Chemistry and metal contents of magmatic gases: the new Tolbachik volcanoes case (Kamchatka)

The gaseous products of new Tolbachik volcanoes were studied during 1975 to 1977 throughout all eruptive stages and during the post eruptive activity. In investigations the northern break-out gases emitted during the eruption from the moving and consolidated lava flows there have been detected H₂O (the main component), H₂, HF, HCl, SO₂ and H₂S, CO₂, CO, NH₃, CH₄ and other hydrocarbons, NH₄Cl predominated in compositions of condensates and subtimates on lava flows and the most characteristic microcomponents were Zn, Cu, Pb, Sn, Ag and others. Sampling of gases and condensates at the southern break-out was conducted immediately from the flowing melt. In gases there have been detected H₂O (98 mol. %). HCl and H₂ (0.9 mol. % each) as well as HF, SO₂, H₂S, CO₂ and in small quantities O₂ and N₂, Gases reached the equilibrium state atT andP sampling and were characteristic of gas composition of the southern break-out magma. HCl, HF and H₂SO₄ were predominant during condensate and sublimate mineralization. The major raicrocomponents were represented by Pt, Sb, As, Zn, Cu, Pb, Ni, Co and others. Comparison of compositions of gases and of products of their reactions at the northern and at the southern break-outs allows us to assume the presence of the deeper magma source at the northern break-out and of shallow magma source at the southern break-out.     

Forecast of volcanic eruptions by chemical methods

From the magmatic emanations differentiation point of view it is possible to calculate some ratios such as F/CO₂, Cl/CO₂, SO₂/CO₂, SO₂/H₂S, H₂S/CO₂ and CO₂/N₂ in the tumarolic gases for the forecasting of volcanic activity. In order to predict the cruptions of a volcano it is needed to select several fumaroles or hot springs having different regimes of variation of the above ratios. The study of some fumaroles composition at the Asama. Mihara, Kirishima and other volcanoes in Japan showed a close connection between volcanic gas compositions and state of the volcanoes.     

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».     

Evolution of fumarolic gases — boundary conditions set by measured parameters: case study at Vulcano,Italy

Physical, chemical and isotopic parameters were measured in fumaroles at the Vulcano crater and in drowned fumaroles near the beach. The data were used to define boundary conditions for possible conceptual models of the system.Crater fumaroles: time variations of CO₂ and SO₂ concentrations indicate mixing of saline gas-rich water with local fresh water. Cl/Br ratios of 300– 400 favour sea-water as a major source for Cl, Brand part of the water in the fumaroles. Cl concentrations and D values revealed, independently, amixing of 0.75 sea-water with 0.25 local freshwaterin furmarole F-5 during September 1982.Patterns of parameter correlation and mass balances reveal that CO₂, S, NH₃ and B originate from sources other than sea water. The CO₂ value of ¹³C = – 2%_o favours, at least partial, origin from decomposition of sedimentary rocks rather than mantle-derived material. Radiogenic⁴He(1.3 × lO^–3 ccSTP/g water) and radiogenic⁴⁰Ar(10.6 × 10^–4 ccSTP/g water) are observed, (⁴He/⁴⁰Ar)_radiogenic = 1.2, well in the range of values observed in geothermal systems.Drowned fumaroles: strongly bubbling gas at a pond and at the beachappears to have the same origin and initial compositionas the crater fumaroles (2 km away). The fumarolic gas is modified by depletion of the reactive gases, caused by dissolution in shallow-water. Atmospheric Ne, Ar, Kr and Xe are addeden route, some radiogenic He and Ar are maintained. The Vulcano system seems to be strongly influenced by the contribution of sea-water and decomposition of sedimentary rocks. Evidence of magmatic contributions is mainly derived from heat.     

