Gas Exsolution Research Articles

Dynamics of diffusive bubble growth and pressure recovery in a bubbly rhyolitic melt embedded in an elastic solid

We present a model of gas exsolution and bubble expansion in a melt supersaturated in response to a sudden pressure drop. In our model, the melt contains a suspension of gas bubbles of identical sizes and is encased in a penny‐shaped crack embedded in an elastic solid. The suspension is modeled as a three‐dimensional lattice of spherical cells with slight overlap, where each elementary cell consists of a gas bubble surrounded by a shell of volatile‐rich melt. The melt is then subjected to a step drop in pressure, which induces gas exsolution and bubble expansion, resulting in the compression of the melt and volumetric expansion of the crack. The dynamics of diffusion‐driven bubble growth and volumetric crack expansion span 9 decades in time. The model demonstrates that the speed of the crack response depends strongly on volatile diffusivity in the melt and bubble number density and is markedly sensitive to the ratio of crack thickness to crack radius and initial bubble radius but is relatively insensitive to melt viscosity. The net drop in gas concentration in the melt after pressure recovery represents only a small fraction of the initial concentration prior to the drop, suggesting the melt may undergo numerous pressure transients before becoming significantly depleted of gases. The magnitude of pressure and volume recovery in the crack depends sensitively on the size of the input‐pressure transient, becoming relatively larger for smaller‐size transients in a melt containing bubbles with initial radii less than 10−5 m. Amplification of the input transient may be large enough to disrupt the crack wall and induce brittle failure in the rock matrix surrounding the crack. Our results provide additional basis for the interpretation of volume changes in the magma conduit under Popocatépetl Volcano during Vulcanian degassing bursts in its eruptive activity in April–May 2000.

Abrupt transitions during sustained explosive eruptions: examples from the 1912 eruption of Novarupta, Alaska

Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 μm vesicle diameter and cumulative number densities ranging from 107–109 cm–3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h.

Computing sand production under foamy oil flow in porous media via least-squares finite elements

Tiny gas bubbles together with sand grains are produced whenever a pressure drawdown is induced in an oil well. The resulting foamy oil flow and sand production seem to enhance the production of oil under certain conditions and environments such as the heavy oil solution gas drive reservoirs in Western Canada and Venezuela. The purpose of this paper is to propose a mathematical model in which gas exsolution and gas bubble dispersions together with sand production are described in a multiphasic porous medium. Gas bubble formation and dispersion are described in a macroscopic manner through equations of states and relaxation, whereas sand production is viewed as an erosional hydrodynamics problem with geomechanical considerations. The formulation of the proposed model leads to a set of highly non-linear governing equations with primary field unknowns such as reservoir fluid pressure, volume fraction of dispersed gas bubbles, concentration of fluidized solid, porosity and solid displacement. The numerical solutions of these equations are challenging and require special treatment such as least-squares finite element techniques in order to ensure stability and accuracy in results. Numerical examples in which foamy oil flow occurs with concomitant sand production and failure are presented to demonstrate the performance of the proposed model.

Attenuation in gas-charged magma

Low frequency seismic events observed on volcanoes, such as Soufriere Hills Volcano, Montserrat, are thought to be caused by a resonating system. The modelling of seismic waves in gas-charged magma is critical for the understanding of seismic resonance effects in conduits, dykes and cracks. Seismic attenuation, which depends mainly on magma viscosity, gas and crystal content, is an essential factor in such modelling attempts. So far only two-phase gas–melt systems with the assumption of no diffusion and transport of volatiles between the melt and the gas bubbles have been considered. In this study, we develop a method of quantifying attenuation within gas-charged magma, including the effects of diffusion and exsolution of gas into the bubbles. The results show that by including such bubble growth processes attenuation levels are increased within magma. The resulting complex behaviour of attenuation with pressure and frequency indicates that two factors are controlling attenuation, the first due to viscous hindrance or the melt, and the second due diffusion processes. The level of attenuation within a gas-charged magma conduit suggests an upper limit on the length of a resonating conduit section of just a few hundred meters.

