The yield strength of subliquidus basalts - experimental results

Yield strength is an important property of particle-fluid suspensions. In b asaltic lavas that crystallize during Row emplacement, the onset of yield s trength may result in threshold transitions in flow behavior and Row surfac e morphology. However, yield strength-crystallinity relations are poorly kn own, particularly in geologic suspensions, where difficulties of experiment al and field measurements have limited data acquisition in the subliquidus temperature range. Here we describe two complementary experimental approach es designed to examine the effect of particle shape on the low-shear yield strength of subliquidus basalts. The first involves melting cubes of holocr ystalline basalt samples with different initial textures to determine the t emperature (crystallinity) at which these samples lose their cubic form. Th ese experiments provide information on the minimum crystal volume fractions (0.20 < phi < 0.35) required to maintain the structural integrity of the c ube. The second set of experiments uses suspensions of corn syrup and neutr ally buoyant particles to isolate the effect of particle shape on yield str ength development. From these experiments, we conclude that the shape is im portant in determining the: volume fraction range over which suspensions ex hibit a finite yield strength. As anisotropic particles may orient during f low, the effect of particle shape will be controlled by the orientation dis tribution of the constituent particles. We find that the so-called 'exclude d volume can be used to relate results of experiments on anisotropic partic les to those of suspensions of spherical particles. Recent measurements of yield strength onset in basaltic melts at crystal volume fractions near 0.2 5 are consistent with our observations that crystal frameworks develop at l ow to moderate crystal volume fractions when crystals are anisotropic (e.g. plagioclase). We further suggest that conditions leading to yield strength onset at low crystallinities include rapid cooling (increased crystal anis otropy), heterogeneous nucleation (which promotes extensive crystal cluster ing and large cluster anisotropy) and static conditions (random crystal ori entations). (C) 2001 Elsevier Science B.V. All rights reserved.