MICROSTRUCTURES IN OBSIDIAN

Obsidian contains microstructures with a range of sizes. The orientation of micron scale crystallites –mircrolites -- can be used to infer the style of deformation, e.g., simple vs pure shear, and the total strain. When the spatial context of samples is known, it is then possible to use microlites to study the emplacement history of obsidian flows. The deformation of bubbles with diameters ranging in size from microns to 100s of microns can be used to determine not only the style of deformation, but also the deformation rate. These bubbles thus contain unique information about at least some aspects of ascent dynamics within volcanic conduits. At still larger scales, there are mm to m size bands defined by variations in microlite or bubble concentration. The deformation and size distribution of bands provide insight into the deformation and fragmentation history of obsidian. Obsidian is not a bland volcanic glass: it contains a range of microscopic inclusions and textures. Quantitative characterizations of these features, combined with fluid mechanics and structural models, provide a means to obtain quantitative information about the kinematics and dynamics of evolved magmas.