The role of fluidisation in the formation of volcaniclastic kimberlite: Grain size observations and experimental investigation

摘要

Deep, steep-sided kimberlite pipes in southern Africa are normally partially filled with multiple units of a characteristically massive, structureless volcaniclastic rock, here termed massive volcaniclastic kimberlite (MVK). These units can be arranged in a pseudo-concentric, nested 'pipes-within-pipes' structure. Several key observations on typical MVK from the Venetia kimberlite pipes, South Africa, indicate that fluidisation may have been an important process during its emplacement. These include: (1) the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and; (2) a matrix composed of void-filling minerals related to hydrothermal metamorphism (mainly diopside and serpentine), which indicates that the primary deposit had a high porosity ( > 50%); and (3) particle size distributions poor in fine ash ( < 0.0625 mm) and coarse lithic fragments ( > 10 cm). Analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of MVK. High-pressure gas, fed through the bed from a point source, resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand. Strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins. Analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a Gaussian distribution of vertical velocity in the cross-flow direction. A decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the 'pipes-within-pipes' structures in kimberlite pipes. The results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes. We propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep, wide conduit. This bed is subject to quasi-sustained fluidisation by the erupting gas-particle flow, which imparts on it the distinctive characteristics of a massive well-mixed, poorly sorted deposit.