Numerical models of the onset of yield strength in crystal-melt suspensions

The formation of a continuous crystal network in magmas and lavas can provi de finite yield strength, tau (y), and can thus cause a change from Newtoni an to Bingham theology. The theology of crystal-melt suspensions affects ge ological processes, such as ascent of magma through volcanic conduits, flow of lava across the Earth's surface, melt extraction from crystal mushes un der compression, convection in magmatic bodies, and shear wave propagation through partial melting zones. Here, three-dimensional numerical models are used to investigate the onset of 'static' yield strength in a zero-shear e nvironment. Crystals are positioned randomly in space and can be approximat ed as convex polyhedra of any shape, size and orientation. We determine the critical crystal volume fraction, phi (c), at which a crystal network firs t forms. The value of phi (c) is a function of object shape and orientation distribution, and decreases with increasing randomness in object orientati on and increasing shape anisotropy. For example, while parallel-aligned con vex objects yield phi (c) = 0.29, randomly oriented cubes exhibit a maximum phi (c) of 0.22. Approximations of plagioclase crystals as randomly orient ed elongated and flattened prisms (tablets) with aspect ratios between 1:4: 16 and 1:1:2 yield 0.08 < phi < 0.20, respectively. The dependence of phi ( c) on particle orientation implies that the flow regime and resulting parti cle ordering may affect the onset of yield strength. phi (c) in zero-shear environments is a lower bound for phi (c). Finally the average total exclud ed volume is used, within its limitation of being a 'quasi-invariant', to d evelop a scaling relation between tau (y) and phi for suspensions of differ ent particle shaper. (C) 2001 Elsevier Science B.V. All rights reserved.