Frequency dependent rheology of vesicular rhyolite

Abstract

Frequency dependent rheology of magmas may result from the presence of inclusions (bubbles, crystals) in the melt and/or from viscoelastic behavior of the melt itself. With the addition of deformable inclusions to a melt possessing viscoelastic properties one might expect changes in the relaxation spectrum of the shear stresses of the material (e.g., broadening of the relaxation spectrum) resulting from the viscously deformable geometry of the second phase. We have begun to investigate the effect of bubbles on the frequency dependent rheology of rhyolite melt. The present study deals with the rheology of bubble‐free and vesicular rhyolite melts containing spherical voids of 10 and 30 vol %. We used a sinusoidal torsion deformation device. Vesicular rhyolite melts were generated by the melting (at 1 bar) of an Armenian obsidian (Dry Fountain, Erevan, Armenia) and Little Glass Mountain obsidian (California). The real and imaginary parts of shear viscosity and shear modulus have been determined in a frequency range of 0.005–10 Hz and temperature range of 600°–900°C. The relaxed shear viscosities of samples obtained at low frequencies and high temperatures compare well with data previously obtained by parallel plate viscometry. The relaxed shear viscosity of vesicular rhyolites decreases progressively with increasing bubble content. The relaxation spectrum for rhyolite melt without bubbles has an asymmetric form and fits an extended exponent relaxation. The presence of deformable bubbles results in an imaginary component of the shear modulus that becomes more symmetrical and extends into the low‐frequency/high‐temperature range. The internal friction Q−1 is unaffected in the high‐frequency/low‐temperature range by the presence of bubbles and depends on the bubble content in the high‐temperature/low‐frequency range. The present work, in combination with the previous study of Stein and Spera (1992), illustrates that magma viscosity can either increase or decrease with bubble content, depending upon the rate of style of strain during magmatic flow.

Viscous heating in silicate melts: An experimental and numerical comparison

“…It is thus confirmed upon approaching the structural relaxation time scale that magmas may self heat and exhibit a pseudo non‐Newtonian behavior as their viscosity decreases as a result [ Hess et al , 2008]. Notably however the viscous heating effect does not follow the slope of the classic non‐Newtonian curve (see the purple curve in Figure 7) explained by structural effects as observed in the linear stress–strain regime of frequency domain experiments [ Bagdassarov and Dingwell , 1993; Webb and Dingwell , 1995]. This difference was predictable as the structural theory is directly proportional to the strain rate see (13) while the viscous heating theory is function of the strain rate squared see (6).…”

Effect of boron, phosphorus and fluorine on shear stress relaxation in haplogranite melts

“…1 and Kampfmann & Berckhemer, 1985;Bagdassarov & Dingwell, 1993), comprises a high temperature horizontal Pt-Rh furnace with an as sembly consisting of a cylindrical samples (S) be tween two alumina torsion-bars (Alj and Al 2 ). The temperature is measured by a type-S thermo couple.…”

Oscillatory rheology measurements of particle‐ and bubble‐bearing fluids: Solid‐like behavior of a crystal‐rich basaltic magma

“…The oscillatory rheology measurements are briefly described in this section and have been previously defined in the literature [e.g., Bagdassarov and Dingwell , ; Larson , ]. To generate a sinusoidally oscillating strain, , with a small amplitude of γ 0 and an angular frequency of ω , a sinusoidal, time‐dependent stress is required: σ ( t ) = G ∗ γ ( t ).…”