Magmatic Gas Phase Research Articles

Evaluating bubble chain phenomena as a mechanism for open system degassing in basaltic systems

Passive degassing – i.e., the open-system degassing of a magmatic gas phase that is decoupled from the melt phase – is common at many basaltic volcanic systems and consistently makes a greater contribution to total volcanic gas emissions than eruptive degassing. However, the mechanism for passive degassing is not fully understood. We investigate the feasibility of permeable gas flow through connected bubbles or pathways using experiments with aqueous solutions of hydroxyethyl cellulose (HEC), a non-Newtonian analogue material for magma. Stable chains of connected bubbles have previously been reported in similar non-Newtonian polymer solutions. We observe a range of bubble chain phenomena and identified five regimes, numbering in order of increasing gas flow rate: 1) small individual bubbles; 2) chains of rounded bubbles; 3) chains of elongate bubbles; 4) pipe-like ‘winding flue’; 5) large individual bubbles. The bubble chain phenomena (regimes 2–4) are observed over a restricted interval of gas flow rates. We determine the rheology of the solutions and conclude that the HEC solutions that produced bubble chain phenomena in our experiments are well scaled to shear-thinning and viscoelastic magmas, hence bubble chain phenomena could form in magmas. Our analysis also suggests that the viscoelastic rheology of HEC plays a fundamental role in the observed bubble chain phenomena.

Potassium silicate alteration in porphyry copper‑gold deposits: a case study at the giant maar-diatreme hosted Grasberg deposit, Indonesia

Potassium silicate alteration is a hallmark of the porphyry copper deposits that supply two thirds of the world's annual copper demand. These deposits formed in the cores of calc-alkaline to alkaline volcanic systems from the flux of magmatic gas that transported copper and other metals from source to the surface. The giant 3.2 Ma Grasberg CuAu deposit formed within a maar-diatreme complex following a resurgence in magmatism. The defined resources of this deposit occur from a few hundred metres depth to 1.7 km below the paleosurface which, uniquely in this deposit, is partially preserved as a section of maar tuffs.Potassium silicate alteration has commonly been interpreted as being the result of the addition of potassium to the porphyry copper host rocks via pervasive interaction with a large volume of a potassium-rich brine that is commonly presumed to be of magmatic origin. However, the data reported here show that alteration at the deposit scale is essentially isochemical with respect to the major rock-forming components and that only sulphur and the economic metals (Cu, Mo, Au, etc.) are added by flux of reactive magmatic gas containing SO2 and HCl from source intrusions at depth. Silicate solubilities are very low so that only a minor fraction of the total alkalis in the host rock are extracted by alteration reactions and then discharged at the paleo-surface. Reaction of the magmatic gas phase with plagioclase results in the coupled deposition of anhydrite (CaSO4) and disproportionation of SO2 to release H2S. The in-situ release of H2S immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the Cu-, Fe- and other sulphides that, in sufficient concentration, make up the economic reserve available to mining.The sequestration of Ca into anhydrite, along with deposition of silica into early quartz veins, increases the concentration of the other major components (K2O, Na2O, MgO, etc) in the remaining silicate assemblage within the porous host rock. The result is the development of intermingled potassium-enriched silicate and sulphur-rich (anhydrite-sulphide) sub-assemblages that constitute the mineralised phyllic or potassic alteration zones. These crystallise according to their pressure and depth into alteration assemblages dominated by potassic phyllosilicates, quartz and pyrite in the phyllic alteration zone, and alkali feldspar and phlogopitic-biotite plus minor andalusite and corundum in the central potassic zone. Dissolution and recrystallisation of primary magmatic biotite in the host rock releases K as well as Fe, the latter (along with amphibole and feldspar) providing iron for the formation of chalcopyrite, bornite and pyrite. The in-situ release of H2S through anhydrite formation, immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the CuFe sulphides that, in sufficient concentration, make up the economic reserve available to mining. Understanding of the alteration processes during porphyry copper formation also provides insights into gas-solid reactions processes inside active magmatic arc volcanoes but the magnitude of copper mineralisation is dependent on the original metal content of the source of the magmatic gas phase.

