Strain-induced magma degassing: insights from simple-shear experiments on bubble bearing melts
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.
KeywordsMagma deformation Strain-induced degassing Bubble deformation Obsidian banding
This project was financed by an INSU grant and the Electrovolc project, which is funded by the French national agency for research (ANR): contract JC05-42707 to Fabrice Gaillard and by the NERC grant (NE/G012946/1) to Luca Caricchi. We are grateful to the local contacts Peter Modregger and Federica Marone at Swiss Light Source to make possible the x-ray tomography experiments and for their help with the 3D reconstructions and data treatment. The scientific discussions with Fabrice Gaillard, Remi Champallier, Alison Rust and the technical support of Philip Teulat were greatly appreciated. The comments of Satoshi Okumura and two anonymous reviewers greatly improved the manuscript.
- Dingwell DB, Webb SL (1992) The fluxing effect of fluorine at magmatic temperatures (600-800-degrees-C)—a scanning calorimetric study. Amer Mineralog 77:30–33Google Scholar
- IAPWS (1997) Revised release on the IAPS formulation 1985 for the viscosity of ordinary water substance. International Association for the Properties of Water and Steam, Erlangen, 15Google Scholar
- Moore G, Vennemann T, Carmichael ISE (1998) An empirical model for the solubility of H2O in magmas to 3 kilobars. Amer Mineralog 83:36–42Google Scholar
- Stampanoni M, Groso A, Isenegger A, Mikuljan G, Chen Q, Bertrand A, Henein S, Betemps R, Frommherz U, Bohler P, Meister D, Lange M, Abela R (2006) Trends in synchrotron-based tomographic imaging: the SLS experience. Dev X-Ray Tomogr V 6318:U199–U212Google Scholar
- Terzaghi K (1945) Stress conditions for the failure of saturated concrete and rock. Proc Am Soc Test Mater 45:777–792Google Scholar