Study of effect of magma pocket on mixing of two magmas with different viscosities and densities by analogue experiments

We conducted analogue experiments to examine the effect of a magma pocket on magma mixing processes at both high and low Reynolds numbers. Our idea is that when mafic and silicic magmas are simultaneously injected into a magma pocket, gravitationally unstable layers may be formed at both high and low Reynolds numbers. Under certain conditions, the Rayleigh–Taylor instability occurs and mixing of the two magmas is promoted in the pocket even at a low Reynolds number. This shows that gravitational force overcomes viscous force, and the annular flow of the magmas in the pocket is disrupted. If the viscous force is large, the annular flow is maintained, and it passes through the pocket. At a critical condition, two liquids with different densities and viscosities become unstable while mixing in the pocket. This condition is represented by a dimensionless parameter P (P = µU/(gΔρab/π)), where µ is the viscosity, U is the flow velocity, g is the acceleration due to gravity, Δρ is the density difference between the two liquids, and a and b are the base dimensions of the apparatus. When P < 0.1, the two liquids are gravitationally unstable and they mix in the pocket. However, when P > 0.1, the annular flow of the two liquids is maintained, and they do not mix with each other. The critical condition is also dependent on the flux of the liquids and the height of the pocket, and is applicable to the present experimental conditions. The presence of the magma pocket is one of the factors responsible for the disruption of the annular flow, leading to the mixing of the two magmas. We conclude that magmas can mix thoroughly if they flow through several magma pockets.