Scanning Transmission Electron Microscope (STEM) features of glass in crystal fractures

Small crystals (below 100 microns ca.) in pyroclasts from explosive volcanic eruptions are often fractured, with fractures that are both empty or filled with the same groundmass that surrounds the microlite, usually made of glass. Textural observations and experiments show that microlites are fractured during the same process of fragmentation that causes the transition from a continuum of liquid magma to a gas-pyroclast dispersion. However, the timing and dynamics of the fracturing process are still poorly defined. Aiming at constraining how and when crystals are fractured, we investigated the chemical and textural features of glass that entered into fractures within crystals. Under Scanning Transmission Electron Microscope (STEM), these glasses reveal inhomogeneities that are otherwise invisible. In particular, nanometer-sized patches of variable composition are outlined, which are present in the glass only inside the fractures but are lacking outside of them. These features, tentatively interpreted as incipient immiscibility leading to nanolite crystallization, testify local disequilibrium conditions between the melt and the newly-formed crystal fracture surface, and could provide a mean to define the timing intervening between crystal (and magma) fracturing and pyroclast quenching upon eruption.