Direct nanoscale observations of degassing-induced crystallisation in felsic magmas

Abstract

Water degassing plays a major role in magma transport and eruption by increasing liquidus temperatures, bubble and crystal volume fractions, and strongly affecting the viscosity of bulk magma. High spatial resolution textural analysis detailing the dynamics of bubble and crystal growth is key to unravelling the swift changes in magma crystallinity and gas content that affect the conditions of magma flow, fragmentation, and eruption. Ex situ observation of samples from a previous experimental study of magma degassing reveals that vesicles are surrounded by chemically heterogeneous residual glass that may be produced by newly formed minerals that are not observable at the microscale. Here, we present new in situ high-temperature (500–1100 °C), time-elapsed (every ~ 20 min at 200–800 °C, ~ 10 min at 900–1000 °C, and ~ 5 min at 1100 °C) observations of degassing of synthesised, hydrous (4.2 wt.% H₂O) dacite glasses using scanning transmission electron microscopy at 0.4 nm resolution. The experiments reproduce degassing of a silicic melt by high-temperature heated stage mounted in the analytical instrument. We monitor the dynamics of nucleation and growth of nanobubbles that experience coalescence and formation of microbubbles and trigger the nucleation and growth of nanolites of plagioclase, clinopyroxene, Fe-Ti oxides, and quartz, at the expense of the residual melt. The ability to image degassing and crystallisation at nanoscale reveals a sequence of complex physical and chemical changes of the residual melt and shows that the kinetics of crystallisation in silicic melts is modulated by the melt’s ability to exsolve fluids that help form mineral nuclei and nanolites. Finally, we highlight that the competition between gas retention and crystallisation is initiated at the nanoscale and may anticipate the role of microlites in controlling rates of magma ascent in a volcanic conduit and modulating the style of the consequent volcanic eruption.