281-9 Detailed Examination of Bubble-Crystal Interactions During Magma Degassing in Decompression Experiments

Session: Petrology, Volcanology, and Mantle Plumes across the Solar System, Part II

Presenting Author:

Authors:

Abstract:

Gas exsolution and escape from ascending and decompressing magmas is one of the primary controls on volcanic eruption style. Many current models of magma degassing consider magma as a two-phase (liquid+gas) suspension, but recent studies have shown that even a modest amount of crystals (~20 vol. %), either grown during storage as phenocrysts or during decompression as microlites, have a significant impact on bubble coalescence and development of permeability. Here, we re-examine both 2D and 3D properties of bubbles grown during decompression experiments presented in [1] and [2] to better understand the nucleation, growth, and coalescence of bubbles in the presence of crystals.

Back-scattered electron images were used to quantify bubble size distributions (BSDs) and bubble number densities (BNDs), and micro-CT images were used to visualize 3D distributions of bubbles and crystals. BND within the samples is closely tied with number density of Fe-Ti oxides, and bubbles in 3D images are dominantly in contact with oxides, indicating that bubble nucleation was dominantly heterogeneous during these experiments. Cumulative BSDs show that both crystal-free and crystal-bearing samples experience similar continuous bubble nucleation and growth degassing until bubbles grow large enough to interact with crystals; then, BSDs from crystal-bearing samples show a shift to larger bubble sizes from enhanced coalescence, and that shift is greater for higher crystal contents. Both 2D and 3D imagery show that the spatial distribution of large, coalescence-enhanced bubbles is tied to the distribution of crystals within samples. The large-sample spanning bubbles that contribute most to permeability tend to form in locally crystal-rich areas while locally crystal-poor areas show much smaller, uncoalesced bubbles. In addition, 3D imagery reveals that the formation of these sample-spanning interconnected bubble networks from several isolated bubble groupings occurs within 5 MPa of decompression, which is also tied to development of permeability within the samples. These results have implications for complex degassing-outgassing relationships in crystal-rich intermediate magmas that frequently exhibit effusive-explosive eruption transitions associated with Vulcanian eruption style. As crystals may be distributed unequally within the conduit during an eruption, permeable outgassing is either enhanced or impeded based on local crystal distribution and content, leading to shifts in explosive-effusive behavior.

[1] deGraffenried et al. (2019). G-Cubed.

[2] Graham et al. (2023). EPSL.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10734

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