Volcanic Vapors Research Articles

Hydrothermal activity and associated subsurface processes at Niuatahi rear-arc volcano, North East Lau Basin, SW Pacific: Implications from trace elements and stable isotope systematics in vent fluids

Hydrothermal activity is abundant in the area between the North Eastern Lau Spreading Center and the Tofua intra-oceanic island arc with multiple active sites in the rear-arc at the Mata and Niuatahi volcanoes. We report geochemical data for high-temperature vent fluids sampled from within the caldera of Niuatahi volcano. Hydrothermal fluids were sampled from three vent sites: South Central, Southwestern Cone and Northern Cone located in water depths between 1607 and 1699 m. Maximum temperatures of 334 °C were measured and pH values were as low as 2.8. The vent fluids were characterized by depletions in Mg, SO4 and U as well as an enrichment of (trace) metals (e.g., Fe, Mn, K, Li) and dissolved gases (e.g., H2S, CO2, H2) relative to seawater. Water-rock ratios calculated based on concentrations (K, Li, Rb, Cs, REE) and isotope ratios (δ7Li, δ11B, 87Sr/86Sr) suggest fluid-rock interactions under rock-dominated conditions at all three vent sites.The South Central vents lie closest to the site of most recent volcanic activity in the Niuatahi caldera. Vent fluids are characterized by relatively low Cl concentrations (as low as 292 mmol/kg) that are indicative of sub-critical phase separation. These fluids also had the lowest pH values (2.8–3.1), highest H2S and lowest H2, CH4 and CO2 concentrations of the three sites. The δD and δ18O values suggest that H2O and CO2 were added in small amounts by subduction-related volcanic vapors. However, there was no evidence for magmatic SO2 input in the vent fluids at the time of sampling in 2018. Vent fluids from the Northern and Southwestern Cone sites on the caldera ring fault had a similar chemical composition, despite being situated at opposite sides of the caldera. Fluids from these sites had lower Fe/Mn ratios (<1) and H2S concentrations than those from South Central suggesting that they were affected by subsurface cooling and sulfide precipitation. This study indicates variations of the Niuatahi hydrothermal vent sites depending on the location within the caldera with variable effects of fluid-rock interaction and magmatic input on fluid compositions in agreement with previous work on fluid S isotopes and sulfides.

Exploration Implications of Multiple Formation Environments of Advanced Argillic Minerals

Abstract Advanced argillic minerals, as defined, include alunite and anhydrite, aluminosilicates (kaolinite, halloysite, dickite, pyrophyllite, andalusite, zunyite, and topaz), and diaspore. One or more of these minerals form in five distinctly different geologic environments of hydrolytic alteration, with pH 4–5 to &amp;lt;1, most at depths &amp;lt;500 m. (1) Where an intrusion-related hydrothermal system, typical of that associated with porphyry Cu ± Au deposits, evolves to white-mica stability, continued ascent and cooling of the white-mica–stable liquid results in pyrophyllite (± diaspore) becoming stable near the base of the lithocap. (2) A well-understood hypogene environment of formation is vapor condensation near volcanic vents, where magmatic SO2 and HCl condense into local groundwater to produce H2SO4 and HCl-rich solutions with a pH of 1–1.5. Close to isochemical dissolution of the host rock occurs because of the high solubility of Al and Fe hydroxides at pH &amp;lt;2, except for the SiO2 component, which remains as a siliceous residue because of the relatively low solubility of SiO2. This residual quartz, commonly with a vuggy texture, is largely barren of metals because of the low metal content in high-temperature but low-pressure volcanic vapor. Rock dissolution causes the pH of the acidic solution to increase, such that alunite and kaolinite (or dickite or pyrophyllite at higher temperatures) become stable, forming a halo to the residual quartz. This initially barren residual quartz, which forms a lithocap horizon where permeable lithologic units are intersected by the feeder structure, may become mineralized if a subsequent white-mica–stable liquid ascends to this level and precipitates copper and gold. (3) Boiling of a hydrothermal liquid generates vapor with CO2 and H2S. Where the vapor condenses above the water table, atmospheric O2 in the vadose (unsaturated) zone causes oxidation of H2S to sulfuric acid, forming a steam-heated acid-sulfate solution with pH of 2–3. In this environment, kaolinite and alunite form in horizons above the water table at &amp;lt;100°C. Silica derived within the vadose zone will precipitate as amorphous silica at the water table, as the condensate follows the hydraulic gradient, causing opal replacement above and at the aquifer. (4) By contrast, where condensation of this vapor occurs below the water table, the CO2 in solution forms carbonic acid (H2CO3), leading to a pH of 4–5. This marginal carapace of condensate, with temperatures up to 150°–170°C, commonly acts as a diluent of the ascending parental NaCl liquid. This steam-heated liquid forms intermediate argillic alteration of clays, kaolinite, and Fe-Mn carbonates; this kaolinite, which can be present at depths of several hundreds of meters, can potentially be mistaken as having been caused by a steam-heated acid-sulfate or supergene overprint. (5) The final setting is supergene, caused by posthydrothermal weathering and oxidation of mainly pyrite, locally creating pH &amp;lt;1 liquid because of high concentrations of H2SO4 within the vadose zone and forming kaolinite, alunite, and Fe oxyhydroxides. This genetic framework of formation environments of advanced (and intermediate) argillic alteration provides the basis to interpret alteration mineralogy, in combination with alteration textures and morphology plus zonation, including the overprint of one alteration style on another. This framework can be used to help focus exploration for and assessment of hydrothermal ore deposits, including epithermal, porphyry, and volcanic-hosted massive sulfide.

