Pre- and post-fragmentation conditions during the 1999 Vulcanian activity at Guagua Pichincha volcano (Ecuador) revealed by textural analysis of breadcrust bombs

Breadcrust bombs are typical products of Vulcanian eruptions. The exteriors of such pyroclasts experience rapid quenching after fragmentation and hence preserve a dense crust with no or limited textural change. The core of breadcrust bombs can instead stay above the glass transition temperature for a long time and evolve texturally as vesicles nucleate and grow. Breadcrust bombs are thus key pyroclasts that provide information about (i) pre-eruptive textural, chemical, and pressure conditions in the conduit at the time of fragmentation, (ii) the link between radial cooling history of the bomb and dynamics of bubble formation and (iii) transition from closed- to open-system degassing in natural magma at near-atmospheric pressure and in the presence of variable amounts of crystals. We analyzed two breadcrust bombs from 1999 Vulcanian activity at Guagua Pichincha volcano, Ecuador, previously described by Wright et al. (2007). We performed a quantitative textural analysis along radial profiles from the exterior surface to the interior of the bombs in two dimensions using Scanning electron microscopy, and in three dimensions using synchrotron-based X-ray micro-tomography. This analysis yielded the porosity and vesicle number density change with radial distance. We coupled this textural analysis to a model of viscosity, bubble growth rate and diffusion timescales to shed light on the timing of vesiculation and cooling across the rim to core profile. Our results revealed three vesicle populations: (i) a pre-existing population of large vesicles with a low vesicle number density that was formed prior to fragmentation, (ii) a halo of small and isolated, syn-eruptive vesicles with high vesicle number density formed around these large vesicles in the crust and (iii) vesicles that did not have time to form in the crust and nucleated and grew after fragmentation in the initially vesicle-free groundmass towards the core. We interpret these textures in the light of pre-eruptive conditions in the conduit prior to Vulcanian eruptions preserved in the crust and post-fragmentation vesiculation and transition from closed- to open-system degassing in the core.