The dynamics of magma mixing in a rising magma batch

Abstract

The conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid-dynamical experiments. The results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number $I = \mu _1 U/g\Delta \rho R_{}^2$ where μ ₁ is the viscosity of the fluid, U is the velocity, g is the acceleration due to gravity, Δρ is the density difference between the two fluids, and R is the radius of the tube. The parameter I represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls, and the buoyancy forces which tend to keep the interface horizontal. The experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube. They show that overturning of the initial density stratification and mixing occur when I>order 10^-1; the two fluids remain stratified when I<order 10^-3. Transitional states are observed when 10^-3<I<10^-1. These results are nearly independent of Reynolds number $(Re_1 = \rho _1 UR/\mu _1 )$ and viscosity ratio $(\mu _1 /\mu _2 )$ in the range of $10^{_{}^{ - 2} } < \mu _1 /\mu _2 < 10^{_{}^3 }$ and Re ₁<300. Applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch. This mechanism can explain some occurrences of small-volume mixed lava flows.