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Abstract

Magma fragmentation is a key process in explosive volcanic eruptions, but the mechanisms of fragmentation at the microscale remain elusive. A key challenge is that viscous deformation of melt may overprint some signatures of fragmentation in erupted pyroclasts. Conversely, solid crystals preserve features due to magma fragmentation. SEM revealed broken crystals within the intact glass of pyroclasts from explosive eruptions ranging from Strombolian to Plinian in style and from basanite to rhyolite in bulk magma composition. Increasing abundance of broken crystals in smaller pyroclasts and at their edges, their occasional association with sutured fractures in the melt, and nearly-identical features reproduced by high-temperature fragmentation experiments, reveal that the crystals are broken during fragmentation by the passage of brittle cracks through the magma. Subsequent viscous deformation of the magma then heals the cracks in the melt, effectively masking fragmentation and ultimately affecting the grain size distribution of eruption products. Features of broken crystals mirrors the state of magma at fragmentation. Concentrations of broken crystals with large, empty cracks define ‘damage zones’ in pyroclasts. There, limited viscous healing of the melt suggests an origin relatively close to quenching temperature. Micro-XCT reveals cracks with complex morphologies and almost ubiquitous connections with vesicles. EBSD results show limited misalignment in the lattice of broken crystals, and large (up to 10°) misalignment in unbroken but bent ones, suggesting stress accumulation in the magma (and crystals) and its release by cracks. Damage zones likely formed in narrow areas of a locally stressed and already vesicular magma.