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.

Lava balloons—peculiar products of basaltic submarine eruptions

Between December 1998 and April 2001, a submarine basaltic eruption occurred west of Terceira Island, Azores (Portugal) in water depths between 300 and 1,000?m. Physical evidence for the eruption was provided by the periodic occurrence of hot lava “balloons” floating on the sea surface. The balloons consisted of a large gas-filled cavity surrounded by a thin shell (a few centimetres thick). The shells of the collected balloons are composed of two layers, termed the outer layer and the inner layer, defined by different bubble number density, bubble sizes and crystal content. The inner layer is further divided into three sublayers defined by more subtle differences in vesicularity. The outer layer is glassy, golden-coloured and highly porous. It shows signs of fluidal deformation and late-stage extension cracks. Interstitial glass contains 0.29?wt% H₂O and CO₂ is below detection. Melt inclusions contain up to 1.18?wt% H₂O and 1,500?ppm CO₂ (from different inclusions). Cooling rates of the outermost glass of the outer layer are found to be as high as 1,259?K/s. During ascent of low viscosity magma to the ocean floor, volatiles, dominated by CO₂, exsolved from the magma (melt + crystals). The buoyancy of the vapour phase that accumulated below a thin crust on lava ponded at the vent caused bulging and ultimately cracking of the crust. This allowed large bubbles (central cavity) surrounded by a film of vesicular magma (balloon shell) to leak into the water column. On contact with the seawater, the outermost part of the outer layer of the shell hyperquenched. If an entirely closed shell was produced during detachment, the trapped gas inside allowed buoyant rise. Only balloons with the right balance of physical properties (e.g. size and bulk density) rose all the way to the sea surface.     

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.     

Equilibrium calculations in volcanic gaseous systems

Equilibria calculations of high-temperature volcanic gases from lava lakes are carried out on the basis of best volcanic gas samples. The equilibrium gas composition at temperatures from 800° to 1400°K and pressures up to 25 kilobars (in ideal gas system) was calculated using the free energy minimization model as well as the Newton-Raphson methods. It is shown that the juvenile «magmatic gas » of basaltic magma consists of three components: H₂O, SO₂, CO₂; the water vapor being about 60%. The increase of temperature under constant pressure results in the increase of the SO₂ concentration and in the simultaneous decrease of H₂S. Under the same conditions the ratios CO/CO₂ and H₂/H₂O are found to increase. Methane cannot be a component of «magmatic gas» corresponding to the elemental composition of basaltic lava gases. The calculated values of \(P_{O_2 } \) are in good agreement with the experimental data obtained from direct measurements of \(P_{O_2 } \) in lava lakes and experiments with basaltic melts.     

Hydrogen emissions from Erebus volcano, Antarctica

The continuous measurement of molecular hydrogen (H₂) emissions from passively degassing volcanoes has recently been made possible using a new generation of low-cost electrochemical sensors. We have used such sensors to measure H₂, along with SO₂, H₂O and CO₂, in the gas and aerosol plume emitted from the phonolite lava lake at Erebus volcano, Antarctica. The measurements were made at the crater rim between December 2010 and January 2011. Combined with measurements of the long-term SO₂ emission rate for Erebus, they indicate a characteristic H₂ flux of 0.03?kg s^–1 (2.8?Mg? day^–1). The observed H₂ content in the plume is consistent with previous estimates of redox conditions in the lava lake inferred from mineral compositions and the observed CO₂/CO ratio in the gas plume (～0.9 log units below the quartz–fayalite–magnetite buffer). These measurements suggest that H₂ does not combust at the surface of the lake, and that H₂ is kinetically inert in the gas/aerosol plume, retaining the signature of the high-temperature chemical equilibrium reached in the lava lake. We also observe a cyclical variation in the H₂/SO₂ ratio with a period of ～10?min. These cycles correspond to oscillatory patterns of surface motion of the lava lake that have been interpreted as signs of a pulsatory magma supply at the top of the magmatic conduit.     

Probing the magma plumbing of Erebus volcano,Antarctica, by open-path FTIR spectroscopy of gas emissions

Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H₂O, CO₂, CO, SO₂, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO₂ and H₂O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H₂O/CO₂ proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO₂ flux determinations and measured gas ratios. In the case of CO₂ and water, ∼ 1 and ∼ 0.4 m³ s^− 1, respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water.     

Erta'ale lava lake: heat and gas transfer to the atmosphere

Data on uncontaminated samples of volcanic gases can be counted on the fingers of one hand, yet estimation of total volcanic gas flow cannot be made without such data. In this paper the flux of gas from the lava lake to the atmosphere is calculated by a heat budget based on the excess heat loss caused by combustion of H₂ and CO and by the mass rate of loss of other gases on the basis of their ratios to H₂ and CO in the unoxidized gas samples. The estimated rates of loss of H₂O, CO₂, SO₂ and HCl are consistent with the rate of loss of heat if this heat is generated by crystallization and if the initial magma contains concentrations of gas appropriate for submarine basalt from oceanic ridges. The moderate activity of permanent degassing from the two active lava ponds studied gives a lower flux than that of other volcanoes.     

Mechanism of energy transfer in the lava lake of Niragongo (Zaire), 1959–1977

Heat and mass transfer rates were studied at the Niragongo lava lake during two expeditions directed by H. Tazieff in 1959 and 1972. The results of this study are as follows:Heat is transferred to the surface of the lake by the movement of lava; gas discharge is a result and not the cause of convection. The chemical composition of the gases and magma has changed very little between 1959 and 1972, whereas the mass and energy outputs differ by an order of magnitude. In 1977 a catastrophic explosion seems to have been caused by tectonic factors, stopping the slow convection of magma under the volcano and hence reducing surface manifestations in the form of the lava lake and escaping fumarolic and magmatic gases. The gas discharge was, in tons day⁻¹, 5000 for H₂O, 11,000 for CO₂, 1000 for SO₂ in 1959, and in 1972 7700 for H₂O, 180,000 for CO₂ and 23,000 for SO₂. These values correspond to an energy transfer of 0.9 × 10⁹ W in 1959 and 16 × 10⁹ W in 1972.     

Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano,Ecuador

Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4–0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H₂O contents of >∼0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H₂O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H₂O content of 3.1 wt% establishes the maximum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H₂O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO₂ (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open-system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressurization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.     

Deep sea explosive activity on the Mid-Atlantic Ridge near 34°50′N: Magma composition, vesicularity and volatile content

Submersible observations and sampling were carried out in the rift valley of the Mid-Atlantic Ridge (MAR) near 34°40′N–35°N. The 4-km-wide rift valley consists of a Neo Volcanic Zone (NVZ) (<1 km wide) bounded at the west by a Median Ridge (MR) (5 km wide and 20 km long) and at the east by the first scarps of the eastern wall. The MR and the eastern wall are characterized by volcanic cones about 200–300 m height culminating at depths of 1500–1900 m which are made up of volcaniclastic deposits (pyroclasts and hyaloclasts) suggestive of explosive volcanism. Based on their surface morphology, degree of vesicularity, and composition, the erupted deposits are classified into four groups: (1) poorly vesicular (<15% vesicles) N- and T-MORBs (K/Ti <0.25, Na₂O+K₂O<2.9%) consisting of sheet flows and pillows formed during fissure eruptions in the NVZ at 2000–2300 m depths; (2) vesicular (15–30% vesicles) E-MORBs (K/Ti=0.25−0.45,Na₂O+K₂O>2.8−3.2%) and alkali basalts (K/Ti=0.45−0.70,Na₂O+K₂O>3.3−4) made up mainly of pillows; (3) highly vesicular (>35% vesicles) pillow lava and pyroclastic (scoria-like) alkali basalts (K/Ti>0.45−0.80,Na₂O+K₂O>3−4%); and (4) hyaloclastites consisting of glassy shards of alkali basalt composition. The total water and carbon contents of the deposits increase with the incompatible element concentrations. The estimated initial H₂O content for the N- and T-MORBs is less than 3500 ppm, whereas for the E-MORBs and alkali basalts the H₂O content is near 4000 and 7000 ppm, respectively. While the H₂O is mainly in the melt, the carbon is in the form of CO₂ filling vesicles. The vesicles are formed from magma with an initial carbon content of 1000–3000 for the N- and T-MORBs, 3000–6500 ppm for the E-MORBs and higher than 1 wt% for the alkali basalts.The various lava types were derived from a heterogeneous mantle source composed of enriched and depleted components during sequential eruptions of N-, T- and E-MORBs and alkali basalts (K/Ti>0.7). The amount of CO₂ and H₂O in equilibrium with the dissolved species present in the vesicles indicates that CO₂ (XCO₂=1−0.84) was the main exsolved compound responsible for bubble nucleation. The increase in the degree of vesicularity and pressure of the volatile phases is mainly due to the early exsolution of CO₂ from an alkali melt. The exsolution of significant amounts of dissolved water occurred only for the alkali basalt a few hundred meters beneath the seafloor and contributed to late bubble expansion. This subsequent addition of magmatic water to the vesicles increased the gas pressure and triggered explosions. An alternative hypothesis for the explosive volcanism is based on field observations. During crater collapsed, seawater could have been trapped in fractured volcanic conduits and later sealed by hydrothermal fluid circulation and precipitation. In such an environment, this seawater will be heated and vaporized during renewed magmatic upwelling. Both scenarios give rise to fragmented debris (hyaloclasts and pyroclasts) and the explosive events create turbulent flows followed by differential gravity settling of the particles (shards versus lapilli) through the seawater.     

