Volcanic Gases Research Articles

Research on optimization design method of horizontal well of volcanic gas reservoir in Daqing Xushen deep natural gas field

The volcanic gas reservoir in Daqing Xushen deep natural gas field has low porosity, low permeability, strong heterogeneity, more bottom water layer, low production of vertical wells in most blocks and other characteristics. The development of horizontal well can better improve the development effect of gas reservoir. The optimization design method of “One choice, two determination and three optimization” horizontal well is established based on the gas reservoir geology and dynamic new achievements on the fine identification of volcanic bodies, prediction of well control range of gas wells and so on according to the characteristics of volcanic bodies controlling reservoir development. This method shall be applied to the optimization design of horizontal wells, which makes the production capacity of the well increase significantly and better ensures the continuous and stable exploitation of gas reservoir.

Experimental constraints on metal transport in fumarolic gases

Metals are efficiently transported in a gas phase and the gas composition strongly affects transport rates of these metals in the gas phase. Here we present a new approach to systematically investigate metal transport and deposition from volcanic gases of different compositions: We test the effect of sulfur and chlorine on the volatilization of 30 metal oxides and the phases deposited along a temperature gradient of 1250–300 °C in evacuated silica glass tubes in water-free systems. In total, we observe 30 different deposited phases, including oxides, sulfides and chlorides. The experimental results are compared with closed system and open system thermochemical equilibrium calculations. The closed system calculations provide insights into the metal speciation in the gas phase, and the open system calculations provide thermodynamic predictions of the temperature ranges over which metals deposited from the gas phase. Both approaches only partially reproduce the experimental observations of the deposited phases in the silica glass tubes. However, when we compare our experimental results with records from fumaroles on three different volcanoes (Kudriavy volcano, Russia, La Fossa crater on Vulcano, Italy, and Momotombo, Nicaragua), we find that our experiments agree very well with natural observations. We suggest that similar experimental investigations are powerful tools in the study of metal transport in ore forming systems involving gas phases, and results may be applied to various processes in terrestrial and planetary environments.

Discovery and exploration inspiration of deep volcanic gas reservoirs in eastern sag of Liaohe depression

Discovery and exploration inspiration of deep volcanic gas reservoirs in eastern sag of Liaohe depression

Using the composition of fluid seepage from the magmatic-hydrothermal system of Kawah Ijen volcano, Indonesia, as a monitoring tool

The lake waters of Kawah Ijen volcano are concentrated hyperacidic brines that obtain their composition from volcanic gases and water-rock interaction. Lake water and hydrothermal fluids seep through the western flank of the volcano where they precipitate gypsum and form the hyperacid Banyu Pahit river. In this contribution, we present compositional data from bi-weekly samples taken from the Banyu Pahit between 2015 and 2018 to evaluate whether these fluids can be used to monitor the hydrothermal system, and compare the fluids to the record of variability documented by the precipitated gypsum. Understanding the causal links between variability of the Banyu Pahit and the magmatic-hydrothermal processes operating in the Kawah Ijen system, and establishing a baseline for quiescence are pre-requisites to using the river for monitoring. Fluid samples were analysed at the Ijen observatory for SO4/Cl ratio using BaSO4 turbidity and AgCl gravimetry, and subsequently sent for ICP-MS and ICP-OES analyses. The data shows a first order seasonal variation that can be explained by variable input of neutral groundwater, but is underlain by more subtle trends. Temporally, there is a shift in the system from 2015–2016 to 2017 when the fluid contains a higher proportion of elements derived from rock alteration. The shift occurs at the peak of the rainy season and coincides with a substantial rainfall increase in 2017 compared to 2015–2016. At the start of 2018 there is a marked increase in Cu, coinciding with a gas burst in March 2018. We suggest that an increased meteoric water input led to an increase in water-rock interaction and dissolution of a Cu-sulphide phase in the hydrothermal system. The variability of the fluids over the 3-year sampling period does not exceed that of the longer record preserved by the gypsum precipitates, indicating that it is typical for the current style of activity.

