Abstract:
Yield strength is an important property of particle-fluid suspensions. In basaltic lavas that crystallize during flow emplacement, the onset of yield strength
may result in threshold transitions in flow behavior and flow surface morphology. However, yield strength-crystallinity relations are poorly known,
particularly in geologic suspensions, where difficulties of experimental and field measurements have limited data acquisition in the subliquidus temperature
range. Here we describe two complementary experimental approaches designed to examine the effect of particle shape on the low-shear yield strength of
subliquidus basalts. The first involves melting cubes of holocrystalline basalt samples with different initial textures to determine the temperature
(crystallinity) at which these samples lose their cubic form. These experiments provide information on the minimum crystal volume fractions
(0.20 < phi < 0.35) required to maintain the structual integrity of the cube. The second set of experiments uses suspensions of corn syrup and neutrally
buoyant particles to isolate the effect of particle shape on yield strength development. From these experiments, we conclude that the shape is important in
determining the volume fraction range over which suspensions exhibit a finite yield strength. As anisotropic particles may orient during flow, the effect of
particle shape will be controlled by the orientation distribution of the constituent particles. We find that the so-called `excluded volume' can be used to
relate results of experiments on anisotropic particles to those of suspensions of spherical particles. Recent measurements of yield strength onset in
basaltic melts at crystal volume fractions near 0.25 are consistent with our observations that crystal frameworks develop at low 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 anisotropy), heterogeneous nucleation (which promotes extensive crystal clustering and large cluster anisotropy)
and static conditions (random crystal orientations).