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The timing, geometry, compositions and volumes of fluid fluxes associated with metamorphism of carbonates and pelites from east-central Vermont during the Acadian orogeny (380 Ma ago) are determined using petrological observations, calculation of petrogenetic grids, time-integrated flux calculations and construction of strontium isotope profiles across carbonate horizons. Qualitative petrological observations suggest flow to have been largely layer-parallel and focussed along permeability contrasts such as carbonate/pelite boundaries. Vein-deformation relationships constrain timing, implying that fluid flow occurred to some degree throughout the metamorphic event, but that the majority of mineral reaction driven by fluid flow occurred to the thermal peak of metamorphism. Textures also record both pre- and post- metamorphic events. Carbon distribution and isotope values may indicate that carbon deposition occurred in certain locations in response to the mixing of fluids of different compositions. Pseudosections based on the petrogenetic grid constructed in the system KCaNaFMASCH predict observed sequences of assemblages and reactions. Calculations studying the effect of alkali metal metasomatism on assemblages show that increasing potassium content stabilises biotite over amphibole at high temperatures, while addition of sodium stabilises plagioclase at the expense of all other phases, particularly chlorite and muscovite. Detailed temperature - X CO<SUB>2</SUB> work on four outcrops identifies gradients in fluid composition across carbonate bands on < 1m scales at biotite grade and on 5 -15 m scales at kyanite grade. Time-integrated fluid flux calculations, performed using an expression derived from the mass-continuity equation for this study, show that observed reaction progress could have been driven by fluid fluxes between 10<SUP>4</SUP> moles m<SUP>-2</SUP> if flow was layer-perpendicular and 10<SUP>8</SUP> moles m<SUP>-2</SUP> if flow was layer-parallel. Fluid release was continuous but varied in rate by up to two orders of magnitude, with the bulk of reaction and fluid release occurring over small (5-10°C) temperature intervals.

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A set of thermodynamic models suitable for mantle mineral phases, accompanied by a mafic melt model, has been calibrated in the system CaO-MgO-Al­₂O₃-SiO₂ (CMAS) and its subsystems, over the pressure range 0.50 kbar. The models are able to reproduce phase assemblages and reactions observed by experiment. Having a fundamentally thermodynamic form, they can be interpolated reliably between the pressures, temperatures and compositions of the experiments. The models may be used with the phase equilibrium calculation software THERMOCALC, and they incorporate end-members from its internally consistent dataset for greater rigour of calibration. The mafic melt model was the focus of model development. It takes a simple regular solution formulation, in common with many of the solid solution models used in THERMOCALC. However a number of modifications had to be made in order to attain a suitably flexible model melt, since natural mafic liquids cover a very much larger compositional range than any solid solution, and have many more degrees of structural freedom. Faced with these intrinsic problems in melt modelling, the approach taken has been to calibrate the model first in small systems of one, two and three components, combining these to make larger systems. Such an approach leads to a more comprehensive model calibration, exploiting the information available in very simple systems about liquid behaviour. In the CMAS system, model fit is excellent in the pressure range 15-50 kbar. During the fitting process, a case was made for applying a pressure adjustment of -15% to one group of calibration experiments. If this is appropriate, model calculations reproduce the melting reactions well within the experimental error of ±10°C, otherwise, the reactions are calculated at temperatures up to 50°C too high. Calculated phase relations at lower pressure require further attention. Liquid and solid solution compositions are difficult to determine experimentally and probably have large unquantified errors; calculated values typically match those of the calibration experiments to within 20%. The models are able to mimic subtle features of experimental melting relations in CMAS, mostly arising from the interaction of clinopyroxene solid solution with liquid. A preliminary extension of the models was made into the system Na₂O-CaO-FeO-MgO-Al₂O₃-Fe₂O₃-SiO₂-Cr₂O₃ (NCFMASCrO), producing a reasonable fit to experimentally determined oxide trends. A set of sample calculations produced with the CMAS models is presented, demonstrating the modelling of fractional and batch melting and crystallisation. Further calculations take the form of pseudosections: maps of the phase assemblages in <i>P-T </i>space drawn for a single bulk composition, and contoured for phase composition. Pseudosections are powerful means of investigating thermodynamic equilibrium in a rock, since they incorporate the natural constraint of bulk composition – however they cannot produce meaningful calculations without sophisticated and reliable thermodynamic phase models. The propagation of uncertainties in fitted model parameters into pseudosection calculations is explored for the first time using Monte Carlo techniques.

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This thesis describes the investigation of fluid dynamic processes involved in maintained explosive volcanic eruptions. The thesis is divided into chapters relating to dynamical processes in a volcanic system: evolution and evacuation of a reservoir of molten rock, flow in a narrow conduit to the Earth's surface, and subsequent transport in the atmosphere. In chapter 2, we calculate the mass erupted, prior to caldera collapse, from a chamber as the pressure changes from a certain overpressure to a specified underpressure at which wall collapse occurs. The compressibility of the magma increases significantly as the pressure falls and the magma becomes saturated in volatiles. Magma saturation exerts a dominant control on the amount of magma erupted. We also examine the effects on mass erupted of the chamber shape, size and depth beneath the Earth's surface, the magma composition and the strength of country rock. Finally, we demonstrate applications of our results to various historical eruptions, including the eruption at Vesuvius in 79A.D. and the eruption at Mt St Helens in 1980. During maintained explosive volcanic eruptions, fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10-100 atm and at the speed of sound of the mixture. In chapter 3, we review previous models of magma ascent up a conduit and identify some new scalings for the exit velocity as a function of the speed of sound of the mixture. In chapter 4, we combine models of evolution of the magma chamber with models of ascent of magma up the conduit to make estimates of the duration of the eruption and examine the rate of change in eruption rate with time under conditions of decreasing chamber pressure, changing magma volatile content and conduit widening due to erosion. Finally, we demonstrate an application of our results to the historical eruptions at Vesuvius in 79A.D. and at Mt St Helens in 1980. After decompression, the bulk of the material may ascend as a larger convecting eruption column or collapse to form a dense fountain which sheds ash flows around the vent. In chapter 5, we model the decompression of jets beyond the vent. We describe a jet freely decompressing into the atmosphere or into a crater, coupling our results with models of eruption column formation. We show that decompression through a crater may cause collapse at relatively small eruption rates, while it may promote formation of buoyant eruption columns at higher eruption rates. If a crater grows through erosion during an eruption, then typically a transition in eruption style may occur from an eruption column to column collapse.

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No description available.

No description available.