APPLICATION OF LAND SURFACE TEMPERATURE DERIVED FROM ASTER TIR TO IDENTIFY VOLCANIC GAS EMISSION AROUND BANDUNG BASIN

Gas emission in volcanic areas is one of the features that can be used for geothermal exploration and to monitor volcanic activity. Volcanic gases are usually emitted in permeable zones in geothermal fields. The use of thermal infrared radiometers (TIR) onboard of advanced spaceborne thermal emission and reflection radiometers (ASTER) aims to detect thermal anomalies at the ground surface related to gas emissions from permeable zones. The study area is located around Bandung Basin, West Java (Indonesia), particularly the Papandayan and Domas craters. This area was chosen because of the easily detected land surface temperature (LST) following emissivity and vegetation corrections (Tcveg). The ASTER TIR images used in this study were acquired by direct night and day observation, including observations made using visible to near-infrared radiometers (VNIR). Field measurements of volcanic gases composed of SO2 and CO2 were performed at three different zones for each of the craters. The measured SO2 concentration was found to be constant over time, but CO2 concentration showed some variation in the craters. We obtained results suggesting that SO2 gas measurements and Tcveg are highly correlated. At Papandayan crater, the SO2 gas concentration was 334.34 ppm and the Tcveg temperature was 35.67 °C, results that are considered highly anomalous. The same correlation was also found at Domas crater, which showed an increased SO2 gas concentration of 35.39 ppm located at a high-anomaly Tcveg of 30.65 °C. Therefore, the ASTER TIR images have potential to identify volcanic gases as related to high Tcveg.

Volatile transport of metals and the Cu budget of the active White Island magmatic-hydrothermal system, New Zealand

Evidence of metal transport by volcanic gases is found, amongst others, in volcanic plumes and sublimates growing around fumaroles. The transport mechanisms (i.e. volatile metal compounds) are, however, not well constrained. Here, we present results on the volatile transport of metals at the scale of a magmatic-hydrothermal system, White Island volcano. We studied metal emissions from the main vent through the crater lake covering the conduit. Volcanic gases injected in the lake are responsible for the waters and sediments enrichment in various metals, while elemental sulfur forming at the bottom appears as the main sink for chalcophile metals. Metal chloride complexes are the dominant species in the hot and acidic crater lake waters. Nearby low-temperature fumarolic gases are lower in metals than higher temperature gases reported in the 1970s, emphasizing precipitation at depth and sequestration by condensing elemental sulfur. Efficient gaseous transport of metals by HCl and HBr is revealed by thermodynamic modeling, whereas the role of sulfur is restrained to controlling redox conditions. A copper budget for White Island magmatic-hydrothermal system is assessed, taking into consideration all the potentials sinks. About 3900 tons of copper are estimated to be retained in the subsurface environment every year. Over the 10,000 years lifetime of the magmatic-hydrothermal system, this adds up to ~40 megatons, depicting White Island as a potential Cu porphyry forming deposit.

Strombolian eruptions and dynamics of magma degassing at Yasur Volcano (Vanuatu)

Open vent basaltic volcanoes account for a substantial portion of the global atmospheric outgassing flux, largely through passive degassing and mild explosive activity. We present volcanic gas flux and composition data from Yasur Volcano, Vanuatu collected in July 2018. The average volcanic plume chemistry is characterised by a mean molar CO2/SO2 ratio of 2.14, H2O/SO2 of 148 and SO2/HCl of 1.02. The measured mean SO2 flux in the period of 6th to 9th July is 4.9 kg s−1. Therefore, the mean fluxes of the other species are 7.5 kg∙s−1 CO2, 208 kg∙s−1 H2O and 4.8 kg∙s−1 HCl. The degassing regime at Yasur volcano ranges from ‘passive’ to ‘active’ styles, with the latter including Strombolian activity and spattering. Gases emitted during active degassing are enriched in SO2 over HCl and CO2 over SO2 relative to passive degassing, with CO2/SO2 ratios of 2.85 ± 0.17, SO2/HCl of 1.6 ± 0.22, and H2O/SO2 of 315 ± 78.8. Gases emitted during passive degassing have CO2/SO2 ratios of 1.96 ± 0.12, SO2/HCl of 0.50 ± 0.07 and H2O/SO2 of 174 ± 43.5. We use a model of volatile degassing derived from melt inclusion studies (Metrich et al., 2011), combined with our observations of chemical variations in the outgassing bubbles to propose a mechanism for magma degassing in the conduit at Yasur. We envisage a shallow conduit filled with crystal-rich magma, forming a viscous and mobile plug that develops an effective yield strength from the surface to a depth of at least 2000 m, in which bubbles are trapped, grow, ascend towards the surface and burst in a typical Strombolian eruption. Deeper bubbles released during active degassing are enriched in CO2 and SO2 compared to bubbles released during ‘passive degassing’, which are sourced from close to the surface, and are, consequently, HCl-rich.