ブルカノ式噴火の発生要因としての火道浅部マグマの増圧過程

Vulcanian eruptions are relatively frequent eruptive phenomena in island arc volcanism. The high frequency provides us many opportunities to observe the eruptive behavior and to sample the erupted materials. Vulcanian activity of the Sakurajima volcano located at southern Kyushu in Japan has continued since 1955 and been precisely monitored by Sakurajima Volcano Observatory. Therefore, the vulcanian eruptions of Sakurajima volcano can be a common, interesting target to study the eruptive behavior from the various viewpoints of material analysis, geophysical observation and numerical modeling. Recently, many researchers have focused on pressurization processes of magmas in shallow conduits to cause vulcanian eruptions. In this paper, we review several studies on observations and models of generation of excess pressure in shallow conduit magmas just before vulcanian eruptions. We evaluate the possibility of pressurization processes by dehydration-induced crystallization and buoyancy of vesicular conduit magmas. The results of this study indicate that the dehydration-induced crystallization is not effective for excess pressure generation, if crystallizing magma in shallow conduit contains a significant amount of exsolved gas phase. Instead, excess pressure can be generated in a shallow conduit by buoyancy of vesicular conduit magma, when partially solidified magma at the vent of conduit prevents magma ascent and the conduit is filled with vesicular magmas from magma chamber located at ca. 3 km depth to the shallow conduit.

Gas saturation prediction and effect of low frequencies on acoustic impedance images at Foinaven Field

ABSTRACTLow frequencies are necessary in seismic data for proper acoustic impedance imaging and for petrophysical interpretation. Without lower frequencies, images can be distorted leading to incorrect reservoir interpretation and petrophysical predictions. As part of the Foinaven Active Reservoir Management (FARM) project, a Towed Streamer survey and an Ocean Bottom Hydrophone (OBH) survey were shot in both 1995 and 1998. The OBH surveys contain lower frequencies than the streamer surveys, providing a unique opportunity to study the effects that low frequencies have on both the acoustic impedance image along with petrophysical time‐lapse predictions. Artefacts that could easily have been interpreted as high‐resolution features in the streamer data impedance volumes can be distinguished by comparison with the impedance volumes created from the OBH surveys containing lower frequencies.In order to obtain results from the impedance volumes, impedance must be related to saturation. The mixing of exsolved gas, oil and water phases involves using the Reuss (uniform) or Voigt (patchy approximation) mixing laws. The Voigt average is easily misused by assuming that the end‐points correspond to 0% and 100% gas saturation. This implies that the patches are either 0% gas saturation or 100% gas saturation, which is never the case. Here, the distribution of gas as it comes out of solution is assumed to be uniform until the gas saturation reaches a sufficiently high value (critical gas saturation) to allow gas to flow. Therefore, at low gas saturations the distribution is uniform, but at saturations above critical, it is patchy, with patches that range from critical gas saturation to the highest gas saturation possible (1 minus residual oil and irreducible water saturation).

Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions

Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing.

Effect of Oil Sands Core Disturbance Induced by Gas Exsolution on Geotechnical and Hydraulic Properties Measurements

Abstract Oil sand cores tend to expand when they are recovered from deep heavy oil formations. Computer tomography scans of these cores show that discrete tensile fractures are induced within the cores. Formation of these fractures is attributed to the gas nucleation and exsolution process in the viscous heavy oil under the overburden stress relief in coring and retrieval. In core tubes with perforated holes, the core dilation depends on the clearance gap between the inner diameters of the core barrel and core tube. This paper proposes a recompaction method to restore these dilated cores to a state close to its initial porosity (some of locked structure may not be recovered). Geotechnical and hydraulic tests along with computer tomography imaging technique were conducted on these recompacted core specimens. Test results of the recompacted specimens were compared to those of the intact and reconstituted specimens to illustrate the effectiveness of this recompaction method. Introduction Oil sand is a very dense uncemented granular material with locked fabric(1,2). The main mineral composition is dominated by quartz. Its in situ porosity varies from 0.30 to 0.33. Bitumen filling the intergranular spaces does not contribute to the geotechnical properties of the material. Because of its uncemented nature and the presence of dissolved gas in the pore fluid, core disturbance is unavoidable if conventional coring techniques are used. Core disturbance disrupts the locked structure affecting not only the geotechnical, but also the hydraulic properties of the oil sand matrix. This paper examines the mechanism of core disturbance encountered during coring, retrieval, and handling. Based on the mechanism, a recompaction method is developed to restore some of the oil sand fabric inside the disturbed cores. In addition, this paper assesses how the core disturbance affects the measured geotechnical and hydraulic properties of oil sand based on the results of tests on intact, recompacted, and reconstituted specimens. Responses (Core Disturbance) of Oil Sand Cores in Drilling, Retrieval, and Preservation Dusseault(3) and Wong et al.(4) demonstrated that core disturbance resulting from gas exsolution was significant in gassy unconsolidated oil sands. Dusseault and van Domselaar(5) developed an index to quantify the degree of core disturbance for oil sands. The index, termed as "Index of Disturbance," ID is defined as: Equation (Available In Full Paper) where Φe is the porosity of the test specimen and Φ?is the in situ porosity of the specimen (determined from downhole petrophysical logs or from an undisturbed core with zero gas saturation under in situ stresses). This index, a scalar parameter, does not reflect the changes in fabric due to core disturbance. It is of practical importance to quantify the degree of core disturbance that has resulted from different stages in coring and handling. Under in situ conditions (Figure 1a), the oil sand formation is subjected to its overburden total stress (σ) and reservoir pore pressure (PL). Coring induces a reduction in total stress due to the difference between the formation and drilling fluid unit weights, thereby causing a decrease in the pressure in pore fluid.

