Disequilibrium Rheology and Crystallization Kinetics of Basalts and Implications for the Phlegrean Volcanic District

Large volcanic eruptions are frequently triggered by the intrusion of hot primitive magma into a more evolved magma-chamber or -mush zone. During intrusion into the cooler mush zone, the basaltic magma undergoes crystallization, which in turn can release heat and volatiles to the mush. This should cause a drop in bulk mush-viscosity, potentially leading to its mobilization and even eruption. The non-linear changes in the transport properties of both magmas during this interaction also modulate how the magmas accommodate deformation during both interaction and ascent. As such, this interaction represents a complex disequilibrium phenomenon, during which the material properties guiding the processes (dominantly viscosity) are in constant evolution. This scenario highlights the importance of non-isothermal sub-liquidus processes for the understanding of natural magmatic and volcanic systems and underlines the need for a rheological database to inform on, and to model, this interaction process. Here we present new experimental data on the disequilibrium rheology of the least evolved end-member known to be involved in magma mixing and eruption triggering as well as lava flow processes in the Phlegrean volcanic district (PVD). We measure the melt's subliquidus rheological evolution as a function of oxygen fugacity and cooling rate and map systematic shifts in its rheological "cut off temperature;T-cutoff" (i.e., the point where flow ceases). The data show that (1) the rheological evolution and solidification behavior both depend on the imposed cooling-rate, (2) decreasing oxygen fugacity decreases the temperature at which the crystallization onset occurs and modifies the kinetics of melt crystallization and (3) the crystallization kinetics produced under dynamic cooling are significantly different than those observed at or near equilibrium conditions. Based on the experimental data we derive empirical relationships between the environmental parameters and T-cutoff. These empirical descriptions of solidification and flow may be employed in numerical models aiming to model lava flow emplacement or to reconstruct the thermomechanical interaction between basalts and magma mush systems. We further use the experimental data in concert with existing models of particle suspension rheology to derive the disequilibrium crystallization kinetics of the melt and its transition from crystallization to glass formation

Fault rheology beyond frictional melting

During earthquakes, comminution and frictional heating both contribute to the dissipation of stored energy. With sufficient dissipative heating, melting processes can ensue, yielding the production of frictional melts or “pseudotachylytes.” It is commonly assumed that the Newtonian viscosities of such melts control subsequent fault slip resistance. Rock melts, however, are viscoelastic bodies, and, at high strain rates, they exhibit evidence of a glass transition. Here, we present the results of high-velocity friction experiments on a well-characterized melt that demonstrate how slip in melt-bearing faults can be governed by brittle fragmentation phenomena encountered at the glass transition. Slip analysis using models that incorporate viscoelastic responses indicates that even in the presence of melt, slip persists in the solid state until sufficient heat is generated to reduce the viscosity and allow remobilization in the liquid state. Where a rock is present next to the melt, we note that wear of the crystalline wall rock by liquid fragmentation and agglutination also contributes to the brittle component of these experimentally generated pseudotachylytes. We conclude that in the case of pseudotachylyte generation during an earthquake, slip even beyond the onset of frictional melting is not controlled merely by viscosity but rather by an interplay of viscoelastic forces around the glass transition, which involves a response in the brittle/solid regime of these rock melts. We warn of the inadequacy of simple Newtonian viscous analyses and call for the application of more realistic rheological interpretation of pseudotachylyte-bearing fault systems in the evaluation and prediction of their slip dynamics

Volcanic ash melting under conditions relevant to ash turbine interactions

The ingestion of volcanic ash by jet engines is widely recognized as a potentially fatal hazard for aircraft operation. The high temperatures (1,200-2,000 degrees C) typical of jet engines exacerbate the impact of ash by provoking its melting and sticking to turbine parts. Estimation of this potential hazard is complicated by the fact that chemical composition, which affects the temperature at which volcanic ash becomes liquid, can vary widely amongst volcanoes. Here, based on experiments, we parameterize ash behaviour and develop a model to predict melting and sticking conditions for its global compositional range. The results of our experiments confirm that the common use of sand or dust proxy is wholly inadequate for the prediction of the behaviour of volcanic ash, leading to overestimates of sticking temperature and thus severe underestimates of the thermal hazard. Our model can be used to assess the deposition probability of volcanic ash in jet engines

Volcanic ash melting under conditions relevant to ash turbine interactions

The ingestion of volcanic ash by jet engines is widely recognized as a potentially fatal hazard for aircraft operation. The high temperatures (1,200-2,000 degrees C) typical of jet engines exacerbate the impact of ash by provoking its melting and sticking to turbine parts. Estimation of this potential hazard is complicated by the fact that chemical composition, which affects the temperature at which volcanic ash becomes liquid, can vary widely amongst volcanoes. Here, based on experiments, we parameterize ash behaviour and develop a model to predict melting and sticking conditions for its global compositional range. The results of our experiments confirm that the common use of sand or dust proxy is wholly inadequate for the prediction of the behaviour of volcanic ash, leading to overestimates of sticking temperature and thus severe underestimates of the thermal hazard. Our model can be used to assess the deposition probability of volcanic ash in jet engines

Volcanic ash versus thermal barrier coatings of jet engines – a holistic experimental approach

