Abstract
During magma ascent, bubbles nucleate, grow, coalesce, and form a variably permeable porous network. The volcanic system opens and closes as bubble walls reorganize, seal or fail. In this contribution we cause obsidian to nucleate and grow bubbles to high gas volume fraction at atmospheric pressure by heating samples to 950 oC for different times and we image the growth through a furnace. Following the experiment, we imaged the internal pore structure of selected samples in 3D and then dissected for analysis of textures and dissolved water content remnant in the glass. We demonstrate that in these high viscosity systems, during foaming and subsequent foam-maturation, bubbles near a free surface resorb via diffusion to produce an impermeable skin of melt around a foam. The skin thickens nonlinearly through time. The water concentrations at the outer and inner skin margins reflect the solubility of water in the melt at the partial pressure of water in atmospheric and water-rich bubble conditions, respectively. In this regime, mass transfer of water out of the system is diffusion limited and the sample shrinks slowly. In a second set of experiments in which we polished off the skin of the foamed samples and placed them back in the furnace, we observe rapid sample contraction and collapse of the connected pore network under surface tension as the system efficiently outgasses. In this regime, mass transfer of water is permeability limited. The mechanisms described here are relevant to the evolution of pore network heterogeneity in permeable magmas. We conclude that diffusion-driven skin formation can efficiently seal connectivity in foams. When rupture of melt film around gas bubbles (i.e. skin removal) occurs, then rapid outgassing and consequent foam collapse modulate gas pressurisation in the vesiculated magma.
Highlights
During ascent and eruption, magma vesiculates due to changes in pressure and temperature (e.g., Zhang, 1999)
Magma vesiculates due to changes in pressure and temperature (e.g., Zhang, 1999). This vesiculation is initiated by the oversaturation of volatile components that exsolve through the nucleation and growth of bubbles, which may produce a magmatic foam (Sparks, 1978)
In experiments exceeding 5 h, a discernible bubble-free skin develops around the bubbly core
Summary
Magma vesiculates due to changes in pressure and temperature (e.g., Zhang, 1999) This vesiculation is initiated by the oversaturation of volatile components that exsolve through the nucleation and growth of bubbles, which may produce a magmatic foam (Sparks, 1978). Collapse of the bubble network is driven by stress (Ashwell et al, 2015), and/or shear (Okumura et al, 2008, 2009) and surface tension (Kennedy et al, 2016) This densification may lead to the formation of a less permeable magmatic conduit fill that can plug the volcanic system leading to an overpressurization (Michaut et al, 2009; Ashwell et al, 2015)
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