Nonequilibrium magma degassing: Results from modeling of the ca. 1340 A.D. eruption of Mono Craters, California

We present results from a numerical conduit model of nonequilibrium magma degassing. We show that CO 2 / H 2O concentration ratios in pyroclastic obsidian from the ca. 1340 A.D. Mono Craters eruption may record nonequilibrium degassing during magma ascent. Our results also indicate that permeability-controlled, open-system gas loss is consistent with obsidian formation and promotes nonequilibrium degassing at shallow depths. Because of the low diffusivity of CO 2 relative to H 2O, we find that CO 2 concentrations can remain above equilibrium during magma ascent. To reproduce Mono Craters volatile concentrations, our model does not require volatile supersaturation, or an exsolved gas phase, prior to magma eruption. We estimate magmatic volatile contents of approximately 400 ppm CO 2 and 4.6 wt.% H 2O. However, nonequilibrium is contingent upon low rates of bubble nucleation to shallow depths, so that average bubble number densities are of order 10 11 m − 3 during most of the magma ascent. If bubble number densities during ascent of the Mono Craters magma were greater than 1011 m − 3 , degassing was in equilibrium and a CO 2-rich vapor buffer is required to reproduce observed CO 2 / H 2O concentration ratios.

Effects of a Cretaceous structural inversion and a postulated high heat flow event on petroleum system of the western Lower Saxony Basin and the charge history of the Apeldorn gas field

AbstractThermal history and evolution of the Palaeozoic petroleum system of the western Lower Saxony Basin as well as charge history of the Apeldorn gas field was reconstructed using 2-D forward basin modelling software. The Apeldorn gas field is located on an inverted western rim of the Lower Saxony Basin (LSB) and belongs with its relatively anomalous nitrogen content of 73.9 vol. % to one of the most unique gas accumulations in North Germany. Based on thermal calibration studies utilising both, vitrinite reflectance and corrected bottom hole temperatures as calibration parameters, a shallow burial model and an anomalous event of Coniacian high heat flow of 80 to 120 mW/m2was derived. As result, Lower Triassic and younger successions became subjected to slight changes in thermal maturity as opposed to Carboniferous and Permian successions, which show no assessable impact of the high heat flow event on the coalification pattern. The deep burial model in contrary to the shallow burial model is not supported by the structural reconstruction and backstripping in this more marginal setting. According to the modelling results, the key charge of the present Apeldorn gas field began in Tithonian (late Upper Jurassic) during the major phase of rifting in the Lower Saxony Basin. The present Westphalian coal-derived gas accumulations of the Lower Triassic Buntsandstein reservoir were sourced directly from modelled methane pools at top Rotliegend level. The hydrocarbon potential of the Westphalian source rocks became exhausted in Oxfordian (early Upper Jurassic). Reduction of the hydrostatic pressure during the Coniacian high heat flow event together with uplift during the Coniacian-Santonian inversion led to an extensive free gas exsolution. The resulting gas mixture between the exsolved free gas and the Westphalian coal-derived gas reached and saturated Buntsandstein reservoir. The structural trap became destroyed in course of the inversion leading to a sharp decrease of methane and nitrogen saturation.

