Sintering of vesiculating pyroclasts

Hot volcanic pyroclasts can sinter, vesiculate, and outgas in concert – a combination of processes which remains poorly constrained. And yet this combination of processes can occur coincidently during deposition from pyroclastic density currents, in conduit-filling pyroclastic debris, and in tuffisites. In many of these settings, it is the sintering-driven evolution of permeability that is key to gas transport through the evolving deposit. Here, we experimentally and theoretically investigate the evolution of the permeable networks during sintering of hot fragmental volcanic systems, which are hydrous and oversaturated at the experimental conditions. Firstly, we find that vesiculation results in shutting of the inter-granular porous network as bubble growth drives expansion of the particles into one another, destroying interconnected pores. Secondly, we observe that degassing by diffusion out of the particle edge results in contraction of the vesicular particles, re-opening pore spaces between them. Therefore, we find that vesiculation, and diffusive outgassing compete to determine both the intra-fragment vesicularity and the permeability during sintering. The development of intra-fragment vesicularity directly impacts the inter-fragment pore space and its connectivity, which decreases during vesiculation and subsequently increases during diffusive outgassing, prompting complex, non-linear permeability evolution.The relative dominance of these processes is fragment size dependent; proportionally, fine fragments lose gas at a higher rate than coarser fragments during diffusive outgassing due to larger surface area to volume ratios. As the systems progress, larger fragments retain a higher proportion of gas and so attain greater vesicularities than finer ones – and therefore, the coarse fragmental pyroclasts experience a greater, yet transient, reduction in connected porosity and permeability. We suggest that where vesiculation is sufficient, it can lead to the complete loss of connected porosity and the sealing of permeable pathways much earlier than in a sintering-only system. Our results suggest that classical sintering models must be modified to account for these vesiculation and diffusive degassing processes, and that only a combined vesiculation, sintering, and diffusive outgassing model can resolve the evolution of permeability in hot clastic volcanic systems

Self-consistent thermodynamic description of silicate liquids, with application to shock melting of MgO periclase and MgSiO 3 perovskite

We develop a self-consistent thermodynamic description of silicate liquids applicable across the entire mantle pressure and temperature regime. The description combines the finite strain free energy expansion with an account of the temperature dependence of liquid properties into a single fundamental relation, while honouring the expected limiting behaviour at large volume and high temperature. We find that the fundamental relation describes well previous experimental and theoretical results for liquid MgO, MgSiO 3 , Mg 2 SiO 4 and SiO 2 . We apply the description to calculate melting curves and Hugoniots of solid and liquid MgO and MgSiO 3 . For periclase, we find a melting temperature at the core–mantle boundary (CMB) of 7810 ± 160 K , with the solid Hugoniot crossing the melting curve at 375 GPa, 9580 K , and the liquid Hugoniot crossing at 470 GPa, 9870 K . For complete shock melting of periclase we predict a density increase of 0.14 g cm −3 and a sound speed decrease of 2.2 km s −1 . For perovskite, we find a melting temperature at the CMB of 5100 ± 100 K with the perovskite section of the enstatite Hugoniot crossing the melting curve at 150 GPa, 5190 K , and the liquid Hugoniot crossing at 220 GPa, 5520 K . For complete shock melting of perovskite along the enstatite principal Hugoniot, we predict a density increase of 0.10 g cm −3 , with a sound speed decrease of 2.6 km s −1 .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75103/1/j.1365-246X.2009.04142.x.pd

