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

A rheological investigation of vesicular rhyolite

The rheology of vesiculating rhyolitic systems exerts a strong control on the transport of silicic magmas in the subvolcanic to volcanic environments. We present here an investigation of vesiculating and vesiculated rhyolites using dilatometric methods. This study examines the effect of vesicle content on the viscosity of a natural supercooled rhyolitic liquid with 0–70% vesicles. The experimental samples of rhyolitic glass are derived from fusion of a natural obsidian from Little Glass Butte, Oregon. Crystal-free rhyolite glasses of varying porosity were prepared by fusing obsidian powder in a Pt crucible. Differing porosities were obtained by varying the temperature (1300—1650°C) and duration (0.5–6 h) of the fusions. Cylindrical samples of the resulting vesiculated rhyolites were cored from the crucible using diamond tools and their ends were ground flat and parallel for dilatometry. The porosity of each sample was determined from Archimedean buoyancy density determinations and comparison with bubble-free rhyolite (2.331 g/cm3, porosity = 1 - p/po). The density of foamed samples was determined using their mass, volume and regular geometry. Viscosities were determined in the parallel plate mode at stresses of 5 × 103 to 105 Pa. The viscosimeter was calibrated using NBS 711 glass. The bubble contents were microscopically investigated using a video-reflected light system and image analysis software. Distribution functions of the size, orientation, aspect ratio and surface porosity were obtained. The viscosity of rhyolite decreases with increasing bubble content. A general relationship of the form: η(|) = η(0)/(1 + C|), describes the effect of porosity, | (in volume fraction) on the viscosity, η, where C is a dimensionless constant (= 22.4 ± 2.9) and log10η(0) = 10.94 ± 0.04 Pa s at 850°C

Deformation of foamed rhyolites under internal and external stresses: an experimental investigation

The style of magma eruption depends strongly on the character of melt degassing and foaming. Depending on the kinetics of these processes the result can be either explosive or effusive volcanism. In this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been investigated in time series experiments (2 min-24 h) followed by quenching the samples. The volumetric gas-melt ratio has been estimated through the density measurements of foamed samples. The variation of gas volume (per unit or rhyolite melt volume) with time may be described by superposition of two exponentials responsible for gas generation and gas release processes respectively. An observed difference in foaming style in this study is interpreted as the result of variations in initial contents of microlites that serve as bubble nucleation centers during devolatilization of the melts. Quantitatively the values of the gas generation rate constants (k g) are more than an order of magnitude higher in microlite-rich obsidian than in microlite-free obsidian. Possible origins of differences in the degassing style of natural magmas are discussed in the light of bubble nucleation kinetics in melts during foaming. In a complementary set of experiments the mechanical response of vesicular melt to external shear stress has been determined in a concentric cylinder viscometer. The response of vesicular melt to the pulse of shear deformation depends on the volume fraction of bubbles. The obtained response function can be qualitatively described by a Burgers body model. The experimental shear stress response function for bubble-bearing melt has an overshoot due to the strain-dependent rheology of a twophase liquid with viscously deformable inclusions

Thermal properties of vesicular rhyolite

Thermal diffusivity of rhyolite melt and rhyolite foam (70–80% porosity) has been measured using the radial heat transfer method. Cylindrical samples (length 50–55 mm, diameter 22 mm) of rhyolite melt and foam have been derived by heating samples of Little Glass Mountain obsidian. Using available data on heat capacity and density of rhyolite melt, the thermal conductivity of samples has been determined. The difference in thermal conductivity between rhyolite melt and foam at igneous temperatures ( 1000°C) is about one order of magnitude. The effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

Effect of lattice volume and strain on the conductivity of BaCeY-oxide ceramic proton conductors

In-situ electrochemical impedance spectroscopy was used to study the effect of lattice volume and strain on the proton conductivity of the yttrium-doped barium cerate proton conductor by applying the hydrostatic pressure up to 1.25 GPa. An increase from 0.62 eV to 0.73 eV in the activation energy of the bulk conductivity was found with increasing pressure during a unit cell volume change of 0.7%, confirming a previously suggested correlation between lattice volume and proton diffusivity in the crystal lattice. One strategy worth trying in the future development of the ceramic proton conductors could be to expand the lattice and potentially lower the activation energy under tensile strain

Frequency Dependent Rheology of Vesicular Rhyolite

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

Experimental strategies for the investigation of low temperature properties in granitic and pegmatitic melts

The physical behavior of silicate melts during the final stages of intrusion in the earth's crust are poorly understood. In particular, the low temperature limit of igneous petrogenesis is poorly constrained. The extreme differentiates of granitic magmatism that lead to pegmatite genesis span a very large range of composition not normally considered to be within the domain of igneous melt compositions. This combination of very low petrogenetic temperatures and extreme chemistries requires a concentrated effort for the determination of melt properties under conditions of pressure, temperature and composition appropriate to these systems. An experimental strategy for the determination of melt properties under appropriate conditions is presented. The determination of individual melt properties at very low temperatures is described with the aid of three examples, heat capacity, volume and viscosity. In this way the physical behavior of an important component of the earth's crust will become accessible

Settling and compaction of olivine in basaltic magmas: an experimental study on the time scales of cumulate formation

A series of centrifuge-assisted settling experiments of 30 vol% olivine in 70 vol% basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates. The settling experiments were performed in a centrifuging piston cylinder at 200-1,500g, 1,270-1,280°C, and 0.8-1.1GPa on previously annealed and texturally equilibrated samples. The mechanical settling of the dense olivine suspension occurs at about 1/6 the speed of simple Stokes settling, resulting in a sedimentation exponent n=4.1(6) in agreement with predictions from analogue systems. The porosity (φ m ) of the orthocumulate resulting from gravitational settling of crystals is about 54% and formation times of olivine orthocumulates result to 0.1-10mday−1 (for an initial crystal content of the melt of 1-5% and grain sizes of 2-10mm). After mechanical settling, olivine grains rest on each other, and further compaction occurs through pressure dissolution at grain contacts, olivine reprecipitation where olivine is in contact with melt, and concomitant expulsion of excess liquid from the cumulate layer. With centrifugation at 400g for 50h, porosities as low as 30.3 vol% were achieved. The olivine content at the bottom of the gravitational cumulate is 1−φm~log(Δρ·h·a·t), where Δρ is the density difference between crystals and melt, h the crystal layer thickness, a the acceleration, and t the time of centrifuging. Compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction. The compaction limit, that is the lowermost porosity to be reached by this mechanism, is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3-5% porosity for the experiments. Crystal size distribution curves and a growth exponent n of 3.1(3) indicate that diffusion-controlled Ostwald ripening is the dominant crystal growth mechanism. The above relationship, combined with a linear scaling for grain size as appropriate for reaction-controlled pressure solution creep, allows calculation of formation times of adcumulates. If chemical compaction is dissolution-reprecipitation limited, then single layers of natural olivine adcumulates of ½ m thickness with 70-75 vol% olivine at the base (as observed in the Rhum layered intrusion) would have typical formation times of 0.4-3years for grain sizes of 2-10mm. This time scale compares favourably with characteristic cooling times of sills. If a greater than20-m-thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt, then compaction proceeds to the compaction limit within a few years. It can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades, a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5%. In conclusion, gravitational settling and gravitation-driven chemical compaction are feasible cumulate-forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusion