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.     

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.     

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.     

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.     

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.     

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.     

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.     

Restoration of new volcanic gas analyses from basalts of the afar region: Further evidence of CO2 -degassing trends

The restored compositions for approximately 70 new analyses reported recently for Erta'Alelava lake (LeGuern et al., 1979) are in good agreement with restored compositions (Gerlach, 1980a) based on previously published data. The results confirm earlier indications that gas collections taken at different times from the lava lake are related principally by variations in CO₂ content. Restored compositions for gas samples collected in the final stages of a November 1978 Ardoukoba eruption along the Asal Rift spreading axis resemble the Erta'Ale gases except for a much lower CO₂ content. The Ardoukoba gases fall close to a CO₂-decreasing control line for gases with initial compositions similar to the 1971–1973 Erta'Ale gases. These results suggest that gases released from basaltic lava along zones of crustal spreading follow compositional trends dominated by changes in CO₂ content.     

Geochemistry and mineralogy of the phonolite lava lake,Erebus volcano,Antarctica: 1972–2004 and comparison with older lavas

Mount Erebus, Antarctica, is a large (3794 m) alkaline open-conduit stratovolcano that hosts a vigorously convecting and persistently degassing lake of anorthoclase phonolite magma. The composition of the lake was investigated by analyzing glass and mineral compositions in lava bombs erupted between 1972 and 2004. Matrix glass, titanomagnetite, olivine, clinopyroxene, and fluor-apatite compositions are invariant and show that the magmatic temperature (∼ 1000°C) and oxygen fugacity (ΔlogFMQ = − 0.9) have been stable. Large temperature variations at the lake surface (~ 400–500°C) are not reflected in mineral compositions. Anorthoclase phenocrysts up to 10 cm in length feature a restricted compositional range (An_10.3–22.9Ab_62.8–68.1Or_11.4–27.2) with complex textural and compositional zoning. Anorthoclase textures and compositions indicate crystallization occurs at low degrees of effective undercooling. We propose shallow water exsolution causes crystallization and shallow convection cycles the anorthoclase crystals through many episodes of growth resulting in their exceptional size. Minor variations in eruptive activity from 1972 to 2004 are decoupled from magma compositions. The variations probably relate to changes in conduit geometry within the volcano and/or variable input of CO₂-rich volatiles into the upper-level magma chamber from deeper in the system.     

Geochemical monitoring of the 2002–2003 eruption at Stromboli volcano (Italy): precursory changes in the carbon and helium isotopic composition of fumarole gases and thermal waters

Significant changes in the helium and carbon isotopic composition of shallow thermal waters vs. gas and a crater fumarolic gas have been recorded at Stromboli prior and during the 2002–2003 eruption. The³He/⁴He ratios corrected for air contamination (Rc/Ra), and δ¹³C of fumarolic gases gradually increased from May to November 2002 before the eruption onset. These variations imply early degassing of a gas-rich magma at depth that likely fed both the intense Strombolian activity and small lava overflows recorded during that period. The lava effusion of late December 2002 was shortly preceded by a marked Rc/Ra decrease both in water and fumarolic gases. Comparison of He/CO₂ and CH₄/CO₂ ratios in dissolved gas and with values rules out the Rc/Ra decrease due to an increasing input of radiogenic⁴He. The Rc/Ra decrease is attributed to the He isotope fractionation during rapid magma ascent and degassing. A new uprising of ³He-rich magma probably occurred in January to February 2003, when Rc/Ra ratios displayed the highest values in dissolved gases ever measured before (4.56 Rc/Ra). The increase in He/CO₂ and CH₄/CO₂ ratios and decrease in δ¹³C of dissolved CO₂ was recorded after the 5 April 2003 explosive paroxysm, likely caused by enhanced gas-water interaction inducing CO₂ dissolution. No anomalous Rc/Ra values were recorded in the same period, when usual Strombolian activity gradually resumed.Editorial responsibility: H Shinohara     

Excessive degassing of Izu-Oshima volcano: magma convection in a conduit

Excess degassing of magmatic H₂O and SO₂ was observed at Izu-Oshima volcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1×10⁴ kg/s using emission rates of magmatic H₂O and SO₂, and H₂O and S contents of the magma. The minimum total volume of magma degassed during the 27-month period is estimated to be 2.6×10⁸ m³. This volume is 20 times larger than that of the magma ejected during the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a magma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed magma spheres through a conduit filled with degassed magma. This process is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles.     