Environmental changes occurred during the Early Danian at the Rio Grande Rise, South Atlantic Ocean

Early Danian environmental perturbations related to carbon isotope negative excursions (CIEs) were recognized at Deep Sea Drilling Project (DSDP) Hole 516F, Rio Grande Rise. Within the first ~160 kyr of the Danian, environmental stability is evidenced by well‑oxygenated bottom waters, good carbonate preservation, and relatively high primary production. The preservation of carbonates and the oxygenation of the deep water decreased progressively, leading to the formation of red layers in the sediments between 65.86 Ma and 65.71 Ma. The onset of carbonate dissolution and the reduction in dissolved oxygen concentrations occurred approximately 50 kyr before the onset of the Dan-C2 event, and was probably related to the third volcanic event of the Deccan Traps (DT). Our results support the link between enhanced emissions of volcanic gasses and deep ocean carbonate dissolution and deoxygenation during the Early Danian. The CIEs at the Rio Grande Rise are positioned within the planktonic foraminiferal biozone P1b, suggesting that the first appearance of Subbotina triloculinoides within Chron C29r (at 65.89 Ma) is a diachronous datum.

Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen.

Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during theEarth's accretion versus that subducted from the Earth's surface is unclear1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7-10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12-15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ15N values may both be dominantly primordial features.

Active Volcanism Revealed from a Seismicity Conduit in the Long-resting Tatun Volcano Group of Northern Taiwan

Abundant earthquakes clustered within a particular zone often reflect an active geological feature, such as clustering seismicity along a fault zone and a huge number of volcanic-earthquakes around the erupting conduit. Herein we perform a double-difference tomographic inversion and relocate the seismicity at the long-resting Tatun volcano group (TVG) in northern Taiwan. A dramatic improvement of the earthquake location model surprisingly show that, from 2014 to 2017, two clustered seismic zones are identified in the TVG. One major group of events (>1000) persistently clustered within a ~500 m diameter vertical conduit with a ~2 km height. The clustering seismicity conduit is just located nearby Dayoukeng, one of the strongest fumaroles in the TVG, and is connected to a fracture zone characterized by low Vp/Vs in the shallow crust. The other group of events is clustered within a sphere-like zone beneath Mt. Chihsin around the depths between 0.5 km and 2 km. Both seismic zones are probably triggered by the significantly volcanic gases and fluids ascending from the deep magma reservoir. Combined with a variety of results from literature, the seismicity conduit near the strong fumarole is the evidence for an active volcano and also identifies a likely pathway for ascending magma if the TVG erupts again in the future. But possibility of developing different magma pathways at other clustered seismic zones such as beneath Mt. Chihsin may not be totally excluded.

