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The vesiculation process is a key factor influencing the kinetics of an ascending magma. Explosive as well as effusive eruptions, associated with magma degassing, are coupled to the intensity of magma vesiculation, the decompression rate and volume expansion. Therefore it is a scientific challenge to characterize and constrain the physical properties of ascending magma under geologically realistic conditions. Here we present a dilatometric in-situ device (Fig. 3.7-5) which enables us to measure volume expansion of water bearing silicate melts in order to characterize the vesiculation process.

Fig. 3.7-5: Schematic illustration of the dilatometer that can be easily inserted into an autoclave.

Figure 3.7-6 shows the principle of this method. The parallel polished sample is placed in a high-temperature stainless steel container and matches exactly its inner dimensions. During the measurements the samples are heated at a rate of 10 K/min and exposed to temperatures just above the glass transition for different lengths of time. In dry melts no volume expansion is observed at constant temperatures whereas volatile bearing melts become oversaturated and a gas phase (H₂O) nucleates within the melt after a certain amount of time. This leads to an increase of volume that is recorded by a linear voltage displacement transducer (LVDT). Knowing the previous water content of the sample we are able to calculate the activation energy of the degassing process by the use of the Avrami equation. First measurements at room pressure have been conducted successfully. Further research will investigate volume expansion at elevated pressures.

Fig. 3.7-6: Sketch explaining the dilatometric setup of the volume expansion measurements.