Processes responsible for variations in the isotopic composition (ΔD and Δ18O) of thermal waters of the Kuril islands arc

Many active volcanoes of the Kuril Islands host hydrothermal systems. Their surface manifestations are represented by numerous thermal springs showing diverse chemical composition and physical-chemical parameters. Four main isotopic shifts relative to the local meteoric water line can be observed in the corresponding δD vs. δ18O diagrams. For the acid Cl-SO4 waters there is a clear mixing trend between meteoric water and volcanic vapor. The acid SO4waters demonstrate trends indicating kinetic fractionation at temperatures close to the boiling-point. Isotopic composition of the coastal springs tend to march the mixing line between meteoric and seawater. The δ18O-shift for deep thermal water is accounted to of isotopic exchange with host rock. The latitude effect revealed for meteoric waters also observed in the isotopic composition of the thermal waters.

Processes Responsible of Variations in the δ18O – δD values of Thermal Waters from the Kuril Islands (Russia)

Many active and dormant volcanoes of the Kuril Islands host hydrothermal systems which discharge acid to ultra-acid SO4-Cl (Cl-SO4) and SO4 waters. On some island, hot near-neutral Na-Cl waters can be found discharging in coastal hot springs. Four main different isotopic shifts relative to the local meteoric water line can be observed in the corresponding δD vs. δ18O diagram. For the acid Cl-SO4 waters discharging within thermal fields on volcano slopes, there is a clear mixing trend between meteoric water and volcanic vapor. Steam-heated SO4 waters demonstrate trends indicating kinetic fractionation at temperatures close to the boiling-point. For the coastal springs, the trend is apparently a mixing line between meteoric and seawater. The δ18O-shift for deep thermal waters is related to isotopic exchange with host rock but there is also a clear latitude effect in the isotopic composition of the meteoric endmember.

Geochemistry of thermal waters in the Southern Volcanic Zone, Chile – Implications for structural controls on geothermal fluid composition

Thermal waters in the Southern Volcanic Zone (SVZ) of Chile between 36°–41°S were studied using major and selected trace element relationships to characterize their geochemistry, formation mechanisms, and to explore the influence of regional structural controls on fluid composition. Three sets of waters were identified based on physicochemical characteristics: (i) NaCl waters, (ii) acid-sulfate waters, and (iii) bicarbonate (HCO3) waters. NaCl waters are the most abundant type in the studied region and their chemistry is controlled by significant water-rock interaction. They are characterized by an alkaline pH (7.2–9.3), generally lower temperatures (avg: 55°C), and relatively high concentrations of Cl, Na, B, As, Li, and Cs. Acid-sulfate waters are typically associated with volcanoes and have a strong magmatic/volcanic component due to the absorption of magmatic vapors. They are acidic (pH<4), generally higher in temperature (avg: 85°C), and have elevated concentrations of SO4, Mg, and Ba. Bicarbonate waters are characterized by the highest concentrations of HCO3 (>892ppm) in the region and are similar in temperature (<47°C) and pH (>6.2) to NaCl waters. They have elevated concentrations of most cations (Ca, K, Na, Mg, Ba, Sr) as a result of intense shallow cation leaching due to the absorption of CO2-rich volcanic vapors on the peripheries of geothermal systems. The thermal waters were also characterized according to their spatial relation with the dominant fault systems of the region: the NNE-striking intra-arc Liquiñe-Ofqui Fault System (LOFS) and the WNW-striking Andean Transverse Faults (ATF). The inherent differences in fault nature between these fault systems constitutes the primary structural control influencing geothermal fluid development in the SVZ. The chemistry of waters spatially associated with the LOFS as a whole is defined by high vertical permeability networks and lack magmatic reservoir development. Therefore, these waters tend to have higher Cl/B ratios and strong correlations between trace alkali metals and Cl due to rapid, efficient upflow pathways. In contrast, waters spatially associated with the ATF have lower Cl/B ratios and show no correlation between trace alkali metals and Cl due to degassing magma chambers and decreased vertical permeability. The relationship between water type and structural domain in Cl/B ratios and trace metal behavior provides evidence that fault geometry and kinematics exert a fundamental control on geothermal fluid development in the SVZ of Chile.

