Microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure

The relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high- temperature high- pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0% to 76% solid fraction ( alumina grains). Torsion experiments were conducted at 300 MPa confining pressure, temperatures ranging from 475 degrees C to 1000 degrees C and shear strain rates ranging from 2.0 x 10(-5) to 2.1 x 10(-3) s(-1) up to total strains of 21.3. Flow is Newtonian for a solid fraction of phi(s) = 0.0 - 0.16 with a log dynamic viscosity eta = 10.3 Pa s ( T = 500 degrees C). A deviation from Newtonian behavior is observed for phi(s) > 0.16 with an increase in apparent viscosity of about 1 order of magnitude between phi(s) = 0.16 and 0.54. The shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction. Both shape fabric and deviation from Newtonian behavior originate from the increase in the number of particle clusters in the suspension. The apparent viscosity increases by 1.5 orders of magnitude between phi(s) = 0.54 and 0.65, and extrapolation of the data suggests a very sharp increase in apparent viscosity for phi(s) >= 0.65. At T = 550 degrees C and phi(s) = 0.54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains. At T >= 550 degrees C and phi(s) = 0.65, intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles. The resulting bulk extensional fabric is almost parallel to the shortening direction.