Volcanic gas emissions from the main low-temperature fumaroles (around 100°C) of one of the most dangerous volcanoes in the Lesser Antilles, La Soufrière de Guadeloupe, are measured routinely using direct sampling with NaOH bottles (for H2O, CO2, H2Stot, H2, CH4, CO, N2, He, Ar, O2), with P2O5 bottles (for H2O, CO2, H2S, SO2, H) and MULTIGAS (for HO, CO HS, SO, H). This allows us to perform a study about an intercomparison of volcanic gas monitoring methologies. The results show some good overlap and similar temporal evolution between the dataset of the different methologies, which is of utmost of importance for monitoring purposes. However, there is no good agreement between the three methodologies for the same chemical ratio. The differences could be explained by a time- dependant diffusive phenomenon that affects the concentrations of sampled gases for two methods of the three (P2O5 bottles and MULTIGAS). P2O5-based analyses are affected because effusion on rapid decompression and fast sampling affects the gas volume in the pipeline connecting the vent outlet to the sampling. Although MULTIGAS is a continuous, flow-through, sampling method, results are likely affected because of the sampling architecture involving filter and pump. Moreover, sulphur reaction (precipitation of elemental S upon exit from the fumarole, conversion of H2S to SO2, chemical reaction with the P2O5 powder that release H2S) and water condensation can also explain some discrepancies.
Abstract The 2014 Bárðarbunga rifting event in Iceland resulted in a 6‐month long eruption at Holuhraun. This eruption was characterized by high lava discharge rate and significant gas emission. The SO 2 flux for the first 3 months was measured with satellite sensors and the petrologic method. High‐resolution time series of the satellite data give 1200 kg/s that concurs with 1050 kg/s obtained from melt inclusion minus degassed lava sulfur contents scaled to the mass of magma produced. A high‐purity gas sample, with elevated S/Cl due to limited chlorine degassing, reveals a similar degassing pattern of trace metals as observed at Kīlauea (Hawai'i) and Erta Ale (Ethiopia). This suggests a common degassing mechanism at mantle plume‐related volcanoes. The trace metal fluxes, calculated from trace element to sulfur ratios in the gas sample and scaled to the sulfur dioxide flux, are 1–2 orders of magnitude stronger at Holuhraun than Kīlauea and Erta Ale. In contrast, volcanoes at convergent margins (Etna and Stromboli, Italy) have 1–2 orders of magnitude higher trace element fluxes, most likely caused by abundant chlorine degassing. This emphasizes the importance of metal degassing as chlorine species. Short‐lived disequilibria between radon daughters, 210 Pb‐ 210 Bi‐ 210 Po measured in the gas, suggest degassing of a continuously replenished magma batch beneath the eruption site. Earlier and deep degassing phase of carbon dioxide and polonium is inferred from low ( 210 Po/ 210 Pb) in the gas, consistent with magma transfer rate of 0.75 m/s.
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.