Volcanic gas emissions from Taftan and Damavand, the Iranian volcanoes

Volcanic gas sampling and SO2 flux measurements were performed on Taftan volcano (3920 m height, SE Iran, Makran volcanic arc) and Damavand volcano (5610 m height, Northern Iran, Alborz Mountains). Both volcanoes possess near-summit fumarolic fields with moderately intensive gas jets and temperatures up to 160 °C (Taftan) and 175 °C (Damavand). Gases of both volcanoes contain (mmol/mol): H2O (910–930), CO2 (50–80), SO2 (3–7), H2S (2–5.5), HCl (5–8.2) and HF (0.1–0.13). Both volcanoes are also similar in terms of minor gas species (mmol/mol): He (0.0005–0.0011), H2 (0.00077–0.0046), N2(ex) (0.15–0.30) and Ar(ex) (0.0006–0.0012), where subscript “ex” denotes non-atmospheric fraction. δD–δ18O systematics has shown that Damavand fumarolic gases contain 60–65% of magmatic fraction whereas the Taftan emissions correspond to almost pure magmatic (“andesitic”) vapor. Low amount of nitrogen in Taftan gases and its isotopic composition may be explained by low input of nitrogen-containing organic matter with the subducting slab due to specific geometry of the Makran subduction zone, namely, by the presence of an accretionary prism of a considerable size. The origin of high CH4 content in Damavand gases and very low concentrations of C2H6 and higher hydrocarbons (0.65 mmol/mol; CO2/CH4 ~ 80; CH4/C2H6 ~ 104) is unclear. Fumaroles of Taftan volcano are poor in CH4, similar to many other arc volcanoes (CO2/CH4 ~ 105). 3He/4He isotopic composition expressed as R/Ra was measured at 7.0–7.5 for Taftan and 6.65 for Damavand, corresponding to the CO2/3He ratio of ~1.0E+10. The δ34S of the total sulfur is +7.6 ± 2‰ (2σ) for Taftan fumaroles and + 8.1 ± 2‰ (2σ) for Damavand fumaroles; δ13C (CO2) is −5.9 ± 2.0‰ (Damavand) and − 4.3 ± 0.15‰ (Taftan). Concentrations of major gas species (H2O/CO2/S/HCl ratios) and isotopic data (R/Ra, CO2/3He, δ34S) show that the volcanic gas composition on both volcanoes have a distinct arc signature. This conclusion is especially important for the intraplate Damavand volcano, which has an uncertain tectonic affinity but according to the latest geochemical studies was considered having hotspot/rift origin. Mini-DOAS measurements of the SO2 fluxes showed 20 ± 12 t/d SO2 at Taftan and 43 ± 20 t/d SO2 at Damavand, which places both volcanoes as small SO2 emitters.

Enigmatic diamonds from the Tolbachik volcano, Kamchatka

Abstract Approximately 700 diamond crystals were identified in volcanic (mainly pyroclastic) rocks of the Tolbachik volcano, Kamchatka, Russia. They were studied with the use of SIMS, scanning and transmission electron microscopy, and utilization of electron energy loss spectroscopy and electron diffraction. Diamonds have cube-octahedral shape and extremely homogeneous internal structure. Two groups of impurity elements are distinguished by their distribution within the diamond. First group, N and H, the most common structural impurities in diamond, are distributed homogeneously. All other elements observed (Cl, F, O, S, Si, Al, Ca, and K) form local concentrations, implying the existence of inclusions, causing high concentrations of these elements. Most elements have concentrations 3–4 orders of magnitude less than chondritic values. Besides N and H, Si, F, Cl, and Na are relatively enriched because they are concentrated in micro- and nanoinclusions in diamond. Mineral inclusions in the studied diamonds are 70–450 nm in size, round- or oval-shaped. They are represented by two mineral groups: Mn-Ni alloys and silicides, with a wide range of concentrations for each group. Alloys vary in stoichiometry from MnNi to Mn2Ni, with a minor admixture of Si from 0 to 5.20–5.60 at%. Silicides, usually coexisting with alloys, vary in composition from (Mn,Ni)4Si to (Mn,Ni)5Si2 and Mn5Si2, and further to MnSi, forming pure Mn-silicides. Mineral inclusions have nanometer-sized bubbles that contain a fluid or a gas phase (F and O). Carbon isotopic compositions in diamonds vary from –21 to –29‰ δ13CVPDB (avg. = –25.4). Nitrogen isotopic compositions in diamond from Tolbachik volcano are from –2.32 to –2.58‰ δ15NAir. Geological, geochemical, and mineralogical data confirm the natural origin of studied Tolbachik diamonds from volcanic gases during the explosive stage of the eruption.

Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition

Abstract Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0—coupled substitution of Al3+ charge-balanced by interstitial Na+—which are typical of cristobalite. However, new electron probe microanalysis (EPMA) element maps show individual crystals to have impurity concentrations that systematically decline from crystal cores-to-rims, and atom probe tomography reveals localized segregation of impurities to dislocations. Impurity concentrations are inversely correlated with degrees of crystallinity [observed by electron backscatter diffraction (EBSD), hyperspectral cathodoluminescence, laser Raman, and transmission electron microscopy (TEM)], such that crystal cores are poorly crystalline and rims are highly ordered tetragonal α-cristobalite. The VPC-plagioclase interfaces show evidence that dissolution-reprecipitation reactions between acidic gases and plagioclase crystals yield precursory amorphous SiO2 coatings that are suitable substrates for initial deposition of impure cristobalite. Successive layers of cubic β-cristobalite are deposited with impurity concentrations that decline as Al-bearing gases rapidly become unstable in the vapor cooling within pores. Final cooling to ambient temperature causes a displacive transformation from β→α cristobalite, but with locally expanded unit cells where impurities are abundant. We interpret this mechanism of VPC deposition to be a natural proxy for dopant-modulated Chemical Vapor Deposition, where halogen-rich acidic gases uptake silica, react with plagioclase surfaces to form suitable substrates and then deposit SiO2 as impure cristobalite. Our results have implications for volcanic hazards, as it has been established that the toxicity of crystalline silica is positively correlated with its purity. Furthermore, we note that VPC commonly goes unreported, but has been observed in silicic lavas of virtually all compositions and eruptive settings. We therefore suggest that despite being metastable at Earth's surface, cristobalite may be the most widely occurring SiO2 polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies.

Changes in Atmospheric, Meteorological, and Ocean Parameters Associated with the 12 January 2020 Taal Volcanic Eruption

The Taal volcano erupted on 12 January 2020, the first time since 1977. About 35 mild earthquakes (magnitude greater than 4.0) were observed on 12 January 2020 induced from the eruption. In the present paper, we analyzed optical properties of volcanic aerosols, volcanic gas emission, ocean parameters using multi-satellite sensors, namely, MODIS (Moderate Resolution Imaging Spectroradiometer), AIRS (Atmospheric Infrared Sounder), OMI (Ozone Monitoring Instrument), TROPOMI (TROPOspheric Monitoring Instrument) and ground observations, namely, Argo, and AERONET (AErosol RObotic NETwork) data. Our detailed analysis shows pronounced changes in all the parameters, which mainly occurred in the western and south-western regions because the airmass of the Taal volcano spreads westward according to the analysis of airmass trajectories and wind directions. The presence of finer particles has been observed by analyzing aerosol properties that can be attributed to the volcanic plume after the eruption. We have also observed an enhancement in SO2, CO, and water vapor, and a decrease in Ozone after a few days of the eruption. The unusual variations in salinity, sea temperature, and surface latent heat flux have been observed as a result of the ash from the Taal volcano in the south-west and south-east over the ocean. Our results demonstrate that the observations combining satellite with ground data could provide important information about the changes in the atmosphere, meteorology, and ocean parameters associated with the Taal volcanic eruption.

Investigation of the effect of Cl on the transport and sublimation of volatile trace metals in volcanic gases using benchtop fumarole experiments

Volatile trace metals emitted from volcanoes are frequently complexed with ligands and sublime from gases as halides, sulfides, sulfates and oxides. We investigate the effect of Cl as a ligand on the sublimation process by degassing broadly phonolitic composition melts with added trace metals (V, Cu, Zn, As, Y, Mo, Cd, Sn, Yb, Pb and Bi) and Cl in a benchtop fumarole apparatus at 100 kPa. Sublimates forming from the gas phase at 25–900 °C along a silica glass tube suspended above degassed melts are characterized by electron microscopy, Raman spectroscopy and solution nebulization inductively coupled plasma mass spectrometry (SN ICP-MS). Silica polymorphs are ubiquitous at all temperatures and halite forms below 600 °C. Szenicsite (Cu3(MoO4)(OH)4) and molybdofornacite (CuPb2(MoO4)(AsO4)(OH)) occur at ~500 °C. The latter phases may have formed from post-experiment hydration of original anhydrous molybates or arsenates, or alternatively are their Cl−-bearing analogues. Arsenate and vanadate minerals such as bradaczekite (NaCu4(AsO4)3) and leningradite (PbCu3(VO4)2Cl2), known in natural fumaroles, also occur in the experiments. Chloride sublimates can host substantial Pb, Tl, As, Cu, Bi and Zn. The addition of Cl to the melt in the experiments increases the concentration of Li, Rb, Cs, Ag, Cr, Cu, Mo and W in the sublimates compared to Cl-free experiments. In Cl-bearing volcanic gases, molybdates and chlorides are the most likely hosts of volatile trace metals, and when adhered to ash particles, are potential sources of trace metals to the atmosphere and oceans.

