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，对于确定从淬灭浮石碎屑中获得的结构信息所能告诉我们的爆炸喷发期间岩浆脱气过程是至关重要的。聚结、奥斯特瓦尔德成熟、熔体中浓度梯度的影响和多次成核事件等过程都是由单一成核事件产生的最初均匀的气泡大小分布演变成更像浮石碎屑中观察到的多模态分布的可能途径。我们建议通过实验量化这些过程在减压速率、压力、温度和含水量下影响流纹岩和声纹岩熔体中BSD的速率和条件，这些速率和条件适用于破碎极限之前的岩浆上升。我们的研究将通过比较天然高硅流纹岩和phonolite组成（分别为h ~109和105）的结果来解决熔体粘度在浮石囊泡大小分布中所起的作用。我们的目标是利用这些实验来约束在适当条件下的预破碎效应，以便更好地约束喷发过程中BSD的修饰如何产生淬硬浮石中VSD的存在。我们的目标是建立一个BSD在上升岩浆破碎前演化的实验约束模型，并为浮石中观察到的囊泡大小分布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.