Water chemistry of Lake Quilotoa (Ecuador) and assessment of natural hazards

A geochemical survey carried out in November 1993 revealed that Lake Quilotoa was composed by a thin (14 m) oxic epilimnion overlying a 200 m-thick anoxic hypolimnion. Dissolved CO₂ concentrations reached 1000 mg/kg in the lower stratum. Loss of CO₂ from epilimnetic waters, followed by calcite precipitation and a consequent lowering in density, was the apparent cause of the stratification.The Cl, SO₄ and HCO₃ contents of Lake Quilotoa are intermediate between those of acid–SO₄–Cl Crater lakes and those of neutral-HCO₃ Crater lakes, indicating that Lake Quilotoa has a ‘memory’ of the inflow and absorption of HC1- and S-bearing volcanic (magmatic) gases. The Mg/Ca ratios of the lake waters are governed by dissolution of local volcanic rocks or magmas, but K/Na ratios were likely modified by precipitation of alunite, a typical mineral in acid–SO₄–Cl Crater lakes.The constant concentrations of several conservative chemical species from lake surface to lake bottom suggest that physical, chemical and biological processes did not have enough time, after the last overturn, to cause significant changes in the contents of these chemical species. This lapse of time might be relatively large, but it cannot be established on the basis of available data. Besides, the lake may not be close to steady state. Mixing of Lake Quilotoa waters could presently be triggered by either cooling epilimnetic waters by 4°C or providing heat to hypolimnetic waters or by seismic activity.Although Quilotoa lake contains a huge amount of dissolved CO₂ (3×10¹¹ g), at present the risk of a dangerous limnic eruption seems to be nil even though some gas exsolution might occur if deep lake waters were brought to the surface. Carbon dioxide could build up to higher levels in deep waters than at present without any volcanic re-awakening, due to either a large inflow of relatively cool CO₂-rich gases, or possibly a long interval between overturns. Periodical geochemical surveys of Lake Quilotoa are, therefore, recommended.     

CO2 output and δ13C(CO2) from Mount Etna as indicators of degassing of shallow asthenosphere

 An estimated average CO₂ output from Etna's summit craters in the range of 13±3 Mt/a has recently been determined from the measured SO₂ output and measured CO₂/SO₂ molar ratios. To this amount the CO₂ output emitted diffusely from the soil (≈ 1 Mt/a) and the amount of CO₂ dissolved in Etna's aquifers (≈ 0.25 Mt/a) must be added. Data on the solubility of CO₂ in Etnean magmas at high temperature and pressure allow the volume of magma involved in the release of such an amount of this gas to be estimated. This volume of magma (≈ 0.7 km³/a) is approximately 20 times greater than the volume of magma erupted annually during the period 1971–1995. On the basis of C-isotopic data of CO₂ collected in the Etna area and of new hypotheses on the source of Mediterranean magmas, significant contributions of CO₂ from non-magmatic sources to the total output from Etna are unlikely. Such large outputs of CO₂ and also of SO₂ from Etna could be due to an anomalously shallow asthenosphere beneath the volcano that allows a continuous escape of gases toward the surface, even without migration of magma. Received: 7 August 1996 / Accepted: 9 November 1996     

Volcanic plume thermal radiation: 1972 eruption of piton de la fournaise, Réunion Island

Using an infrared radiometer, the authors measured the infrared radiation emitted by the plume during the 1972 Piton de la Fournaise (Réunion Island) eruption in the 9.1- to 11.9-μm range.Eight histograms summarize twenty hours of continuous measurements and show a flux between 0.557 and 1.279 W/cm² with an average of 0.88 W/cm². The greatest number of events correspond to the flux emitted by lava fountains, while the high and rare flux values (4–6 W/cm²) correspond to detonations of chemical explosions, probably of H₂ and O₂.After calibration in the laboratory on a reference black body and in the field by correlation with field observations, such a technique can be used to follow changes of volcanic activity in remote and dangerous spots. Information on the number of explosions, their magnitude, and the presence and relative significance of hot magma can be transmitted continuously to a safe place.     

