Chemically driven growth and resorption of bubbles in a multivolatile magmatic system

A carbon dioxide-rich fluid from a deep source fluxes in a water-rich shallow-stored magma and causes a volatile exchange with the melt. We carried out hydrothermal experiments on both bubble-free and -bearing rhyolitic melts in an H2O–CO2 system to explore the kinetics of chemical re-equilibration between the fluid and the magma. In the bubble-free experiment, a water-saturated rhyolitic melt was first synthesized at 800 °C and 100 MPa, and this interacted with a carbon dioxide-rich fluid under the same PT conditions for a period of 3 h. The melt temporarily became undersaturated upon the volatile exchange, as water quickly diffused out of the melt, while dissolution of the carbon dioxide was much slower. In the bubble-bearing experiments, the rhyolitic melt was saturated with H2O–CO2 fluids at 800 °C and 100 MPa, decompressed to 50 MPa, and then kept for up to 5.56 h. Water-rich bubbles were generated by decompression, and initially grew, but then dissolved in the melt during the retention time. This bubble resorption was attributed to water depletion of the melt upon re-equilibration with the carbon dioxide-rich surrounding fluid. Such water depletion and resulting bubble resorption are specific to multivolatile systems, such as H2O–CO2, in which the diffusivity and solubility differ among the end components. The bubble resorption occurring in this mechanism may explain the genesis of carbon dioxide-rich, bubble-poor obsidian pyroclasts, which are considered to have undergone carbon dioxide-rich fluid fluxing. We also discuss possible consequences of this effect on the bulk magma density. The dissolution of a small amount of carbon dioxide may cause expulsion of a much larger amount of water from the hydrous melt, which may increase fluid fraction and thus decrease the bulk density by up to 188 and 104 kg/m3 at 100 and 200 MPa, respectively, for rhyolitic melts. We suggest that such a process has the potential to trigger a volcanic eruption.