Vesiculation and densification of pyroclasts: A clast-size dependent competition between bubble growth and diffusive outgassing

During volcanic eruptions, bubble growth and outgassing determine the porosity, buoyancy, and rheological evolution of magmas, which in turn, dictates the potential for explosive eruption. The processes which lead to magmatic fragmentation have received substantial attention, whereas the subsequent modification of fragmented pyroclasts remains poorly constrained. Here, we present the results of experimentation using obsidian cylinders in the ash to lapilli size range (1-12 mm diameter), for which we find that closed system bubble growth is progressively suppressed by fragment size-dependent, diffusive outgassing. We find that when the volatile partial pressure is lower in the ambient exterior gas than in the melt-hosted bubbles, the volatiles diffusively outgas. This volatile loss produces a bubble-free dehydrated rind, which thickens proportional to the diffusion length-scale. We show that more outgassing is possible from fragments with higher surface area to volume ratios, and therefore, pyroclasts with a smaller initial radius develop a higher proportion of dehydrated rind, densify faster, and attain more subdued vesicularities. We find that this diffusive outgassing process can produce fully dense, non-vesicular pyroclasts, effectively erasing the textural evidence of the vesiculation event altogether. Using an analytical approximate approach to the evolving clast geometry, we show that current closed system bubble growth models and diffusion models can be combined to estimate vesicularity in pyroclasts surrounded by a free gas of relatively low partial pressure of H2O. Our analyses highlight that a single explosive eruptive episode with disequilibrium volatile partial pressures may produce both dense and vesicular pyroclasts depending on their grain size.