Monitoring rapid temporal change in a volcano with coda wave interferometry

Multiply–scattered waves typically dominate the late part of the seismic coda in local earthquake seismograms. Small medium changes that have no detectable influence on the first arrivals are amplified by multiple scattering and may thus be readily observed in the coda. We exploit this idea using Coda Wave Interferometry to monitor temporal changes at Mount Erebus Volcano, Antarctica. Erebus is one of the few volcanoes on Earth with a long–lived convecting lava lake. Large exsolved gas bubbles generate impulsive Strombolian explosions that provide a repeating seismic source of seismic energy propagating through the strongly scattering geology of the volcano. We examined these signals during a particularly active eruptive two–month period between December, 1999 and February, 2000. Early seismograms are highly reproducible throughout this period. During the first month this is also the case for the coda. Approximately midway through this period, however, the seismic coda decorrelates rapidly over a period of several days. This indicates a rapid change in the scattering properties of the volcano, likely reflecting subtle changes in the near–summit magma/conduit system that would not be discernible using direct– or single–scattered seismic wave methods.

Controls on conduit magma flow dynamics during lava dome building eruptions

Lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity. We develop a transient model of the magma flow in a volcanic conduit from an open‐system magma chamber with continuous replenishment. The model incorporates gas exsolution, bubble growth, gas escape through the magma, and decompression‐induced crystallization and considers variations in magma temperature, water content, conduit diameter, phenocryst size, chamber volume, and magma rheology. Calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime, when discharge rate is low and crystals grow efficiently leading to high magma viscosity, to another stable state, when discharge rate is high and crystallization is negligible. The difference in discharge rates between these regimes can be several orders of magnitude. Periods are similar to the observed timescales and mainly depend on the chamber volume. The system shows strongly nonlinear responses to the variation of governing parameters. If magma has a Bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of Newtonian rheology. Large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature, water content, phenocryst size distribution, or conduit diameter. The system can fluctuate between low and high discharge rates with transitions to explosive activity.

Mafic microgranular enclave swarms in the Chenar granitoid stock, NW of Kerman, Iran: evidence for magma mingling

Mafic microgranular enclaves (MME) are common in the Early to Middle Miocene Chenar granitoid stock, northwest of Kerman, which is a part of Central Iranian Eocene volcanic belt. They occur individually and in homogeneous or heterogeneous swarms. The MME form a number of two-dimensional structural arrangements, such as dykes, small rafts, vortices, folded lens-shapes and late swarms. The enclaves are elongated, rounded to non-elongated and subrounded in shape and often show some size-sorting parallel to direction of flow. Variation in the elongation of enclaves could reflect variations in the viscosity of the enclave, the time available for enclave deformation and differential strain during flow of the host granitoid magma. The most effective mechanism in the formation of enclave swarms in the Chenar granitoid stock was velocity gradient-related convection currents in the granitoid magma chamber. Gravitational sorting and the break-up of heterogeneous dykes also form MME swarms. The MME (mainly diorite to diorite gabbro) have igneous mineralogy and texture, and are marked by sharp contacts next to their host granitoid rocks. The contact is often marked by a chilled margin with no sign of solid state deformation. Evidence of disequilibrium is manifested in feldspars by oscillatory zoning, resorbed rims, mantling and punctuated growth, together with overgrowth of clinopyroxene/amphibole on quartz crystals, the acicular habit of apatites and the development of Fe–Ti oxides along clinopyroxene cleavages. These observations suggest that the MMEs are derived from a hybrid-magma formed as a result of the intrusion of a mafic magma into the base of a felsic magma chamber. The density contrast between hybrid-magma and the overlying felsic magma was reduced by the release of dissolved fluids and the ascent of exsolved gas bubbles from the mafic magma into the hybrid zone. Further convection in the magma chamber dispersed the hybridized magma as globules in the upper parts of magma chamber where their entrainment and coalescence eventually formed swarms. Magma mingling between felsic and hybrid-magmas led to the formation of MMEs and swarms.

