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Abstract:
At silicic volcanoes, the eruptive style is strongly influenced by the evolution of the magma’s permeable, outgassing pathways (e.g. fractures, connected pores). Studies have revealed that ascending magmas can undergo multiple cycles of fragmentation (or fracture) and healing (or sintering) due to stress fluctuations imparted by shear, vesiculation, gas flow, etc. Fragmentation and healing cycles result in transient permeability; fracturing rapidly increases permeability, whilst sintering progressively shuts permeable pathways. Tuffisite veins are a fossil record of this process and further evidence that sintering may be accompanied by vesiculation and diffusive outgassing of pyroclasts. A recent study showed that in contrast to sintering, which causes progressive densification as particles agglutinate, the occurrence of vesiculation and diffusive outgassing causes hysteretic volume and rheological changes that momentarily inhibit sintering, prompting fluctuations in porosity and permeability. Thus, pyroclast sintering models must be expanded and experimentally verified to represent the full spectrum of pyroclastic deposits, considering the impacts of volatile saturation, crystallinity, spatial limits (volume containment), stress and pressure. Here, we investigate the evolution of the permeable network during sintering of vesiculating and diffusively outgassing melt fragments of different grain sizes, mixed with variable amounts of crystals to examine the efficacy of sintering in these complex systems. We perform isothermal sintering experiments in variably confined spaces and stresses, and we quantify the evolution of the permeable porous network over a series of time scales. We examine how these constraints need to be included in future models aiming to resolve the evolution of fragmental magmatic systems.