Elevated CO2 Emissions during Magmatic-Hydrothermal Degassing at Awu Volcano, Sangihe Arc, Indonesia

Awu is a remote and little known active volcano of Indonesia located in the northern part of Molucca Sea. It is the northernmost active volcano of the Sangihe arc with 18 eruptions in less than 4 centuries, causing a cumulative death toll of 11,048. Two of these eruptions were classified with a Volcanic Explosivity Index (VEI) of 4. Since 2004, a lava dome has occupied the centre of Awu crater, channelling the fumarolic gas output along the crater wall. A combined Differential Optical Absorption Spectroscopy (DOAS) and Multi-component Gas Analyzer System (Multi-GAS) study highlight a relatively small SO2 flux (13 t/d) sustained by mixed magmatic–hydrothermal emissions made-up of 82 mol.% H2O, 15 mol.% CO2, 2.55 mol.% total S (ST) and 0.02 mol.% H2. The CO2 emission budget, as observed during a short observation period in 2015, corresponds to a daily contribution to the atmosphere of 2600 t/d, representing 1% of the global CO2 emission budget from volcanoes. The gas CO2/ST ratio of 3.7 to 7.9 is at the upper limit of the Indonesian gas range, which is ascribed to (i) some extent of S loss during hydrothermal processing, and perhaps (ii) a C-rich signature of the feeding magmatic gas phase. The source of this high CO2 signature and flux is yet to be fully understood; however, given the peculiar geodynamic context of the region, dominated by the arc-to-arc collision, this may result from either the prolonged heating of the slab and consequent production of carbon-rich fluids, or the recycling of crustal carbon.

Continuous monitoring of hydrogen and carbon dioxide at Stromboli volcano (Aeolian Islands, Italy)

The geochemistry of volcanic gases has been fruitfully applied to identify important changes in the volcanic activity. This study reviews the dataset of the volcanic gas survey performed during 2009 and 2010 at Stromboli (Aeolian Islands – Italy). Dry gas collection occurred discontinuously at the crater fumaroles for subsequent chemical and isotopic (δ13C-CO2) analyses in the laboratory.A tailor-made device enabled continuous survey of H2 molar fraction and CO2 flux on the same site. Evaluation of the raw data was performed in accordance with air temperature, atmospheric pressure, wind speed and wind direction dataset. Both MLR (Multiple Linear Regression) and FFT (Fast Fourier Transform) analyses allowed filtering the dataset from the environmental effects. The MLR analysis indicated that wind speed and air temperature affected the CO2 flux. Changes in the atmospheric pressure promoted pumping effect of the fumarole gas and caused changes in the H2 molar fraction. The power spectral analysis revealed daily cycles in both gases. A digital signal filtering procedure enabled minimizing the environmental effects.This study confirmed that gas emissions from the crater fumaroles have both chemical ad isotopic composition similar to the magmatic gas phase. The results of the continuous survey showed that changes of both H2 and CO2 correlated with changes of the volcanic activity. Therefore, H2 and CO2 resulted effective tracers of the dynamics involving the plumbing system of Stromboli. Identification of changes in the gas emissions at open conduit volcanoes offers a great advance over the ground gas survey. The results of the continuous survey at Stromboli showed that H2 could apply as an auxiliary parameter of the CO2 flux in the surveillance programs of active volcanoes.