Porosity and permeability development in evolving fragmental volcanic systems

The capacity for fluid percolation in volcanic environments may be considered a double-edged sword. On the one hand, high permeabilities offer both a pressure release valve to mitigate the explosive potential of a magma, and access to valuable clean energy in geothermal reservoirs. Conversely, low permeabilities are associated with dangerous, highly pressurised magma which have a propensity to violently erupt. In geothermal systems, impervious rocks may prohibit resource utilisation, or provide the necessary cap rock to seal-in and maintain the desired high temperature fluids. In both active volcanic environments and hydrothermal systems, fluid flow is heavily concentrated through fracture networks and fragmental systems, such that they play a central role in determining the style of volcanism and the potential for geothermal energy production. In this thesis, I investigate how the evolution of several fragmental systems may impact their porosity and permeability development. Specifically, I analyse how dehydration affects juvenile fragmental melts which remain in viscous environments and how altered fragmental deposits respond to thermal stress. Before exploring the evolution of the aggregate fragmental systems, I first assess individual melt fragments which are dehydrating in an open system, such that volatiles may escape the melt into vesicles, via vesiculation, or into the surrounding atmosphere, via diffusion out of the sample. Where volatiles move into vesicles, the isolated bubble growth expands the fragment volume, whereas volatiles released from the fragment do not directly impart a geometry change. I demonstrate that in pyroclasts, vesiculation will strive towards equilibrium with the closed system conditions, and so it is a fragment size independent process; volatiles continually diffuse into vesicles until the water content of the melt drops to the melt solubility limit, such that the closed-system vesicularity of fragments can be assessed using bubble growth models. On the other hand, diffusive outgassing equilibrates the melt with the conditions of the open, surrounding gas, and so the effectiveness of diffusive volatile loss is determined by the surface area of the melt-atmosphere interface. I observe that dehydration caused by diffusive outgassing progressively impacts deeper into the fragment, where it causes exsolved volatiles to resorb and the vesicles to shrink and be lost. Accordingly, a dense and dehydrated rind forms at the sample margin, which thickens with the lengthscale of diffusion. I show that where vesiculation and diffusive outgassing occur in a melt fragment concomitantly, the processes compete to expand and densify the fragment. Because the rate of diffusive outgassing is determined by the melt surface area, the size of pyroclasts controls this competition, such that, as fragment size decreases, the vesicularity moves increasingly away from the closed-system bubble growth models. I find that smaller fragments attain lower vesicularity profiles than larger fragments, and that over time, fragments of all sizes will densify and eventually lose all vesicularity. In fragmental aggregate systems, this evolution in individual melt fragments is likely inversed in the volume of the inter-fragment pore space, leading to implications for the porosity and permeability of the system. I monitor the evolution of vesicularity, connected porosity, and permeability in open fragmental melts with various grain sizes, to assess how the concurrent processes of vesiculation, diffusive outgassing, and sintering interact. I find that as melt vesiculates, the expansion of fragments causes a commensurate loss in the inter-fragment pore space, which causes a reduction in permeability. However, this process is transient whilst the system remains open to the atmosphere, as diffusive outgassing causes fragment contraction, which reverses the porosity and permeability impact of vesiculation. Overprinting these processes, sintering continues to densify the melt and will ultimately close the permeable network. From the complex fragment size controls for these processes, I establish regimes which determine the general evolution of porosity and permeability during sintering of hydrous melts. Finally, I assess the impact of dehydration in hydrothermally altered volcaniclastic reservoir rocks. I explore the thermal stability of hydrous minerals in hyaloclastites and investigate how the dehydration and dissolution of matrix constituents influences the porosity, permeability, and mechanical behaviour of the bulk rock. I find that at relatively low temperatures, which are applicable for geothermal resources, smectite dehydrates, causing the mineral lattices to densify and ultimately, collapse. This dissolution creates pore volume, which is increasingly connected, such that thermal stress increases permeability without necessitating the formation of fractures. The increase in porosity reduces the compressive and tensile strength of the hyaloclastites. I show that rocks containing phyllosilicate minerals may be susceptible to thermal fluctuations, and that this enhances porosity and permeability and reduces strength, which may then facilitate mechanical compaction at lower stresses, with significant implications for geothermal reservoir rocks and magmatic host rocks. Through these studies I highlight that dehydration in fragmental volcanic systems can produce complex porosity and permeability evolution. If these systems are to be well understood, a careful assessment of their compositions (particle size distributions and mineralogies) is required and their thermal, chemical, and physical environmental conditions should be well constrained

Bubble growth and resorption in magma: insights from dissolved water distributions in volcanic glass

