Progressive excavation of gas pathways in hot silicic lava through fluxing of Cl-rich corrosive gas

Abstract:

Gas exsolution and transport occurring in hot silicic lava are crucial to the development of interior structure and surface explosion of the lava, yet details concerning these processes are still debated. To address this issue I analysed texture and Cl-content distribution in groundmass glass of a dacitic lava flow from Naruko Volcano, NE Japan. This lava typically showed an interlayered structure composed of dense, glassy region and powdery, crystalline region. In the glassy region, small, deformed bubbles produced gas channels. SiO₂ minerals (possibly cristobalite) and plagioclase crystals were formed along this channel. The crystalline region was composed of SiO₂ mineral, plagioclase, and a small amount of glass fragments. This region is porous (52 vol%) and the 2D connectivity is 91%, indicating that it is highly permeable. The outline of each glass fragment is rough and wavy. The interface between the glassy region and crystalline region were also wavy and irregularly curved, indicating that glass in contact with the gas is being corroded. The Cl content of glass at these wavy interfaces was strongly elevated (1600 to 2400 ppm) compared to background groundmass (1150 ppm). Based on these results, I propose an idea that a Cl-rich, highly corrosive gas flowed through originally glassy lava and formed permeable crystalline region. SiO₂ and plagioclase crystals may have formed as a result of devitrification of reacted glass. Once the corrosive gas flows in the glassy region, the gas widens the pre-existing pathways, further increasing permeability. Because of this positive-feedback effect, the interior structure of the lava may develop rapidly. The Cl-rich gas is considered to have originated from the magma chamber, because the Cl content at the corrosion interfaces is comparable to that of melt inclusions (1500 to 2200 ppm).