Bubble growth in rhyolitic melts: experimental and numerical investigation

 Bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically. Rhyolitic melts were hydrated at 150 MPa, 780–850  °C to uniform water content of 5.5–5.3 wt%. The pressure was then dropped and held constant at 15–145 MPa. Upon the drop bubbles nucleated and were allowed to grow for various periods of time before final, rapid quenching of the samples. The size and number density of bubbles in the quenched glasses were recorded. Where number densities were low and run duration short, bubble sizes were in accord with the growth model of Scriven (1959) for solitary bubbles. However, most results did not fit this simple model because of interaction between neighboring bubbles. Hence, the growth model of Proussevitch et al. (1993), which accounts for finite separation between bubbles, was further developed and used to simulate bubble growth. The good agreement between experimental data, numerical simulation, and analytical solutions enables accurate and reliable examination of bubble growth from a limited volume of supersaturated melt. At modest supersaturations bubble growth in hydrated silicic melts (3–6 wt% water, viscosity 10⁴–10⁶ Pa·s) is diffusion controlled. Water diffusion is fast enough to maintain steady-state concentration gradient in the melt. Viscous resistance is important only at the very early stage of growth (t<1 s). Under the above conditions growth is nearly parabolic, R²=2Dtρ_m(C₀–C_f)/ρ_g until the bubble approaches its final size. In melts with low water content, viscosity is higher and maintains pressure gradients in the melt. Growth may be delayed for longer times, comparable to time scales of melt ascent during eruptions. At high levels of supersaturation, advection of hydrated melt towards the growing bubble becomes significant. Our results indicate that equilibrium degassing is a good approximation for modeling vesiculation in melts with high water concentrations (C₀>3 wt%) in the region above the nucleation level. When the melt accelerates and water content decreases, equilibrium can no longer be maintained between bubbles and melt. Supersaturation develops in melt pockets away from bubbles and new bubbles may nucleate. Further acceleration and increase in viscosity cause buildup of internal pressure in the bubbles and may eventually lead to fragmentation of the melt. Received: 19 June 1995 / Accepted: 27 December 1995     

The evolution of bubble size distributions in volcanic eruptions

We review observations of bubble size distributions (BSDs) generated during explosive volcanic eruptions and laboratory explosions, as inferred from vesicle size distributions found in the end products. Unimodal, polymodal, exponential and power law BSDs are common, even in the absence of coalescence, and both power law and exponential distributions have been generated in the same eruption. To date theoretical models have proposed incompatible mechanisms for producing the various distributions. We here present a unifying mechanism. Data from our laboratory analogue experiments suggest that power law distributions are associated with highly non-equilibrium degassing. A numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations, where, in a non-equilibrium degassing scenario, the volatile concentration remains high. This process causes the BSD to evolve from unimodal, through exponential, into a power law. The exponent of the power law is a measure of the number of nucleation events, or the duration of the nucleation period compared with the timescale of bubble growth. The mathematical inevitability of the evolution from unimodal (Poissonian) to power law is discussed. The findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non-equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions. The process of ongoing nucleation is the mechanism whereby the volcanic system maintains near-equilibrium in the case of rapid depressurisation and slow volatile diffusion.     

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.     

Modelling the dynamics and thermodynamics of volcanic degassing

 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO₂–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s^–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO₂ chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s^–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO₂ chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998     

The influence of trace amounts of water on the viscosity of rhyolites

 As a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas. This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650  °C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300  °C, 3 kbar) obsidians. Ten natural rhyolitic obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained <0.2 wt.% H₂O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts. In addition, a marked non-linear decrease in η exists with increasing water content. In contrast to the case for peralkaline rhyolites, 0.1–0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity, largely based on synthetic systems. Received: 7 July 1997 / Accepted: 6 April 1998     

Mafic enclaves in the rhyolitic products of Lipari historical eruptions; relationships with the coeval Vulcano magmas (Aeolian Islands, Italy)

