GMPV6.2

The characterization of volcanic hydrothermal systems (VHS) is fundamental for the early detection of precursors to phreatic or magmatic eruptions and for understanding hazards related to slope instabilities. Since these phenomena can be related to pore-fluid dynamics within the volcanic edifice, monitoring the spatial distribution of the different fluid phases (water, air, vapor) is of great importance. Ambient noise seismic interferometry has been employed for this task, correlating temporal changes in seismic velocities with variations in the water table depth in volcanic areas. However, this technique usually considers that above this depth, the pore space within the rock is totally occupied by a gaseous phase. This, in turn, implies that the body wave velocities and the density of the unsaturated zone are constant values, which is expected to impact on the determination of the water table depth. In this work, we assess the influence of partial saturation in the unsaturated zone of a VHS on ambient seismic noise, by employing a comprehensive rock physics model based on a saturation profile given by the Van Genuchten model. We focus on the sensitivity of Rayleigh waves, which are usually considered to be the most important contribution to the ambient seismic noise.

We consider an altered andesite layer overlying a half-space consisting of a relatively unaltered andesite representing the volcanic basement, which is representative of the VHS of La Soufrière de Guadeloupe volcano (Eastern Caribbean, France). We base our rock physics model on Gassmann’s equations to compute body wave velocities as a function of fluid saturation. We compute Rayleigh wave phase velocities for different positions of the water table and analyze their relative difference with respect to a reference scenario that corresponds to the mean value of the water table depth in this region. Our results suggest that the existence of a partial water saturation distribution could affect the Rayleigh wave velocities, and that this effect depends on the range of frequencies considered and the degree of hydrothermal alteration of the medium. For highly altered andesite, characterized by a higher porosity and a lower rock-mass stiffness, the relative variation in velocity obtained with a partial saturation distribution can be up to twice or down to half of the variation for the scenario corresponding to a constant saturation, depending on the frequency range considered. The depth sensitivity kernels of the Rayleigh wave phase velocities exhibit significant variations within regions with variable water content for frequencies between 4 and 6 Hz, which indicates that these seismic waves are able to distinguish the presence of a partial saturation distribution. These results indicate that relative velocity differences derived from ambient noise interferometry provide the possibility of inferring spatial distributions of water content and, therefore, density variations within VHS. This technique therefore emerges as a useful tool to inform on volcanic hazards related to hydrothermal activity, such as erratic explosions and partial flank collapses.

How to cite: Castromán, G., Rosas-Carbajal, M., Barbosa, N., Burtin, A., Heap, M., and Zyserman, F.: Rayleigh wave sensitivity to partial water saturation in highly altered volcanic hydrothermal systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-385, https://doi.org/10.5194/egusphere-egu22-385, 2022.

The Kemp Caldera is a submarine arc caldera volcano and belongs to the southernmost part of the South Sandwich island arc, located in the Scotia Sea. In 2009, the caldera was discovered by the R/V James Clark Ross research cruise JR224 during a geophysical survey. At this time, first hydrothermal activities were observed within the caldera. Around a resurgent cone in the center of the caldera, extinct chim­neys and whi­te smo­ker vent fiel­ds are found. A special feature of the Kemp Caldera hydrothermal system is the occurrence of elemental sulfur (S⁰) at uncommonly high pH values. Sulfur samples of the white smoker vent fields “Great Wall” and “Toxic Castle” at the eastern flank of the resurgent cone were recovered with a remotely operated vehicle during the R/V Polarstern PS119 expedition in 2019. These two sites are no more than 80 m apart, but the occurrence of S⁰ is different: at Great Wall, the sulfur is crystalline, while at Toxic Castle the sulfur is liquid and forms amorphous, pearl-like structures. Both sites are characterized by fluids with pH_{25 °C} values > 5 and show a temperature range from 63 to > 200 °C. Most interesting, however, are the δ³⁴S values of elemental sulfur, ranging from +5.2 to +5.8 ‰.

Disproportionation of magmatic SO₂ commonly explains the formation of S⁰ in arc/back-arc systems. Elemental sulfur precipitates from highly acidic hydrothermal fluids with pH-values ≤ 1 and show negative δ³⁴S values due to isotope fractionation. However, this formation mechanism cannot explain the moderate pH of the fluids and the lack of significantly negative δ³⁴S values for sulfur that would indicate SO₂ disproportionation. We suggest that the formation of sulfur in the Kemp Caldera is a result of SO₂ and H₂S synproportionation. From a thermodynamic point of view, this formation mechanism is possible, but it has not yet been demonstrated that it actually takes place in hydrothermal systems. Our study focuses on the formation of elemental sulfur in the Kemp Caldera hydrothermal system and shows that the diversity of hydrothermal arc/back-arc systems may be greater than previously assumed.