<p>Fumarolic gas composition and temperature record deep processes that generate and transfer heat and mass towards the surface. &#160;These processes are a result of the emplacement, degassing and cooling of magma and the overturning of the above hydrothermal system. &#160;A reasonable expectation, and too often an unproved assumption, is that fumarole temperatures and the deep heat sources vary on similar timescales. &#160;Yet signals from deep and shallow processes have vastly different temporal variations.&#160; This indicates that signals arising from deep activity may be masked or modified by intervening hydrothermal processes, such as fluid-groundrock reactions in which secondary minerals play a major role. &#160;Clearly, this complicates the interpretation of the signals such as the joint variation of fumarole vent temperature and geochemical ratios in terms of what is occurring at depth. &#160;So what do the differences between the timescales governing deep and shallow processes tell us about the intervening transport mechanisms?</p><p>At the volcanic dome of La Soufri&#232;re de Guadeloupe, the Observatoire Volcanologique et Sismologique de la Guadeloupe has performed weekly-to-monthly in-situ vent gas sampling over many years. &#160;These analyses reliably track several geochemical species ratios over time, which provide important information about the evolution of deep processes. &#160;Vent temperature is measured as part of the in-situ sampling, giving a long time series of these measurements. &#160;Here, we look to exploit the temporal variations in these data to establish the common processes, and also to determine why these signals differ. &#160;By fitting sinusoids to the gas-ratio time series we find that several of the deep signals are strongly sinusoidal. &#160;For example, the He/CH<sub>4</sub> and CO<sub>2</sub>/CH<sub>4</sub> ratios, which involve conservative components and mark the injection of deep and hot magmatic fluids, oscillate on a timescale close to 3 years. We also analyse the frequency content of the temperature measurements since 2011 and find that such long signals are not seen. &#160;This may be due to internal buffering by the hydrothermal system, but other external forcings are also present. &#160;From these data we build up a more informed model of the heat-and-mass supply chain from depth to the surface. &#160;This will potentially allow us to predict future unrest (e.g. thermal crises, seismic swarms), and distinguish between sources of unrest.</p>
<p>Signals of volcanic unrest do not usually provide insights into the timing, size and style of future eruptions. However, analysis of past eruptions provides useful information in order to understand the evolution, magma storage and onset of future eruptions, Here, we examine basaltic-andesitic to andesitic eruption deposits from La Soufri&#232;re de Guadeloupe, covering a range of eruption styles, ages and magnitudes. Our work is timely given unrest at this system has increased over the last 25 years, with a potential eruption capable of directly impacting ~70,000 people in Southern Basse-Terre.</p><p>Here, we study the geochemistry of pre-eruptive magmas and timescales of magmatic processes preceding four explosive eruptions: 1657 Cal. CE (Vulcanian), 1010 Cal. CE (Plinian), ca. 341 Cal. CE (Strombolian) and 5680 Cal. BCE (Plinian). Using diffusion timescale studies of orthopyroxene phenocrysts, we constrain the timing of magma injections into the La Soufri&#232;re de Guadeloupe magmatic reservoir. These range from 35 &#177; 0.37 to 848 &#177; 0.4 days before eruption. Diffusion timescales do not appear to correlate with eruption style/size, but may correlate with other parameters (e.g., magma interactions in the reservoir and/or volatile content of the magma).</p><p>Major element concentrations in whole rock (WR), groundmass glasses (GM) and melt inclusions (MI) show a strong linear trend. However, this evolution cannot be resolved through fractional crystallisation alone, as there is no clear temporal trend. MIs reveal a relatively homogenous melt composition from the first to the most recent eruptions, ranging from 63.6 &#8211; 78.7 wt% SiO2. Volatiles, including H&#173;<sub>2</sub>O (2.3-4.4 wt%), CO<sub>2 </sub>(35-866 ppm) and sulphur (30-202 ppm), are also consistent across the various eruptions. MIs are often more evolved than the GM, indicating they cooled prior to their entrainment. This, along with the different crystal populations observed, suggests a recharge magma has intruded through a mush system and entrained crystals stored there. Crystals in different regions of the mush therefore experience different interactions with magmatic processes.</p><p>The major element compositional homogeneity across the eruptions indicates that composition does not have a large control on eruption style at this system. However, MI pre-eruptive volatile contents are more concentrated in the larger Plinian eruptions (e.g., CO&#173;<sub>2</sub> &#8211; 866 ppm) than the smaller Vulcanian and Strombolian eruptions (e.g., CO&#173;<sub>2</sub> &#8211; 674 ppm). Volatile emissions calculated through the petrologic method also differ, with higher total volatile emissions observed in the Plinian eruptions (12 Mt) than the smaller eruptions (0.1 Mt). The Plinian eruptions also have a faster magma ascent rate (0.3-22 m/s) than the vulcanian eruptions (3 m/s) as calculated from mass flux estimates.&#160;</p><p>Though the composition of the La Soufri&#232;re de Guadeloupe system has remained constant over time, changes in eruption style can result from variations: (i) in the way magma interacts with the mush system, (ii) in the pre-eruptive volatile contents and (iii) in the ascent rates. Understanding the controls on eruption style is important, especially during the current phase of unrest, in order to improve early-warning system efficiency, forecast models, eruption scenario crisis response and long-term risk reduction planning.</p>
