Effects of State and Decompression Rate on the Decompressive Response of Volatile- and Crystal-Bearing Analogue Magmas.

Abstract:

Volcanic eruptive styles are influenced both by the physical properties of the ascending magma as well as the decompression rates involved. Systematic experimental investigations of both state and rate have been performed here on analogue basaltic systems. Controlled decompression experiments were performed in a shock tube system, using a silicon oil basis as the basalt proxy. The samples were saturated with 10 MPa of Ar for 72h, followed by controlled decompression. Four series of experiments were performed: 1) Pure liquids with viscosities ranging from 1 to 1000 Pa s were used to map the liquid response. 2) Micrometric spherical particles were added to the liquid to evaluate the effect of crystal fraction. 3) The role of crystal shape was examined by using particles with different aspect ratios. 4) Finally, the effects of saturation time and of pressure were examined via a series of experiments at 24 h, performed over a range of saturation pressure.

The dynamics of foaming and flow of the bubbly fluid during decompression were constrained using image analysis, by measuring the height of the expanding column, as well as analyzing the bubble size distribution. At the onset a delayed nucleation event is observed. When the amount of nucleated bubbles approaches a critical thickness, a foam develops. Finally, the foam reaches equilibrium, and starts oscillating in response to the balance between foam disruption and growth. These observation may have important implications for oscillatory eruptive phenomena observed in active volcanoes (i.e. gas piston activity).

Finally, the effect of crystals was investigated. In their presence, heterogeneous nucleation enhances the number of bubble nuclei, even at low crystal fractions. As a consequence, the foam develops earlier, and is able to ascend with major upward speed, in comparison to the pure oil.

Experimental decompression of silicon oil has proven to be a unique tool to unravel the hidden dynamics of magma into the conduit.