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

Fluidal pyroclasts reveal the intensity of peralkaline rhyolite pumice cone eruptions

This work is a contribution to the Natural Environment Research Council (NERC) funded RiftVolc project (NE/L013932/1, Rift volcanism: past, present and future) through which several of the authors are supported. In addition, Clarke was funded by a NERC doctoral training partnership grant (NE/L002558/1).Peralkaline rhyolites are medium to low viscosity, volatile-rich magmas typically associated with rift zones and extensional settings. The dynamics of peralkaline rhyolite eruptions remain elusive with no direct observations recorded, significantly hindering the assessment of hazard and risk. Here we describe uniquely-preserved, fluidal-shaped pyroclasts found within pumice cone deposits at Aluto, a peralkaline rhyolite caldera in the Main Ethiopian Rift. We use a combination of field-observations, geochemistry, X-ray computed microtomography (XCT) and thermal-modelling to investigate how these pyroclasts are formed. We find that they deform during flight and, depending on size, quench prior to deposition or continue to inflate then quench in-situ. These findings reveal important characteristics of the eruptions that gave rise to them: that despite the relatively low viscosity of these magmas, and similarities to basaltic scoria-cone deposits, moderate to intense, unstable, eruption columns are developed; meaning that such eruptions can generate extensive tephra-fall and pyroclastic density currents.Publisher PDFPeer reviewe

Viscoelastic behaviour of basaltic lavas.

The rheological properties of basaltic lavas from Etna, Hawai′i and Vesuvius have been investigated at temperatures between 500 and 1150°C using a small-strain oscillatory shear. The viscoelastic response of the lavas to small, forced, sinusoidal torques (109 Pa s) could be measured at strain rates less than 10−2–10−1 s−1. At 800°C, temporal variations in complex shear modulus and internal friction suggest that, over durations of up to 120 h, structural adjustments were occurring within some of the samples. This time-varying behaviour of lava samples may be attributed to the slow closing (healing) of microcracks and small pore spaces, resulting in the apparent stiffening of lava samples under annealing. Thus, those parts of lava flows that undergo slow cooling will have more elastic properties. Regions that cool faster possess smaller shear moduli and higher internal friction due to thermal microcracking and less cohesion between crystals and the bulk glassy matrix

A review and investigation of the Non-Newtonian properties of lavas based on laboratory experiments with analogue materials.

Realistic lava flow models require a comprehensive understanding of the rheological properties of lava under a range of stress conditions. Previous measurements have shown that at typical eruption temperatures lavas are non-Newtonian. This is commonly attributed to the formation and destruction of crystal networks. In the present study, the effects of bubbles on the time-dependent, non-Newtonian properties of vesicular melts are investigated experimentally using analogue materials. The shear-thinning behaviour of bubbly liquids is shown to be dependent on the previous shearing history. This thixotropic behaviour, which was investigated using a rotational vane-viscometer, is caused by delayed bubble deformation and recovery when subjected to changes in shear stress. The viscoelastic transition and the transient flow behaviour of analogue fluids were studied using both a rotational vane-viscometer and oscillatory shear apparatus. These experiments have shown that vesicular suspensions are viscoelastic fluids with a yield strength, power law rheology, and a non-zero shear modulus. These properties are also found in polymer fluids commonly used as analogue materials for lava such as gum rosin. We show that, when materials with this rheology are accelerated in channels, they may be fragmented, and when they flow through a narrowing conduit, pulsating flow can develop as a consequence of a transition from slip to non-slip conditions at the conduit wall. This has important implications both for effusive and explosive volcanic eruptions

Vesiculation processes in a water-rich calc-alkaline obsidian

The effect of temperature and viscosity on the kinetics of bubble growth was measured for a natural water-rich rhyolite melt. The change in volume of a natural crystal-poor calc-alkaline rhyolitic obsidian with an initial water content of = 1.8 wt% H,O was determined in the temperature interval 520-624°C. Shear viscosity was measured on the natural sample using the micropenetration method in the temperature interval 450-590°C and water contents before and after viscometry were monitored by FTIR. The time-dependence of the volume increase as a result of vesiculation approximates the Avrami equation: AV(t) = 1 - exp[ -(t/t)°]. At temperatures close to the glass transition temperature (Ts), growing bubbles ruptured the surrounding melt. During diffusion of water from the hydrous melt with = 1.8 wt% H,O into a bubble, the deltaTg of the melt at the bubble wall increases by = 315°C. In addition, the viscosity of the melt at the bubble wall increases by 5-6 orders of magnitude. The increasing elastic component of shear stress on the bubble wall as a result of bubble growth at about Tg may exceed the bubble wall yield strength, resulting in cracking. No time lag in the onset of bubble growth occurred for this water-rich rhyolite. In those parts of volcanic edifices where water contents are of several weight percent (e.g., within the upper parts of volcanic conduits), the probability of melt fracturing due to the degassing of water at about Tg increases