Volcanic eruptions are driven by the growth of gas bubbles in magma, which grow and shrink as volatile species exsolve from and dissolve back into the melt in response to changes in the local environment, particularly in pressure and temperature. This movement of volatiles, particularly water, is recorded in the glass around vesicles and recent studies have used this record to interpret natural samples. This thesis investigates the processes that control bubble growth and resorption in magma, by measuring the distribution of dissolved H2O in experimentally-vesiculated volcanic glasses. H2O concentration profiles obtained using SIMS-calibrated BSEM imaging and H2O speciation data obtained using FTIR spectroscopy, are interpreted in the context of the known pressure and temperature history of the samples. Samples are found to have undergone partial bubble resorption during the quench to glass at the end of experiments, as a result of increasing H2O solubility with decreasing temperature. Analysis of the lengthscale and timing of the resulting H2O concentration profiles demonstrates that the majority of resorption occurs above the glass transition. This quench resorption is associated with a reduction in bubble volumes which creates characteristic textures, such as buckled melt films between adjacent vesicles and reoriented cracks around resorption halos. Highly disequilibrium H2O speciation ratios within resorption halos are found to be diagnostic of quench resorption and can preserve evidence of pre-quench bubble growth Quench resorption can increase sample H2O concentrations and H2Om:OH ratios and reduce bubble volumes and sample porosities. Studies based on these parameters must therefore consider the potential impact of quench resorption, which is expected to be greatest for samples with high H2O concentrations, slow quench and low initial sample porosities. H2O speciation data offer a way to investigate these impacts in unconstrained natural samples and could provide a tool for forensic interrogation of their eruptive history

Water and CO2 solubilities in rhyolitic to basaltic melts : experimental determination and calibration for IR spectroscopy = Wasser und CO2 Löslichkeiten in rhyolitischen bis basaltischen Schmelzen : experimentalle Bestimmung und Kalibration für IR Spektroskopie

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Atypical planetary lavas: rheological evolution of cooling and crystallizing flows from lunar impact melts and cryovolcanic processes

Volcanism is common to many of the solid planets and moons throughout the solar system. On Earth, volcanic research is mainly targeted at hazard assessment and prediction but volcanism on other worlds helps us understand how planetary bodies evolve and what that evolution means for the Earth and its future. Understanding the volcanic process on our world and others yields information about heat and mass transport processes, and about interior and surface evolution.One way of furthering our understanding of the volcanic process is by investigating the erupted products. Lavas in particular make up a large portion of planetary surfaces, however, some lavas in the solar system are very different to what we expect on Earth. Both impact events and ice volcanism (cryovolcanism) in the outer solar system can create molten material of very different compositions to the silicate volcanism on Earth, at very different conditions (e.g., temperature and pressure). Despite this, many planetary features share common morphologies with terrestrial volcanism, suggesting similar physical processes driving emplacement. In this work, I draw comparisons between composition and formation mechanism for impact melts, cryovolcanism, and silicate volcanism by investigating their rheology – the flow behavior that links material properties to morphology.I measured the rheology of lunar simulants for both highland and mare compositions to investigate how lunar impact melts evolve as they flow. Crystallization happens rapidly upon crossing the liquidus for highland compositions but mare compositions require undercooling before rapid crystallization occurs. This leads to shorter, thicker flows in the highlands and longer, thinner flows in the mare. This pattern may explain why more highland impact melt sheets are observed, because the thinner impact melts in the mare are more readily erased by impact gardening resulting in a preservation bias in the rock record. I also synthesized a wide range of aqueous solutions as analog cryolavas to measure their viscosity. I developed a new viscosity model, based on the Vogel-Fulcher-Tammann (VFT) equation commonly used in silicate rheology, to predict viscosity of aqueous solutions a function of both temperature and concentration for binary systems. This model provides better extrapolation down to cryogenic temperatures than previous models and can be scaled up to more complicated multicomponent systems. I then developed a new model for cryovolcanic flow evolution to investigate emplacement. This model simultaneously tracks the physical, chemical, and thermal state of the flow and allows entrainment of the solid fraction rather than surface accumulation. These are all improvements over several previous models. I found that the heat loss from vaporization of the flow in the low-pressure environment of many icy worlds was the dominant heat flux and that aspect ratios predicted match well with observed features

Micropetrology: Are Inclusions Grains of Truth?

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Storage conditions and degassing processes of low-K and high-Al tholeiitic island-arc magmas : experimental constraints and natural observations for Mutnovsky volcano, Kamchatka

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Experimental study on vesiculation and formation of groundmass microlites induced by decompression : constraints on processes related to magma ascent at Unzen volcano

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