Magma degassing and basaltic eruption styles: a case study of 2000 year BP Xitle volcano in central Mexico

To investigate the relationship between volatile abundances and eruption style, we have analyzed major element and volatile (H₂O, CO₂, S) concentrations in olivine-hosted melt inclusions in tephra from the 2000 yr BP eruption of Xitle volcano in the central Trans-Mexican Volcanic Belt. The Xitle eruption was dominantly effusive, with fluid lava flows accounting for 95% of the total dense rock erupted material (1.1 km³). However, in addition to the initial, Strombolian, cinder cone-building phase, there was a later explosive phase that interrupted effusive activity and deposited three widespread ash fall layers. Major element compositions of olivine-hosted melt inclusions from these ash layers range from 52 to 58 wt.% SiO₂, and olivine host compositions are Fo_84–86. Water concentrations in the melt inclusions are variable (0.2–1.3 wt.% H₂O), with an average of 0.45±0.3 (1σ) wt.% H₂O. Sulfur concentrations vary from below detection (50 ppm) to 1000 ppm but are mostly ≤200 ppm and show little correlation with H₂O. Only the two inclusions with the highest H₂O have detectable CO₂ (310–340 ppm), indicating inclusion entrapment at higher pressures (700–900 bars) than for the other inclusions (≤80 bars). The low and variable H₂O and S contents of melt inclusions combined with the absence of less soluble CO₂ indicates shallow-level degassing before olivine crystallization and melt inclusion formation. Olivine morphologies are consistent with the interpretation that most crystallization occurred rapidly during near-surface H₂O loss. During cinder cone eruptions, the switch from initial explosive activity to effusive eruption probably occurs when the ascent velocity of magma becomes slow enough to allow near-complete degassing of magma at shallow depths within the cone as a result of buoyantly rising gas bubbles. This allows degassed lavas to flow laterally and exit near the base of the cone while gas escapes through bubbly magma in the uppermost part of the conduit just below the crater. The major element compositions of melt inclusions at Xitle show that the short-lived phase of renewed explosive activity was triggered by a magma recharge event, which could have increased overpressure in the storage reservoir beneath Xitle, leading to increased ascent velocities and decreased time available for degassing during ascent.     

Magmatic degassing at Erta 'Ale volcano,Ethiopia

Here we report measurements of the chemical composition and flux of gas emitted from the central lava lake at Erta 'Ale volcano (Ethiopia) made on 15 October 2005. We determined an average SO₂ flux of ∼ 0.69 ± 0.17 kg s^− 1 using zenith sky ultraviolet spectroscopy of the plume, and molar proportions of magmatic H₂O, CO₂, SO₂, CO, HCl and HF gases to be 93.58, 3.66, 2.47, 0.06, 0.19 and 0.04%, respectively, by open-path Fourier transform infrared (FTIR) spectrometry. Together, these data imply fluxes of 7.3, 0.7, 0.008, 0.03 and 0.004 kg s^− 1 for H₂O, CO₂, CO, HCl and HF, respectively. These are the first FTIR spectroscopic observations at Erta 'Ale, and are also some of the very few gas measurements made at the volcano since the early 1970s (Gerlach, T.M., 1980b. Investigation of volcanic gas analyses and magma outgassing from Erta 'Ale lava lake, Afar, Ethiopia. Journal of Volcanology and Geothermal Research, 7(3–4): 415–441). We identify significant increases in the proportion of H₂O in the plume with respect to both CO₂ and SO₂ across this 30-year interval, which we attribute to the depletion of volatiles in magma that sourced effusive eruptions during the early 1970s and/or to fractional magma degassing between the two active pit craters located in the summit caldera.     