The recent finding of mafic enclaves in the Rocche Rosse (RR) lava flow, the last magmatic product on Lipari (Aeolian Islands, Italy) (AD 1230 ± 40), opens the possibility to investigate in detail the most recent magmatic system of the island, an important issue for the volcanic hazard assessment of the area. The RR lava flow is an aphyric rhyolitic coulée consisting of grey and black pumice and black and grey obsidian. Enclaves have ellipsoidal to spheroidal shape and vary from mm-sized in the central portion of the flow, to cm-sized, at the top and in the flow front, where they are also more abundant. Enclaves are shoshonitic-latitic (group A) and trachytic (group B) in composition. The mineralogy of group A consists of dominant clinopyroxene crystals with minor abundance of feldspar (plagioclase > K-feldspar), olivine and biotite, while group B is composed of feldspar (K-feldspar > plagioclase) with minor clinopyroxene, olivine and biotite. Geochemical modeling suggests that the host rhyolitic rocks could be the product of AFC (Assimilation plus Fractional Crystallization) of a magma compositionally similar to the associated shoshonitic-latitic enclaves, which, in turn, could be obtained, through an AFC process, from the primitive melts erupted as olivine hosted melt inclusions during the last 15 ka at Vulcano. The already-known last 42 ka relationship between Lipari and Vulcano Islands is here reinforced until historical time, especially for the last 1 ka. The geochemical and petrological overlap between Lipari and Vulcano is interpreted to reflect the existence of a similar magmatic system underneath the two islands. The nearly aphyric RR rhyolites are interpreted to be the products of a superheated (temperature far above the liquidus) and initially water-undersaturated magma that underwent degassing close to the surface inhibiting microlite crystallization.     

Vesiculation path of ascending magma in the 1983 and the 2000 eruptions of Miyakejima volcano, Japan

The vesiculation of magma during the 1983 eruption of Miyakejima Volcano, Japan, is discussed based on systematic investigations of water content, vesicularity, and bubble size distribution for the products. The eruption is characterized by simultaneous lava effusion and explosive sub-plinian (‘dry’) eruptions with phreatomagmatic (‘wet’) explosions. The magmas are homogeneous in composition (basaltic andesite) and in initial water content (H₂O = 3.9±0.9 wt%), and residual groundmass water contents for all eruption styles are low (H₂O <0.4 wt%) suggestive of extensive dehydration of magma. For the scoria erupted during simultaneous ‘dry’ and ‘wet’ explosive eruptions, inverse correlation was observed between vesicularity and residual water content. This relation can be explained by equilibrium exsolution and expansion of ca. 0.3 wt% H₂O at shallow level with different times of quenching, and suggests that each scoria with different vesicularity, which was quenched at a different time, provides a snapshot of the vesiculation process near the point of fragmentation. The bubble size distribution (BSD) varies systematically with vesicularity, and total bubble number density reaches a maximum value at vesicularity Φ ∼ 0.5. At Φ  ∼ 0.5, a large number of bubbles are connected with each other, and the average thickness of bubble walls reaches the minimum value below which they would rupture. These facts suggest that vesiculation advanced by nucleation and growth of bubbles when Φ < 0.5, and then by expansion of large bubbles with coalescence of small ones for Φ > 0.5, when bubble connection becomes effective. Low vesicularity and low residual water content of lava and spatter (Φ  < 0.1, H₂O  < 0.1 wt%), and systematic decrease in bubble number density from scoria through spatter to lava with decrease in vesicularity suggest that effusive eruption is a consequence of complete degassing by bubble coalescence and separation from magma at shallow levels when magma ascent rate is slow.

Crystallization of microlites and degassing during magma ascent: Constraints on the fluid mechanical behavior of magma during the Tenjo Eruption on Kozu Island, Japan

