the fragmentation limit. Our study will address the role that melt viscosity plays in the production of pumice vesicle size distributions by comparing results for natural high-silica rhyolite and phonolite compositions (h ~109 and 105 respectively). Our goal is to use these experiments to constrain pre-fragmentation effects at applicable conditions in order to better constrain how BSD modification may occur during eruptions to produce the VSD's present in quenched pumice. Our goal for this proposed work is an experimentally constrained model of the evolution of BSD's in ascending magma prior to fragmentation, and provide better constraints on the processes that contribute to vesicle size distributions observed in pumice.