G-Cubed: Bubble Coalescence and Percolation Threshold in Expanding Rhyolitic Magma

Thomas Giachetti, Helge M. Gonnermann, James E. Gardner, Alain Burgisser, Sahand Hajimirza, Tobias C. Earley, Nathan Truong, and Pamela Toledo

Geochem. Geophys. Geosyst. 20 (2019) 1054-1074.

DOI: 10.1029/2018GC008006


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 1014 and 1015 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.

JGR-Solid Earth: Predicting Homogeneous Bubble Nucleation in Rhyolite

Sahand Hajimirza, Helge M. Gonnermann, James E. Gardner, and Thomas Giachetti
J. Geophys. Res.-Solid Earth 124 (2019) 2395-2416.
Bubble nucleation is the critical first step during magma degassing. The resultant number density of bubbles provides a record of nucleation kinetics and underlying eruptive conditions. The rate of bubble nucleation is strongly dependent on the surface free energy associated with nucleus formation, making the use of bubble number density for the interpretation of eruptive conditions contingent upon a sound understanding of surface tension. Based on a suite of nucleation experiments with up to >1016 bubbles per unit volume of melt, and using numerical simulations of bubble nucleation and growth during each experiment, we provide a new formulation for surface tension during homogeneous nucleation of H2O bubbles in rhyolitic melt. It is based on the Tolman correction with a Tolman length of δ = 0.32 nm, which implies an increase in surface tension of bubbles with decreasing nucleus size. Our model results indicate that experiments encompass two distinct nucleation regimes, distinguishable by the ratio of the characteristic diffusion time of water, τdiff, to the decompression time, td. Experiments with >1013 m−3 bubbles are characterized by τdiff/td≪ 1, wherein the nucleation rate predominantly depends on the interplay between decompression and diffusion rates. Nucleation occurs over a short time interval with nucleation rate peaks at high values. For experiments with comparatively low bubble number density the average distance between adjacent bubbles and the diffusion timescale is large. Consequently, τdiff/td≫ 1 and nucleation is nearly unaffected by diffusion and independent of decompression rate, with bubbles nucleating at an approximately constant rate until the sample is quenched.