A 550‐year‐old Plinian eruption at El Chichón Volcano, Chiapas, Mexico: Explosive volcanism linked to reheating of the magma reservoir

Some 550 years ago (1320–1433 A.D.), a powerful Plinian eruption at El Chichón Volcano in southern Mexico produced a widespread pumice fall deposit. We subdivided the deposit into three parts on the basis of structural and textural characteristics, pumice lithology and density, granulometry, and petrologic‐geochemical attributes. The deposit covers an area of 1500 km2 within the 1‐cm isopach and has a minimum estimated bulk volume of 2.8 km3 (1.1 km3 dense rock equivalent (DRE)); its eruptive column reached an altitude of ∼31 km. Consideration of field evidence, the presence and nature of mafic enclaves, and chemical data strongly suggest that the 550 year B.P. eruption is linked with the intrusion of a high‐temperature basaltic magma into preexisting but stagnated trachyandesitic magma beneath El Chichón. Thorough mixing of the two magmas produced a compositionally uniform hybrid trachyandesite magma (average SiO2 55.3 wt %), which subsequently underwent crystal growth and gas exsolution, ultimately overpressurizing the zoned magmatic system to erupt explosively. On the basis of El Chichón's known eruptive history, the intrusion‐mixing event occurred sometime after the 900 year B.P. eruption. The hybrid magma had a preeruption temperature of 820–830°C and was water undersaturated (5–6 wt % H2O) at pressures of ∼2–2.5 kbar.

Hydrological response to earthquakes in the Haibara well, central Japan - II. Possible mechanism inferred from time-varying hydraulic properties

SUMMARY 28 coseismic groundwater level decreases have been observed at the Haibara well, Shizuoka prefecture, central Japan, from 1981 to 1997. These groundwater level changes cannot be explained as the poroelastic response to coseismic static strain. We use the atmospheric pressure and tidal responses of the well, rock properties measured on core samples from the same formation and pumping test results to characterize the hydraulic and mechanical properties of the aquifer. The responses of the Haibara well to the M2 Earth tide constituent and to atmospheric pressure have varied over time. In particular, increasing amplitude and decreasing phase lags were observed after the 1993 pumping test, as well as after earthquakes that caused coseismic water level changes. The tidal response, together with the surface load efficiency derived from the atmospheric pressure response, is used to estimate the mechanical properties of the aquifer. The largest amplitude of the M2 constituent, 2.2 mm, is small enough to imply that pore fluid in this system is approximately twice as compressible as water, possibly due to the presence of a small amount of exsolved gas. Diffusion of a coseismic pressure drop near the well could account for the observed time histories of the water level changes. The time histories of the water level drops are well matched by the decay of a coseismic pressure drop at least 80 m away from the well. Removal of a small amount of gas from the formation in that location might in turn explain the coseismic pressure drops.

Dynamics of active magmatic and hydrothermal systems at Taal Volcano, Philippines, from continuous GPS measurements

A dense network of continuous single‐ and dual‐frequency GPS receivers at Taal Volcano, Philippines, reveals four major stages of volcanogenic deformation: deflation, from installation in June 1998 to December 1998; inflation, from January to March 1999; deflation, from April 1999 to February 2000; and inflation, from February to November 2000. The largest displacements occurred during the February–November 2000 period of inflation, which was characterized by ∼120 mm of uplift of the center of Volcano Island relative to the northern caldera rim at average rates up to 216 mm/yr. Deformation sources were modeled using a constrained least squares inversion algorithm. The source of 1999 deflation and inflation in 2000 were modeled as contractional and dilatational Mogi point sources centered at 4.2 and 5.2 km depth, respectively, beneath Volcano Island. The locations of the inflationary and deflationary sources are indistinguishable within the 95% confidence estimates. Modeling using a running 4‐month time window from June 1999 to March 2001 reveals little evidence for source migration. We suggest that the two periods of inflation observed at Taal result from episodic intrusions of magma into a shallow reservoir centered beneath Volcano Island. Subsequent deflation may result from exsolution of magmatic fluids and/or gases into an overlying, unconfined hydrothermal system.

