Vesicle shrinkage in hydrous phonolitic melt during cooling

The ascent of hydrous magma prior to volcanic eruptions is largely driven by the formation of H2O vesicles and their subsequent growth upon further decompression. Porosity controls buoyancy as well as vesicle coalescence and percolation, and is important when identifying the differences between equilibrium or disequilibrium degassing from textural analysis of eruptive products. Decompression experiments are routinely used to simulate magma ascent. Samples exposed to high temperature (T) and pressure (P) are decompressed and rapidly cooled to ambient T for analysis. During cooling, fluid vesicles may shrink due to decrease of the molar volume of H2O and by resorption of H2O back into the melt driven by solubility increase with decreasing T at P < 300 MPa. Here, we quantify the extent to which vesicles shrink during cooling, using a series of decompression experiments with hydrous phonolitic melt (5.3–3.3 wt% H2O, T between 1323 and 1373 K, decompressed from 200 to 110–20 MPa). Most samples degassed at near-equilibrium conditions during decompression. However, the porosities of quenched samples are significantly lower than expected equilibrium porosities prior to cooling. At a cooling rate of 44 K·s−1, the fictive temperature Tf, where vesicle shrinkage stops, is up to 200 K above the glass transition temperature (Tg), Furthermore, decreasing cooling rate enhances vesicles shrinkage. We assess the implications of these findings on previous experimental degassing studies using phonolitic melt, and highlight the importance of correctly interpreting experimental porosity data, before any comparison to natural volcanic ejecta can be attempted

A validated numerical model for the growth and resorption of bubbles in magma

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 spatial distribution and speciation of water 'frozen into' the glass in eruptive products. Accurate modelling of growth and resorption is therefore essential both for forward modelling of eruptive processes, and for inverse modelling to reconstruct pre-eruptive history. We present the first experimentally-validated numerical model for bubble growth and resorption in magma. The model includes the kinetics of speciation, allows for arbitrary temperature and pressure pathways, and accounts for the impact of spatial variations in water content on diffusivity and viscosity. We validate the model against three sets of data. (1) Continuous vesicularity-time data collected using optical dilatometry and in-situ synchrotron-source x-ray tomography of natural and synthetic magma during thermally-induced vesiculation and resorption at magmatic temperatures and ambient pressure. This represents approximately isobaric bubble growth and resorption under disequilibrium conditions. (2) Final vesicularity data from decompression experiments at magmatic temperatures and pressures. This represents isothermal, decompression-driven bubble growth from equilibrium to strongly disequilibrium conditions. (3) Speciation data from diffusion-couple experiments on synthetic haplogranites at magmatic temperatures and pressures. The numerical model closely reproduces all experimental data, providing validation against equilibrium and disequilibrium bubble growth/resorption and speciation scenarios. The validated model can be used to predict the growth and resorption of bubbles, and associated changes in magma properties, for arbitrary eruption pathways. It can also be used to reconstruct pressure-temperature-time pathways from textures and volatile contents of eruptive products. This will open up new ways of accessing the dynamics of magma ascent and eruption in unobserved volcanic eruptions

Quantifying Microstructural Evolution in Moving Magma

Many of the grand challenges in volcanic and magmatic research are focused on understanding the dynamics of highly heterogeneous systems and the critical conditions that enable magmas to move or eruptions to initiate. From the formation and development of magma reservoirs, through propagation and arrest of magma, to the conditions in the conduit, gas escape, eruption dynamics, and beyond into the environmental impacts of that eruption, we are trying to define how processes occur, their rates and timings, and their causes and consequences. However, we are usually unable to observe the processes directly. Here we give a short synopsis of the new capabilities and highlight the potential insights that in situ observation can provide. We present the XRheo and Pele furnace experimental apparatus and analytical toolkit for the in situ X-ray tomography-based quantification of magmatic microstructural evolution during rheological testing. We present the first 3D data showing the evolving textural heterogeneity within a shearing magma, highlighting the dynamic changes to microstructure that occur from the initiation of shear, and the variability of the microstructural response to that shear as deformation progresses. The particular shear experiments highlighted here focus on the effect of shear on bubble coalescence with a view to shedding light on both magma transport and fragmentation processes. The XRheo system is intended to help us understand the microstructural controls on the complex and non-Newtonian evolution of magma rheology, and is therefore used to elucidate the many mobilization, transport, and eruption phenomena controlled by the rheological evolution of a multi-phase magmatic flows. The detailed, in situ characterization of sample textures presented here therefore represents the opening of a new field for the accurate parameterization of dynamic microstructural control on rheological behavior

