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Experimental and model constraints on degassing of magma during ascent and eruption

By
James E. Gardner
James E. Gardner
Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA
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Alain Burgisser
Alain Burgisser
Institut des Sciences de la Terre d'Orleans, 45071 Orleans, France
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Matthias Hort
Matthias Hort
Institut für Geophysik, Universität Hamburg, 20146 Hamburg, Germany
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Malcolm Rutherford
Malcolm Rutherford
Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
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Published:
January 01, 2006

Surface volcanic gases may reflect volatile budgets in magma and forecast impending eruptions, and their release to the atmosphere may affect climate. The dynamics of magma degassing is complicated, however, by differences in the solubility, partitioning, and diffusion of the various volatiles, all of which can vary with pressure, temperature, and melt composition. To constrain possible gas outputs, we carried out experiments to determine how Cl partitions between water bubbles and silicate melt, and decompression experiments to examine how Cl behaves during closed- and open-system degassing. We incorporated our findings and those from the literature for CO2 and S into a steady, isothermal, and homogeneous flow model to estimate fluxes of gases at the vent from ascending water-rich magma, assuming different scenarios for the onset and development of permeability in bubbly magma. We find that, for given permeability scenarios, total gas fluxes vary with magma flux, but ratios of gas species do not change. The S/Cl and SO2/CO2 ratios do change, however, depending on whether the magma is oxidized or reduced. After magma fragments into a Plinian eruption column, gases continue to escape from cooling pumice in the plume, but here the rate of gas release is controlled by diffusion, which varies with temperature. Degassing of pumice and ash was modeled by linking a steady-state plume model, which gives the vertical variation of mean temperature and velocity of particles inside the plume, to a conductive cooling model of pumices, which controls diffusion of Cl, CO2, and S in pumice. We find that gas loss increases with column height (mass flux) and initial temperature, because in both cases pumices cool over a longer time period, allowing more gas to diffuse out of the matrix glass. The amount of gas released also depends on the size distribution of particles in the erupting mixture, with less being released for a finely skewed distribution.

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GSA Special Papers

Neogene-Quaternary Continental Margin Volcanism: A perspective from Me´xico

Claus Siebe
Claus Siebe
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José Luis MacíasGerardo
José Luis MacíasGerardo
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J. Aguirre-Díaz
J. Aguirre-Díaz
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Geological Society of America
Volume
402
ISBN print:
9780813724027
Publication date:
January 01, 2006

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