In order to investigate bubble formation and growth at 1 atmosphere, degassing experiments using a Stromboli basalt with dissolved H2O or H2O + CO2 were performed in a custom furnace on a beamline at the Advanced Photon Source. The glasses were synthesized at 1250°C and 1000 MPa, with ~3.0 wt%, ~5.0 wt%, or ~7.0 wt% H2O or with mixtures of H2O + CO2, ~3.0 wt% H2O and ~440 ppm CO2, ~5.0 wt% H2O and 880 ppm CO2, ~7.0 wt% H2O and ~1480 ppm CO2, then heated on the beamline while recording the bubble growth. The 3D bubble size distributions in the quenched samples were then studied with synchrotron X-ray microtomography. / The experimental results show that bubble nucleation and growth are volatile-concentration dependent. Bubbles can easily nucleate in melts initially containing high volatile concentrations. CO2 has no significant effect on bubble formation and growth because of low CO2 concentrations. Multiple nucleation events occur in most of these degassing samples, and they are more pronounced in more supersaturated melts. Bubble growth is initially controlled by viscosity near glass transition temperatures and by diffusion at higher temperatures where melt viscous relaxation occurs rapidly. Bubble foam forms when bubbles are highly connected due to coalescence, and bubbles begin pop, 10 to 20 seconds after the foam is developed. The degree of bubble coalescence increases with time, and bubble coalescence can significantly change the bubble size distribution. Bubble size distributions follow power-law relations at vesicularities of 1.0% to 65%, and bubble size distributions evolve from power-law relations to exponential relations at vesicularities of 65% to 83%. This evolution is associated with the change from far-from-equilibrium degassing to near-equilibrium degassing. / The experimental results imply that during basaltic eruptions both far-from-equilibrium degassing and near-equilibrium degassing can occur. The far-from-equilibrium degassing generally generates the power-law bubble size distributions whereas the near-equilibrium degassing produces exponential bubble size distributions Bubbles begin to pop when the vesicularities attain 65% to 83%. Bubble expansion in the foam possibly accounts for the mechanism of magma fragmentation. / Afin d'étudier la formation et la croissance de bulle; sous pression d'une atmosphère, desexpériences de dégazage sur un basalte de Stromboli, avec HiO ou H20 + CO2 dissouts,ont été exécutées dans un four pilote sous rayonnement synchrotron à l'APS (AdvancedPhoton Source). Les verres ont été synthétisés à une température de 1250°C et unepression de 1000 MPa, avec des teneurs en eau dissoute de ~ 3.0, ~ 5.0 ou ~ 7.0% (enpoids), et des mélanges H20 + C02 à teneurs de ~ 3.0% H20 (en poids) et 440 ppm CO2,~ 5% H20 et 880 ppm CO2, et de ~ 7.0% H20 et 1480 ppm CO2. La croissance des bullesest enregistrée pendant le chauffage du mélange en utilisant le rayonnement synchrotron.Les distributions tridimensionnelles de la taille des bulles dans les échantillons trempésont été étudiées par microtomographie à rayon X synchrotron.

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