Bubble and crystal textures provide information with regard to the kinetics of the vesiculation and crystallization processes. They also provide insights into the fluid mechanical behavior of magma in a conduit. We performed textural (bubble and crystal) and compositional analyses of pyroclasts that were obtained from the Tenjo pyroclastic flow, which resulted on account of the eruption in 838 A.D. on Kozu Island, about 200 km south of Tokyo, Japan. Pyroclasts in one flow unit (300∼2,060 kg/m³; average density 1330 kg/m³) can be classified into three types on the basis of vesicle textures. Type I pyroclasts have small isolated spherical bubbles with higher vesicularities (67–77 vol.%) and number density (10.8–11.7 log m⁻³). Type II pyroclasts have vesicularities similar to type I (61–69 vol.%), but most bubbles exhibit evidences of bubble coalescence, and lower number densities than type I (8.9–9.5 log m⁻³). Type III pyroclasts contain highly deformed bubbles with lower vesicularities (16–34 vol.%) and number densities (8.2–9.0 log m⁻³). The microlite volume fraction (DRE converted) also changes consistently across type I, type II, and type III as 0.06, 0.08, and 0.10–0.15, respectively. However, the number density of the microlites remains nearly invariant in all the pyroclast types. These facts indicate that the variation in the microlite volume fraction is controlled not by the number density (i.e., nucleation process), but by the size (i.e., growth process); the growth history of each type of microlite was different. Water content determinations show that the three types of pumices have similar H₂O contents (2.6±0.2 wt%). This fact implies that all three types were quenched at nearly the same depth (35±5 MPa, assuming that the magma was water-saturated) in the conduit. If the crystal sizes are limited only by growth time, a variation in this parameter can be related to the residence time, which is attributed to the flow heterogeneity in the conduit. By assuming a laminar Poiseuille-type flow, these textural observations can be explained by the difference in ascent velocity and shearing motion across the conduit, which in turn results in the differences in growth times of crystals, degrees of deformation, and bubble coalescence. Consequently, for crystals in the inner part of the conduit, the crystal growth time from nucleation to quenching is shorter than that near the conduit wall. The vesicle texture variation of bubbles in types I, II, and III results from the difference in the deformation history, implying that the effect of degassing occurred primarily towards the conduit wall.     

Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā

 Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150  °C) cooled and crystallized rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14  °C over the first 2 km of transport. At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50  °C/h. Cooling rates this high can be explained by radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel. Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50% per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼10⁴/cm³s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137  °C and crystallinity of ∼15%, and a flow surface temperature of ∼1100  °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins, where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization. Received: 3 September 1998 / Accepted: 12 April 1999     

Strain-induced magma degassing: insights from simple-shear experiments on bubble bearing melts

Experiments have been performed to determine the effect of deformation on degassing of bubble-bearing melts. Cylindrical specimens of phonolitic composition, initial water content of 1.5 wt.% and 2 vol.% bubbles, have been deformed in simple-shear (torsional configuration) in an internally heated Paterson-type pressure vessel at temperatures of 798–848 K, 100–180 MPa confining pressure and different final strains. Micro-structural analyses of the samples before and after deformation have been performed in two and three dimensions using optical microscopy, a nanotomography machine and synchrotron tomography. The water content of the glasses before and after deformation has been measured using Fourier Transform Infrared Spectroscopy (FTIR). In samples strained up to a total of γ ∼ 2 the bubbles record accurately the total strain, whereas at higher strains (γ ∼ 10) the bubbles become very flattened and elongate in the direction of shear. The residual water content of the glasses remains constant up to a strain of γ ∼ 2 and then decreases to about 0.2 wt.% at γ ∼ 10. Results show that strain enhances bubble coalescence and degassing even at low bubble volume-fractions. Noticeably, deformation produced a strongly water under-saturated melt. This suggests that degassing may occur at great depths in the volcanic conduit and may force the magma to become super-cooled early during ascent to the Earth’s surface potentially contributing to the genesis of obsidian.     

