Grain size distribution controls sintering of hydrous pyroclasts

Hot hydrous pyroclasts vesiculate, diffusively outgas, and sinter during deposition which leads to continuous evolution of their porous-permeable networks. Welded ignimbrites, tuffisites, and sintered in-conduit pyroclasts, all exhibit variable degrees of sintering as well as a broad grain size distribution. How this polydispersity in grain sizes influences evolution has not yet been understood. Where polydisperse models for sintering exist, they are limited to relatively small ash particles (1–50 μm ) and do not account for vesiculation that occurs in relatively larger pyroclasts. Here, we perform high-temperature sintering experiments using relatively coarse ash-to-lapilli (0.5–2.5 mm) clasts. First, we find that polydisperse systems exhibit more efficient initial packing relative to monodisperse systems. Second, polydisperse samples with fine grains between larger grains may undergo differential size-dependent sintering and vesiculation. Third, for larger grains, particle-particle contact areas are enhanced by vesiculation-driven clast expansion onto neighbouring grains, with some contact flattening. Because both sintering and diffusive outgassing are grain-size dependent, any population at a given time will contain grains that differ in internal vesicularity, H 2 O content, and size; each evolving on different timescales. We compare our results to sintering models which account for H 2 O loss and we augment these by empirically capturing the effect of a time-dependent particle radius due to vesiculation. To distinguish between the three dominant processes – sintering, vesiculation, and diffusive outgassing – we introduce a regime diagram. Models for sintering that accommodate the complexity of processes occurring in volcanic environments will be particularly impactful for our conceptual understanding of the conditions under which sintering in conduits or tuffisites may influence eruptive behaviour as a whole. • The evolution of permeability during pyroclastic sintering is strongly dependent on grain size. • Polydispersivity of grain sizes affects the system's propensity to sinter, vesiculate, and diffusively outgas. • Porosity and permeability models must account for clast growth during vesiculation and clast shrinkage during resorption/outgassing. • A regime diagram is proposed, governed by two fundamental parameters that dictate the system behaviour.