Evolution of bubble microstructure in sheared rhyolite: Formation of a channel-like bubble network

The microstructure of bubbles in magma controls the dynamics of magma flow in a volcanic conduit. To investigate the shear-induced evolution of bubble microstructures, we performed a series of deformation experiments on vesiculated rhyolitic melts by twisting columnar obsidians at a temperature of 975 degrees C and at rotation rates of 0.3-0.5 rpm for up to 10 rotations. Three-dimensional shapes of the run products were analyzed using X ray-computed tomography. The experimental results demonstrate that shear strain and strain rate control the degree of bubble deformation and coalescence. The bubble connectivity, defined as the ratio of the volume of the largest bubble to the total bubble volume, starts to increase for a vesicularity of 20-30 vol%. A connectivity of >80% was achieved for a vesicularity of approximately 40 vol% and 10 rotations at 0.5 rpm, corresponding to a maximum strain of 30 at a strain rate of 0.025 s(-1). These conditions are identical to those in a volcanic conduit near the conduit wall, and a vesicularity of 40 vol% is accomplished below a depth of several hundred meters. Therefore, we infer that the bubbles form large interconnecting channelized networks near the conduit wall before the magma reaches to this depth. Below this, the fracturing of magma by shear stress cannot occur under a realistic strain rate. This implies that brittle fracture of magma should occur after the formation of channel-like bubble networks, and permeable degassing along this network would be the only effective degassing mechanism at such a depth.