The Difficulty of Measuring the Absorption of Scattered Sunlight by H2O and CO2 in Volcanic Plumes: A Comment on Pering et al. “A Novel and Inexpensive Method for Measuring Volcanic Plume Water Fluxes at High Temporal Resolution,” Remote Sens. 2017, 9, 146

In their recent study, Pering et al. (2017) presented a novel method for measuring volcanic water vapor fluxes. Their method is based on imaging volcanic gas and aerosol plumes using a camera sensitive to the near-infrared (NIR) absorption of water vapor. The imaging data are empirically calibrated by comparison with in situ water measurements made within the plumes. Though the presented method may give reasonable results over short time scales, the authors fail to recognize the sensitivity of the technique to light scattering on aerosols within the plume. In fact, the signals measured by Pering et al. are not related to the absorption of NIR radiation by water vapor within the plume. Instead, the measured signals are most likely caused by a change in the effective light path of the detected radiation through the atmospheric background water vapor column. Therefore, their method is actually based on establishing an empirical relationship between in-plume scattering efficiency and plume water content. Since this relationship is sensitive to plume aerosol abundance and numerous environmental factors, the method will only yield accurate results if it is calibrated very frequently using other measurement techniques.

Geochemistry of volcanic gas at the 2012–13 New Tolbachik eruption, Kamchatka

We report measurements of the chemical and isotopic composition of gas emitted from the lava flow at the 2012–13 New Tolbachik eruption. Gas and condensate samples were taken from two vents over a lava tube in May 2013. The 1030°C gas sample was collected in evacuated Giggenbach bottle from a periodically pumping-to-venting outlet above active lava flow ~300m from Naboko cone. Concentrations of major components in the 1030°С gas sample are (mol%): 95.5 H2O, 0.47 CO2, 2.01 SO2, 1.18 HCl, 0.34 HF that are within a range of gas compositions for subduction zone volcanoes. Isotopic analysis of He gives a corrected to atmosphere R/Ra ratio=7.24 (He/Ne ratio=1.41) that is close to MORB values. The 1030°C condensate contained 9.7ppm Cu, 2.5ppm Zn, 1.5ppm Tl, 20ppb Re and 3ppb Au, and can be considered as a representative sample for the metal composition of exsolved magmatic gases at the 2012–13 Tolbachik eruption. Isotopic data on the 1030°C condensate (δ18O=6.4‰, δD=−32‰) indicate a magmatic source. Another condensate sample taken at 690°C was found to be drastically different from the magmatic 1030°C condensate. We suggest that the disproportional enrichment in trace elements of this 690°C condensate as compared to the 1030°C condensate could result from evaporation at forced pumping during sampling and possible dissolution of earlier precipitated sublimates in the gas conduit. Unusual isotopic composition of the 690°C condensate (δ18О=18.9‰, δD=−68.5‰) can be explained by the isotopic exchange between volcanic vapor and atmospheric O2 (δ18О=23.5‰).

Gold recycling and enrichment beneath volcanoes: A case study of Tolbachik, Kamchatka

Magmas supply metals to hydrothermal ore deposits, although typical arc basalts may be unable to produce a gold-rich ore-forming fluid, as such basalts rarely exceed 5 ppb Au. Consistent with this, the occurrence of native gold of magmatic origin is extremely rare, and only a few finds of micron-sized gold particles in unaltered basalts have been documented. Surprisingly, some lava flows and scoria cones of the historic basaltic eruptions of Tolbachik volcano (Kamchatka) are unusually gold-rich. Tolbachik basalts contain up to 11.6 ppb Au based on whole rock analyses, nuggets of gold (electrum) up to 900 μm in size and native gold droplets up to 200 μm, plus numerous vapor-deposited gold crystals within fumarolic incrustations and directly on surfaces of basaltic lapilli. Our results demonstrate that the gold nuggets in Tolbachik basalt are of hydrothermal origin and were physically scavenged from epithermal veins hosted by country rocks during intrusion of mafic magmas. Depending on the melt temperature and/or time span of the melt-rock interaction, gold was ejected by the erupting volcano either in the form of abraded nuggets or liquid droplets, or was fully assimilated (dissolved) into the shallow long-lived magma chamber to provide a 4-fold increase in gold content over background concentration of 2.7 ppb Au, characteristic of mafic volcanic rocks in Kamchatka. Upon the end of the eruption, the continued discharge of volcanic vapors enriched in gold deposited abundant crystals of gold on cooling lava and scoria. Similar to Tolbachik, recycling of metals from prior accumulations (ore deposits) in the shallow crust may take place in other long-lived magma reservoirs, thus upgrading the gold and other metal contents and contributing to the ore-forming potential of a magma.