Geobarometry of ultramafic xenoliths from Loihi Seamount,Hawaii, on the basis of CO2 inclusions in olivine

Abundant fluid inclusions in olivine of dunite xenoliths (～1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO₂ (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions.Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO₂ liquid and gas, with an aggregate ? = ～ 0.5–0.75 g cm^?3, and represent trapped globules of dense supercritical CO₂ (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO₂ blebs. Spherical sulfide blebs having widely variable volume ratios to CO₂ and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present.Assuming olivine growth at ～ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO₂P-V-T data indicates that the primary inclusions were trapped at ～ 220–470 MPa (2200–4700 bars), or ～ 8–17 km depth in basalt magma of ? = 2.7 g cm^?3. Because the temperature cannot change much during the rise to eruption, the range of CO₂ densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.     

Interpretation of volcanic gas data from tholeiitic and Alkaline Mafic Lavas

The analyses of approximately 100 high temperature gas samples from erupting lavas of Surtsey, Erta Ale, Ardoukoba, Kilauea, Mount Etna and Nyiragongo exhibit erratic compositions resulting from analytical errors, condensation effects, reactions with sampling devices, and contamination by atmospheric gases, meteoric water and organic material. Computational techniques have been devised to restore reported analyses to compositions representative of the erupted gases. The restored analyses show little evidence of short-term variations. The principal species are H₂O, CO₂, SO₂, H₂, CO, H₂S, S₂, and HCl. The O₂ fugacities range from nickel-nickel oxide to a half order of magnitude below quartz-magnetite-fayalite. There is no evidence for a unique magmatic gas composition; instead, the erupted gases show regular compositional trends characterized by decreasing CO₂ with progressive outgassing. The gases from more alkaline lavas (Etna, Nyiragongo) are distinctly richer in CO₂, while those from less alkaline (Surtsey) or tholeiitic lavas (Erta Ale, Ardoukoba) tend to be richer in H₂O. Kilauean gases range from CO₂-rich to H₂O-rich. The total sulfur contents of the erupted gases show an excellent positive correlation with lava O₂ fugacity. All restored analyses are significantly lower in H₂O and enriched in sulfur and CO₂ compared to the «excess volatiles».     

Evaluation of volcanic gas analysis from surtsey volcano, iceland 1964–1967

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H₂O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H₂O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H₂S, S₂, and H₂. The estimated gas compositions are relatively CO₂-rich, low in total sulfur and reduced. They contain approximately 35–50% CO₂ 45–55% H₂O, 1–2% SO₂, 1–2% H₂., 2–3% CO, 1.5–2.5% H₂S, 0.5% S₂ and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f_O2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f_O2 buffered by the rock, results in gas compositions very rich in CH₄ (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH₄-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.     

Micromodel investigations of CO2 exsolution from carbonated water in sedimentary rocks

In this study, carbon dioxide exsolution from carbonated water is directly observed under reservoir conditions (9 MPa and 45 °C). Fluorescence microscopy and image analysis are used to quantitatively characterize bubble formation, morphology, and mobility. Observations indicate the strong influence of interfacial tension and pore-geometry on bubble growth and evolution. Most of the gas exhibits little mobility during the course of depressurization and clogs water flow paths. However, a snap-off mechanism mobilizes a small portion of the trapped gas along the water flow paths. This feature contributes to the transport of the dispersed exsolved gas phase and the formation of intermittent gas flow. A new definition of critical gas saturation is proposed accordingly as the minimum saturation that snap-off starts to produce mobile bubbles. Low mobility of the water phase and CO₂ phase in exsolution is explained by formation of dispersed CO₂ bubbles which block water flow and lack the connectivity to create a mobile gas phase.     

CO2-Hydrocarbon fluids of the Jade hydrothermal field in the Okinawa trough: fluid inclusion evidence

The Okinawa trough is a spreading back-are basin featuring emitting hydrothermal solutions (black chimney type) and modem sulfide precipitation on the sea floor. The study of fluid inclusions in water-rock interaction products in the Jade hydrothermal field indicates that the deep hydrothermal system beneath the sea floor is fairly rich in gas and there are two independent and coexisting fluids-CO₂-hydrocarbon fluid and salt aqueous fluid. On the whole, the composition of CO₂-hydrocarbon fluid inclusions is similar to that of the fluid inclusions in natural gas fields. The dominant composition of the inclusions in aqueous fluid is H₂O with CO₂ and CH₄ being oversaturated. The salt aqueous fluid of the Jade hydrothermal system might be emitted through a black chimney, whereas CO₂-rich fluids discharge CO₂ bubbles and CO₂ hydrate through fissures. Hydrocarbons in gas phase or in fluid might be enclosed somewhere under the sea. Large storage of CO₂-CH₄-H₂S gas or fluid and reaction of this gas or fluid with salt water will lead to commercial sulfide deposits.     

