Analogue experiments to examine the possibility of forming chromite layers by kinetic sieving during slumping of crystal slurry.

English Abstract:

A series of experiments were conducted to investigate if the magmatic layering could be forms through the slumping of semi-consolidated cumulate pile in an unstable magmatic chamber, forming crystal slurries and leading to sorting of crystals based on density and/or size differences. Using analogue modelling, we have identified the required conditions leading to the sorting of particles from a homogeneous mixture.

In our experiments we use a perplex box fixed on an inclinable apparatus. In order to accurately model crystals in a magmatic chamber, the box is filled with an ambient fluid (glycerine) representative of tholeiitic basaltic magma with 1.8% water at a temperature of 1282°C. Crystals within the melt are simulated with a mixture of particles with three different sizes and densities: silicone balls, polyacetal balls and glass beads replicating plagioclase, pyroxene and chromite crystals, respectively. The experiments were conducted at different inclination angles with two different proportions of particles.

During the run, once the angle of repose is reached, the flow begins and particles became segregated with larger lighter particles at the top of the flow and smaller denser ones at the bottom. The final deposits are characterized by three regions that evolve with time, slope and the amount of smaller particles. Additionally, in certain experiments we observe the formation of levees and front flow, typically found in granular flows. A particle velocity gradient is also observable.

During flow, the smaller particles percolate through the voids between the larger particles in a process known as kinetic sieving. Through these experiments, we observe that the efficiency of this mechanism depends on the angle of inclination, the proportion of smaller particles (i.e. the speed and thickness of the flow) and the relative size and shape of particles. The morphology of deposits was then compared to several naturally occurring analogues found in layered intrusions.