Carbon dioxide transport in crustal magmatic systems

The volatile chemistry of juvenile volcanic glasses has suggested that shallow-stored crustal magmas often suffer the open-system addition of a carbon-dioxide-rich fluid from below, probably from a mantle-derived basaltic source (“carbon dioxide fluxing”). However, the actual mechanism of such a fluid transport is poorly understood. To constrain the volatile transport mechanism, we formulated this phenomenon as a reactive transport process and clarified the fundamental characteristics of chemical exchange in the system. The model assumes that a carbon-dioxide-rich fluid is introduced into a water-rich rhyolitic magma column from below and ascends at a constant velocity whilst a volatile exchange takes place between the fluid and melt. Two types of exchange modes were examined. One is the equilibrium mode where the volatile exchange is instantaneously achieved at all column depths. The second is the diffusive mode where the volatile exchange is rate limited by diffusion. In the equilibrium mode, the extent of re-equilibration of the entire column is controlled solely by the mass ratio of the integrated fluid to the melt. In the diffusive mode, the extent of re-equilibration is controlled by the Damköhler number, a dimensionless parameter representing the ratio of the advection time to the diffusion time. When the Damköhler number for carbon dioxide exceeds 10, the diffusive exchange becomes indistinguishable from the equilibrium exchange. Both exchange modes produce a negative correlation between the concentrations of carbon dioxide and water in the melt, which cannot be explained by conventional degassing models without significant crystallisation. The fluid emitted from the column as a volcanic gas changes its composition from carbon dioxide rich to water rich, and the emission rate decreases monotonically during fluxing. The simulation enables us to constrain the mechanism of fluid transport. For the melt inclusion data from the Bishop Tuff (Wallace et al., 1999; Anderson et al., 2000), fluid velocity in this magma was estimated to be 10− 6–10− 7 m/s. The corresponding mechanism of fluid transport may include permeable flow with a permeability of ~ 10− 15 m2 or a buoyant ascent of individual bubbles with a radius of 4–7 mm.

Research highlights
► Carbon dioxide fluxing in magmas is formulated as a reactive transport process.
► Characteristic compositional trends found in nature were quantitatively reproduced.
► We determine the parameters controlling the evolution of the volatile composition.
► We predict a volcanic gas character showing ongoing fluxing beneath active volcanoes.
► The gas flow mode in a huge magma chamber is a permeable flow or buoyant bubble rise.