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3.9 c. Centrifuge modelling of bubble ascent in viscous magmas (N.S. Bagdassarov and A. Dorfman)

The vertical ascent of bubbles in magma chambers and lavas may be used as a paleoslope indicator of volcanic rock orientation. The interaction of vesicles and their deformation in a shear flow regime can be employed as a flow-direction and intensity factor. The distortion of vesicles from a spherical shape results in a number of flow instability phenomena which are responsible for the rate of gas exsolution from lava surfaces.

The ascent rate of vesicle swarms or cavity plumes differs significantly from the Stokes rise velocity of individual bubbles because of bubble-bubble interaction and the plume tail. An umbilical conduit of a cavity plume develops as a tail of small distorted bubbles, which is present in the region behind the plume head, slowing down the ascent velocity. The ascent rate of a cavity plume may approach an asymptotic power law dependency between L (the path length of the plume) and time t. For example, for the ascent of a cavity plume equivalent to a sphere, the path-time relation is L∝ V2/3 t, where V is the initial volume of the cavity. In the case when the cavity is equivalent to a cylinder, L ∝ V1/2 t1/2 . When the cavity plume is treated as a composite body consist of a stable head and an elongate cylindrical tail, the path-time correlation is L∝ V3/5 t4/5.

Our modelling experiments on bubble interaction during their ascent have been performed in a centrifuge furnace operating at an acceleration of 1000 g and temperature of 1300 °C. The centrifuge furnace provides a high Bond number (a measure of the ratio of buoyancy to surface forces 10²-10³) in a melt with high viscosity (the Reynolds number Re=10-8-10-9). The rotating furnace has a small temperature gradient zone in which the alumina capsule (5 mm in diameter and 15 mm in length) is placed. Viscous melts for experiments are HPG8 + 20 wt% Na2O (haplogranite composition with the addition of sodium oxide) and NBS711 (National Bureau of Standards, USA). The viscosity of these melts at high temperatures are well known. At the temperature of the experiments (650° - 700 °C) the viscosities of the melts are logη(in dPas) = 6.5-7, respectively. The centrifuge acceleration in experiments is 1000 g (gravity). Cylindrical samples have been cored from the prepared batch of glasses. In experiments with bubble ascent, cylindrical channels (∅ 1 mm, length 1-2 mm) have been drilled in the base of the glass samples. The ascent of bubbles has been simulated using air vesicles developed from the initial cylindrical cavity. The interaction of a sinking rigid sphere with ascending bubbles has been studied by using a Pt sphere of 0.6 mm diameter. The development of bubble-bubble interactions, bubble-rigid sphere interactions and the evolution of a cavity diapir with time have been observed in time-step experiments on quenched samples. Quenched samples have been studied using optical microscopy combined with an Image Analysis System. The tail of an ascending cavity diapir consists of a large number of distorted vesicles. The most distorted and elongated bubbles are in the boundary layer of the plume head and in the boundary layer of the tail. The sinking Pt-sphere results in a strain rate of viscous flow 10 times larger than the strain rate due to the vesicle ascent (the difference in the buoyancy force is about 10 times. In this case the swarm of large bubbles in the tail of the sinking sphere evolves relatively fast in a thin trace consisting of small non-distorted vesicles. The interaction and coalescence of two vertically ascending vesicles has been simulated by using a second cavity plume in a tail of the leading one. The velocity of the trailing vesicle is larger than of the leading vesicle and after a certain time the trailing vesicle deforms the leading one. The leading bubble becomes flattened (oblate distortion) due to the viscous stresses of the flow produced by the trailing bubble. This oblate shape of the leading drop is unstable and with time evolves into a number of small spherical bubbles.

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