Simulating bubble number density of rhyolitic pumices from Plinian eruptions: constraints from fast decompression experiments

Decompression experiments of a crystal-free rhyolitic liquid with ≈ 6.6 wt. % H₂O were carried out at a pressure range from 250 MPa to 30–75 MPa in order to characterize effects of magma ascent rate and temperature on bubble nucleation kinetics, especially on the bubble number density (BND, the number of bubbles produced per unit volume of liquid). A first series of experiments at 800°C and fast decompression rates (10–90 MPa/s) produced huge BNDs (≈ 2 × 10¹⁴ m⁻³ at 10 MPa/s ; ≈ 2 × 10¹⁵ m⁻³ at 90 MPa/s), comparable to those in natural silicic pumices from Plinian eruptions (10¹⁵–10¹⁶ m⁻³). A second series of experiments at 700°C and 1 MPa/s produced BNDs (≈ 9×10¹² m⁻³) close to those observed at 800°C and 1 MPa/s (≈ 6 × 10¹² m⁻³), showing that temperature has an insignificant effect on BNDs at a given decompression rate. Our study strengthens the theory that the BNDs are good markers of the decompression rate of magmas in volcanic conduits, irrespective of temperature. Huge number densities of small bubbles in natural silicic pumices from Plinian eruptions imply that a major nucleation event occurs just below the fragmentation level, at which the decompression rate of ascending magmas is a maximum (≥ 1 MPa/s).     

Volcanological evolution of the Rivi–Capo Volcanic Complex at Salina,Aeolian Islands: magma storage processes and ascent dynamics

Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ～65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.     

The impact of pre-existing gas on the ascent of explosively erupted magma

Most, if not all, magmas contain gas bubbles at depth before they erupt. Those bubbles play a crucial role in eruption dynamics, by allowing magma to degas, which causes the magma to accelerate as it ascends towards the surface. There must be a limit to that acceleration, however, because gas bubbles cannot grow infinitely fast. To explore that limit, a series of experiments was undertaken to determine the maximum rate at which bubbly high-silica rhyolite can decompress. Rhyolite melt that was hydrated at 150 MPa with ~5.3 wt.% dissolved water and contained 7 to 18 vol.% bubbles can degas in equilibrium at 875°C when decompressed at rates up to 1.2 MPa s⁻¹ from 150 to 78 MPa, and up to 1.8 MPa s⁻¹ when decompressed further to 42 MPa. In contrast, that same rhyolite cannot degas in equilibrium at 750°C if decompressed faster than 0.015–0.025 MPa s⁻¹. When combined with other published experiments, the maximum rate of decompression for equilibrium degassing is found to increase by a factor of ten for every 50–75°C increase in temperature. When compared to predictions from conduit flow models that assume equilibrium degassing, it is found that such models greatly over-estimate the rate at which relatively cold rhyolite can decompress, whereas that assumption is largely correct for hot rhyolite, and thus for most other magmas, all of which are less viscous than rhyolite. In addition, most bubbles that were 20–30 μm in size at high pressure were lost from the population at low pressure. That absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre-eruptive bubbles, even if small bubbles were originally present at depth.     

Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

3 [magma volume (DRE): 24 ± 5 km³]. The main phase (ca. 95 vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25 km and thus entered the stratosphere. A late phase (5 vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain  ∼ 3, and the trachytes 10–20 vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H₂O concentration of the comenditic matrix glass is 1.5 wt.% (probably largely secondary) and of the corresponding melt inclusions  ∼ 5.2 wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992 ppm F) and fluorine-poor group (2431 ppm F), averaging 3853 ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of  ≥ 250 ppm S. The difference between pre- and post-eruptive concentration of H₂O is at least 3.7 ± 0.6 wt.% H₂O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 ± 90 ppm Cl, and 1863 ± 280 ppm and 302 ± 40 ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455 ppm S) and the detection limit, resulting in 205 ± 40 ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 ± 453 megatons for H₂O, 45 ± 10 megatons for chlorine, 42 ± 11 megatons for fluorine, and 2 ± 0.6 megatons for sulfur. The 969 ± 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000 years, is thought to have had a substantial but possibly short-lived effect on climate. Received: 25 July 1998 / Accepted: 8 September 1999     

Bubble growth in highly viscous melts: theory, experiments, and autoexplosivity of dome lavas