The effect of initial H2O concentration on decompression-induced phase separation and degassing of hydrous phonolitic melt

Supersaturation of H2O during magma ascent leads to degassing of melt by formation and growth of vesicles that may power explosive volcanic eruptions. Here, we present experiments to study the effect of initially dissolved H2O concentration (cH2Oini) on vesicle formation, growth, and coalescence in phonolitic melt. Vesuvius phonolitic melts with cH2Oini ranging between 3.3 and 6.3 wt% were decompressed at rates of 1.7 and 0.17 MPa·s−1 and at temperatures ≥ 1323 K. Decompression started from 270 and 200 MPa to final pressures of 150–20 MPa, where samples were quenched isobarically. Optical microscopy and Raman spectroscopic measurements confirm that the glasses obtained were free of microcrystals and Fe-oxide nanolites, implying that the experiments were superliquidus and phase separation of the hydrous melt was homogeneous. A minimum number of the initially formed vesicles, defined by the number density normalized to vesicle-free glass volume (VND), is observed at ~ 5 wt% cH2Oini with a logVND of ~ 5 (in mm−3). The logVND increases strongly towards lower and higher cH2Oini by one order of magnitude. Furthermore, an important transition in evolution of vesiculation occurs at ~ 5.6 wt% cH2Oini. At lower cH2Oini, the initial VND is preserved during further decompression up to melt porosities of 30–50%. At higher cH2Oini, the initial vesicle population is erased at low melt porosities of 15–21% during further decompression. This observation is attributed to vesicle coalescence favored by low melt viscosity. In conclusion, cH2Oini determines the VND of initial phase separation and the evolution of vesiculation during decompression that controls the style of volcanic eruptions.German Science Foundatio

Vesicle shrinkage in hydrous phonolitic melt during cooling

The ascent of hydrous magma prior to volcanic eruptions is largely driven by the formation of H2O vesicles and their subsequent growth upon further decompression. Porosity controls buoyancy as well as vesicle coalescence and percolation, and is important when identifying the differences between equilibrium or disequilibrium degassing from textural analysis of eruptive products. Decompression experiments are routinely used to simulate magma ascent. Samples exposed to high temperature (T) and pressure (P) are decompressed and rapidly cooled to ambient T for analysis. During cooling, fluid vesicles may shrink due to decrease of the molar volume of H2O and by resorption of H2O back into the melt driven by solubility increase with decreasing T at P < 300 MPa. Here, we quantify the extent to which vesicles shrink during cooling, using a series of decompression experiments with hydrous phonolitic melt (5.3–3.3 wt% H2O, T between 1323 and 1373 K, decompressed from 200 to 110–20 MPa). Most samples degassed at near-equilibrium conditions during decompression. However, the porosities of quenched samples are significantly lower than expected equilibrium porosities prior to cooling. At a cooling rate of 44 K·s−1, the fictive temperature Tf, where vesicle shrinkage stops, is up to 200 K above the glass transition temperature (Tg), Furthermore, decreasing cooling rate enhances vesicles shrinkage. We assess the implications of these findings on previous experimental degassing studies using phonolitic melt, and highlight the importance of correctly interpreting experimental porosity data, before any comparison to natural volcanic ejecta can be attempted.German Science Foundatio

Growth and Resorption of Bubbles in Magma

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

In situ observation of the percolation threshold in multiphase magma analogues

Magmas vesiculate during ascent, producing complex interconnected pore networks, which can act as outgassing pathways and then deflate or compact to volcanic plugs. Similarly, in-conduit fragmentation events during dome-forming eruptions create open systems transiently, before welding causes pore sealing. The percolation threshold is the first-order transition between closed- and open-system degassing dynamics. Here, we use time-resolved, synchrotron-source X-ray tomography to image synthetic magmas that go through cycles of opening and closing, to constrain the percolation threshold ΦC at a range of melt crystallinity, viscosity and overpressure pertinent to shallow magma ascent. During vesiculation, we observed different percolative regimes for the same initial bulk crystallinity depending on melt viscosity and gas overpressure. At high viscosity (> 106 Pa s) and high overpressure (~ 1–4 MPa), we found that a brittle-viscous regime dominates in which brittle rupture allows system-spanning coalescence at a low percolation threshold (ΦC~0.17) via the formation of fracture-like bubble chains. Percolation was followed by outgassing and bubble collapse causing densification and isolation of the bubble network, resulting in a hysteresis in the evolution of connectivity with porosity. At low melt viscosity and overpressure, we observed a viscous regime with much higher percolation threshold (ΦC > 0.37) due to spherical bubble growth and lower degree of crystal connection. Finally, our results also show that sintering of crystal-free and crystal-bearing magma analogues is characterised by low percolation thresholds (ΦC = 0.04 – 0.10). We conclude that the presence of crystals lowers the percolation threshold during vesiculation and may promote outgassing in shallow, crystal-rich magma at initial stages of Vulcanian and Strombolian eruptions.Paul Scherrer Institut http://dx.doi.org/10.13039/501100004219European Research Council http://dx.doi.org/10.13039/501100000781NERCDeutsche Forschungsgemeinschaf

