Attrition in the kimberlite system

Abstract

The sustained transportation of particles in a suspension commonly results in particle attrition leading to grain size reduction and shape modification. Particle attrition is a well-studied phenomenon that has mainly focussed on sediments produced in aeolian or fluvial environments. Here, we present analogue experiments designed to explore processes of attrition in the kimberlite system; we focus on olivine as it is the most abundant constituent of kimberlite. The attrition experiments on olivine use separate experimental set-ups to approximate two natural environments relevant to kimberlites. Tumbling mill experiments feature a low energy system supporting near continual particle-particle contact and are relevant to re-sedimentation and dispersal processes. Experiments performed in a fluidized particle bed constitute a substantially higher energy environment pertinent to kimberlite ascent and eruption. The run-products of each experiment are analysed for grain size reduction and shape modification and these data are used to elucidate the rates and extents of olivine attrition as a function of time and energy. Lastly, we model the two experimental datasets with an empirical rate equation that describes the production of daughter products (fines) with time. Both datasets approach a fines production limit, or plateau, at long particle residence times; the fluidized system is much more efficient producing a substantially higher fines content and reaches the plateau faster. Our experimental results and models provide a way to forensically examine a wide range of processes relevant to kimberlite on the basis of olivine size and shape properties.

Modification of Mantle Cargo by Turbulent Ascent of Kimberlite

“…Group D experiments that showed transitional behavior were calculated using H f because this is where most of the particles reside. In general, the attrition rate (b) is expected to increase with increasing φ and increasing KE (Bemrose and Bridgwater, 1987;Jones and Russell, 2018). However, in our experiments we show that these simple general expectations are not always valid; rather, there is a clear "tradeoff " between these parameters (Figure 8B).…”

“…A series of tubes, connectors, and a water reservoir ensure the successful recovery of all the products. Figure modified from Jones and Russell (2018). prevent any sample loss.…”

“…Previous work (e.g., Gwyn, 1969;Bemrose and Bridgwater, 1987) has shown that attrition is dependent on a minimum of four governing parameters: concentration of suspended particles; feed particle size distribution; particle/gas velocity; and particle residence time. Attrition is typically promoted by high particle concentrations, poorly sorted feed distributions, high differential velocities, and long residences times (e.g., Bemrose and Bridgwater, 1987;Jones and Russell, 2018). The question now is, how can we relate the attrition model parameters (i.e., a and b) to physical quantities relevant to kimberlite ascent?…”

“…mode of attrition wherein the rough edges or asperities of the parent (and daughter) particles are removed and particles become rounded. Importantly, abrasion is accompanied by the production of very fine particles-commonly chips (Figure 4) derived from the surficial asperities on the parent grains (Bemrose and Bridgwater, 1987;Kueppers et al, 2012;Jones and Russell, 2018).…”

“…Here, we present data from a new series of analog experiments designed to explore attrition driven by particleparticle interactions in a fluidized system. We build on the previous experimental work of Jones and Russell (2018) who compared two different environments hosting particle interaction-a fluidized bed and a tumbling mill. In this study we investigate olivine attrition (scaled for an ascending cargoladen kimberlite dyke) as a function of particle concentration, time, and energy.…”