Bubble Nucleation, Coalescence and Outgassing Induced by Crystallization: Insights into Their Contribution to Seismic Properties of Magmas.

Abstract:

Seismic tomography of potentially hazardous volcanoes is a prime tool to assess the location and dimensions of magmatic reservoirs. Magma rheology and volcanic eruptive style are to a first order controlled by processes occurring within the conduit or in the magma chamber, such as crystallization and bubble exsolution. Seismic velocities are strongly affected by these processes, but the limited number of constrained measurements does not allow yet establishing a firm link between seismic tomography and the textural and hence rheologic state of volcanic systems. Elastic parameters of vapor-saturated, partially molten systems are thus providing fundamental information for the identification of such reservoirs under volcanoes.

We investigated a chemically simplified melt analogous to trachyte, which undergoes plagioclase crystallization and bubble exsolution. A Paterson-type apparatus was employed to measure the seismic velocities at a constant pressure of 250 MPa and at a frequency of 0.1 MHz. The temperature was decreased at a rate of 0.5 or 0.1 °C/min from 850 to 700 °C and velocities were recorded every 45 minutes. In order to characterize the microstructure evolution, we conducted series of cold-seal experiments at identical pressure conditions but with rapid-quenching at each of the recorded temperatures.

Magmatic processes such as crystallization, bubble nucleation and coalescence have been recognized throughout the measurements of seismic velocities in the laboratory. Compression and shear wave velocities increase non-linearly during crystallization. At crystal fraction exceeding 45 vol%, the formation of a crystal network favors the propagation of seismic waves through magmatic liquids. However, bubble nucleation induced by crystallization leads to an increase of magma compressibility resulting in a lowering of the wave propagation velocities. These two processes occurring simultaneously have thus competing effects on the seismic properties of magmas. In addition, when the bubble fraction is less than 10 vol%, the decrease in seismic velocities is more pronounced than for higher bubble fractions. The effect of bubble coalescence on elastic properties is thus lower than the effect of bubble nucleation.