We examine the physics of growth of water bubbles in highly viscous melts. During the initial stages, diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the initial pressure at nucleation. Growth is controlled by melt viscosity and supersaturation pressure and radial growth under constant pressure is approximately exponential. At later stages, internal pressure falls, radial growth decelerates and follows the square-root of time. At this stage it is controlled by diffusion. The time of transition between the two stages is controlled by the decompression, melt viscosity and the Peclet number of the system. The model closely fit experimental data of bubble growth in viscous melts with low water content. Close fit is also obtained for new experiments at high supersaturation, high Peclet numbers, and high, variable viscosity. Near surface, degassed, silicic melts are viscous enough, so that viscosity-controlled growth may last for very long times. Using the model, we demonstrate that bubbles which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation. We suggest that tiny bubbles observed in melt pockets between large bubbles in pumice represent a second nucleation event shortly before or after fragmentation. The presence of such bubbles is an indicator of the conditions at fragmentation. The water content of lavas extruded at lava domes is a key factor in their evolution. Melts of low water content (<0.2 wt%) are too viscid and bubbles nucleated in them will not grow to an appreciable size. Bubbles may grow in melts with 0.4 wt% water. The internal pressure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome rocks and the formation of pyroclastic flows.     

Equilibrium and disequilibrium degassing of a phonolitic melt (Vesuvius AD 79 “white pumice”) simulated by decompression experiments

Equilibrium and disequilibrium degassing of a volatile phase from a magma of K-phonolitic composition was investigated to assess its behavior upon ascent. Decompression experiments were conducted in Ar-pressurized externally heated pressure vessels at superliquidus temperature (1050 °C), in the pressure range 10–200 MPa using pure water as fluid phase. All experiments were equilibrated at 200 MPa and then decompressed to lower pressures with rates varying from 0.0028 to 4.8 MPa/s. Isobaric saturation experiments were performed at the same temperature and at 900–950 °C to determine the equilibrium water solubility in the pressure range 30–250 MPa. The glasses obtained from decompression experiments were analyzed for their dissolved water content, vesicularity and bubble size distribution. All decompressed samples presented a first event of bubble nucleation at the capsule–melt interface. Homogeneous bubble nucleation in the melt only occurred in fast-decompressed experiments (4.8 and 1.7 MPa/s), for ΔP ≅ 100 MPa. For these decompression rates high water over-saturations were maintained until a rapid exsolution was triggered at ΔP > 150 MPa. For slower rates (0.0028, 0.024, 0.17 MPa/s) the degassing of the melt took place by diffusive growth of the bubbles nucleating at the capsule–melt interface. This process sensibly reduced water over-saturation in the melt, preventing homogeneous nucleation to occur. For decompression rates of 0.024 and 0.17 MPa/s low water over-saturations were attained in the melt, gradually declining toward equilibrium concentrations at low pressures. A near-equilibrium degassing path was observed for a decompression rate of 0.0028 MPa/s. Experimental data combined with natural pumice textures suggest that both homogeneous and heterogeneous bubble nucleations occurred in the phonolitic magma during the AD 79 Vesuvius plinian event. Homogeneous bubble nucleation probably occurred at a depth of ∼ 3 km, in response to a fast decompression of the magma during the ascent.     

Effects of eruption and lava drainback on the H2O contents of basaltic magmas at Kilauea Volcano

 Lava drainback has been observed during many eruptions at Kilauea Volcano: magma erupts, degasses in lava fountains, collects in surface ponds, and then drains back beneath the surface. Time series data for melt inclusions from the 1959 Kilauea Iki picrite provide important evidence concerning the effects of drainback on the H₂O contents of basaltic magmas at Kilauea. Melt inclusions in olivine from the first eruptive episode, before any drainback occurred, have an average H₂O content of 0.7±0.2 wt.%. In contrast, many inclusions from the later episodes, erupted after substantial amounts of surface degassed lava had drained back down the vent, have H₂O contents that are much lower (≥0.24 wt.% H₂O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39–0.51 wt.% H₂O in tephra from high fountains to 0.10–0.28 wt.% H₂O in spatter from low fountains. The low H₂O contents of many melt inclusions from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal that prior to crystallization of the enclosing olivine host, the melts must have exsolved H₂O at pressures substantially less than those in Kilauea's summit magma reservoir. Such low-pressure H₂O exsolution probably occurred as surface degassed magma was recycled by drainback and mixing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H₂O for differentiated tholeiitic magma in Kilauea's summit magma storage reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995) and melt inclusions from Kilauea Iki demonstrate that primary Kilauean tholeiitic magma has an H₂O/K₂O mass ratio of ∼1.3. At transition zone and upper mantle depths in the Hawaiian plume source, H₂O probably resides partly in a small amount of hydrous silicate melt. Received: 31 March 1997 / Accepted: 17 November 1997     