Reaction path models of magmatic gas scrubbing

Gas–water–rock reactions taking place within volcano-hosted hydrothermal systems scrub reactive, water-soluble species (sulfur, halogens) from the magmatic gas phase, and as such play a major control on the composition of surface gas manifestations. A number of quantitative models of magmatic gas scrubbing have been proposed in the past, but no systematic comparison of model results with observations from natural systems has been carried out, to date. Here, we present the results of novel numerical simulations, in which we initialized models of hydrothermal gas–water–rock at conditions relevant to Icelandic volcanism. We focus on Iceland as an example of a “wet” volcanic region where scrubbing is widespread. Our simulations were performed (using the EQ3/6 software package) at shallow (temperature<106°C; low-T model runs) and deep hydrothermal reservoir (200–250°C; high-T model runs) conditions. During the simulations, a high-temperature magmatic gas phase was added stepwise to an initial meteoric water, in the presence of a dissolving aquifer rock. At each step, the chemical compositions of coexisting aqueous solution and gas phase were returned by the model. The model-derived aqueous solutions have compositions that describe the maturation path of hydrothermal fluids, from immature, acidic Mg-rich waters, toward Na–Cl-rich mature hydrothermal brines. The modeled compositions are in fair agreement with measured compositions of natural thermal waters and reservoir fluids from Iceland. We additionally show that the composition of the model-generated gases is strongly temperature-dependent, and ranges from CO2(g)-dominated (for temperatures ≤80°C) to H2O(g)-dominated (and more H2S(g) rich) for temperatures >100°C. We find that this range of model gas compositions reproduces well the (H2O-CO2-STOT) compositional range of reservoir waters and surface gas emissions in Iceland. From this validation of the model in an extreme end-member environment of high scrubbing, we conclude that EQ3/6-based reaction path simulations offer a realistic representation of gas–water–rock interaction processes occurring underneath active magmatic-hydrothermal systems.

Open-path FTIR spectroscopy of magma degassing processes during eight lava fountains on Mount Etna

In June–July 2001 a series of 16 discrete lava fountain paroxysms occurred at the Southeast summit crater (SEC) of Mount Etna, preceding a 28-day long violent flank eruption. Each paroxysm was preceded by lava effusion, growing seismic tremor and a crescendo of Strombolian explosive activity culminating into powerful lava fountaining up to 500 m in height. During 8 of these 16 events we could measure the chemical composition of the magmatic gas phase (H2O, CO2, SO2, HCl, HF and CO), using open-path Fourier transform infrared (OP-FTIR) spectrometry at ∼1–2 km distance from SEC and absorption spectra of the radiation emitted by hot lava fragments. We show that each fountaining episode was characterized by increasingly CO2-rich gas release, with CO2/SO2 and CO2/HCl ratios peaking in coincidence with maxima in seismic tremor and fountain height, whilst the SO2/HCl ratio showed a weak inverse relationship with respect to eruption intensity. Moreover, peak values in both CO2/SO2 ratio and seismic tremor amplitude for each paroxysm were found to increase linearly in proportion with the repose interval (2–6 days) between lava fountains. These observations, together with a model of volatile degassing at Etna, support the following driving process. Prior to and during the June–July 2001 lava fountain sequence, the shallow (∼2 km) magma reservoir feeding SEC received an increasing influx of deeply derived carbon dioxide, likely promoted by the deep ascent of volatile-rich primitive basalt that produced the subsequent flank eruption. This CO2-rich gas supply led to gas accumulation and overpressure in SEC reservoir, generating a bubble foam layer whose periodical collapse powered the successive fountaining events. The anti-correlation between SO2/HCl and eruption intensity is best explained by enhanced syn-eruptive degassing of chlorine from finer particles produced during more intense magma fragmentation.

Abiogenic hydrocarbon isotopic signatures in granitic rocks: Identifying pathways of formation

The stability and isotopic composition of hydrocarbons formed in the mantle are controversial subjects. Knowing the range in isotopic compositions of abiogenically-derived hydrocarbons is important for recognising biological signatures in ancient and extra-terrestrial materials. In an effort to enhance this database, stable isotope results are reported here for hydrocarbon-bearing fluid inclusions hosted in two peralkaline igneous complexes from Lovozero, Russia and Strange Lake, Canada. Based on the distribution of isotopic compositions, we propose three pathways for abiogenically-generated hydrocarbons. Type-1 (δ13CCH4>δ13CCO2 and δ13CC2+) represents mantle-derived hydrocarbons generated in equilibrium with hyperagpaitic magmas in the upper mantle, and suggests that mantle CH4 and C2H6δ13C and δ2H could have values of ~−5.3 and ~−112‰, and ~−10.8 and −162‰, respectively. Type-2 (δ13CCO2>δ13CCH4>δ13CC2+) represents Fischer–Tropsch-type hydrocarbon generation in quartz-bearing peralkaline rocks with CO2 as the initial magmatic gas phase. For Type-2, δ13CCO2 values are ~−2‰, whereas δ13CCH4 and δ2HCH4 values range from −32 to −20‰ and −170 and −160‰, respectively. The δ13CC2H6 values are slightly lower than associated δ13CCH4 values. Type-3 (δ13CCO2>δ13CCH4<δ13CC2+) represents an overprint of Type-2, formed during low temperature alteration of quartz-bearing peralkaline rocks. Here, δ13CCO2 values are variable, −14.0 to −9.5‰, δ13CCH4 values range from −30.8 to −20.8‰ while δ13CC2H6 values are higher than associated δ13CCH4 values. Both Type-2 and Type-3 isotopic compositions mimic patterns normally considered to be thermogenic in origin, thus demonstrating that isotopic data alone cannot be used reliably to distinguish between hydrocarbons of abiogenic versus biogenic origin.

