Physical properties of crystal-and bubble-bearing magmas

Laboratory experiments at high temperature and/or high pressure were conducted to determine the physical properties of magmas with focus on the rheological behavior of bubble- and crystal-bearing suspensions. Two experimental investigations have been performed: one focused on magma rheology; the other centered on studying the magma vesiculation in-situ.

The rheology of three-phase magmas, composed of carbon dioxide-bearing gas bubbles, quartz crystals and haplogranitic melt, was investigated by deformation experiments at high pressure (200-250 MPa) and temperature (673-1023 K). The three-phase samples for these experiments were first synthesized to produce specimens with different volumetric proportions of crystals (24-65 vol.%) and bubbles (9-12 vol.%). The rheological and microstructural results have been combined to explain specific physical processes, such as the generation of shear bands in magmas at high crystallinity and outgassing in magmas at low crystallinity. The same results have been also used to provide empirical equations describing the general rheological behavior of three-phase magmas. Such results compose a solid background to better understand the physical processes occurring within magmatic chambers and along volcanic conduits.

High-temperature vesiculation of silicate melts has been tested using in-situ synchrotron experiments, to directly observe and quantify the dynamics of such a process. Water-bearing silicate glasses were heated up to 1297 K to promote bubble nucleation and growth, while acquiring ultrafast (13 seconds) three-dimensional tomographic scans. This novel technique permitted to quantify the gas overpressure within gas bubbles, the surface tension between bubbles and silicate melt and the effective viscosity of magmas vesiculating in real time. Particularly, the determination of gas overpressure in magmas results fundamental because this parameter represents a key factor that can trigger hazardous explosive eruptions. Although the limited spatial and temporal scale of the experiments, such an in-situ experimental technique is a new frontier for the investigation of the physical processes responsible for the rheological behavior of magmas within magmatic chambers and along volcanic conduits.