Shear-enhanced dynamic permeability development of magma vesiculating in cylindrical conduits

During magma vesiculation, permeability is established when growing bubbles form connected networks which allow fluids to percolate. This percolation threshold controls the relative rates between magma ascent and volatile outgassing, which in turn dictate eruptive style. Percolation in crystal-poor magmas is controlled primarily by vesicularity and shear. We investigate the effect of realistic shear conditions on system-spanning connectivity via vesiculation experiments on rhyolitic melts in a cylindrical, conduit-like geometry. The amount of shear experienced is controlled by varying the sample and confining diameters to allow for various degrees of free (isotropic) followed by confined (anisotropic) expansion. We observe two regimes of behavior for samples dominated by (1) initial isotropic vesiculation, in which the onset of permeability corresponds with the beginning of shear deformation that connects existing vesicles and (2) anisotropic vesiculation which shows low percolation thresholds (< 20%) followed by continuous deformation. We find distinct trends in porosity and permeability for sheared and unsheared samples. Furthermore, We recover the bulk dynamic permeability time series in the shearing melts which requires maintaining connectivity both within bubbles and to the exterior. The development of anisotropy is ubiquitous in vesiculating magmatic networks and our data highlight the importance of considering the in situ shear conditions when investigating the percolation threshold.