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

了解岩浆上升过程中气泡大小分布(BSD)的演化，以确定从淬火浮石碎屑中获得的结构信息对我们了解岩浆在爆炸喷发期间的脱气过程是至关重要的。聚结、Ostwald熟化、熔体中浓度梯度的影响以及多次成核事件等过程都是从单一成核事件得到的最初均匀的气泡尺寸分布演变为更像在浮石碎屑中观察到的多峰分布的可能方式。我们建议在实验上量化这些过程在减压速率、压力、温度和水含量下影响流纹岩和声石岩熔体中BSD的速率和条件，这些因素适用于岩浆在碎裂极限之前的上升。我们的研究将通过比较天然高硅流纹岩和声石成分(分别为h~109和105)的结果来说明熔体粘度在浮石泡孔尺寸分布中所起的作用。我们的目标是利用这些实验在适当的条件下限制预破碎效应，以便更好地限制在喷发过程中如何发生BSD修改，从而在淬火的浮石中产生VSD。我们这项拟议工作的目标是在实验上限制BSD在碎裂之前在上升岩浆中的演化模型，并对在浮石中观察到的囊泡尺寸分布的过程提供更好的约束。

Faust LarsenEAR-0074127It 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.