Some effects of viscosity on the dynamics of replenished magma chambers

Abstract

Some aspects of the dynamical behavior of magma chambers, replenished from below with hotter but denser magma, have been modeled in a series of laboratory experiments. In previously reported work the fluids used were aqueous solutions of comparable viscosity, and thus the results should be applicable to basaltic magma chambers, in which the magmas do not vary greatly in viscosity. In that case, the lower layer cools by convective heat transfer to the fluid above, and crystallization causes the density of the residual liquid in the lower layer to decrease. When the density becomes equal to that in the upper layer, sudden overturning and intimate mixing take place. The present paper reports experimental results that allow us to extend the application to systems in which there is a large viscosity ratio between the resident and the injected fluid, for example, to calcalkaline magmas, where magma viscosity can vary by as much as 5 orders of magnitude. The largest viscosity ratio in our experiments (about 3000) was achieved using cold glycerine for the upper layer, above a hot denser KNO3 solution. The most striking new feature with the very viscous upper layer is that now less dense fluid is released immediately and continuously from the interface and rises as plumes through the upper layer. Further crystallization occurs in the plumes, and the crystals fall out, but there is little mixing, and a layer of depleted KNO3 solution is eventually deposited at the top. The transfer process between the layers is dominated by interfacial effects, with the high‐viscosity upper layer acting as a nearly rigid lid that allows buoyant fluid to accumulate just below the interface and then rise in localized plumes across the interface into the viscous layer. This physical picture is supported by a series of experiments in which the viscosity ratio is varied systematically; the mixing behavior changes gradually between that described above for a large viscosity ratio and the sudden over‐turning characteristic of layers with comparable viscosity. The importance of the viscosity ratio, rather than just an increase in viscosity, is confirmed by experiments in which both viscosities are increased by the same factor; the overturning process is then slower, but symmetrical. Other variations suggested by previous experiments are also described: the release of gas by a chemical reaction, to model the release of volatiles following an overturning event in a magma chamber; the effect of a cold, immiscible layer above the cooling crystallizing fluid; the influence of two viscous layers with a density step between them; and the constraining effects of a density (with corresponding viscosity) gradient in the upper region. The experiments indicate that whatever the stratification, whether it be in layers or continuous, the form of the initial motion in the upper fluid is determined by the viscosity ratio between the two fluids immediately adjacent to the interface. Geological applications are not examined in detail in this paper, but the experiments suggest that both sudden overturning (characteristic of magmas of nearly equal viscosity) and continuous release (when the upper layer is much more viscous) are viable mechanisms for magma mixing in the appropriate circumstances.