On magma chamber evolution during slow effusive eruptions

We explore some of the controls on the evolution of a slow effusive eruption imposed by processes in a shallow magma reservoir. We envisage that the emplacement of basaltic magma into a shallow, evolved crustal reservoir generates sufficient overpressure to trigger the slow effusive eruption of the overlying layer of evolved magma. The subsequent evolution of the system then depends on a number of factors including (1) the volatile contents of the two magmas; (2) the rate of any continuing influx of basalt; (3) the cooling/heating rates of the two layers of magma; and (4) the eruption rate of the silicic magma, where we assume that the crystals and bubbles remain in the layer in which they formed. Any exsolved volatiles in the magma increase the compressibility of the magma. In a waning system, this tends to increase the volume of material, which needs to be erupted to relieve a given overpressure by an amount of order 10–100, and hence increases the duration of the eruption. Typical calculations suggest that eruptions from magma chambers of size 1–10 km3 may persist for times of order 1–10 years if the main body of silicic magma is saturated in volatiles. In addition, cooling of the basalt leads to formation of dense crystals, which lowers the pressure. However, if the magma becomes saturated, exsolution of volatile gases occurs, tending to increase the pressure. Again, in a waning system, this tends to extend the eruption duration and increase the mass erupted from the chamber. For chamber volumes of 1–10 km3, cooling rates of 10−6 K/s can increase the eruption volume by a factor of order 10. Rapid cooling may also lead to an initial increase in the eruption rate, while the chamber pressure is controlled by the rate of production of new bubbles. If there is an influx of basalt to the chamber which is maintained during the eruption, then the evolution of the eruption is primarily dependent on this input rate, although the cooling of the basalt may increase the eruption rate to values in excess of this input rate, especially at an early stage in the eruption. The complex and nonlinear evolution of the eruption rate predicted by our simple model illustrates the sensitivity of the eruption rate to the magma volatile content and magma cooling rates. Such understanding is key in developing quantitative models for hazard assessments of slow effusive eruptions.

Exploring links between physical and probabilistic models of volcanic eruptions: The Soufrière Hills Volcano, Montserrat

Probabilistic methods play an increasingly important role in volcanic hazards forecasts. Here we show that a probability distribution characterized by competing processes provides an excellent statistical fit (>99% confidence) to repose intervals between 75 vulcanian explosions of Soufrière Hills Volcano, Montserrat in September–October, 1997. The excellent fit is explained by a physical model in which there are competing processes operating in the upper volcano conduit on different time scales: pressurization due to rheological stiffening and gas exsolution, and depressurization due to development of permeability and gas escape. Our experience with the Soufrière Hills Volcano eruption sequence suggests that volcanic eruption forecasts are improved by accounting for these different conduit processes explicitly in a single probability model.

Percolating magmas and explosive volcanism

Magma under pressure rises in conduits, depressurizes, forms bubbles by the exsolution of gas and – at void fractions (P) typically of the order of 0.7 – can fragment and explode. The study of overlapping geometrical units – percolation theory – predicts that at a critical volume fraction Pc the size of the largest simply connected region becomes infinite. We apply percolation theory to overlapping bubbles arguing that this geometric singularity at Pc implies a physical singularity in the magma rheology. This would imply that if the magma is under stress, ‐ whether it is ductile or brittle ‐ this rapid development of a network of infinitely long “bubbles” triggers fragmentation and explosion. Classical monodisperse (equal size) continuum percolation theory predicts Pc = 0.2985 ± 0.005 which is far from the observed values. However, it has recently been shown that the bubble distribution is a power law associated with a huge range of bubble sizes. Using Monte Carlo percolation simulations, we show that distributions exhibiting the empirical exponents are very efficient at “packing” the bubbles, drastically raising Pc to the value = 0.70 ± 0.05. Explosive volcanism is thus explained by singular rheology at Pc.

Nanocrystalline diamond from the Earth’s mantle underneath Hawaii

Melt inclusions in a mantle-derived garnet pyroxenite xenolith from Salt Lake Crater (Hawaiian island of Oahu) have been analysed by means of transmission electron microscopy, including energy dispersive X-ray analysis, electron energy-loss spectroscopy and electron diffraction. The melt inclusions represent trapped low-degree melt that metasomatised the Hawaiian mantle lithosphere before the eruption of the host magma. The inclusions consist of a C–Cl–S-rich glass of basaltic composition, an exsolved gas–fluid phase (mainly CO 2 with minor amounts of CO, H 2O, and some unidentified C–H and H–S species) and/or carbonate. Diamond is the major phase in the glass, ranging in size between a few and several hundred nanometres. Two varieties of nanocrystalline diamonds are identified. Polycrystalline aggregates of nanometre-size diamonds are tentatively explained as pseudomorphs after carbonate, while single crystals are suggested to have crystallised from the melt. Other nanocrystalline phases include native Fe and Cu, FeS, FeS 2, ZnS, AgS and several Ti, Nb, Zr, Ir, In, Pd-rich phases of unknown composition. Overall, the observed mineral assemblage suggests that the melt was highly reduced and strongly enriched in volatiles and trace elements of contrasting geochemical affinity. Our findings are the first evidence for a metasomatic impregnation of the Hawaiian mantle by low-degree melts. Here, we report the first occurrence of diamond in the modern oceanic mantle.