Strain-induced magma degassing: insights from simple-shear experiments on bubble bearing melts |
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| Authors: | Luca Caricchi Anne Pommier Mattia Pistone Jonathan Castro Alain Burgisser Diego Perugini |
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| Institution: | 1.Department of Earth Sciences,University of Bristol,Bristol,United Kingdom;2.Centre National de la Recherche Scientifique (CNRS), Institut National des Sciences de l’Univers (INSU), Université d’Orléans, Université Fran?ois Rabelais–Tours,Institut des Sciences de la Terre d’Orléans,Orléans cedex 2,France;3.Department of Earth, Atmospheric and Planetary Sciences,Massachusetts Institute of Technology,Cambridge,USA;4.Institute for Geochemistry and Petrology,ETH Zurich,Zurich,Switzerland;5.School of Geosciences,Monash University,Melbourne,Australia;6.Department of Earth Sciences,University of Perugia,Perugia,Italy |
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| Abstract: | Experiments have been performed to determine the effect of deformation on degassing of bubble-bearing melts. Cylindrical specimens
of phonolitic composition, initial water content of 1.5 wt.% and 2 vol.% bubbles, have been deformed in simple-shear (torsional
configuration) in an internally heated Paterson-type pressure vessel at temperatures of 798–848 K, 100–180 MPa confining pressure
and different final strains. Micro-structural analyses of the samples before and after deformation have been performed in
two and three dimensions using optical microscopy, a nanotomography machine and synchrotron tomography. The water content
of the glasses before and after deformation has been measured using Fourier Transform Infrared Spectroscopy (FTIR). In samples
strained up to a total of γ ∼ 2 the bubbles record accurately the total strain, whereas at higher strains (γ ∼ 10) the bubbles
become very flattened and elongate in the direction of shear. The residual water content of the glasses remains constant up
to a strain of γ ∼ 2 and then decreases to about 0.2 wt.% at γ ∼ 10. Results show that strain enhances bubble coalescence
and degassing even at low bubble volume-fractions. Noticeably, deformation produced a strongly water under-saturated melt.
This suggests that degassing may occur at great depths in the volcanic conduit and may force the magma to become super-cooled
early during ascent to the Earth’s surface potentially contributing to the genesis of obsidian. |
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