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Over the past two decades, La Soufrière volcano in Guadeloupe has displayed a growing degassing unrest whose actual source mechanism still remains unclear. Based on new measurements of the chemistry and mass flux of fumarolic gas emissions from the volcano, here we reveal spatio-temporal variations in the degassing features that closely relate to the 3D underground circulation of fumarolic fluids, as imaged by electrical resistivity tomography, and to geodetic-seismic signals recorded over the past two decades. Discrete monthly surveys of gas plumes from the various vents on La Soufrière lava dome, performed with portable MultiGAS analyzers, reveal important differences in the chemical proportions and fluxes of H2O, CO2, H2S, SO2 and H2, which depend on the vent location with respect to the underground circulation of fluids. In particular, the main central vents, though directly connected to the volcano conduit and preferentially surveyed in past decades, display much higher CO2/SO2 and H2S/SO2 ratios than peripheral gas emissions, reflecting greater SO2 scrubbing in the boiling hydrothermal water at 80−100 m depth. Gas fluxes demonstrate an increased bulk degassing of the volcano over the past 10 years, but also a recent spatial shift in fumarolic degassing intensity from the center of the lava dome towards its SE−NE sector and the Breislack fracture. Such a spatial shift is in agreement with both extensometric and seismic evidence of fault widening in this sector due to slow gravitational sliding of the southern dome sector. Our study thus provides an improved framework to monitor and interpret the evolution of gas emissions from La Soufrière in the future and to better forecast hazards from this dangerous andesitic volcano.

This study was aimed to test a new methodological approach to carry out measurements of gaseous elemental mercury (GEM) diffusively emitted from soils in hydrothermal-volcanic environments. This method was based on the use of a static closed-chamber (SCC) in combination with a Lumex ® RA-915M analyzer that provides GEM measurements in a wide range of concentrations (from 2 to 50,000 ng m −3 ). Gas samples were collected at fixed time intervals from the SCC positioned on the ground (time-series samples). The Lumex ® inlet port was equipped with a three-way Teflon valve allowing the free entrance of air through a carbon trap, in order to: (i) prevent disturbance to the Lumex ® operative flow rate (10 L min −1 ) during the injection of the gas samples from the SCC and (ii) minimize the instability of the baseline signal induced by possible variations of GEM concentrations in air. In the lab, known amounts of GEM, pipetted from a vessel containing an Hg-saturated air in equilibrium with liquid mercury at 27 °C, were injected in the Lumex ® via the modified inlet port to construct a calibration curve. The latter was used to calculate the amount of GEM in the SCC (K SCC ) from the corresponding GEM concentrations measured by the Lumex ® analyzer. The K SCC values of the time-series samples were proportionally increasing with the GEM fluxes (ϕGEM), thus ϕGEM values were computed according to the following equation: ϕGEM = (dK SCC /dt)/A, where A is the basal area of the SCC and dt is the time interval of the time-series sampling. Up to 214 ϕGEM measurements were carried out at Solfatara crater (Campi Flegrei, southern Italy), a hydrothermally altered tuff cone characterized by an anomalous diffuse soil emission of GEM-rich geogenic gases. The measured ϕGEM values varied up to 4 orders of magnitude, from 1,296, corresponding to the sensitivity of the method at the selected sampling time interval (1 min), to 1,957,500 ng m −2 day −1 , and were consistent with those recently measured in this crater using a different method. In the field, 10 replicates were carried out in 5 selected sites, allowing to demonstrate that the proposed method has a high reproducibility (RDS < 4%). The ϕGEM and ϕCO 2 values, the latter being measured in the same 214 sites by using the accumulation chamber method, showed a low correlation, although both gases were originated from the same deep source. This suggests that GEM and CO 2 soil fluxes are differently affected by environmental parameters, such as soil humidity and temperature, which have a strong effect on the release of GEM from the soil, whereas they do not play a significant role in the diffuse degassing of CO 2 . The measured fluxes were used to compute the CO 2 and GEM total outputs (402 and 5.41 × 10 −6 t day −1 , respectively) from the study area (92,000 m 2 ) and to construct contour maps showing the spatial distribution of the ϕCO 2 and ϕGEM values. By modifying the geometry of SCC and the time interval of the sampling series, the proposed method can be applied to the measurements of GEM soil fluxes in other geological systems and man-made environments. [ABSTRACT FROM AUTHOR]