Photodissociation rates in the atmosphere below 100km

In this survey we consider the atmospheric photodissociation rates of the molecules, O₂, O₃, NO, NO₂, N₂O, N₂O₅, HNO₃, HO₂, H₂O, H₂O₂, CO₂, CH₄, CH₂O, SO₂, and H₂S. Data for the absorption cross sections and quantum yields of these molecules are assembled here along with other information pertinent to the determination of photodissociation rates. The most recent techniques for computing atmospheric photodissociation rates are discussed. Photodissociation rates for all of the molecules are given at several solar zenith angles for altitudes up to 100 kilometres. A knowledge of the photodissociation rates of atmospheric molecules is essential to the resolution of many important atmospheric problems. Pollution of the stratosphere by high-flying aircraft, and of the troposphere by other anthropogenic activities, can only be described in terms of complex photochemical-dynamical models in which photolytic processes have a dominant role. A great deal of scientific effort is presently being spent in determining the mechanisms which control ozone, nitric oxide, and excited molecular oxygen concentrations in the mesosphere. Photolytic processes are already known to be important to all of these species. The photodissociation rates presented here can be applied directly to atmospheric problems such as these, or the methodology and data contained in this work can be used to compute photorates as needed.     

A simple method for the collection and analysis of volcanic gas samples

Investigations into the chemistry of volcanic gases depend on the availability of complete and accurate analyses of volcanic exhalations. The wide variety of sampling and analysis methods hitherto used, often supplying only partial analyses of low precision, made intercomparison, and thus a systematic study of volcanic gases, difficult. With the method proposed here, complete volcanic gas samples are obtained permitting the accurate determination of all major species by standard analytical methods without the need for highly specialised ancillary equipment. The samples are collected in evacuated 300 ml pyrex flasks through titanium tubes deeply inserted into the gas vent. Two types of flask are used, a single compartment flask allowing the easy determination of the major constituents and containing 50 ml 4 N NaOH, and a double compartment flask for the separate analysis of the sulfur species and containing 25 ml 0.1 N As₂O₃ in 1 N HClO₄ in the first, and 50 ml 4 N NaOH in the second compartment. Non-absorbed gases are determined by gas chromatography, the rest by standard analytical procedures. The determination of H₂O, CO₂, SO₂, SO₂, S₂, H₂S, HCl, HF, H₂, N₂, O₂, CH₄, CO and NH₂ is described.     

Magmatic gases extracted and analysed from ocean floor volcanics

Magmatic gases extracted and analysed from basaltic rocks collected in the FAMOUS area near 36°50′ N in the Atlantic ocean show that the total amount of gas included in the samples varies between about 500 ppm to 1600 ppm. The main gaseous phases included in the various types of basalts consist of CO₂ (270–700 ppm), CO (150–800 ppm), HCl (100–1000 ppm), H₂ (0–50 ppm), SO₂ (up to 175 ppm), N₂ (up to about 213 ppm) and traces of hydrocarbons (up to about 24 ppm). The relative amount of CO, CO₂ and SO₂ varies with both the degree of crystallinity of the rock and with fractional crystallization and/or fractional melting. The glassy margin of pillow lavas have a higher CO/CO₂ ratio than the more crystalline interior. The most fractionated rocks of the series rich in clinopyroxene are depleted in the CO/CO₂ ratio and have a higher SO₂ content than do the most mafic end members rich in olivine. Early-formed olivine was crystallized in a reducing environment rich in CO and H₂ with respect to later formed mineral associations. It is likely that the carbon and sulfur oxidation is taking place at a relatively shallow depth during magmatic ascent or during volcanism. The ocean floor volcanics when compared to subaerial basalts are depleted in SO₂ and have on the average ten times more H₂.     

Volcanic gases and sublimates from Showashinzan

Volcanic gases from Showashinzan are qualitatively the same as those liberated from igneous rocks when they are heated in vacuum. Their main components are H₂O, CO₂, and H₂. Then follow HCl, HF, N₂, SO₂, H₂S, S, CH₄, CO, Ar, Si, B, Mg, Na, K, Ca, Al, Fe, P, Br, NH₃, As, Zn, Sr, Ba, Cu, Pb, Sn, Sb, Bi, Ge, Ag, Cr, Ni, Mo, Rn, Ra, etc. They come through fumaroles of high temperature (～750°C.). Metallic compounds deposit as sublimates around the outlet of fumaroles. They are fractionated there according to their thermodynamic properties. When the temperature of gases falls, heavy metal elements deposit before reaching the surface of the earth. Ra is among them. Owing to the contribution of Ra thus depositted, Rn content of vapor is larger in low temperature fumaroles than in high temperature ones. Chemical compounds of H, C, N, O, and S vary their composition according to the condition of temperature and pressure. Sulfur exists as SO₂ more than H₂S. As the temperature of gases falls, SO₂ and H₂ decrease and H₂S increases. Mutual relation among them is ruled by the chemical equilibrium: SO₂+3H₂=H₂S+2H₂O. The structure of Showashinzan is not simple. Some deviations from the general rule are explained in connection with ground water.     

