A fluid-dynamical model of differentiation and layering in magma chambers

Abstract

Large igneous intrusions characteristically have a systematic gradation in mineral composition from top to bottom, often associated with a horizontally layered structure on several scales1,2. Current theories3 invoke the settling of denser crystals as the mechanism responsible for separating the various components from the solidifying melt. However, recent laboratory experiments4,5, using crystallization from aqueous solutions to model the essential fluid-dynamical processes, have documented another significant way in which chemical differentiation of the liquid can be caused by relative motion between crystals and the remaining melt. The growth of crystals of a denser component on a side-wall boundary leaves behind less dense fluid. (This corresponds to the behaviour of many common magmas, notably those of the calc-alkaline series. However, in tholeiitic basalt magmas, of the type which commonly form layered intrusions, crystallization lends to an increase in density of the remaining melt.) The lighter fluid rises to the top of the chamber in a boundary layer flow, which builds up a stable density gradient, by the 'filling box' mechanism6,7 previously discussed in other contexts. In the latest experiments reported here, the process of differentiation is demonstrated explicitly: starting with a homogeneous mixture of two solutes in water, crystallization at a vertical boundary produces overall vertical composition gradients, as well as density stratification accompanied by smaller scale layering. Show more