Simulation expérimentale de l'ascension et de la vésiculation des magmas rhyolitiques

The purpose of this thesis was to characterize the kinetics of bubble nucleation in rhyolitic magmas. Isothermal decompression experiments (in externally heated pressure vessels fitted with a rapid quench) were carried out to better understand (i) the kinetics of heterogeneous bubble nucleation in hydrous rhyolitic melts, and (ii) the effect of carbon dioxide on homogeneous bubble nucleation in the rhyolite-H2O-CO2 system. The final objective of our research was to identify textural parameters which could constitute reliable markers of the dynamics of magma ascent. The kinetics of heterogeneous nucleation was studied in rhyolite-hematite-H2O and rhyolite-magnetite-H2O systems, using glasses saturated in water at 200 MPa and 775-825°C (≈ 6 wt% H2O), and comprising different number densities of crystals. We carried out several experiments at two decompression rates (⎪dP/dt⎪ = 27.8 and 1000 kPa/s). Our main results are as follows: (1) The degree of supersaturation ΔPN required for nucleation is strongly dependent on the mineral species present in the liquid (ΔPN is the difference between the water saturation pressure and the bubble nucleation pressure): in the presence of hematite, ΔPN is very large (≈ 135 MPa) like in the case of homogeneous nucleation, whereas it is reduced to 15-35 MPa in presence of magnetite crystals. (2) For a given system, ΔPN does not vary with the decompression rate. (3) During decompression, the bubble number density n3D first increases rapidly, and then stabilizes to a value strongly dependent on the decompression rate. The number of bubbles is not controlled by the number of crystals in the liquid but by the relative values of surface tension, water diffusivity and decompression rate, as predicted by Toramaru (1995, 2006). (4) Once the nucleation pressure is crossed, the degree water supersaturation rapidly decreases and the water content in the liquid approaches the equilibrium value, independently of the decompression rate. (5) A second event of nucleation was observed in the rhyolite-magnetite-H2O system, and was ascribed to a slight increase of the degree of water supersaturation at low pressure due to a decrease of water diffusivity. Homogeneous nucleation in the rhyolite-H2O-CO2 was studied at decompression rates of 27.8, 167 and 1000 kPa/s, using starting glasses saturated in volatiles at 250 MPa and 800°C and containing ≈ 5.2 wt% H2O and ≈ 590 ppm CO2. The main results are as follows: (i) the nucleation pressure is almost independent of the decompression rate; (ii) ΔPN is reduced at 115 MPa in the presence of CO2, compared to 150 MPa in the rhyolite-H2O system; and (iii) for a given decompression rate, bubble number densities are two orders of magnitude larger than those measured in the rhyolite-H2O system. The major outcome of this research is a very strong correlation between decompression rate and bubble number density: the larger ⎪dP/dt⎪, the larger n3D. The principal volcanological implication is that the textural study of natural pumices, and in particular the measurement of bubble number densities, could be used for velocimetric applications and to provide information on the dynamics of magma ascent in volcanic conduits.L'étude du processus de nucléation des bulles dans les magmas rhyolitiques a été abordée au cours de cette thèse. Des expériences de décompression isothermes en autoclave à chauffage externe et trempe rapide ont été réalisées afin de mieux comprendre : (1) les effets de différentes populations cristallines sur la cinétique de nucléation des bulles d'eau, et (2) les effets du CO2 sur la nucléation homogène des bulles. L'objectif ultime de nos travaux était d'identifier les paramètres texturaux qui pourraient constituer des marqueurs robustes de la dynamique d'ascension des magmas rhyolitiques. Le résultat majeur de cette étude est la démonstration que la relation très forte entre [dP/dt] et n3D tient aussi dans le cas de la nucléation hétérogène. La principale implication volcanologique est que l'étude texturale des ponces naturelles pourrait servir à des applications vélocimétriques et fournir des renseignements sur la dynamique d'ascension des magmas dans les conduits volcanique