Mantle cooling causes more reducing volcanic gases and gradual reduction of the atmosphere

Mantle cooling causes more reducing volcanic gases and gradual reduction of the atmosphere

AGU Centennial Grand Challenge: Volcanoes and Deep Carbon Global CO2 Emissions From Subaerial Volcanism—Recent Progress and Future Challenges

AbstractQuantifying the global volcanic CO2 output from subaerial volcanism is key for a better understanding of rates and mechanisms of carbon cycling in and out of our planet and their consequences for the long‐term evolution of Earth's climate over geological timescales. Although having been the focus of intense research since the early 1990s, and in spite of recent progress, the global volcanic CO2 output remains inaccurately known. Here we review past developments and recent progress and examine limits and caveats of our current understanding and challenges for future research. We show that CO2 flux measurements are today only available for ~100 volcanoes (cumulative measured flux, 44 Tg CO2/year), implying that extrapolation is required to account for the emissions of the several hundred degassing volcanoes worldwide. Recent extrapolation attempts converge to indicate that persistent degassing through active crater fumaroles and plumes releases ~53–88 Tg CO2/year, about half of which is released from the 125 most actively degassing subaerial volcanoes (36.4 ± 2.4 Tg CO2/year from strong volcanic gas emitters, Svge). The global CO2 output sustained by diffuse degassing via soils, volcanic lakes, and volcanic aquifers is even less well characterized but could be as high as 83 to 93 Tg CO2/year, rivaling that from the far more manifest crater emissions. Extrapolating these current fluxes to the past geological history of the planet is challenging and will require a new generation of models linking subduction parameters to magma and volatile (CO2) fluxes.

The role of sub-continental mantle as both “sink” and “source” in deep Earth volatile cycles

The extent to which Earth’s sub-continental lithospheric mantle modulates the flux of volatile elements from our planet’s deep interior to its atmosphere (via volcanism) is poorly constrained. Here, we focus on “off-craton” sub-continental lithospheric mantle because this long-lived reservoir potentially acts as both a volatile “sink” and “source” during major heating and rifting events. The sub-continental lithospheric mantle is primarily formed of peridotites with subordinate amounts of pyroxenites. While both lithologies are dominated by nominally-volatile-free mantle minerals, some of these phases have been shown to contain non-negligible amounts of H2O (e.g. 100’s of ppmw in clinopyroxene). Data for volatile elements other than Li are, however, limited. We present new, high-precision, in-situ Secondary Ion Mass Spectrometry analyses of H, F, Cl, Li and B in olivine and pyroxenes from well-characterised garnet- and spinel-bearing peridotites and pyroxenites (from southern Patagonia and the Antarctic Peninsula). Our study confirms that clinopyroxene is the main host of H2O and F. The maximum F contents we report (up to 154 ppmw) are higher than those in previous studies and occur in Ti-Cr diopsides in highly-metasomatised peridotites and Ti-Al augites from clinopyroxenite veins. Water contents of clinopyroxenes (up to 615 ppmw) are within the range previously published for continental mantle. Lithium concentrations are low (<5 ppmw) in all analysed phases and both Cl and B are below detection levels (14 ppmw and 0.03 ppmw, respectively).Unique to our study is the large variation in major- and trace-element concentrations of the clinopyroxenes, which allows us to place quantitative constraints on how volatiles are stored in the mantle. We demonstrate that: (i) F contents of clinopyroxenes closely correlate with Ti and (ii) DHCpx-Opx and DFCpx-Opx is systematic and inversely correlated with temperature. Despite the redistribution of volatiles during sub-solidus re-equilibration, we show that the first order control on the concentration of volatiles in clinopyroxene is the style of metasomatism, i.e. channellised flow versus reactive percolation. The mean bulk volatile contents of peridotites from Pali Aike and the Antarctic Peninsula (H2O = 89 ± 31 ppmw, F = 16 ± 11.2 ppmw and Li = 2 ± 0.7 ppmw) are within the range previously published for continental “off-craton” mantle. The pyroxenites have significantly higher mean bulk concentrations of H2O (260 ± 59 ppmw), F (86 ± 43 ppmw) and Li (1.0 ± 0.35 ppmw). While the greater capacity of mantle pyroxenites to host H2O relative to the associated peridotites has previously been observed in global “off-craton” mantle xenolith suites (e.g. Oahu, Hawaii; eastern China and the Rio Grande Rift, SW USA), here we show for the first time that pyroxenites are also major hosts of F (but not Cl, Li or B). Because of their relatively low solidus temperatures, pyroxenites in “off-craton” settings will be readily re-mobilised during lithospheric extension (and heating). We suggest these pyroxene-rich mantle lithologies may be responsible for the elevated concentrations of H2O and F observed in basalts and volcanic gasses from major continental rift zones and flood basalt provinces, and hence an important consideration in models of global volatile cycles.

