Flow banding in obsidian: A record of evolving textural heterogeneity during magma deformation

We perform a quantitative textural analysis of banding in obsidian from Big Glass Mountain (BGM), Medicine Lake Volcano, California and from Mayor Island (MI), New Zealand. The samples are compositionally homogeneous, with banding defined by variable microlite content (BGM) or vesicularity (MI). The spatial distribution of alternating light and dark bands has a 1/wavenumber power-spectral density, as well as multifractal characteristics. Through the use of simple mathematical models, we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma. Our results indicate that changes in crystallinity (vesicularity) were concurrent with magma deformation and are random multiples of the total amount of crystallinity (vesicularity) that was already present at any given time. Banded obsidian from BGM and MI can be found in close spatial proximity to brittle deformational textures. We therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation. Fragmentation can result in variable rates of degassing of fragments, which in turn can lead to nonuniform crystal (vesicle) nucleation and growth rates. Repeated fragmentation, welding and subsequent viscous deformation can thus lead to banding formation.