Fumarole/plume and diffuse CO2 emission from Sierra Negra caldera, Galapagos archipelago

Measurements of visible and diffuse gas emission were conducted in 2006 at the summit of Sierra Negra volcano, Galapagos, with the aim to better characterize degassing after the 2005 eruption. A total SO₂ emission of 11?±?2?t day^?1 was derived from miniature differential optical absorption spectrometer (mini-DOAS) ground-based measurements of the plume emanating from the Mini Azufral fumarolic area, the most important site of visible degassing at Sierra Negra volcano. Using a portable multigas system, the H₂S/SO₂, CO₂/SO₂, and H₂O/SO₂ molar ratios in the Mina Azufral plume emissions were found to be 0.41, 52.2, and 867.9, respectively. The corresponding H₂O, CO₂, and H₂S emission rates were 562, 394, and 3?t day^?1, respectively. The total output of diffuse CO₂ emissions from the summit of Sierra Negra volcano was 990?±?85?t day^?1, with 605?t day^?1 being released by a deep source. The diffuse-to-plume CO₂ emission ratio was about 1.5. Mina Azufral fumaroles released gasses containing 73.6?mol% of H₂O; the main noncondensable components amounted to 97.4?mol% CO₂, 1.5?mol% SO₂, 0.6?mol% H₂S, and 0.35?mol%?N₂. The higher H₂S/SO₂ ratio values found in 2006 as compared to those reported before the 2005 eruption reveal a significant hydrothermal contribution to the fumarolic emissions. ³He/⁴He ratios measured at Mina Azufral fumarolic discharges showed values of 17.88?±?0.25?R _A , indicating a mid-ocean ridge basalts (MORB) and a Galapagos plume contribution of 53 and 47?%, respectively.     

Fumarole compositions and mercury emissions from the Tatun Volcanic Field,Taiwan: Results from multi-component gas analyser,portable mercury spectrometer and direct sampling techniques

Gas emissions from Tatun volcanic group, northern Taiwan, were studied for the first time using a multi-component gas analyser system (Multi-GAS) in combination with Giggenbach flask methods at fumaroles and mud pools at Da-you-keng (DYK) and Geng-tze-ping (GZP). CO₂/S molar ratios observed at DYK ranged from 3–17, similar ratios were observed using a Multi-GAS sensor box of 8–16. SO₂ at GZP was low, higher concentrations were observed at DYK where SO₂/H₂S ratios were close to 1 for both methods. A lower CO₂/H₂S ratio was measured via Giggenbach flask sampling (7.2) than was found in the plume using the gas sensor at GZP (9.2). This may reflect rapid oxidation of H₂S as it mixes with background air. Gaseous elemental mercury (GEM) levels were observed in the fumarole gases using a portable mercury spectrometer. These are the first such measurements of mercury at Tatun. Mean GEM concentrations in the fumarole plumes were ∼ 20 ng m^− 3, with much higher concentrations observed close to the ground (mean [GEM] 130 and 290 ng m^− 3 at DYK and GZP, respectively). The GEM in the fumarole plume was elevated above concentrations in industrial/urban air in northern Taiwan and the increase in GEM observed when the instrument was lowered suggests high levels of mercury are present in the surrounding ground surface. The GEM/CO₂ (10^− 8) and GEM/S (10^− 6) ratios observed in the fumarole gases were comparable to those observed at other low-temperature fumaroles. Combining the Hg/CO₂ ratio with a previous CO₂ flux value for the area, the annual GEM flux from the Tatun field is estimated as 5–50 kg/year.     

