SEM-EBSD reveals crystal plasticity in pyroclasts

Electron BackScatter Diffraction can be used to measure misalignment in the lattice of crystals. Non-reversible crystal lattice deformation by migration of dislocations, i.e., crystal plasticity, has been observed and recreated under controlled experimental conditions in effusive volcanic rocks. There, crystal plasticity increases with increasing stressing of the magma, and precedes crystal failure. Here, we present the first observation of crystal plasticity in pyroclasts. We found lattice misalignment mainly in plagioclase microlites within ash- and lapilli-sized pyroclasts of mafic composition from explosive eruptions of Etna (Italy), Paricutin (Mexico), and Cumbre Vieja (Spain) volcanoes. Misalignment is found both in visibly curved and apparently undeformed crystals, its magnitude increasing with increasing aspect ratio of the crystals. Plastically deformed crystals are unevenly distributed within the pyroclasts, and often clustered. Similar to observations made in effusive rocks, crystal plasticity in pyroclasts is marked by the presence of broken crystals. But unlike the effusive case, fragments of broken crystals in pyroclasts show little or no plastic deformation, while more deformed crystals are found around zones with broken crystals. Textural evidence from natural and experimental samples show that these zones underwent localized damage by brittle fracturing. Coexisting broken and plastically deformed crystal reveal millimetre-scale patterns of stress localisation in magma during fragmentation