Abstract: Chemical and isotopic compositions have been measured for CO2-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed 3He/4He ratios (2–3RA) and δ13C values of CO2 (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO2/3He ratios are in wide range of (0.4–97×109). Based on the apparent mixing trend in a 3He/4He and δ13C of CO2 diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has 3He/4He, δ13C and CO2/3He value of ∼3.2RA, ∼−4.6‰ and ∼6×1010, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO2 from the adjacent Changbaishan volcanic area. [Copyright &y& Elsevier]

Abstract: We have addressed the origins of methane in fumarolic, thermal and cold emissions discharging from a diversity of geological settings spread all over the Italian Peninsula and at Panarea and Pantelleria islands. Fluid discharges from the main geothermal systems of the peri-Tyrrhenian area (including Mt. Amiata, Larderello, Latera and Manziana) show δ13C-CH4 and δD-CH4 values >−28‰ V-PDB and >−160‰ V-SMOW, respectively, and C1/C2+ concentration ratios >1000. These patterns suggest that CH4 discharged from these systems is primarily generated by abiogenic reduction of CO2 and/or CO. Methane seems to have, at least partially, a different origin than the higher hydrocarbons that might be entirely related to the thermal decomposition of organic matter or, alternatively, to methane polymerization. Fumaroles from the volcanic systems of Solfatara, Vesuvio, Panarea and Pantelleria have C1/C2+ concentration ratios in a wide range (from 17 to 4300), but δ13C-CH4 and δD-CH4 values >−21‰ V-PDB and >−120‰ V-SMOW, respectively. Such isotopic values are very similar to those reported for CH4 discharging from ultramafic hydrothermal systems. In agreement with recent evidence provided for Solfatara and Pantelleria we, therefore, infer that the CH4 in the volcanic fumaroles is mainly abiogenic, too. The δ13C-CH4 (from −21.6 to −36.9‰ V-PDB) and δD-CH4 (from −113 to −164‰ V-SMOW) values, as well as the C1/C2+ concentration ratios (from 19 to 2240), in the CO2-rich fluid discharges located between the geothermal-volcanic systems and the Apennine sedimentary chain tend to decrease eastwards as the temperatures of the fluid reservoirs progressively diminish, suggesting that the CH4 production is mainly related to thermogenic processes. According to this general trend, the CH4-dominated fluids from mud volcanoes located eastwards of the CO2-rich emissions are fed by an almost pure thermogenic source. The easternmost fluid discharges from the Po valley and the Adriatic coast, where organic matter evolves at relatively shallow depths in the presence of a low geothermal gradient, record the lowest δ13C-CH4 and δD-CH4 values, i.e. as low as −69‰ V-PDB and <−190‰ V-SMOW, respectively, and C1/C2+ concentration ratios up to 5500, suggesting that the CH4 production is mainly from microbially driven processes. [Copyright &y& Elsevier]

Over the past two decades, La Soufrière volcano in Guadeloupe has displayed a growing degassing unrest whose actual source mechanism still remains unclear. Based on new measurements of the chemistry and mass flux of fumarolic gas emissions from the volcano, here we reveal spatio-temporal variations in the degassing features that closely relate to the 3D underground circulation of fumarolic fluids, as imaged by electrical resistivity tomography, and to geodetic-seismic signals recorded over the past two decades. Discrete monthly surveys of gas plumes from the various vents on La Soufrière lava dome, performed with portable MultiGAS analyzers, reveal important differences in the chemical proportions and fluxes of H2O, CO2, H2S, SO2 and H2, which depend on the vent location with respect to the underground circulation of fluids. In particular, the main central vents, though directly connected to the volcano conduit and preferentially surveyed in past decades, display much higher CO2/SO2 and H2S/SO2 ratios than peripheral gas emissions, reflecting greater SO2 scrubbing in the boiling hydrothermal water at 80–100 m depth. Gas fluxes demonstrate an increased bulk degassing of the volcano over the past 10 years, but also a recent spatial shift in fumarolic degassing intensity from the center of the lava dome towards its SE–NE sector and the Breislack fracture. Such a spatial shift is in agreement with both extensometric and seismic evidence of fault widening in this sector due to slow gravitational sliding of the southern dome sector. Our study thus provides an improved framework to monitor and interpret the evolution of gas emissions from La Soufrière in the future and to better forecast hazards from this dangerous andesitic volcano. [ABSTRACT FROM AUTHOR]