Differentation of magmatic emanation

Chemical properties of magmatic emanation can be estimated roughly by i) volatiles from rocks by heating at various temperatures, ii) volcanic emanations, iii) residual magmatic emanations, iv) calculation from chemical equilibrium between volatile matters and magmas. Magmatic emanation is assumed to consist all of the volatile matters in magmas such asH ₂ O, HCl, HF, SO ₂ H ₂ S, H ₂,CO 2,N ₂ and others (halides, etc.) at about 1200°C, although various kinds of magmatic emanations can be formed at different conditions. Magmatic emanation separated from magmas will change their chemical properties by many factors such as changes of temperature and pressure (displacement of chemical equilibrium), and reactions with other substances and it will differentiate into volcanic gases, volcanic waters, volcanic sublimates, and hydrothermal deposits (hot spring deposits). At temperatures above the critical point of water, separation of solid phase (sublimates), liquid phase, and displacement of chemical equilibrium may take place, and gaseous phase will gradually change their chemical properties as will be seen at many fumaroles. Chloride, hydrogen, andSO ₂ contents will gradually decrease along with lowering temperature. Once aqueous liquid phase appears below the critical point of water, all the soluble materials may dissolve into this hydrothermal solution. Consequently, the gaseous phase at this stage must have usually a little hydrogen chloride as is observed at many fumaroles. Aqueous solutions must be of acidic nature by dissolution of acid forming components, and by hydrolysis (Chloride type). When a self-reduction-oxidation reaction of sulfurous acid takes place, an aqueous solution of sulfate type will be formed. At this stage, solid phases consist of the remained sublimates which are difficultly soluble in aqueous solution, and deposits formed by reaction in the hydrothermal solutions. The gaseous phases below the boiling point of water, have usually a little water, and consist mainly ofCO ₂ type,H ₂ S type,N ₂ type, and mixed type owing to elimination or addition of components by reactions with waters or wall rocks according to their geological conditions. Aqueous solutions which was of acidic nature must be changed into alkaline solutions by reaction with wall rocks for a long time. When the oxidation of sulfur compounds takes place, an aqueous solution of sulfate type will be formed. Hydrogen sulfide type of water will be formed by reaction of sulfides with acid waters or absorption of hydrogen sulfide. Carbonate type of water will be formed whenCO ₂ is absorbed. Solid phases at this stage consist usually of hydrothermal deposits except for that at solfatara or mofette. The course of differentiation of magmatic emanation could take place in more complicated ways than that of magmatic differentiation.     

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.     

UV camera measurements of fumarole field degassing (La Fossa crater,Vulcano Island)

The UV camera is becoming an important new tool in the armory of volcano geochemists to derive high time resolution SO₂ flux measurements. Furthermore, the high camera spatial resolution is particularly useful for exploring multiple-source SO₂ gas emissions, for instance the composite fumarolic systems topping most quiescent volcanoes. Here, we report on the first SO₂ flux measurements from individual fumaroles of the fumarolic field of La Fossa crater (Vulcano Island, Aeolian Island), which we performed using a UV camera in two field campaigns: in November 12, 2009 and February 4, 2010. We derived ~ 0.5 Hz SO₂ flux time-series finding fluxes from individual fumaroles, ranging from 2 to 8.7 t d^?1, with a total emission from the entire system of ~ 20 t d^?1 and ~ 13 t d^?1, in November 2009 and February 2010 respectively. These data were augmented with molar H₂S/SO₂, CO₂/SO₂ and H₂O/SO₂ ratios, measured using a portable MultiGAS analyzer, for the individual fumaroles. Using the SO₂ flux data in tandem with the molar ratios, we calculated the flux of volcanic species from individual fumaroles, and the crater as a whole: CO₂ (684 t d^?1 and 293 t d^?1), H₂S (8 t d^?1 and 7.5 t d^?1) and H₂O (580 t d^?1 and 225 t d^?1).     

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.     

Crystallization temperature and gas phase composition of alkaline effusives as indicated by primary melt inclusions in the phenocrysts

