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. 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 the authors' experiments (about 3000) was achieved using cold glycerine for the upper layer, above a hot denser KNO//3 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 KNO//3 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.