c : photograph of the apparatus
In a cooling magma, rheological changes are primary controlled by the progressively increasing crystallinity and related melt fraction threshold(s) marking changes in deformation mechanisms. We present a series of simple analogue experiment, which aims to constrain the minimum liquid fraction below which the deformation of the suspension must be accommodated by additional crystal deformation.
The measured particle locking threshold is about 30-35 % liquid at best, and is consistent with previously published, classical values. Particle shape have little effect on the observed threshold, in contrast with the particle size.
The minimum liquid fraction is not reached by continuously shearing the suspension, because of Reynold dilatancy, but by alternating short periods of
rotation of the apparatus piston in opposite senses, or by shaking the suspension.
As shown earlier, particle shape preferred orientations in concentrated suspensions (magmatic mushes) are nearly steady-state and cannot be correlated to finite strain. The present experiments suggest that shape preferred orientations preferentially record an obliquity with the shear plane that can be used as a shear sense indicator in the field, provided the shear plane can be properly identified, this obliquity being primarily due to mechanical interactions between particles.
References
Ildefonse, B., Michibayashi, K., Cecchi, E. Analogue experiments on compaction and shearing of wet granular media. Application to crystallizing magmas. EUG99, Strasbourg, J. Conf. Abstracts, 4, 617. Download poster Paper in revision (email for a copy) by Benoit Ildefonse, Katsuyoshi Michibayashi, Emmanuelle Cecchi