Minerals formed during magma crystallization trap droplets of melt that are preserved as primary or secondary inclusions. Depending on the rate of cooling, the droplets may solidify as glass, or crystallize. Inclusions may contain one or more bubbles, or none. When inclusions are heated the glass or crystalline material are melted and the inclusion expands, the size of bubbles diminishes, and homogenization of the inclusion occurs. It is possible to observe these transformations by means of high-temperature cameras which permit visual observations to 1600°C and above. The possibility of using the homogenization of inclusions to determine the temperature of formation of the host mineral has been demonstrated experimentally, using inclusions in artificial diopside formed at 1300 ± 10°. Melt inclusions in phenocrysts from nepheline basalt, fergusite porphyry, and tephrite were investigated. In the leucite-bearing rocks leucite crystallized at 1600° or above, and clinopyroxene in the range 1380–1250°. The central part of olivines in nepheline basalt formed at 1290–1270° and the peripheral zones at 1160–1120°; nepheline formed at 1290–1250°; the central part of pyroxenes at 1280–1250° and the peripheral zones at 1160–1120°. These temperatures suggest almost dry magma. Gas from the bubbles of individual inclusions has been analyzed. The predominant gaseous component of the early crystallization stage of the nepheline basalt and fergusite porphry was CO₂, H₂S, SO₂, NH., HCl, HF, and H. comprise less than 5 volume percent except in olivine of olivine basalt in which the total content of these gases was on average 6.22 volume percent, and in leucite of fergusite porphyry in which H₂ was on average 12.7 volume percent. The main gas component in the crystallization of the leucite tephrite were nitrogen and rare gases. Liquid hydrocarbons in the secondary inclusions in pyroxene from nepheline basalt can be accounted for by their assimilation by the magma from enclosing rocks during its rise.     

Volatile emissions and gas geochemistry of Hot Spring Basin,Yellowstone National Park,USA

We characterize and quantify volatile emissions at Hot Spring Basin (HSB), a large acid-sulfate region that lies just outside the northeastern edge of the 640 ka Yellowstone Caldera. Relative to other thermal areas in Yellowstone, HSB gases are rich in He and H₂, and mildly enriched in CH₄ and H₂S. Gas compositions are consistent with boiling directly off a deep geothermal liquid at depth as it migrates toward the surface. This fluid, and the gases evolved from it, carries geochemical signatures of magmatic volatiles and water–rock reactions with multiple crustal sources, including limestones or quartz-rich sediments with low K/U (or ^40?Ar/^4?He). Variations in gas chemistry across the region reflect reservoir heterogeneity and variable degrees of boiling. Gas-geothermometer temperatures approach 300 °C and suggest that the reservoir feeding HSB is one of the hottest at Yellowstone. Diffuse CO₂ flux in the western basin of HSB, as measured by accumulation-chamber methods, is similar in magnitude to other acid-sulfate areas of Yellowstone and is well correlated to shallow soil temperatures. The extrapolation of diffuse CO₂ fluxes across all the thermal/altered area suggests that 410 ± 140 t d^− 1 CO₂ are emitted at HSB (vent emissions not included). Diffuse fluxes of H₂S were measured in Yellowstone for the first time and likely exceed 2.4 t d^− 1 at HSB. Comparing estimates of the total estimated diffuse H₂S emission to the amount of sulfur as SO₄²⁻ in streams indicates ~ 50% of the original H₂S in the gas emission is lost into shallow groundwater, precipitated as native sulfur, or vented through fumaroles. We estimate the heat output of HSB as ~ 140–370 MW using CO₂ as a tracer for steam condensate, but not including the contribution from fumaroles and hydrothermal vents. Overall, the diffuse heat and volatile fluxes of HSB are as great as some active volcanoes, but they are a small fraction (1–3% for CO₂, 2–8% for heat) of that estimated for the entire Yellowstone system.     

Leaching characteristics of ash from the May 18, 1980, eruption of Mount St. Helens volcano,Washington

Leaching of freshly erupted air-fall ash, unaffected by rain, from the May 18, 1980, eruption of Mount St. Helens volcano, Washington, shows that Ca²⁺, Na⁺, Mg²⁺, SO ₄ ^2? , and Cl^? are the predominant chemical species released on first exposure of the ash to water. Extremely high correlation of Ca with SO₄ and Na with Cl in water leachates suggests the presence of CaSO₄ and NaCl salts on the ash. The amount of water soluble material on ash increases with distance from source and with the weight fraction of small (less than 63 micrometers) ash particles of high-surface area. This suggests that surface reactions such as adsorption are responsible for concentrating the soluble material. CaSO₄, NaCl, and other salts are probably formed as microscopic crystals in the high-temperature core of the eruption column and are then adsorbed by silicate ash particles. The environmentally important elements Zn, Cu, Cd, F. Pb, and Ba are released by a water leach in concentrations which could pose short-term hazards to some forms of aquatic life. However, calculated concentrations are based on a water-to-ash ratio of 4:1 or less, which is probably an underestimation of the regionally operative ratio. A subsequent leach of ash by warm alkaline solution shows dramatic increases, in the amount of dissolved SiO₂, U, and V, which are probably caused by increased dissolution of the glassy component of ash. Glass dissolution by alkaline ground water is a mechanism for providing these three elements to sedimentary traps where they may coaccumulate as uraniferous silica or U-V minerals. Leaching characteristics of ash from Mount St. Helens are comparable to characteristics of ash of similar composition from volcanoes in Guatemala. Ashes from each locality show similar ions predominating for a given leachate and similar fractions of a particular element in the ash removed on contact with the leach solution.     