Radionuclides and sulfur content in Mount Etna plume in 1983–1995: new constraints on the magma feeding system

Radionuclide activities (²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po) were investigated in Mount Etna plume from 1983 to 1995. At SE crater the long-term observation (12 years) of the ²¹⁰Po/²¹⁰Pb ratio shows that it can behave as a degassing vent not directly related to the main magma reservoir depending on the magma level inside the volcano. Since 1992, the simultaneous determination of radionuclides and sulfur in the main plume results in new constraints on the degassing model of Lambert et al. (Earth Planet. Sci. Lett., 76 (1986) 185). The ²¹⁰Po/SO₂ and ²¹⁰Pb/SO₂ ratios enable us to identify two sources of ²¹⁰Po in the plume: one is magmatic, correlated with SO₂, the other one is an additional component issued from the decay of ²¹⁰Pb in the shallow degassing cell, and depends directly on the residence time of the gases before their emission. Estimations of the volume of degassing magma, the residence time of the gases and the proportion of undegassed magma renewing the shallow degassing cell are given for the period 1992–1995. During the 1992 eruption, the rate of degassing magma volume is estimated to have been as high as 5×10⁶ m³/day, and the volume of the shallow degassing magma reservoir about 0.5 km³. In 1994 and 1995 the rate of non-erupted degassing magma volume was estimated to have been about 0.18 km³/year. During the entire 1983–1995 period, only 15–20% of the degassed magma was erupted.     

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».     

Mantle-derived magmatic gas releasing features at the Rehai area, Tengchong county, Yunnan Province, China

This paper deals with the chemical and isotopic compositions of escaped gases from the Rehai geothermal area in Tengchong county of Yunnan Province. Results indicate that there is the mantle-derived magmatic intrusion in shallow crust at this area. Modern mantle-derived magmatic volatiles are being released currently in a steady stream by way of active faults. The escaped gases are mostly composed of CO₂, together with subordinate amounts of H₂S, N₂, H₂, CH₄, SO₂, CO and He. At the studied area, the north-south directed fault is the deepest, and it may be interlinked with the deep-seated thermal reservoir that would be directly recharged by the mantle-derived magmatic volatile. The He, C isotopic evidence reveals that the modern active magma beneath Rehai area may originate from the historical mantle-derived magma which caused the latest eruptive activity of volcanoes in that region.     

Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung,Bali, Indonesia

 The 1963 eruption of Gunung Agung produced 0.95 km³ dense rock equivalent (DRE) of olivine±hornblende-bearing, weakly phyric, basaltic andesite tephra and lava. Evidence for magma mixing in the eruptive products includes whole-rock compatible and incompatible trace element trends, reverse and complex compositional zoning of mineral phases, disequilibrium mineral assemblages, sieve-textured plagioclase phenocrysts, and augite rims on reversely zoned orthopyroxene. Basalt magma mixed with pre-existing andesite magma shortly before eruption to yield basaltic andesite with a temperature of 1040–1100  °C at an assumed pressure of 2 kb, f O₂>NNO, and an average melt volatile content (H₂O±CO₂) of 4.3 wt.%. Magma-mixing end members may have provided some of the S and Cl emitted in the eruption. Glass inclusions in phenocrysts contain an average of 650 ppm S and 3130 ppm Cl as compared with 70 ppm and 2220 ppm, respectively, in the matrix glass. Maximum S and Cl contents of glass inclusions approach 1800 and 5000 ppm, respectively. Application of the petrologic method to products of the 1963 eruption for estimating volatile release yields of 2.5×10¹²g (Mt) of SO₂ and 3.4 Mt of Cl released from the 0.65 km³ of juvenile tephra which contributed to stratospheric injection of H₂SO₄ aerosols on 17 March and 16 May, when eruption column heights exceeded 20 km above sea level. An independent estimate of SO₂ release from atmospheric aerosol loading (11–12 Mt) suggests that approximately 7 Mt of SO₂ was injected into the stratosphere. The difference between the two estimates can be most readily accounted for by the partitioning of S, as well as some Cl, from the magma into a water-rich vapor phase which was released upon eruption. For other recent high-S-release eruptions of more evolved and oxidized magmas (El Chichón, Pinatubo), the petrologic method gives values two orders of magnitude less than independent estimates of SO₂ emissions. Results from this study of the Agung 1963 magma and its volatile emissions, and from related studies on eruptions of more mafic magmas, suggest that SO₂ emissions from eruptions of higher-S-solubility magma may be more reliably estimated by the petrologic method than may those from more-evolved magma eruptions. Received: 29 June 1994 / Accepted: 25 April 1996     