A gas-chromatograph, continuous flow-isotope ratio mass-spectrometry method for δ13C and δD measurement of complex fluid inclusion volatiles: Examples from the Khibina alkaline igneous complex, northwest Russia and the south Wales coalfields

A new method is described for on-line extraction of complex mixtures of fluid inclusion volatiles for compound specific carbon- and hydrogen-isotope determination by gas-chromatography–continuous flow mass-spectrometry. Reproducibility for multiple aliquots of gas released from single samples and for duplicate samples is generally ≤ ± 0.7‰ for δ 13C measurements of hydrocarbon and CO 2 gases to concentrations as low as 20 nmol and ≤ ± 7‰ for δD measurements of hydrocarbon gases to concentrations as low as 100 nmol. This approach significantly improves upon the detection limits for trace gas analysis in fluid inclusions and enables rapid stable isotope measurements of multiple gas species in a single run. Two examples are evaluated and discussed in their geological context. First, stable isotope results for CH 4, CO 2 and C 2–C 5 fluid inclusion gases hosted in the undersaturated peralkaline igneous complex of Khibina in northwest Russia indicate that CH 4 was the juvenile magmatic gas phase ( δ 13C = − 13 to − 8‰, δD = − 120 to − 50‰). The δ 13C values of the C 2–C 5 hydrocarbons are lower relative to CH 4 (by up to 12‰), confirming their abiogenic origin. The δ 13C values of CO 2 are consistently lower than CH 4 (by up to 9‰) and the δD values of C 2H 6 are 70–100‰ lower than the δD CH 4 values. This pattern indicates that the CO 2 and higher hydrocarbons were generated during sub-solidus, kinetic CH 4-oxidation fractionation and polymerisation reactions. Second, CH 4, CO 2 and C 2H 6 fluid inclusion gases were analysed from quartz veins associated with anthracite from the Nant Helen mine in the south Wales coalfields. The δ 13C values of CH 4 (− 27.9 ± 0.1‰), CO 2 (− 3.1 ± 0.8‰) and C 2H 6 (− 20.7 ± 0.1‰) are typical of mature thermogenic coal-associated natural gases, as is the δD CH 4 values (− 148 ± 1‰). Assuming equilibrium, the Δ 13C CO 2–CH 4 value (+ 24.8‰) suggests that temperatures may have reached ∼ 300 °C in the anthracite zone.