Links between arc volcanoes and porphyry-epithermal ore deposits

Porphyry and epithermal ore deposits, which are the products of magmatic hydrothermal fluids, are intimately associated with volcanoes in continental and island arcs above subduction zones, but the exact nature of this relationship has remained enigmatic. Although metal deposition is usually thought to occur during the waning stages of volcanism, numerous ore deposits have been demonstrated to be synvolcanic. Here we show how the formation of these deposits is tied to volcanic cycles. We relate the chemical variations in vapors from Merapi volcano, Indonesia, to different stages of its eruptive cycle. The chemical compositions of volcanic vapors from subduction zone volcanoes are then compared globally to those of fluid inclusions from porphyry-epithermal deposits. These data show that adiabatic decompression is the principal control on mineralization. The data also suggest that volcanic and subvolcanic magmatic-hydrothermal systems are under lithostatic pressure during quiescence but decompress rapidly during injections of mafic magma and explosive eruptions. During quiescence, the magma evolves through fractional crystallization and devolatilization, gradually becoming oxidized and enriched in gold and other incompatible metals. Upon the injection of sulfur-rich mafic magmas, subvolcanic intrusions brecciate the overlying rocks, the systems are depressurized, the volcanoes erupt explosively, supercritical fluids unmix into vapor and brine, and base metal sulfides precipitate.

Timescales and mechanisms of formation of amorphous silica coatings on fresh basalts at Kīlauea Volcano, Hawai'i

Young basalts from Kīlauea Volcano, Hawai'i, frequently feature opaque surface coatings, 1–80μm thick, composed of amorphous silica and Fe–Ti oxides. These coatings are the product of interaction of the basaltic surface with volcanically-derived acidic fluids. Previous workers have identified these coatings in a variety of contexts on Hawai'i, but the timescales of coating development, coating growth rates, and factors controlling lateral coating heterogeneity were largely unconstrained. We sampled and analyzed young lava flows (of varying ages, from hours to ~40years) along Kīlauea's southwest and east rift zones to characterize variation in silica coating properties across the landscape. Coating thickness varies as a function of flow age, flow surface type, and proximity to acid sources like local fissure vents and regional plumes emitted from Kīlauea Caldera and Pu'u Ō'ō. Silica coatings that form in immediate proximity to acid sources are more chemically pure than those forming in higher pH environments, which contain significant Al and Fe. Incipient siliceous alteration was observed on basalt surfaces as young as 8days old, but periods of a year or more are required to develop contiguous coatings with obvious opaque coloration. Inferred coating growth rates vary with environmental conditions but were typically 1–5μm/year. Coatings form preferentially on flow surfaces with glassy outer layers, such as spatter ramparts, volcanic bombs, and dense pahoehoe breakouts, due to glass strain weakening during cooling. Microtextural evidence suggests that the silica coatings form both by in situ dissolution–reprecipitation and by deposition of silica mobilized in solution. Thin films of water, acidified by contact with volcanic vapors, dissolved near-surface basalt, then precipitated amorphous silica in place, mobilizing more soluble cations. Additional silica was transported to and deposited on the surface by silica-bearing altering fluids derived from the basalt interior.

Modeling the Formation of Advanced Argillic Lithocaps: Volcanic Vapor Condensation Above Porphyry Intrusions

Hypogene advanced argillic alteration, typically quartz-alunite with halos of kaolinite ± dickite and roots of pyrophyllite ± diaspore, forms in the epithermal environment from condensates of magmatic vapor that contain SO2 and HCl, which exsolved from an underlying intrusive source. The most aggressive, nearly isochemical leaching of the host rock by the most acidic condensate, commonly pH ~1, leaves residual silica that recrystallizes to quartz within the flow channel at a high condensate/rock ratio, forming the core of alteration. The alteration characteristically flares upward along feeder structures, and if a permeable lithologic unit is intersected, the alteration zones mushroom to form a subhorizontal blanket due to lateral flow. Where subsequently mineralized, the residual quartz, commonly with a vuggy texture that reflects the texture of the original lithology, has higher Au (and Cu) grades than the quartz-alunite halo. Tonnage in these high sulfidation systems may develop within the subhorizontal lithocap, although the highest grades are typically confined to the structurally controlled feeders. We modeled a typical volcanic vapor condensate, starting with the measured composition of ≤877°C fumaroles from Satsuma Iwojima rhyolite dome, Japan, as it cooled and reacted with a host rhyolite; the predicted hydrothermal mineralogy reproduces the alteration pattern observed in lithocaps that host high sulfidation deposits. The modeling confirms that aluminum-rich minerals (pyrophyllite, diaspore, locally andalusite) are stable at higher temperature at depth, whereas at lower temperature and shallower depth, Na and K alunite become stable. At the lowest temperature ( 10:1), and where SO2 >>H2S in the original volcanic vapor; this SO2-dominant composition is typical of andesitic to rhyolitic volcanoes. The reason for this mineral transition, and the upward flare (widening) of the alteration zone along structures, is related to the dissociation and increased reactivity of H2SO4 and HCl as the temperature decreases. Below ~200°C, only quartz, pyrite, native S, and anhydrite are stable, hence the formation of the dominant quartz from the silica residue. A further check on our modeling is the observation that the calculated composition of the condensate after reaction with fresh Satsuma Iwojima rhyolite and alteration minerals, and cooling to 100°C, is similar (within a factor of two) to that of acidic springs, with pH ~1, that discharge around Satsuma Iwojima and other active volcanoes. The most extensive lithocap alteration, residual quartz and/or quartz-alunite, is commonly offset from the surface projection of the causative intrusion. This observation can be explained by a combination of two factors—hydrology and temperature. Due to hydraulic gradients at shallow depths in a volcanic edifice, the acidic condensate tends to flow along permeable lithologic units away from the locus of the high-temperature vapor plume, which rises directly over the intrusion. Where lateral flow occurs, the most intense leaching and widespread advanced argillic alteration, which develops largely at temperatures of <200° to 250°C based on our modeling, will form away from the near-surface projection of the intrusion, which is the area of highest surface temperature due to ascent of the vapor plume. This potential for lithocap alteration to form on the shoulder of the underlying causative intrusion must be assessed when exploring for high sulfidation ore; the highest ore grades are commonly related to structures that are proximal to the parent intrusion, but they are unlikely to lie directly over the intrusion. In addition, identifying the most likely location of the source intrusion will help the explorer to efficiently assess any potential for deeper porphyry-style mineralization.

