Gas-fluidisation in an experimental tapered bed: Insights into processes in diverging volcanic conduits

Gas-fluidisation is commonly invoked as an important process during the formation of vent-fill deposits in diverging kimberlite pipes. In this study, we performed a series of laboratory experiments to determine the gas-fluidisation behaviour of particles in both confined straight-sided and tapered containers, as an analogue to volcaniclastic materials infilling a kimberlite pipe, in the presence of a gas flow. We investigated the effects of taper angle, bed height and gas flow rate on fluidisation behaviour, focusing on medium-coarse grained particles (106-212 μm). We show that beds in straight-sided containers become homogeneously fluidised, whereas beds in tapered containers become heterogeneous, with fluidisation limited to a central roughly hyperboloid-shaped region. Either side of the well-mixed fluidised core, marginal wedge-shaped regions remain unfluidised, and the width of unfluidised regions decreases with increasing gas flux. The unfluidised wedges are internally laminated and slip downwards when a critical proportion of the bed is fluidised (∼ 90%). This generates a “conveyor-belt”-type mechanism of particle transport. These experimental observations demonstrate how fluctuations in gas velocity can produce steep internal boundaries between laminated and well-mixed regions. The observations also show how marginal inward dipping layered sequences could slip into deep parts of kimberlite pipes. Our results provide a framework for interpreting the volcaniclastic lithofacies of kimberlite pipes.