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(ωτ), is unaffected in the low-temperature—high-frequency range of crystals, with Q−1 ≈ 1/(ωτ)0.5 (the same as for bubble- and crystal-free rhyolite). For the present type of suspension, the internal friction is practically constant and independent of log(ωτ) in the high-temperature—low-frequency limit (ωτ 1). The shape of the Cole-Cole diagram becomes symmetrical and can be described as a Caputo body with parameter γ ≈ 0.45, whereas for bubble-bearing and inclusion-free rhyolite melts the shape of diagram relates to the β-relaxation exponent with β ≈ 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

Numerical modelling of stress generation and microfracturing of vesicle walls in glassy rocks

In the absence of stress-concentrating flaws such as microfractures, vesicular glassy materials can withstand gas pressures within vesicles in excess of 100 MPa; however, vesicles within such materials are known to decrepitate explosively at much lower internal gas pressures, both in natural systems and in the laboratory. Here we present a model that quantitatively predicts the generation of microfractures in vesicle walls during cooling. Cooling of gas-bearing vesicles in glassy rock has little effect on water solubility in the glass, but leads to a rapid decrease in gas pressure in the vesicles. The drop in pressure causes disequilibrium between the water in the glass and in the vesicle. Dehydration of the glass in a diffusive boundary layer around the vesicle leads to elastic shrinkage. The resulting strain generates large tensile tangential stresses which can exceed the strength of the glass, causing microfracturing. Such microfractures present a possible means by which glassy rock surrounding vesicles could be weakened enough to permit explosive decrepitation at low pore vapor pressures. The results have implications wherever hydrous vesicular glasses are formed. For example rocks formed in shallow subvolcanic intrusions or vent plugs may spontaneously disintegrate with explosive emission of vapor; glassy submarine lavas spontaneously decrepitate upon dredging from the ocean floor ("popping rock"); vesicular glasses produced in laboratory experiments investigating vapor-melt phase equilibria have been observed to contain abundant fractures surrounding vesicles and to dehydrate at anomalously high rates

Internal friction spectroscopy in Li20-2SiO2 partially crystallised glasses.

Oscillatory torsion deformation experiments were performed on partially crystallised Li2O–2SiO2 glasses in the temperature range 350–480 °C and with frequencies between 20 and 0.002 Hz. The experiments were carried out in a torsion deformation apparatus exerting a small strain on cylindrical samples. Data obtained at varying temperatures and frequency were reduced to master plots using a normalised frequency. The frequency shift factor has been taken as a function of temperature in an Arrhenian form, yielding an activation energy of a background Q−1 close to the activation energy of oxygen defect diffusion (=120 kJ/mol). The master curves of real and imaginary components of shear modulus and internal friction indicate a stretched exponential shear stress relaxation with a an exponent of ≈0.45, characteristic of a broadened relaxation spectrum. The dynamic viscosity was estimated at temperatures of 470 and 480 °C. The extrapolation of dynamic viscosity to zero frequency allowed estimation of the relaxed shear viscosity. The presence of crystals increases the relaxed shear viscosity by ≈0.2log(Pa s)/10 vol.% of crystallinity. Dependence of the relative shear viscosity of partially crystallised lithium disilicate melts on crystal content is discussed