Experimental study of acid‐sulfate alteration of basalt and implications for sulfate deposits on Mars

Acid‐sulfate alteration of basalt by SO2‐bearing volcanic vapors has been proposed as one possible origin for sulfate‐rich deposits on Mars. To better define mineralogical signatures of acid‐sulfate alteration, laboratory experiments were performed to investigate alteration pathways and geochemical processes during reaction of basalt with sulfuric acid. Pyroclastic cinders composed of phenocrysts including plagioclase, olivine, and augite embedded in glass were reacted with sulfuric acid at 145 °C for up to 137 days at a range of fluid : rock ratios. During the experiments, the phenocrysts reacted rapidly to form secondary products, while the glass was unreactive. Major products included amorphous silica, anhydrite, and Fe‐rich natroalunite, along with minor iron oxides/oxyhydroxides (probably hematite) and trace levels of other sulfates. At the lowest fluid : rock ratio, hexahydrite and an unidentified Fe‐silicate phase also occurred as major products. Reaction‐path models indicated that formation of the products required both slow dissolution of glass and kinetic inhibitions to precipitation of a number of minerals including phyllosilicates and other aluminosilicates as well as Al‐ and Fe‐oxides/oxyhydroxides. Similar models performed for Martian basalt compositions predict that the initial stages of acid‐sulfate alteration of pyroclastic deposits on Mars should result in formation of amorphous silica, anhydrite, Fe‐bearing natroalunite, and kieserite, along with relict basaltic glass. In addition, analysis of the experimental products indicates that Fe‐bearing natroalunite produces a Mössbauer spectrum closely resembling that of jarosite, suggesting that it should be considered an alternative to the component in sulfate‐rich bedrocks at Meridiani Planum that has previously been identified as jarosite.

Geochemistry of volcanic and hydrothermal gases of Mutnovsky volcano, Kamchatka: evidence for mantle, slab and atmosphere contributions to fluids of a typical arc volcano

We report chemical compositions (major and trace components including light hydrocarbons), hydrogen, oxygen, helium and nitrogen isotope ratios of volcanic and geothermal fluids of Mutnovsky volcano, Kamchatka. Several aspects of the geochemistry of fluids are discussed: chemical equilibria, mixing of fluids from different sources, evaluation of the parent magmatic gas composition and contributions to magmatic vapors of fluids from different reservoirs of the Kamchatkan subduction zone. Among reactive species, hydrogen and carbon monoxide in volcanic vapors are chemically equilibrated at temperatures >300°C with the SO2-H2S redox-pair. A metastable equilibrium between saturated and unsaturated light hydrocarbons is attained at close to discharge temperatures. Methane is disequilibrated. Three different sources of fluids from three fumarolic fields in the Mutnovsky craters can be distinguished: (1) magmatic gas from a large convecting magma body discharging through Active Funnel, a young crater with the hottest fumaroles (up to 620°C) contributing ~80% to the total volcanic gas output; (2) volcanic fluid from a separate shallow magma body beneath the Bottom Field of the main crater (96–280°C fumaroles); and (3) hydrothermal fluid with a high relative and absolute concentrations of CH4 from the Upper Field in the main crater (96–285°C fumaroles). The composition of the parent magmatic gas is estimated using water isotopes and correlations between He and other components in the Active Funnel gases. The He-Ar-N2 systematics of volcanic and hydrothermal fluids of Mutnovsky are consistent with a large slab-derived sedimentary nitrogen input for the nitrogen inventory, and we calculate that only ~1% of the magmatic N2 has a mantle origin and <<1% is derived from the arc crust.