Uptake of gaseous thallium, tellurium, vanadium and molybdenum into anhydrous alum, Lascar volcano fumaroles, Chile

Formation of secondary sulfate minerals during the reaction between volcanic gases and rocks modulates the composition and flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (MI MIII (XVIO4)2 with MI = NH4+, Na+, K+; MIII = Al3+, Fe3+ and XVI = S6+, Mo6+) in the 330 °C fumaroles of the Lascar volcano (Chile). The alum occurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rock cations within the crust. The average growth rate is estimated at ca. 0.3 μm/day, based on the 19-year-long activity of the degassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreases towards crust rim. The crust selectively concentrates Tl, V and Te (thousands of μg/g) and to a lesser extent Mo (hundreds of μg/g). The uptake of gaseous Tl, V and Mo is due to the possibility for these elements to enter the MI, MIII and XVI sites of alum, respectively. The process of Te uptake must be related to the incorporation of Tl and V with which Te tightly correlates. Extensive substitution of Tl, V and Te occurs at the surface of the crust where the supply of rock cations is the lowest. Such surface enrichment does not occur for Mo, because Mo substitutes for S, another element from the gas. These findings suggest that the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lack of rock-derived cations. Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that the partition coefficients of Tl (3.3 × 107), V (1.1 × 107) and Te (0.6 × 107) between crust surface and gases are two to four orders of magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alums lose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano (5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrous alums. This study suggests that anhydrous alums significantly immobilize Tl, V and Te in the ground of quiescent volcanoes, reducing the atmospheric emissions of these three elements.

Geochemical constraints on supercritical fluids in geothermal systems

Supercritical fluids with temperatures of ~400–500 °C have been reported from several active geothermal fields worldwide. Although the utilization of such fluids may multiply power production from new and already exploited geothermal systems, the fluid origin and chemical controls on their composition remain unclear. We performed flow-through high-temperature (400–420 °C) experiments at 34–69 bar to study the chemical and mineralogical changes associated with supercritical fluid formation upon boiling of subcritical geothermal fluids of varying chemical composition. Based on geochemical modeling and laboratory results, we propose that an important mechanism of supercritical fluid formation is conductive heating and boiling of subcritical geothermal groundwater by a magmatic intrusion. Such supercritical fluids will display low concentrations of mineral-forming elements (Si, Na, K, Ca, Mg, Al), with their concentrations being controlled by the solubility of salts, oxides, and aluminum silicates in high-temperature (>400 °C) and low-density (ρ < 0.3 g cm−3) fluids. In contrast, supercritical fluids will show elevated concentrations of volatile elements (C, S, B) of crustal and/or mantle origin with their concentrations often being similar to those of subcritical geothermal fluids. Associated mineral deposition, dominated by quartz, aluminum silicates, and salts, may form in the vicinity of the intrusion. Comparison of the modeling and laboratory results with observed chemical composition of natural supercritical fluid discharges indicates that conductive heating and boiling of subcritical geothermal groundwater may indeed be the formation mechanism of such fluids observed for example at Krafla (Iceland), Menengai (Kenya), Los Humeros (Mexico), and Larderello (Italy) with an addition of volcanic gases in many cases. Metal and salt-rich supercritical fluids, for example, at Kakkonda (Japan), may also exist in geothermal systems. However, such supercritical fluids are considered to have been trapped upon crystallization of the magmatic intrusion.