Outgassing from Open and Closed Magma Foams

Abstract

During magma ascent, bubbles nucleate, grow, coalesce, and form a variably permeable porous network. The reorganization, failing and sealing of bubble walls may contribute to the opening and closing of the volcanic system. In this contribution we cause obsidian to nucleate and grow bubbles to high gas volume fraction at atmospheric pressure by heating samples to 950 • C 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 non-linearly 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 to allow open system outgassing, 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. 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 pressurization in the vesiculated magma. The mechanisms described here are relevant to the evolution of pore network heterogeneity in permeable magmas.

Origin of Shallow Volcanic Tremor: The Dynamics of Gas Pockets Trapped Beneath Thin Permeable Media

“…This could be another fractured solidified magma, such that the transport of volatiles occurs through channels produced by bubbles connection or by shear fractures close to the conduit walls (Figure a; Jaupart, ; Sparks, ). Alternatively, the gas pocket may be overlying a fluid‐like magma batch, where the transport of volatiles depends on complex processes involving magma convection, crystallization, volatile exsolution and diffusion, bubble and foam dynamics, and the opening/closure of highly viscous deformable magma (Figure b; e.g., Shinohara, ; Girona et al, ; Girona, Costa, and Schubert, ; von Aulock et al, ). These fluid‐like magma batches can represent a hot magma reservoir or a hot magma column filling the lowest part of the conduit.…”

“…First, while it is widely accepted that volcanic degassing and shallow seismicity are physically related (Conde et al, ; Lesage et al, ; Nadeau et al, ; Salerno et al, ; Zuccarello et al, ), the coupling between outgassing flux and tremor is yet to be quantitatively addressed. Second, although volcanic gases are known to ultimately escape through permeable media (e.g., fractured domes or caps, volcanic edifices; Edmonds et al, ; Campion et al, ; Christenson et al, ; Girona et al, ; von Aulock et al, ), the role that porous flow may play in triggering tremor remains unexplored. Addressing these two issues is especially relevant when dealing with tremor signals sourced beneath active degassing vents, as proposed, for example, at Oshima (Yamaoka et al, ), Sakurajima (Ishihara, ; Maryanto et al, ), the Puu Oo crater of Kilauea (Goldstein & Chouet, ), Ruapehu (Hurst, ), Arenal (Métaxian et al, ), Stromboli (Chouet et al, ), and Láscar (Hellweg, ).…”

Comparative field study of shallow rhyolite intrusions in Iceland: Emplacement mechanisms and impact on country rocks

“…Parameters impacting this magma response include the decompression rate, magma viscosity, volatile content and quench rate (e.g. Sparks, 1978;Papale et al, 1998;Shea et al, 2010;von Aulock et al, 2017). The melt flow deforms vesicles according to the strain rate, which changes depending on the velocity gradient across the intrusion (e.g.…”

Strain‐Dependent Rheology of Silicate Melt Foams: Importance for Outgassing of Silicic Lavas

“…The restricted flow conditions required for strain localization to occur, and the propensity of outgassing pathways to form with a particular orientation to the principal stresses are not the only hindrances to pervasive outgassing: both melt fractures and interconnected bubble networks are susceptible to closure and rapid healing (e.g., Kendrick et al, ; Kolzenburg et al, ; Lindoo et al, ; Rust & Cashman, ; Shields et al, ; Tuffen et al, ; Yoshimura & Nakamura, ). This suggests that other processes, such as diffusive dehydration and volatile resorption (e.g., McIntosh et al, ; Ryan et al, ; von Aulock et al, ; Westrich & Eichelberger, ; Yoshimura et al, ; Yoshimura & Nakamura, ), may be important mechanisms by which lavas, particularly silicic lavas, can outgas and densify (Figure a).…”

“…In addition, fracturing of the melt joins bubbles in some experiments, resulting in localized outgassing (e.g., Kushnir et al, ; Okumura et al, ; Shields et al, ). Finally, other experiments record vesicle shrinkage due to resorption or diffusive loss of the H 2 O fluid within the bubbles (e.g., McIntosh et al, ; von Aulock et al, ; Westrich & Eichelberger, ; Yoshimura & Nakamura, , ). This wide range in behavior observed in parallel experimental studies reflects the complexities of bubble growth and loss in natural melts.…”