Peridotite-CO2-H2O and the low-velocity zone

The properties of the seismic low-velocity zone are consistent with incipient melting of mantle peridotite. Vapor-absent melting of amphibole-peridotite has been used to model the low-velocity zone, but evidence that CO₂ exists in the upper mantle indicates that peridotite-CO₂-H₂O would be a better model. The divariant solidus surface for peridodite-CO₂-H₂O is traversed by a series of univariant lines marking the intersections of divariant subsolidus reactions involving dolomite or magnesite, amphibole, or phlogopite (other hydrous minerals are neglected in this treatment), or combinations of these. The vapor phase compositions are buffered to specific values, which limits the range of vapor compositions that can coexist with peridotite at various pressures. Below about 30 kbar, the vapor phase is buffered by the melting of amphibole-peridotite, with composition ranging from H₂O to high CO₂/H₂O. Above about 25 kbar, the vapor phase is buffered by the melting of dolomite-peridotite, with composition ranging from CO₂ to high H₂O/CO₂ at pressures above 30 kbar. The buffered curve for phlogopite-peridotite intersects the dolomite-peridotite curve, generating another line for phlogopite-dolomite-peridotite; the strong buffering capacity of dolomite forces the vapor on this line to high H₂O/CO₂. Near the buffered curve for the solidus of partly carbonated peridotite there is a temperature maximum on the peridotite-vapor solidus surface. On the CO₂ side of the maximum, above 26 kbar, CO₂/H₂O is greater in liquid than in vapor; on the H₂O side of this maximum, and at all pressures below 26 kbar, CO₂/H₂O is greater in vapor than in liquid. The suboccanic low-velocity zone is caused by incipient melting of amphibole-peridotite in the presence of vapor with high CO₂/H₂O, with generation of forsterite-normative liquid. The subcontinental low-velocity zone, where present, is probably caused by incipient melting of dolomite-peridotite, or phlogopite-dolomite-peridotite, either with H₂O-rich vapor or without vapor, with the generation of CO₂-rich, alkalic, SiO₂-poor liquid (larnite-normative) that in extreme conditions may be carbonatitic.     

Video and seismic observations of Strombolian eruptions at Erebus volcano,Antarctica

Between 1986 and 1990 the eruptive activity of Erebus volcano was monitored by a video camera with on-screen time code and recorded on video tape. Corresponding seismic and acoustic signals were recorded from a network of 6 geophones and 2 infrasonic microphones. Two hundred Strombolian explosions and three lava flows which were erupted from 7 vents were captured on video. In December 1986 the Strombolian eruptions ejected bombs and ash. In November 1987 large bubble-bursting Strombolian eruptions were observed. The bubbles burst when the bubble walls thinned to ∼ 20 cm. Explosions with bomb flight-times up to 14.5 s were accompanied by seismic signals with our local size estimate, “unified magnitudes” (m_u), up to 2.3. Explosions in pools of lava formed by flows in the Inner Crater were comparatively weak.     

Factors influencing the height of Hawaiian lava fountains: implications for the use of fountain height as an indicator of magma gas content

The heights of lava fountains formed in Hawaiian-style eruptions are controlled by magma gas content, volume flux and the amounts of lava re-entrainment and gas bubble coalescence. Theoretical models of lava fountaining are used to analyse data on lava fountain height variations collected during the 1983–1986 Pu'u 'O'o vent of Kilauea volcano, Hawaii. The results show that the variable fountain heights can be largely explained by the impact of variations in volume flux and amount of lava re-entrainment on erupting magmas with a constant gas content of 0.32 wt.% H₂O. However, the gas content of the magma apparently declined by 0.05 wt.% during the last 10 episodes of the eruption series and this decline is attributed to more extensive pre-eruption degassing due to a shallowing of the sub-vent feeder dike. It is concluded that variations in lava fountain height cannot be simply interpreted as variations in gas content, as has previously been suggested, but that fountain height can still be a useful guide to minimum gas contents. Where sufficient data are available on eruptive volume fluxes and extent of lava entrainment, greatly improved estimates can be made of magma gas content from lava fountain height.