Since the heavy interruption of North Atlantic air traffic by volcanic ash in 2010 many experimental investigations have been done in order to better understand the influence of volcanic ash on the functionality of jet engines and in particular thermal barrier coatings (TBCs) on turbine blades within them. Some of these studies used natural volcanic ash while others used a synthetic mixture from the Calcium-Magnesium-Aluminum-Silicon system (CMAS). To this day, a holistic experimental investigation on TBCs, using various natural volcanic ashes, is missing. In the framework of the CORNET research project VAsCo (“Volcanic Ash resistant thermal barrier Coatings for jet engines” – www.vasco-cornet.eu), we are going to close this gap. We use four different volcanic ashes, which represent the chemical range of possibly produced ash by explosive volcanic eruptions. As TBCs, atmospheric plasma sprayed (APS) and electron-beam physical vapor deposited (EBPVD) coatings of yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) were chosen as state-of-the-art materials for first experiments. While YSZ EB-PVD coatings are prone to be fully infiltrated by molten silicates, GZO exhibits a higher resistivity against the infiltration through a rapid re-crystallization of the dissolved coating, thus closing the pathways of infiltration. This contrasting behavior of both materials makes them good candidates to study the influence of different chemical and mineralogical compositions of various volcanic ashes. The experiments are based on static and dynamic experiments: Static experiments include measurements with the heating microscope, to study the wetting and spreading of the molten ash sample on the TBC surface, and muffle furnace experiments with ash covered TBCs to study their chemical interactions. For dynamic experiments we thermally spray the ash on the TBC surfaces in order to simulate real conditions within the combuster/turbine section of a jet engine. Findings are used to modify TBCs and to improve their resistivity against molten volcanic ash. In addition to that, a feasibility study will be conducted in order to create a model synthetic volcanic ash for standard tests in the aviation industry

A Geoscientific Perspective on Silicate Melt Interactions with TBCs

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Rheology of a sodium‐molybdenum borosilicate melt undergoing phase separation

During glass production, phase separation can result in the formation of suspended liquid droplets, which can cause changes in the system rheology. In nuclear waste vitrification context, some new glassy matrices may present this phase separation matter, but the mechanisms controlling the viscosity changes have not yet been determined. Here, we measure the viscosity of a sodium‐borosilicate melt containing dissolved MoO3 at different temperatures and subject to different applied shear strain rates. We observe that (i) the viscosity increases sharply as the temperature decreases and (ii) at any constant temperature below 1000°C, the system presents non‐Newtonian response. Using transmission electron microscope observations coupled with viscosity calculations, we interpret the cause of the observed changes as the result of phase separation. We show that the viscosity increase on cooling is in excess of the predicted temperature dependence for a homogeneous melt of the starting composition. The increase is due to the formation of a second phase and is controlled by chemical and structural modifications of the matrix during the loss of the elements that form the droplets. This work provides insights into the rheology of a system composed of two composition sets due to a miscibility gap

The rheological response of magma to nanolitisation

Viscosity exerts a fundamental control on magmatic kinetics and dynamics, controlling magma ascent, eruptive style, and the emplacement of lava. Nanolites – crystals smaller than a micron – are thought to affect magma viscosity, but the underlying mechanisms for this remain unclear. Here, we use a cylinder compression creep technique to measure the viscosity of supercooled silicate liquids with different amounts of iron (0–20 wt% FeOtot) as a function of temperature, applied shear stress, and time. Sample viscosity was independent on the applied shear stresses, and as expected, melt viscosity decreases as temperature is increased, but only until a critical temperature where a time-dependent increase in viscosity occurs for samples contaning 6.0 wt% FeOtot or more. The magnitude of this increase is controlled by the melt iron content. At constant temperature, these changes are substantial and can reach up to three orders of magnitude for the sample with the most iron. Using transmission electron microscopy, X-ray diffraction, and viscosity modelling, we conclude that this viscosity increase is caused by the formation of nanolites. By using scaling approaches to test suspension effects with and without crystal aggregation, we conclude that the nanolites have only a minimal direct physical effect on the observed viscosity change. Rather, our models show that it is the chemical shift in the groundmass silicate melt composition associated with non-stoichiometric crystallisation that dominates the observed viscosity increase. These findings suggest that iron-rich silicates may encounter chemical viscosity jumps once certain elements are removed from the melt phase to form nanolites. Our work demonstrates an underlying mechanism for the role played by nanolites in viscosity changes of magmas

Can nanolites enhance eruption explosivity?

Degassing dynamics play a crucial role in controlling the explosivity of magma at erupting volcanoes. Degassing of magmatic water typically involves bubble nucleation and growth, which drive magma ascent. Crystals suspended in magma may influence both nucleation and growth of bubbles. Micron- to centimeter-sized crystals can cause heterogeneous bubble nucleation and facilitate bubble coalescence. Nanometer-scale crystalline phases, so-called “nanolites”, are an underreported phenomenon in erupting magma and could exert a primary control on the eruptive style of silicic volcanoes. Yet the influence of nanolites on degassing processes remains wholly uninvestigated. In order to test the influence of nanolites on bubble nucleation and growth dynamics, we use an experimental approach to document how nanolites can increase the bubble number density and affect growth kinetics in a degassing nanolite-bearing silicic magma. We then examine a compilation of these values from natural volcanic rocks from explosive eruptions leading to the inference that some very high naturally occurring bubble number densities could be associated with the presence of magmatic nanolites. Finally, using a numerical magma ascent model, we show that for reasonable starting conditions for silicic eruptions, an increase in the resulting bubble number density associated with nanolites could push an eruption that would otherwise be effusive into the conditions required for explosive behavior

Auto-granulation of particle surfaces during Surtseyan eruptions

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