The dimensions of magmatic inclusions as a constraint on the physical mechanism of mixing

Silicic lavas often bear inclusions of more mafic lava. Several physical processes have been proposed by which these magmas may become mixed. Measurements of the sizes of mafic inclusions in lavas of two calc–alkaline volcanoes are used here to obtain a characteristic length scale for this type of mixing and provide a means to discriminate between different mixing processes. We quantitatively assess the proposal that, in a stratified reservoir, dynamic interactions of rising gas bubbles with the interface between a lower layer of mafic magma and an upper layer of silicic magma lead to the formation of vesiculated blobs of the lower magma in the upper one. This is due to gravitational instability of a foam layer growing at the interface between the two magmas. Our theoretical calculations of this instability predict both the correct order of magnitude for the characteristic size of inclusions and the way in which inclusion size varies with the physical properties of the magmas. This fluid dynamic framework is used to map out different possible flow regimes and place constraints on the fraction of gas in these systems.