Structure and stability of cadmium chloride complexes in hydrothermal fluids

The identity and stability of aqueous species formed by cadmium in H 2O–Na/LiCl–HCl–HNO 3 solutions were investigated using in situ X-ray absorption spectroscopy (XANES and EXAFS) at 20–450 °C and 1–600 bar, combined with solubility measurements of CdO and CdS solids at 350–400 °C and 300–600 bar. Results show that aqueous Cd speciation is dominated by the cation Cd(H 2O) 6 2+ in acidic Cl-free solutions, and by chloride species CdCl m (H 2O) n 2 − m over a wide range of temperatures (20 ≤ T ≤ 450 °C), acidities (1 ≤ pH ≤ 8), and chloride concentrations (0.04 ≤ m Cl ≤ 18 mol/kg H 2O). EXAFS spectra from chloride solutions show that with increasing T and m Cl the average number of Cl atoms increases from 1 to ∼ 3 (± 0.6), accompanied by a decrease of the number of O from 6 to ∼ 1 (± 0.7), in the nearest coordination sphere around Cd. Average Cd–Cl distances in Cd complexes vary in the range 2.41–2.52 (± 0.02) Å, increasing with increasing m Cl and decreasing with increasing T, while Cd–O distances remain constant within errors at 2.30 (± 0.05) Å in the whole T and m Cl range investigated. This evolution of the Cd atomic environment is accompanied by changes in geometry of the dominant Cd–O/Cl species from octahedral-like to tetrahedral-like with increasing T and m Cl, as shown by XANES spectra analyses. These structural data are consistent with the formation of octahedral Cd(H 2O) 6 2+ and CdCl(H 2O) 5 + and tetrahedral CdCl 2(H 2O) 2 0, CdCl 3(H 2O) − and CdCl 4 − species at T ≤ 200–300 °C and m Cl ≤ 18. At T > 300 °C, aqueous Cd speciation is dominated by two complexes, CdCl 2(H 2O) 2 0 and CdCl 3(H 2O) − in a wide m Cl range (0.04–5 m). The stability constants of these complexes, derived from solubility measurements at 400 °C and 600 bar are ∼ 6 orders of magnitude higher than available HKF-model predictions based on low- T data. The new data on aqueous Cd complexes may be used for interpretation of Cd/Zn ratios measured in natural hydrothermal saline fluids and volcanic vapors in which the greater stability of Cd chloride species as compared to their Zn analogs is likely to be responsible for the elevated Cd/Zn ratios in comparison to crustal rocks and Cl-poor waters.

Effects of steam-heating processes on a stratified volcanic aquifer: Stable isotopes and dissolved gases in thermal waters of Vulcano Island (Aeolian archipelago)

We report on a comprehensive study of major-ion chemistry, dissolved gases, and stable isotopes measured in water wells at Vulcano Island since 1988. The work focuses on a quantitative model describing steam condensation and boiling phenomena in shallow water bodies. The model is based on the differences in partition coefficients between liquid water and vapor characterizing oxygen and hydrogen isotopes, as well as volcanic gases (CO 2, S species, and HCl). Based on both physical conditions of aquifers identified during drilling campaigns and the composition of the volcanic vapor, mass and enthalpy balances are applied in a multistep process of steam separation and condensation in shallower aquifers. By comparing the model results with measured data, we infer that (i) strong isotope enrichment observed in some shallow thermal waters can result from an increasing mass rate of condensing deep vapor, even in water meteoric in origin; (ii) the high CO 2 content measured in the fumarolic vapor during 1988–1993 affected the δ 18O value of the steam-heated water due to CO 2–H 2O isotope exchange; (iii) the high pCO 2 measured in the coldest and peripheral waters are explained by the progressive enrichment of this gas in the vapor phase during multistep boiling; and (iv) the high Cl − and SO 4 2−contents in the hottest waters can be attributed to the direct condensation (single-step) of volcanic vapor. The model also takes into account both the mass fluxes and the compositions of the involved endmembers (steam and shallow groundwater), which provides important inferences on the modifications observed or expected during periods of increasing mass and heat input from depth.

Naturally acid waters from Copahue volcano, Argentina

Volcanic acid sulfate–chloride brines form through absorption of volcanic vapors in shallow reservoirs of meteoric water. Reaction with surrounding volcanic rocks leads to partial neutralization of the fluids and precipitation of secondary minerals. Chemical data of such acid waters from Copahue volcano, Argentina, covering 8 years of observations, show evidence for changes in composition related to water rock interaction at depth prior to emergence of the fluids at the surface. The chemical composition changed dramatically during the 2000 eruption of Copahue, with enhanced concentrations and fluxes of Mg, Na, Fe and Al, followed in 2001 by rapidly declining concentrations and element fluxes. The subsequent 5 years saw more variable element ratios and strong depletions in K and Al. Most incompatible elements are released from the rock matrix stochiometrically, whereas some elements are enriched through vapor input from the magma (As, Pb, Zn). Most fluids have LREE enrichments relative to the rock matrix, but during periods of new magma intrusion the LREE enrichment decreases as does the magnitude of the negative Eu anomaly in the fluids. These observations are interpreted assuming early dissolution of plagioclase, olivine and volcanic glass that occurs during intrusion of new magma into the hydrothermal system. The high field strength elements are virtually immobile even in these hot acid fluids, with Nb and Ta more so than Hf and Zr. The mobility of U and Th in these fluids is comparable, at variance with Th behavior in neutral fluids. The local rivers and lakes of Copahue are fertilized by volcanic dissolved P, and most surface waters with pH < 3 have high levels of As. The acid fluids from Copahue may be surficial analogs for deep subduction fluids that evolve below zones of arc magma generation as well as for early Mars environments that are thought to have had large acid lakes.