In situ observation of the percolation threshold in multiphase magma analogues

Magmas vesiculate during ascent, producing complex interconnected pore networks, which can act as outgassing pathways and then deflate or compact to volcanic plugs. Similarly, in-conduit fragmentation events during dome-forming eruptions create open systems transiently, before welding causes pore sealing. The percolation threshold is the first-order transition between closed- and open-system degassing dynamics. Here, we use time-resolved, synchrotron-source X-ray tomography to image synthetic magmas that go through cycles of opening and closing, to constrain the percolation threshold ΦC at a range of melt crystallinity, viscosity and overpressure pertinent to shallow magma ascent. During vesiculation, we observed different percolative regimes for the same initial bulk crystallinity depending on melt viscosity and gas overpressure. At high viscosity (> 106 Pa s) and high overpressure (~ 1–4 MPa), we found that a brittle-viscous regime dominates in which brittle rupture allows system-spanning coalescence at a low percolation threshold (ΦC~0.17) via the formation of fracture-like bubble chains. Percolation was followed by outgassing and bubble collapse causing densification and isolation of the bubble network, resulting in a hysteresis in the evolution of connectivity with porosity. At low melt viscosity and overpressure, we observed a viscous regime with much higher percolation threshold (ΦC > 0.37) due to spherical bubble growth and lower degree of crystal connection. Finally, our results also show that sintering of crystal-free and crystal-bearing magma analogues is characterised by low percolation thresholds (ΦC = 0.04 – 0.10). We conclude that the presence of crystals lowers the percolation threshold during vesiculation and may promote outgassing in shallow, crystal-rich magma at initial stages of Vulcanian and Strombolian eruptions

Viscosity of Palmas-type magmas of the Paraná Magmatic Province (Rio Grande do Sul State, Brazil): Implications for high-temperature silicic volcanism

This paper provides a new parameterization of the temperature and H2O content dependence of the pure liquid viscosities of rhyolitic and dacitic magmas associated with representative volcanic products of the Caxias do Sul and Santa Maria eruptive sequences. The viscosities of silicic volcanic products from the Santa Maria rhyolite (SMr), Caxias do Sul (CSd) and Barros Cassal (BCd) eruptive sequences (Lower Cretaceous volcanism of Parana-Etendeka Large Igneous Province) were measured in the temperature range from ca. 1600 °C to the glass transition (Tg). Anhydrous melt viscosities of representative samples from the main eruptive sequences were determined via concentric cylinder viscometry in the superliquidus regime. The quench products of SMr and CSd were then hydrated using an Internally Heated Pressure Vessel to generate two suites of samples with variable water content of up to 4.41 (CSd) and 5.27 (SMr) wt% as determined by Karl Fischer Titration (KFT). Finally, both anhydrous and hydrous samples were used for micropenetration viscosity measurements near Tg. Both types of samples show a minor amount of Fe-Ti-oxide nanolites identified via Raman spectroscopy, which presence did not substantially interfere with viscosity determinations. Based on the results of the viscosity measurements we parameterized the viscosity dependence as a function of water content using the following Vogel Fulcher Tammann (VFT) expressions accounting for the water and temperature dependence of the viscosity: logη = −4.55 + (10065–176*H2O)/[T-(34.6 + 375.3/(1 + H2O))] for SMr. and logη = −4.55 + (9213–338.1*H2O)/[T-(148.5 + 301.3/(1 + H2O))] for CSd. where η is the viscosity in Pa s, T the absolute temperature and H2O the dissolved water content in wt%. This novel parameterization appears to solve a few inconsistencies associated with the variation of the main descriptive parameters of the effect of H2O, improving the performance of some previous parameterizations. These results are useful for scaling to the conditions extant during ascent and eruption and during flow, emplacement and welding, at temperatures above Tg, for the dacitic and rhyolitic products investigated here