Bubble Coalescence and Percolation Threshold in Expanding Rhyolitic Magma

摘要

Coalescence during bubble nucleation and growth in crystal-free rhyolitic melt was experimentally investigated, and the percolation threshold, defined as the porosity at which the vesicular melt first becomes permeable, was estimated. Experiments with bubble number densities between 10(14) and 10(15)m(-3) were compared to four suites of rhyolitic Plinian pumices, which have approximately equal bubble number densities. At the same total porosity, Plinian samples have a higher percentage of coalesced bubbles compared to their experimental counterparts. Percolation modeling of the experimental samples indicates that all of them are impermeable and have percolation thresholds of approximately 80-90%, irrespective of their porosity. Percolation modeling of the Plinian pumices, all of which have been shown to be permeable, gives a percolation threshold of approximately 60%. The experimental samples fall on a distinct trend in terms of connected versus total porosity relative to the Plinian samples, which also have a greater melt-bubble structural complexity. The same holds true for experimental samples of lower bubble number densities. We interpret the comparatively higher coalescence within the Plinian samples to be a consequence of shear deformation of the erupting magma, together with an inherently greater structural complexity resulting from a more complex nucleation process.