Dynamics of carbon dioxide emissions,crystallization, and magma ascent: hypotheses,theory, and applications to volcano monitoring at Mount St. Helens

 Measurements of CO₂ fluxes from open-vent volcanos are rare, yet may offer special capabilities for monitoring volcanos and forecasting activity. The measured fluxes of CO₂ and SO₂ from Mount St. Helens decreased from July through November 1980, but the record includes variations of CO₂/SO₂ in the emitted gas and episodes of greatly increased fluxes of CO₂. We propose that the CO₂ flux variations reflect two gas components: (a) a component whose flux decreased in proportion to 1/ √t with a CO₂/SO₂ mass ratio of 1.7, and (b) a residual flux of CO₂ consisting of short-lived, large peaks with a CO₂/SO₂ mass ratio of 15. We propose two hypotheses: (a) the 1/ √t dependence was generated by crystallization in a deep magma body at rates governed by diffusion-limited heat transfer, and (b) the gas component with the higher CO₂/SO₂ was released from ascending magma, which replenished the same magma body. The separation of the total CO₂ flux into contributions from known processes permits quantitative inferences about the replenishment and crystallization rates of open-system magma bodies beneath volcanos. The flux separations obtained by using two gas sources with distinct CO₂/SO₂ ratios and a peak minus background approach to obtain the CO₂ contributions from an intermittent source and a continuously emitting source are similar. The flux separation results support the hypothesis that the second component was generated by episodic magma ascent and replenishment of the magma body. The diffusion-limited crystallization hypothesis is supported by the decay of minimum CO₂ and SO₂ fluxes with 1/ √t after 1 July 1980. We infer that the magma body at Mount St. Helens was replenished at an average rate (2.8×10⁶m³d^–1) which varied by less than 5% during July, August, and September 1980. The magma body volume (2.4–3.0 km³) in early 1982 was estimated by integrating a crystallization rate function inferred from CO₂ fluxes to maximum times (20±4 years) estimated from the increase of sample crystallinity with time. These new volcanic gas flux separation methods and the existence of relations among the CO₂ flux, crystallization rates, and magma body replenishment rates yield new information about the dynamics of an open-vent, replenished magma body. Received: 15 February 1995 / Accepted: 30 March 1996     

Effusive natrocarbonatite activity of Oldoinyo Lengai,June 1988

Oldoinyo Lengai in the Tanzanian rift valley is the only active carbonatite volcano in the world and its natrocarbonatitic lavas are unique in composition. The characteristics of effusive natrocarbonatite activity in June 1988 were studied and fresh samples were directly collected from active carbonatitic lava lakes and flows. Analyses of these samples provide the first information on natrocarbonatites since these unusual volcanic rock type was first described from the 1960–1961 eruptions. The analytical results constrain the original chemistry of fresh natrocarbonatite. Temperatures in lava lakes and of carbonatite lava flows range 491–544°C. The natrocarbonatite lava is extremely fluid at these temperatures and reaches incandescence. The most common variety of natrocarbonatite is porphyritic with abundant phenocrysts of nyerereite (Na_0.82K_0.19)₂(Ca, Sr, Ba)_0.975(CO₃)₂ and gregoryite Na_1.74K_0.1(Ca, Sr, Ba)_0.16CO₃, with complex substitution of (CO₃)^2- by (SO₄)^2-, (PO₄)^3-, F^-, and Cl^-. A phenocryst-poor to aphyric natrocarbonatite variety reflects residual liquids separating from the crystal-rich porphyritic flows. Sylvite, fluorite, and Fe-alabandite (Mn_0.7Fe_0.3S) have been identified as additional primary magmatic phases. Rare phases in the matrix are witherite (BaCO₃) and sellaite (MgF₂). Sylvite and gregoryite, and to a lesser extent nyerereite, are water-soluble and are responsible for the immediate decomposition and chemical alteration of natrocarbonatites under atmospheric conditions. A peralkaline combeite-bearing nephelinite lava is closely related to the natrocarbonatite activity, and is isotopically indistinguishable. It is likely that these two magma compositions are related by liquid immiscibility. The unusual hyperalkaline composition of both magma types makes Oldoinyo Lengai an exotic volcano, and its carbonatites have extreme compositions, and are not representative of carbonatites in general.