100th Anniversary Special Paper: Vapor Transport of Metals and the Formation of Magmatic-Hydrothermal Ore Deposits

In most published hydrothermal ore deposit models, the main agent of metal transport is an aqueous liquid. However, there is increasing evidence from volcanic vapors, geothermal systems (continental and submarine), vapor-rich fluid inclusions, and experimental studies that the vapor phase may be an important and even dominant ore fluid in some hydrothermal systems. This paper reviews the evidence for the transport of metals by vapor (which we define as an aqueous fluid of any composition with a density lower than its critical density), clarifies some of the thermodynamic controls that may make such transport possible, and suggests a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapor or a vapor-derived fluid. Analyses of vapor (generally >90% water) released from volcanic fumaroles at temperatures from 500° to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (from ppm to wt %) are observed in vapor inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperatures (400°–650°C). Moreover, concentrations of some metals, notably Cu and Au, are commonly higher in vapor inclusions than they are in inclusions of coexisting hypersaline liquid (brine). Experiments designed to determine the concentration of Cu, Sn, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) partly explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems, and furthermore that it increases sharply with increasing water fugacity and correlates positively with the fugacity of HCl. Thermodynamic analysis shows that metal solubility is greatly enhanced by reaction of the metal with HCl and by hydration, which results in the formation of species such as MeCl m . n H2O. Nonetheless, the concentrations measured by these experiments are considerably lower than those measured in experiments involving aqueous liquids or determined for vapor fluid inclusions. A possible explanation for this and for the apparent preference of metals such as Cu and Au for the vapor over the coexisting brine in some natural settings is suggested by limited experimental studies of metal partitioning between vapor and brine. These studies show that, whereas Cu, Fe, and Zn all partition strongly into the liquid in chloride-bearing sulfur-free systems, Cu partitions preferentially into the vapor in the presence of significant concentrations of sulfur. We therefore infer that high concentrations of Cu and Au in vapor inclusions reflect the strong preference of sulfur for the vapor phase and the formation of sulfur-bearing metallic gas species. Phase stability relationships in the system NaCl-H2O indicate how vapor transport of metals may occur in nature, by showing a range of possible vapor evolution paths for the conditions of porphyry-epithermal systems. At the world-class Bingham Canyon porphyry Cu-Au deposit, evidence from fluid inclusions supports a model in which a single-phase fluid of intermediate to vapor-like density ascends from a magma chamber. On cooling and decompression, this fluid condenses a small fraction of brine by intersecting the two-phase surface on the vapor side of the critical curve, without significantly changing the composition of the expanding vapor. Vapor and brine reach Cu-Fe sulfide saturation as both phases cool below 425°C. Vapor, which is the dominant fluid in terms of the total mass of H2O, Cu, and probably even Cl, is interpreted to be the main agent of metal transport. The evolution of fluids leading to high-grade epithermal gold mineralization is initiated by an H2S-, SO2-, Au-, and variably Cu- and As-rich vapor, which separates from an FeCl2-rich brine in a subjacent porphyry environment. In the early stages of the hydrothermal system, vapor expands rapidly and on reaching the epithermal environment, condenses, producing hypogene advanced argillic alteration and residual vuggy quartz and, in some cases, coeval high-sulfidation precious metal mineralization (e.g., Pascua). More commonly, the introduction of precious metals occurs somewhat later, after the site of magmatic fluid exsolution has receded to greater depth. Because of the relatively high pressure, the vapor separating from brine at this stage cools along a pressure-temperature path above the critical curve of the system, causing it to contract to a liquid capable of transporting several parts per million Au to temperatures as low as 150°C.

Chemical and isotopic composition of fumarolic gases and the SO 2 flux from Volcán de Colima, México, between the 1994 and 1998 eruptions

Volcán de Colima is the most active volcano in Mexico, including a significant effusive eruption in 1991 and a weak explosive event in 1994. Geochemical monitoring of the volcano started in January 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater (Z3, ∼800°C, and Z2, ∼400°C). Chemical compositions of volcanic gases from the Z2 fumarole showed a peak in S/Cl in the middle of 1997, 14–15 months before the start of the 1998 lava eruption. High-temperature volcanic vapors from Z3 changed progressively in water isotopic composition and showed a weak trend of increasing HCl/HF and a peak in S/Cl at the same time. Starting from the summer of 1997, the volcanic water became gradually enriched in deuterium, which probably indicated an increasing contribution from deep, less degassed magma. A negative correlation between δD and Cl content for the Z3 fumarolic gases is explained by shallow degassing of the Colima hot lava plug. This hypothesis is consistent with published data on Cl contents and solubilities in magmas and supported by the observed positive trends of S/Cl and HCl/HF with δD for the Z3 fumarole. Until September 1998, the SO 2 flux from the volcano was at a low level (<100 tons/day), and often below the detection limit of the instrument (∼30 tons/day). A sharp increase in the SO 2 flux was recorded 1 month before the eruption started. It jumped from 400±50 tons/day to 1600 tons/day 2 days before new lava emerged on 20 November 1998. The SO 2 flux correlated well with seismicity.