Main-group pallasites

Abstract We used neutron activation to characterize the metal of 33 main-group pallasites (PMG), widely held to be samples of a core–mantle interface. Most PMG cluster in a narrow range of metal and silicate compositions, but 6 are assigned to an anomalous subset (PMG-am) because of their deviant metal compositions, and 4 others to another anomalous subset (PMG-as) because of their appreciably higher olivine Fa contents. Metal compositions in all PMG are closely related to those in evolved IIIAB irons, and are generally consistent with their formation in the IIIAB parent asteroid. On element-Au diagrams for incompatible elements the normal PMG plot near an extrapolation of IIIAB trends to higher Au concentrations. On element-Au plots of compatible elements such as Ir or Pt the loci of PMG spread out over a broader region explainable by mixing of evolved IIIAB magma with early-crystallized core or mantle-residue solids. Two features of PMG require special models: (1) Ga and Ge contents are generally high (≈1.5×) compared to the IIIAB-based mixing model: and (2) the FeO/(FeO + MgO) ratios span a surprisingly wide range, from 0.11–0.14 in normal PMG to 0.16 to 0.18 in PMG-as This range is larger than expected in a cumulate layer at the base of a mantle. We suggest that both features may be related to the interaction of PMG precursors with a highly evolved magmatic gas phase, and that some or all of these anomalies may have resulted from vapor deposits in voids near the core–mantle interface. An important boundary condition for understanding the detailed PMG origin at the core–mantle interface is the large difference between the solidus temperature of Fa11 olivine (≈2000 K) and the liquidus temperature of an evolved IIIAB melt containing >100 mg/g S and some P (≈1600 K). Following the mixing event that formed the PMG it is therefore reasonable that there would have been olivine rubble floating on top of the IIIAB-like magma, but with appreciable void space present just above the upper level reached by the magma. These voids would have contained gases released from the magma during its flow into the PMG region. We suggest that Ga and Ge, the two most volatile siderophiles in our element set, were added to PMG metal from the magmatic gas. We also suggest that the magmatic gas was oxidizing and that the PMG having high olivine fayalite contents formed in regions where the ratio of void to olivine was high, and that some metallic Fe was oxidized and entered the olivine (or the phosphoran olivine). In support of the latter idea is the observation that both Ni and Co are elevated in the PMG-as (Fa≥16) compared to values predicted by IIIAB trends. We analyzed two Eagle-Station pallasites (PES); after correction for weathering effects in Cold Bay, its composition is found to closely resemble that of Eagle Station but to represent a more evolved composition (i.e., lower Ir, higher Au). Vermillion and Yamato 8451 have been called pyroxene pallasites but have metal compositions (unrelated to those of the PMG or PES) that are too different from each other to even allow assignment to the same grouplet.

"THERMODYNAMICS OF A MAGMATIC GAS PHASE" 50 YEARS LATER: COMMENTS ON A PAPER BY JOHN VERHOOGEN (1949)

In 1949 John Verhoogen published a paper entitled Thermodynamics of a Magmatic Gas Phase in The University of California Publications in the Geological Sciences. This paper documented the first sophisticated application of thermodynamics to a problem in igneous petrology. In that paper, Verhoogen examined many aspects of the behavior of a magmatic gas phase: development and composition of a gas phase, deposition from a gas phase, variation of vapor pressure over a cooling magma. He also discussed two other possible processes in magmas: distillation and crystallization of indifferent states. Distillation occurs when a constituent has a higher concentration in the vapor than in the melt. The volatile constituents of magmas, H2O, CO2, and H2S, obviously fit the definition of constituents that undergo distillation. Verhoogen did not discuss the reverse process, partitioning of a constituent into the melt rather than the gas phase. HF is a constituent that undergoes reverse distillation; its concentration is higher in a melt than in a gas phase. Volatile constituents that undergo reverse distillation cause magmas to vesiculate at shallower depths than those that undergo distillation. Indifferent states occur during processes that change the masses of the phases of a system but leave their compositions unchanged. Univariant systems and azeotropes are familiar examples of indifferent states. At the time Verhoogen wrote his paper, there were limited amounts of experimental data on NaAlSi3O8–H2O and not much else. Thermodynamic databases constructed in the last decade provide the tools needed to explore processes described by Verhoogen 50 years ago.

