Alexander Gennadievich Simakin, Vera Devyatova, Andrey Shiryaev

Flushing of hydrous silicic magmas with crustal carbonic fluid may be an important factor controlling the explosiveness of rhyolitic eruptions. We present combined theoretical and experimental study of the interaction of carbonic fluid with a hydrous silicic melt. The process of diffusional equilibration of a CO2 bubble with a silicic melt was simulated numerically in the spherical approximation. The rapid water transfer from the melt to the bubble is followed by a slower diffusion of CO2 into the melt. The water distribution becomes almost uniform over a period proportional to the diffusional unit of time 0.14W, determined by the initial inter-bubble distance W and the water diffusion coefficient Dw (W =W2/Dw), while the CO2 distribution remains strongly contrasting. This process was modelled experimentally with a hydrous albite melt at P=200 MPa and T=950-1000oC. In the first series of experiments at T=950oC, a glass powder was filled with pure CO2 at the beginning of the experiment, forming numerous bubbles at the run temperature. Micro-FTIR measurements showed that after 40 minutes the water content decreased from 4.9 down to 1.8 wt.% with the maximum CO2 content 500 ppm. After 4 hours, the crystallinity increases to 85%, and the fluid bubbles almost disappear. The second series of experiments CO2 interacted with a 2 mm high column of hydrous albite melt. Diffusion profiles in the quenched glass were measured using EMPA (H2O) and micro-FTIR (CO2 and H2O). The estimated diffusion coefficients for H2O (1.1×10-6 cm2/s) and CO2 (1.5e×10-7 cm2/s) are consistent with published data. Scaling analysis predicts that in natural environment the homogenization of water in rhyolitic magma after an influx of CO2 bubbles few millimeters in size lasts for 1-30 days, i.e. a period compatible with pre-eruptive processes in a magma chamber.