Evolution of microstructure of bubbles and gas permeability in sheared rhyolite (Invited)

The microstructure of bubbles in magmas ascending in volcanic conduits is strongly changed by shear deformation. Because the microstructure controls the magma rheology and degassing behavior, clarifying its evolution in sheared magmas will help to understand the dynamics of magma ascent in volcanic conduits and predict the explosivity and style of volcanic eruptions. We have experimentally investigated the evolution of the microstructure of bubbles and the corresponding change in the gas permeability in shared rhyolite. The deformation experiments were performed using a piston-cylinder-type torsional deformation apparatus. Natural obsidians with a water content of 0.5 wt% were vesiculated in a cell with constant volume (ca. 5 mmΦ and ~5 mm in length) and then twisted at a temperature of around 1000°C. The maximum strain rate and total strain were of the order of 10-2 s-1 and ~35. After the deformation experiments, the microstructure of bubbles in run products was observed using synchrotron radiation X-ray CT at SPring-8 (BL20B2). The gas permeability of the run products was measured using an originally-manufactured gas permeameter. The size distribution and connectivity of bubbles in the run products indicated that shear strongly enhanced bubble coalescence. The coalescence resulted in the formation of bubble networks and an increase in the connectivity with total strain. The shear also elongated bubbles in a direction parallel to the shear. These results imply that shear increases the gas permeability in a direction parallel to the shear, and indeed, the measured gas permeabilities in this direction were two to five orders of magnitude larger than those of non-sheared samples. Once the connectivity and gas permeability increased because of shear deformation, compaction started to occur. During compaction, the connectivity remained large although the vesicularity decreased, and finally, bubble networks were pinched off, leaving elongated and aligned fine bubbles. The compaction of samples with a vesicularity of ~60 vol% resulted in the formation of dense rhyolite with only ~5 vol% vesicularity. Our experimental results imply that under a large shear strain, for example, in a narrow and long volcanic conduit, degassing is enhanced and magma compaction occurs, resulting in the effusion of lava and non-explosive eruptions.