The Role of Viscous Particle Segregation in Forming Chromite Layers from Slumped Crystal Slurries: Insights from Analogue Experiments

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The question of how the mineral layering in layered intrusions forms has been extensively debated for many decades. There are many types of layering and it is of course possible that a number of mechanisms are involved. Of particular interest is how chromite layers form, because these may contain valuable metals (such as platinum-group elements) in addition to Cr. One model for the formation of these layers is that they formed through slumping of semi-consolidated cumulates from the margins of the intrusion into the magma chamber. During this slumping, the grains are sorted by density and/or size differences. This study examines the viability of this process using analogue modelling. Starting materials (beads and glycerine) were scaled to match the density and size of the minerals (chromite, orthopyroxene and plagioclase) present in layered mafic–ultramafic layered intrusions and to match the density and viscosity of the silicate magma. A Perspex flume tank divided by a removable partition at one end was fully filled with glycerine. A homogenized mixture of the beads was placed in the smaller partition of the tank (representing the margins of the magma chamber). The tank was then inclined between 16° and 45°. The partition was removed and the beads flowed into the main part of the box. The experiments were recorded by video camera, allowing us to follow the dynamics of the flow during each run. Segregation of the beads was observed in the final deposits: the larger, less dense beads (representing plagioclase) concentrated at the top of the flow, with the intermediate-sized and medium density beads (representing orthopyroxene) in the middle and the smaller, denser beads (representing chromite) at the bottom, thus mimicking natural examples. In experiments where the angle of inclination was low, long, thin layers formed, such as those found in the Bushveld Complex. In experiments where the angle of inclination was high, thick but short layers formed. A dimensionless analysis allows better understanding of the dynamics of the flow. At the macroscopic scale, the flow regime is strongly influenced by the viscosity of the fluid and is considered macro-viscous, where the role of the interstitial liquid is non-negligible.

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