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 >1016bubbles 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
Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles. Nucleation of those bubbles occurs because pressure decreases as magma rises to the surface. As a consequence, dissolved magmatic volatiles, such as water, become supersaturated and cause bubbles to nucleate. At the same time, diffusion of volatiles into existing bubbles reduces supersaturation, resulting in a dynamical feedback between rates of nucleation due to magma decompression and volatile diffusion. Because nucleation rate increases with supersaturation, bubble number density (BND) provides a proxy record of decompression rate, and hence the intensity of eruption dynamics. Using numerical modeling of bubble nucleation, we reconcile a long-standing discrepancy in decompression rate estimated from BND and independent geospeedometers. We demonstrate that BND provides a record of the time-averaged decompression rate that is consistent with independent geospeedometers, if bubble nucleation is heterogeneous and facilitated by magnetite crystals.
more » « less- NSF-PAR ID:
- 10209563
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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,τ diff, to the decompression time,t d. Experiments with >1013 m−3bubbles are characterized byτ diff/t d≪ 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,τ diff/t d≫ 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. -
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Abstract Bubbles in magmas drive explosive volcanic eruptions. The spatial distribution of bubble nucleation sites in an ascending, decompressing, and supersaturating magma is one of the primary controls on ash morphologies and volcanic hazards. The mechanism of bubble formation is important because it ultimately determines the spatial distribution of bubbles in the fragmenting magma. The initial nucleation of bubbles in a homogeneous magma is problematical because excessive surface tension pressure in very small, nascent bubbles should drive exsolved volatiles back into the melt. This thermodynamic barrier to bubble viability confounds understanding of homogeneous bubble nucleation, yet very small bubbles form, grow, and ultimately drive explosive volcanic eruptions. We refer to this as “the tiny bubble paradox.” Classical nucleation theory typically explains bubble formation and growth, but we propose that a spectrum of bubble‐forming mechanisms may include both homogeneous nucleation and spinodal decomposition (the spontaneous unmixing of phases by uphill diffusion) as end‐member processes. As spinodal decomposition progresses, regularly sized and regularly spaced quasi‐spherical zones form with increasingly high concentration of dissolved water at the centers. Bubble formation occurs as the concentration of water in the interior of the water‐rich zones approaches 100% and the concentration of melt approaches zero. The presence of a broad, diffuse, concentration gradient of water rather than a narrow water‐melt interface means that there is no surface, per se, for surface tension to arise. This is the crux of the solution of the tiny bubble paradox.
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Nucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate (heterogeneous nucleation, <50 MPa), and maximized when no nucleation substrate is present (homogeneous nucleation, >100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (<100 MPa) depths represents an apparent paradox that hints at the presence of a cryptic nucleation substrate. In a pair of studies focusing on Glass Mountain eruptive units from Medicine Lake, California, we characterize titanomagnetite nanolites and ultrananolites in pumice, obsidian, and vesicular obsidian (Brachfeld et al., 2024,more » « less
https://doi.org/10.1029/2023GC011336 ), calculate titanomagnetite crystal number densities, and compare titanomagnetite abundance with the physical properties of pumice to evaluate hypotheses on the timing of titanomagnetite crystallization. Titanomagnetite crystals with grain sizes of approximately 3–33 nm are identified in pumice samples from the thermal unblocking of low‐temperature thermoremanent magnetization. The titanomagnetite number densities for pumice are 10^18 to 10^20 m^−3, comparable to number densities in pumice and obsidian obtained from room temperature methods (Brachfeld et al., 2024,https://doi.org/10.1029/2023GC011336'>https://doi.org/10.1029/2023GC011336). This range exceeds reported bubble number densities (BND) within the pumice from the same eruptive units (average BND ∼4 × 10^14 m^−3). The similar abundances of nm‐scale titanomagnetite crystals in the effusive and explosive products of the same eruption, together with the lack of correlation between pumice permeability and titanomagnetite content, are consistent with titanomagnetite formation having preceded the bubble formation. Results suggest sub‐micron titanomagnetite crystals are responsible for heterogeneous bubble nucleation in this nominally aphyric rhyolite magma. -
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