A model for the rheology of particle-bearing suspensions and partially molten rocks

An edited version of this paper was published by AGU. Copyright (2009) American Geophysical UnionThis contribution presents a semiempirical model describing the effective relative viscosity of crystalbearing magmas as function of crystal fraction and strain rate. The model was applied to an extensive data set of magmatic suspensions and partially molten rocks providing a range of values for the fitting parameters that control the behavior of the relative viscosity curves as a function of the crystal fraction in an intermediate range of crystallinity (30–80 vol % crystals). The analysis of the results and of the materials used in the experiments allows for evaluating the physical meaning of the parameters of the proposed model. We show that the model, by varying the parameters within the ranges obtained during the fitting procedure, is able to describe satisfactory the effective relative viscosity as a function of crystal fraction and strain rate for suspensions having different geometrical characteristics of the suspended solid fraction.PublishedQ030102.3. TTC - Laboratori di chimica e fisica delle rocce3.6. Fisica del vulcanismoJCR Journalreserve

A model for the rheology of particle-bearing suspensions and partially molten rocks

This contribution presents a semiempirical model describing the effective relative viscosity of crystalbearing magmas as function of crystal fraction and strain rate. The model was applied to an extensive data set of magmatic suspensions and partially molten rocks providing a range of values for the fitting parameters that control the behavior of the relative viscosity curves as a function of the crystal fraction in an intermediate range of crystallinity (30–80 vol % crystals). The analysis of the results and of the materials used in the experiments allows for evaluating the physical meaning of the parameters of the proposed model. We show that the model, by varying the parameters within the ranges obtained during the fitting procedure, is able to describe satisfactory the effective relative viscosity as a function of crystal fraction and strain rate for suspensions having different geometrical characteristics of the suspended solid fraction

Electrical conductivity of polycrystalline Mg(OH)2 at 2 GPa: Effect of grain boundary hydration-dehydration

The effect of intergranular water on the conductivity of polycrystalline brucite, Mg(OH)2, was investigated using impedance spectroscopy at 2 GPa, during consecutive heating-cooling cycles in the 298-980 K range. The grain boundary hydration levels tested here span water activities from around unity (wet conditions) down to 10-4 (dry conditions) depending on temperature. Four orders of magnitude in water activity result in electrical conductivity variations for about 6-7 orders of magnitude at 2 GPa and room temperature. Wet brucite samples containing, initially, about 18 wt% of evaporable water (i. e. totally removed at temperatures below 393 K in air), display electrical conductivity values above 10-2-10-3 S/m. A. C. electrical conductivity as a function of temperature follows an Arrhenius behaviour with an activation energy of 0.11 eV. The electrical conductivity of the same polycrystalline brucite material dried beforehand at 393 K (dry conditions) is lower by about 5-6 orders of magnitude at room temperature and possesses an activation energy of 0.8-0.9 eV which is close to that of protonic diffusion in (001) brucitic planes. Above ca. 873 K, a non-reversible conductivity jump is observed which is interpreted as a water transfer from mineral bulk to grain boundaries (i. e. partial dehydration). Cooling of such partially dehydrated sample shows electrical conductivities much higher than those of the initially dry sample by 4 orders of magnitude at 500 K. Furthermore, the corresponding activation energy is decreased by a factor of about four (i. e. 0. 21 eV). Buffering of the sample at low water activity has been achieved by adding CaO or MgO, two hygroscopic compounds, to the starting material. Then, sample conductivities reached the lowest values encountered in this study with the activation energy of 1.1 eV. The strong dependency of the electrical conductivity with water activity highlights the importance of the latter parameter as a controlling factor of diffusion rates in natural processes where water availability and activity may vary grandly. Water exchange between mineral bulk and mineral boundary suggests that grain boundary can be treated as an independent phase in dehydroxylation reactions. © 2011 Springer-Verlag.This work was partly supported by the SYSTER funding program (INSU-CNRS). Travel expenses were covered by the PROCOPE (0938RJ) French–German exchange program. Reviews by J. Renner and C.J. Peach helped substantially improve the manuscript. N. Findling is warmly thanked for his help at running FE-SEM and XRD at ENS. We also acknowledge the technical service of the Instituto de Ciencia de Materiales de Sevilla (CSIC-US).Peer Reviewe

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

Physikalische Eigenschaften teilgeschmolzener Gesteine an der mittelozeanischen Spreizungsachse

National audienceNew experimental data on electrical and anelastic properties of oceanic rocks at high pressures and temperatures are compared with the field data of seismic and electrical anomalies observed at oceanic spreading centers. The laboratory results of electrical conductivity and internal friction on gabbro and peridotites permit to estimate a degree of partial melting and maximum temperature in axial magma chambers (12-15% of melting at 1155-1165°C) and at the upper boundary of a source zone at the depth of 40-90 km (less than 5% of melting in the absence of pyroxene crystals and up to 11-14% in the pyroxene reach rocks at 1360-1370°C) at mid-oceanic ridges