Light hydrocarbons in hydrothermal and magmatic fumaroles: hints of catalytic and thermal reactions

Volcanic gaseous mixtures emitted from active volcanoes frequently show variable amounts of saturated (alkanes), unsaturated (alkenes) and aromatic volatile hydrocarbons. Three major patterns of distributions can be recognized, apparently related to the chemical-physical environment of formation of the gas exhalations: alkane-rich, low-temperature gas emissions from recently active volcanic areas; aromatic-rich hydrothermal manifestations; and alkene-rich, ‘magmatic’ fumaroles on active volcanoes. Thermodynamic data, together with theoretical and practical findings from the petroleum industry, point to two main types of reactions occurring in these volcanic environments: cracking and reforming. Cracking processes, mainly caused by thermal effects, occur when hydrocarbon-bearing hydrothermal fluids enter and mix with a hot and dry, rapidly rising magmatic gas phase. The most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures. The presence of aromatic species in hydrothermal fluids can be linked to reforming processes, catalysed by several possible agents, such as smectites and zeolites, generally present in the hydrothermally altered volcanic terranes, and facilitated by long residence times in a hydrothermal envelope.

Magmatic gas source for the stratospheric SO2 cloud from the June 15,1991, eruption of Mount Pinatubo

Research Article| October 01, 1992 Magmatic gas source for the stratospheric SO2 cloud from the June 15,1991, eruption of Mount Pinatubo H. R. Westrich; H. R. Westrich 1Geochemistry Department 6118, Sandia National Laboratories, Albuquerque, New Mexico 87185 Search for other works by this author on: GSW Google Scholar T.M. Gerlach T.M. Gerlach 2U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661 Search for other works by this author on: GSW Google Scholar Author and Article Information H. R. Westrich 1Geochemistry Department 6118, Sandia National Laboratories, Albuquerque, New Mexico 87185 T.M. Gerlach 2U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (10): 867–870. https://doi.org/10.1130/0091-7613(1992)020<0867:MGSFTS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. R. Westrich, T.M. Gerlach; Magmatic gas source for the stratospheric SO2 cloud from the June 15,1991, eruption of Mount Pinatubo. Geology 1992;; 20 (10): 867–870. doi: https://doi.org/10.1130/0091-7613(1992)020<0867:MGSFTS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A water-rich magmatic gas phase escaped explosively from Mount Pinatubo on June 15, 1991, taking with it a load of crystalline and molten material sufficient to form pumice and tephra deposits with an estimated total dense-rock-equivalent volume of 3-5 km3 (Scott et al., 1991), and carrying in it enough sulfur to form a 20 Mt SO2 cloud in the stratosphere. Application of the "petrologic method" for estimating sulfur degassing during the climactic event from the sulfur content of trapped glass inclusions and matrix glasses in the pumice deposits requires an unacceptably large volume of erupted magma to account for SO2 in the stratospheric cloud. The ubiquitous presence of primary vapor bubbles in glass inclusions and unaltered anhydrite phenocrysts in the pumice suggest that sulfur was present in a separate H2O-rich gas phase of the Pinatubo magma before eruption. Thus, for this eruption, and perhaps others, the petrologic method for estimating sulfur degassing is prone to substantial underestimation of sulfur release and the potential climatic impact of past explosive eruptions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Chlorine in basalts from Iceland

Chlorine has a higher solubility in basaltic magmas than other volatiles. A theoretical degassing model predicts that less than 10% of the chlorine originally present in the magma is lost to the atmosphere during surface degassing. This prediction is born out by strong correlation between chlorine and nonvolatile elements in samples from recent volcanic eruptions in Iceland. It is concluded that the chlorine content of subaerial basaltic lavas is proportional to the chlorine content of the magmas. Assuming a roughly constant ratio between chlorine and water in the magmatic gas phase and assuming further that this ratio approximates the Cl H 2O ratio in sea water and sediments it is possible to assign each basaltic magma a fixed amount of water. The figures thus obtained (0.3–1.2% H 2O) are identical with previously assumed water contents of basaltic magmas. The implied water contents of the melts are discussed in terms of derivation from hydrous mantle phases. It is concluded that such phases can supply only a fraction of the assumed water content, the rest is derived from an additional volatile source in the mantle. The mantle source giving rise to alkali basalts in Iceland has a lower content of volatiles than the tholeiite source.