Sulfur,chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

 The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H₂SO₄ aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods. Received: 30 May 1995 / Accepted: 19 April 1996     

Experimental constraints on degassing and permeability in volcanic conduit flow

This study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions: degassing and permeability in magmas. We studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature. From the data collected, we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium. Equilibrium bubble growth occurs when the decompression rate is slower than 0.1 MPa s^–1, while higher rates cause porosity to deviate rapidly from equilibrium, defining the first disequilibrium regime of degassing. If the deviation is large enough, a critical threshold of super-saturation is reached and bubble growth accelerates, defining the second disequilibrium regime. We studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions. Under these open conditions, we observed that bubbles start to coalesce at ~43 vol% porosity, regardless of decompression rate. Coalescence profoundly affects bubble texture and size distributions, and induces the melt to become permeable. We determined coalescence to occur on a time scale (~180 s) independent of decompression rate. We parameterized and incorporated our experimental results into a 1D conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust. Compared to previous models that assume equilibrium degassing of the melt during ascent, the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by ~25%, the acceleration at high porosities (>50 vol%) by a factor 5, and the associated decompression rate by an order of magnitude. The integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed, and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape.Editorial responsibility: D. Dingwell     

Heterogeneous bubble nucleation and disequilibrium H2O exsolution in Vesuvius K-phonolite melts

This study focuses on constraining bubble nucleation and H₂O exsolution processes in alkalic K-phonolite melts, using “white pumice” of the 79 AD eruption of Vesuvius as starting material. The first set of experiments consisted of H₂O solubility runs at 1153 to 1250 K and pressures between 50 and 200 MPa, to constrain equilibrium water concentrations along the decompression pathways. The decompression experiments were equilibrated with H₂O at 150 MPa and 1173 and 1223 K, and then decompressed at 3 to 17 MPa/s before rapid quenching. Experiments nucleated bubbles within the first 50 MPa pressure drop, producing maximum bubble number densities (N_V), corrected to melt volume, of 3.8 × 10¹⁴ m^− 3 at 1173 K and 4.3 × 10¹³ m^− 3 at 1223 K. Most bubbles were not visibly attached to crystals, except for a subset attached to pyroxenes primarily in the 1173 K experiments. When compared with prior bubble nucleation studies, the reduced nucleation ΔP and relatively low N_V observed indicate predominantly a heterogeneous nucleation mechanism. Melt–vapor–crystal wetting angles measured in 1173 K experiments from bubbles attached to pyroxene crystals are 36 to 69°, which are similar to those measured on titanomagnetite crystals in calc-alkaline dacite melts. The 1223 K experiments have porosities and water concentrations that largely track equilibrium, despite the rapid decompression rate. The 1173 K experiments deviate strongly from equilibrium trends in both porosity and water concentration, and slower H₂O diffusion rates are likely the cause of the inhibited bubble growth. Bubble number densities from 79 AD Vesuvius natural EU2 pumice are relatively high (2 to 4 × 10¹⁵ m^− 3; [Gurioli, L., Houghton, B.F., Cashman, K.V., Cioni, R., 2005. Complex changes in eruption dynamics during the 79 AD eruption of Vesuvius. Bull. Volcanol. 67: 144–159.]) when corrected to vesicularity. In comparison, corrected N_V's from homogeneous and heterogeneous bubble nucleation experiments from this study and prior work are at least factor of 5 lower, indicating perhaps that the natural magmas initially nucleated bubbles in the presence of CO₂. The disequilibrium H₂O exsolution seen in the 1173 K experiments indicates that inhibited bubble growth could lead to delayed exsolution in the conduit in cooler K-phonolite magmas.