Viscoelasticity of crystal- and bubble-bearing rhyolite melts

The effect of non-deformable inclusions on the frequency-dependent rheology of a rhyolite melt plus crystals has been investigated using a sinusoidal torsion deformation device for measurements of shear viscosity and modulus in the frequency range of 5 mHz to 20 Hz at temperatures of 750—1050°C.The relaxed shear viscosity and unrelaxed shear modulus of rhyolite magma (rhyolite melt plus crystals plus bubbles) decreases with increasing bubble content and increases with the addition of crystals. At a crystal concentration of about 45% a relaxed value of the shear viscosity is not attainable. The presence of rigid inclusions results in an imaginary component of the shear modulus that becomes more symmetrical and shifted to the low-frequency—high-temperature range with respect to that for a crystal-free melt. The slope of log(Q 1) (internal friction) as a function of the dimensionless variable log(o T), is unaffected in the low-temperature—high-frequency range of crystals, with ~ 1 1/(wr)°5(the same as for bubbleand crystal-free rhyolite). For the present type of suspension, the internal friction is practically constant and independent of log(wr) in the high-temperature—low-frequency limit (WT << 1). The shape of the Cole—Cole diagram becomes symmetrical and can be described as a Caputo body with parameter y 0.45, whereas for bubble-bearing and inclusion-free rhyolite melts the shape of diagram relates to the /3-relaxation exponent with /3 0.5. The present work demonstrates that magma may or may not follow a power-law rheology depending on the relative volume proportion between crystals and bubbles.