Volcanic-related alteration and geochemistry of Iwodake volcano, Satsuma-Iwojima, Kyushu, SW Japan

Satsuma-Iwojima is located near the northwestern margin of Kikai caldera, and active Mt. Iwodake is one of the post-caldera cones. Inside the summit crater of about 300 m diameter and 50 m depth, volcanic gas with maximum temperature >800°C is discharged. At Iwodake, the rhyolite is strongly altered by volcanic vapor and acid hydrothermal solution inside and around the summit crater. The main alteration mineral assemblages at surface include quartz-tridymite-cristobalite, tridymite-cristobalite-alunite and cristobalite-(alunite) inside the summit crater and on the flank. Molybdenum blue sublimates are forming at the surface around high-temperature fumaroles inside the summit crater. The ground surface is commonly cemented by amorphous silica or cristobalite. The original rhyolite with SiO2 = 71–72 wt% has been leached, resulting in a rock with maximum SiO2 = 97 wt%. All major elements are leached except for TiO2. Although LIL (large-ion lithophile) elements have leached, HFS (high field-strength) elements appear to be immobile. Rare earth elements are considered to have been soluble as a complex with sulfate mainly in the low pH hydrothermal fluid during the silicification of the rhyolite. There is abundant Mo, Pb, Zn, Bi, Sn and much lower As in the volcanic sublimates and altered rocks inside the summit crater. By contrast, the rocks near the acid springs at the foot of the cone are As-rich, but with lower Pb, Zn, Bi and little Mo in comparison with the summit. The fractures and fumarole locations around Iwodake summit and erosional ridges on the flanks reflects a NE-SW trend that is parallel to the trace of the outer ring of Kikai caldera. In addition, this is the same direction as the alignment of Shin-Iwojima, Iwodake and Inamuradake, all of which are post-caldera cones. Zones of structural weakness parallel to the caldera rim may exist, acting as pathways for the ascending volcanic gases and hydrothermal solutions. Fumarolic activity appears to be shifting gradually to the eastern part of the summit crater.

Photochemistry of a Volcanically Driven Atmosphere on Io: Sulfur and Oxygen Species from a Pele-Type Eruption

To determine how active volcanism might affect the standard picture of sulfur dioxide photochemistry on Io, we have developed a one-dimensional atmospheric model in which a variety of sulfur-, oxygen-, sodium-, potassium-, and chlorine-bearing volatiles are volcanically outgassed at Io's surface and then evolve due to photolysis, chemical kinetics, and diffusion. Thermochemical equilibrium calculations in combination with recent observations of gases in the Pele plume are used to help constrain the composition and physical properties of the exsolved volcanic vapors. Both thermochemical equilibrium calculations (Zolotov and Fegley 1999, Icarus 141, 40–52) and the Pele plume observations of Spencer et al. (2000; Science 288, 1208–1210) suggest that S 2 may be a common gas emitted in volcanic eruptions on Io. If so, our photochemical models indicate that the composition of Io's atmosphere could differ significantly from the case of an atmosphere in equilibrium with SO 2 frost. The major differences as they relate to oxygen and sulfur species are an increased abundance of S, S 2, S 3, S 4, SO, and S 2O and a decreased abundance of O and O 2 in the Pele-type volcanic models as compared with frost sublimation models. The high observed SO/SO 2 ratio on Io might reflect the importance of a contribution from volcanic SO rather than indicate low eddy diffusion coefficients in Io's atmosphere or low SO “sticking” probabilities at Io's surface; in that case, the SO/SO 2 ratio could be temporally and/or spatially variable as volcanic activity fluctuates. Many of the interesting volcanic species (e.g., S 2, S 3, S 4, and S 2O) are short lived and will be rapidly destroyed once the volcanic plumes shut off; condensation of these species near the source vent is also likely. The diffuse red deposits associated with active volcanic centers on Io may be caused by S 4 radicals that are created and temporarily preserved when sulfur vapor (predominantly S 2) condenses around the volcanic vent. Condensation of SO across the surface and, in particular, in the polar regions might also affect the surface spectral properties. We predict that the S/O ratio in the torus and neutral clouds might be correlated with volcanic activity—during periods when volcanic outgassing of S 2 (or other molecular sulfur vapors) is prevalent, we would expect the escape of sulfur to be enhanced relative to that of oxygen, and the S/O ratio in the torus and neutral clouds could be correspondingly increased.