BREAD CRUSTED BUBBLES: QUANTIFYING THE BRITTLE-DUCTILE TRANSITION IN VOLCANIC CONDUITS

Here, we describe a new type of pyroclast from the ash sized fraction of a distal, unwelded pyroclastic flow deposit from the ~7 Ma Rattlensake Tuff of eastern Oregon. The clasts comprise discreet, fully intact, hollow spheroids composed of high silica rhyolite glass. We imaged morphological features on individual spheroids using SEM (hemisphere surfaces) and XRCT (whole particles). The particles are typically ~400-1000 microns in diameter. Numerous fractures, collectively resembling bread crust texture on volcanic bombs, are the most prominent surface feature. The number of fractures ranges from <10 to >50 on the imaged hemispheres. Fractures extend for several hundred microns, have apertures of ~50 microns, and do not penetrate through the spheroid walls. Interior spheroid walls are smooth, devoid of noticeable surface disruption. Original crack edges are apparent on particle surfaces as slightly raised ridges running parallel to one another and the crack center; inner faces of those ridges are typically 5-10 microns wide. Notably, multiple generations of fractures are apparent, evidenced by cross-cutting relations.

We interpret that formation of these particles is a result of heterogenous fragmentation of a thick walled magmatic foam that liberated individual intact bubbles. As the erupting mixture decompressed, overpressure inside the bubbles increased until exceeding the critical strain rate of the outermost rind, resulting in brittle deformation (e.g., crack initiation). Ductile flow of the remaining, fully intact, bubble wall thickness accommodated the remaining overpressure in the volumetrically expanding bubble (e.g., crack expansion), resulting in bread crust texture; note, this process ceases immediately if the bubble wall fully ruptures. By tracing individual cracks on each spheroid, we quantify the brittle-ductile strain history recorded in the bread crust texture. Results indicate total strain values for ductile flow in the shells of up to 45%. Bread crusted bubbles indeed capture the brittle-ductile transition shortly after eruption. Using calculated values for viscosity, we exploit deformation timescales of these uniquely preserved pyroclasts to quantify conduit dynamics (i.e., ejection velocity, fragmentation depth) inaccessible through direct observation.