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.     

Fumarolic emissions from Mount St. Augustine,Alaska: 1979–1984 degassing trends,volatile sources and their possible role in eruptive style

Gas samples were collected from high-temperature, rooted summit vents at Mount St. Augustine in 1979, 1982, and 1984. All of the gas samples exhibit various degrees of disequilibrium. Thermodynamic restoration of the analyzed gases permits partial or complete removal of these disequilibrium effects and allows inference of equilibrium gas compositions. Long-term (1979–1984) degassing trends within resampled or adjacent vents are characterized by increases (from 97.4 to 99.8 mole%) in the H₂O fraction and major decreases in the residual gases. Over this same period total gas HCl contents decreased by a factor of 3 to 10 while dry gas (H₂O-free recalculated) HCl contents increased by a factor of 1.6 to 3. Dry gas mole proportions at these sites changed from being CO₂-dominated (46% CO₂, 24% H₂ in 1979) to H₂-dominated (49% H₂, 22% CO₂ in 1984). The overall trends in gas chemistry and the stable isotope patterns in gases and condensates from the summit fumaroles can be explained by progressive magmatic outgassing coupled with increasing proportions of seawater in the fumarole emissions.Studies of the gaseous emissions following the 1976 and 1986 Mount St. Augustine eruptions confirmed the Cl- and S-rich nature of the Mount St. Augustine emanations. Seawater, possibly derived from magmatic assimilation or dehydration of near-surface seawater-bearing sediments, could supply a portion of the outgassed Cl and S. Continued seawater influx through subvolcanic fractures or permeable sediments would recharge the seawater-depleted zone and provide a near-surface Cl and S source for the next eruptive cycle,Various lines of evidence support a phreatomagmatic component in the 1976 and 1986 Mount St. Augustine eruptions. We suggest that seawater may interact with magma or volcanic gases during the early explosive phase of Mount St. Augustine eruptions and that it continues to influence high-temperature fumarole emissions as the volcanic system cools.     

Influence of Atmospheric Solar Radiation Absorption on Photodestruction of Ions at D-Region Altitudes of the Ionosphere

The influence of atmospheric solar radiation absorption on the photodetachment, dissociative photodetachment, and photodissociation rate coefficients (photodestruction rate coefficients) of O^?, Cl^?, O₂ ^?, O₃ ^?, OH^?, NO₂ ^?, NO₃ ^?, O₄ ^?, OH^?(H₂O), CO₃ ^?, CO₄ ^?, ONOO^?, HCO₃ ^?, CO₃ ^?(H₂O), NO₃ ^?(H₂O), O₂ ⁺(H₂O), O₄ ⁺, N₄ ⁺, NO⁺(H₂O), NO⁺(H₂O)₂, H⁺(H₂O) _n for n = 2–4, NO⁺(N₂), and NO⁺(CO₂) at D-region altitudes of the ionosphere is studied. A numerical one-dimensional time-dependent neutral atmospheric composition model has been developed to estimate this influence. The model simulations are carried out for the geomagnetically quiet time period of 15 October 1998 at moderate solar activity over the Boulder ozonesonde. If the solar zenith angle is not more than 90° then the strongest influence of atmospheric solar radiation absorption on photodestruction of ions is found for photodissociation of CO₄ ^? ions when CO₃ ^? ions are formed. It follows from the calculations that decreases in the photodestruction rate coefficients of ions under consideration caused by this influence are less than 2 % at 70 km altitude and above this altitude if the solar zenith angle does not exceed 90°.