Theoretical and Experimental Modeling of Local Scale CO2 Flushing of Hydrous Rhyolitic Magma

Abstract

Flushing of hydrous silicic magmas with crustal carbonic fluid may be an important factor controlling the dynamics 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 shell 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 in the melt becomes almost uniform over a period proportional to the diffusional unit of time 0.14τw, determined by the initial inter-bubble distance W equal the distance between neighbor bubbles centers and the water diffusion coefficient Dw in the melt (τw = W 2/Dw), while the CO2 distribution remains strongly contrasting and the melt remains undersaturated in CO2. This process was modelled experimentally with a hydrous albite melt at P = 200MPa and T = 950–1000 °C. In the first series of experiments at T = 950◦C, 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 in the melt decreased from 4.9 down to 1.8 wt. % with the maximum CO2 content of 500 ppm (below saturation). After 4 hours, the crystallinity increased to 85%, and almost all of the fluid bubbles escaped. 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 in the melt for H2O (1.1 × 10−6 cm2 /s) and CO2 (1.5 × 10−7 cm2 /s) are consistent with published data. Scaling analysis predicts that in the nature, after the influx of CO2 bubbles a few millimeters in size, the maximum dehydration of rhyolitic magma with viscosity near 105 Pa s without a significant increase in CO2 content occurs after 1–30 days, i.e. a period compatible with the minimum duration of pre-eruption processes in the magma chamber.