Variations in the carbon, sulfur and chlorine contents of volcanic gas discharges from White Island, New Zealand

On six occasions, Irom July 1971 to June 1973, discharge samples from twelve selected fumaroles on White Island in the Bay of Plenty, New Zealand, were collected and analysed. The large variations in the absolute and relative proportions of carbon, sulfur, and chlorine containing species, occurring simultaneously over the entire active area of the volcano, suggest that all fumaroles are essentially supplied with one common type of gas made up of at least three components. During quiet periods, with low fumarolic outlet temperatures (≤300), the component with ratios C:S:Cl of 92:3:5 predominates; during heating up periods the relative carbon contents decrease, considered to be due to admixture of a component with C:S:Cl ratios of 44:26:30. Coinciding with maximum outlet temperatures (≈ 800°), however, the component with C:S:Cl ratios of 70:29:1 predominates, causing a rapid and marked rise in the ratio S:Cl from unity to above ten. The latter component with low ratios of C:S of around 2.5 and ratios of S:Cl of above ten is considered to be most closely related to the primary magmatic gas phase.

Some basaltic and andesitic gases

Igneous gases escape from igneous rocks and magmas. They may originate by effervescence or they may bubble through the melt from an external source. In this paper I review data (mostly post‐1963) on H2O, B, CO2, N2, O2, Ne, sulfur gases, Cl, Ar36, Ar40, Br, Kr, I, Xe, Hg, and Rn. The compositions of volcanic gases are compatible with solubility and melt composition for H2O, Cl, and possibly sulfur gases. For other gases there are either inadequate data or conflicting data (CO2). Volcanic gas emitted at surface pressure is generally very rich in H2O because H2O is the principal gas dissolved in most melts. CO2‐rich volcanic gases can be explained by separation of gas from melt at pressures of a few tens of atmospheres and by effervescence of magmas with high ratios of dissolved CO2/H2O (mostly alkaline basalts). There is no single magmatic gas phase. Glassy basalts from the sea floor have substantial concentrations of both active and noble gases. The concentrations of H2O and Cl correlate with the alkali concentration (especially K2O). The recorded concentrations of H2O and CO2 in sea floor basalts are far below saturation at 500 atm. Yet the basalts contain tiny vesicles. It appears likely that magmatic gas of basalts at sea floor pressure is 60% SO2, 20% H2O, 10% H2S, and 10% CO2 at 1200°C. The concentrations of Cl and H2O in basaltic and andesitic glasses trapped in large crystals in pumices from continental margins are about 10 times those of deep sea basalts. Detailed relationships suggest that at least some andesitic magmas develop in vapor‐saturated environments. The restored or potential concentrations of H2O and Cl in andesitic magmas I estimate by allowing for vapor loss during production of andesite from a basaltic precursor. I use the restored concentrations as a basis for estimating the rate of igneous outgassing in island arcs and continental margins. The inferred rates of outgassing are such that the present quantities of gaseous elements in seawater and sediment would be exhaled in 109 to 1010 years for H2O, Cl, Br, S, I, and Hg. The rates of outgassing of CO2 from Kilauea volcano and of SO2 from Pacaya exceed the rate which could be maintained by outgassing of erupted lava. These relations imply the existence of a larger source of these gases than that of the volcanic rocks. Apparently, large subsurface reservoirs of melt discharge gas out of the volcanic vents. It seems probable that volcanic rocks are generally volumetrically small portions of subsurface reservoirs which may be vapor‐saturated. Such a condition has major implications for crustal evolution, the origins of volcanic rocks, eruption mechanisms, and hydrothermal ore deposits.

On the Geochemical Cycle of Fluorine

The average concentration of fluorine in the earth's crust is about 800 gm. per ton. In basalts and gabbros it is present as apatite; in granites, gneisses, and metamorphic rocks it is contained in mica, hornblende, and fluorspar. Much fluorine enters into the magmatic gas phase, which has an acid reaction. Great amounts of fluorine are therefore exhaled by volcanic eruptions. If the magmatic gas phase has opportunity to react with the wall rock, it turns alkalic, and fluorine is extracted (as phosphate?) before it reaches the surface; alkaline hot springs carry no magmatic fluorine. Of the fluorine dissolved by the weathering processes, only 0.2 per cent is found in the ocean. Fluorine is extracted from inland waters and from the sea, forming mineral deposits with a composition resembling fluorapatite.