The rate and timing of bubble growth in magma is an important control on eruption style, determining whether or not magma fragments to produce an explosive eruption. Bubbles nucleate, grow, shrink, and de-nucleate in magma in response to changes in pressure and temperature, and these changes may be recorded in the vesicle textures, and in the spatial distribution and speciation of water ‘frozen into’ the glass in eruption products. We have developed a numerical model for growth and resorption of bubbles in magma, and validated it against experiments across a wide range of conditions. The model allows for arbitrary temperature and pressure pathways, and accounts for the impact of spatial variations in water content on diffusivity and viscosity. Textures in natural, vesicular volcanic rocks are often used to interpret eruptive processes. Similarly, high-pressure, high-temperature experiments are used to probe bubble growth processes, via textural and chemical analysis of the experimental products. However, in both cases, the analysed samples have cooled from magmatic temperatures before analysis, providing a window for thermally-driven bubble shrinkage and resorption to modify the sample. Consequently, interpretations of syn-eruptive and syn-experimental processes must account for changes during cooling. We present results from in situ experiments under synchrotron-source tomography, which demonstrate the thermally driven growth and resorption of bubbles in magma at one atmosphere. The model is applied to 4D textural data, and used to investigate the role of bubble-bubble interactions in modifying growth and resorption behaviour. We also apply the model to re-analyse the results of high-temperature, highpressure experiments, and demonstrate the importance of accounting for thermal resorption during cooling.
Abstract We report the first calorimetric observation of the glass transition for a carbonate melt. A carbonate glass [55K2CO3–45MgCO3 (molar)] was quenched from 780 °C at 0.1 GPa. The activation energy of structural relaxation close to the glass transition was derived through a series of thermal treatments comprising excursions across the glass transition at different heating rates. Viscosities just above the glass transition temperature were obtained by applying a shift factor to the calorimetric results. These viscosity measurements (in the range of 109 Pa·s) at supercooled temperatures (ca. 230 °C) dramatically extend the temperature range of data for carbonates, which were previously restricted to super-liquidus viscosities well below 1 Pa·s. Combining our calorimetrically derived results with published alkaline-earth carbonate melt viscosities at high temperatures yields a highly non-Arrhenian viscosity-temperature relationship and confirms that carbonate liquids are “fragile.” Based on simulations, fragile behavior is also exhibited by Na2CO3 melt. In both cases, the fragility presumably relates to the formation of temperature-dependent low-dimensional structures and Vogel-Fulcher-Tammann (VFT) curves adequately describe the viscosity-temperature relationships of carbonate melts below 1000 °C.
<p>Basaltic volcanism is strongly influenced by magmatic viscosity, which, in turn, is controlled by magma composition, crystallisation, oxygen fugacity and vesiculation. We developed an environmental cell to replicate the pressure and temperature during magma ascent from crustal storage to the surface, while capturing crystallisation using in-situ 4D X-ray computed microtomography. Crystallisation experiments were performed at Diamond Light Source, using monochromatic 53 keV X-rays, a pixel size of 3.2 &#956;m, a sample to detector distance of 2000 mm, 1440 projections per 180 deg, an acquisition time of 0.04 s, and a rotation velocity of 3.125 deg.s-1. The redox conditions were controlled using an oxidised nickel disk for each experiment. Our starting materials were samples made of crystal-free glass cylinders (&#216; 3 mm) from the 2001 Etna eruption with 0.9 and 0.8 wt. % water content. In the experiments, samples and crucibles were sealed initially by applying ~10 N loads. All samples were then heated up above glass transition (between 800 &#176;C and 900 &#176;C) in order to allow sample homogenisation while preventing volatiles exsolution. We then pressurised each sample by applying uniaxial loads (between 80 and 380 N), using high-degree alumina pistons, in order to generate enough internal pressure to maintain bubble-free samples when the desired high temperature was reached. Once at the initial high temperature, we began experiments via dropping the temperature to different target isothermal (from 1210 to 1130 &#176;C or 1180 to 1110 &#176;C) and isobaric conditions (8 and 10 MPa, respectively). For the whole duration of the experiments, we were able to observe directly and record pyroxene crystal nucleation and growth. Specifically, we were able to observe pyroxene nucleation on bubbles at small undercooling (&#8710;T) and epitaxial growth of pyroxene at large &#8710;T. An increase of &#8710;T (up to 50 &#176;C) can be associated with a decompression of a magma chamber or a decompression during magma ascent in the conduit. As &#8710;T = 30 - 50 &#176;C can be reached in most of the basaltic volcanic systems on Earth, our results provide a feasible explanation of which mechanisms control nucleation and growth of pyroxene crystals in hydrous basaltic magmas. In addition, epitaxial growth promotes a faster increase of the crystal volume. As a larger crystal content translates into a higher viscosity, our results have important implications for magma rheology, and are extremely important to improve our understanding of magma ascent dynamics during volcanic eruptions, and our capacity to predict eruptions and mitigate volcanic hazards.</p>
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