How to cite: Kürzinger, V., Bach, W., Diehl, A., Pereira, S. I., Strauss, H., and Bohrmann, G.: Elemental sulfur formation in the Kemp Caldera hydrothermal system, Scotia Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1054, https://doi.org/10.5194/egusphere-egu22-1054, 2022.

In the course of field work on Bolshoy Semyachik in 2020, 14 samples of varying degrees of alteration were taken on two thermal fields (Verhnee thermal field on Burlyashchiy volcano and thermal field of the northern crater of Central Semyachik). Samples included rocks of all stages of alteration from unaltered andesite and basaltic andesite, to completely transformed opalites. Samples were taken from outcrops in various parts of the fields and beyond. Seven samples were taken from each thermal field, and a series of cylinders were prepared from each sample partly in laboratory and partly in field conditions, which were used to measure properties. Unaltered samples from the Verhnee thermal field of Burlyashchy volcano are represented by basaltic andesites, and from Central Semyachik, by andesites. In order to determine the correlation, the properties of 123 cylinders were studied in laboratory conditions, and each of the 14 outcrops was examined with a Schmidt hammer.

In this study Schmidt hammer RGK SK-60 was used (type N). For each sample, 20 measurements were made on each outcrop, which made it possible to calculate the standard values of the elastic rebound in accordance with the methods set in ISRM. At the same time all the prepared cylinders were tested in the laboratory using standard methods to define different physical and mechanical properties.

In the most obvious way, the rebound height turns out to be related to the porosity of the opalized rocks and the dependence turns out to be linear. And porosity determines the indicators of a variety of other physical and mechanical properties. Due to that fact there is also a close relationship between the rebound height and such properties as density, water absorption, velocities of elastic waves, compressive and tensile strength etc.

As a result of the studies carried out, it was found that the height of the elastic rebound, obtained with the Schmidt hammer, is closely related to the physical and mechanical properties of effusive hydrothermally altered rocks. Moreover, the connection turns out to be closer in the rocks of the greatest degree of change, due to the fact that in the process of opalization, due to acid leaching and partial deposition of secondary minerals, the original composition and structure of the rocks completely changes and becomes more similar.

Based on the obtained regularities, it can be concluded that in conditions of strong variability of the geological structure, which occurs in thermal fields characterized by the discharge of acidic thermal waters, the use of the Schmidt hammer as an indirect method for determining the indicators of physical and mechanical properties is very expedient. The revealed interrelationships of the elastic rebound height (R_N) with physical and mechanical properties  make it possible to reliably assess the properties of rocks of varying degrees of alteration even in the field.

How to cite: Bolshakov, I.: Determination of properties of hydrothermally altered rocks using a Schmidt hammer (Bolshoy Semyachik, Kamchatka peninsula), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2275, https://doi.org/10.5194/egusphere-egu22-2275, 2022.

Abstract

The hydrothermal system of the andesitic volcano La Soufrière in the Basse-Terre island of Guadeloupe, is an ever evolving and highly dynamic system that is characterized by a variety of surface manifestations such as thermal springs, fumaroles and the Tarissan acid boiling pond (TAS) (Villemant et al.,2014). These produce halogen-rich surface emissions that makes it difficult to interpret subtle perturbations in the magmatic reservoirs using traditional geochemical and geophysical monitoring techniques (Moretti et al., 2020). A challenging situation for monitoring a volcano that has in recent history experienced six phreatic eruptions, the latest being the 1976-1977 eruptive crisis followed by a renewal of unrest in 1992 with the latest accelerated unrest episode occurring in 2018 (Komorowski et al., 2005, Moretti et al.,2020). Concurrently, TAS exhibited reduced Cl/Br ratios from ~1000 to ~300 from 18 January 2018 to 23 November 2020, suggesting marine water as a possible salinity source into the hydrothermal system. Hence, there is a critical need to fully conceptualize and appreciate the sources, full evolution and dynamic response of the hydrothermal system at La Soufrière. The research begins with investigating the potential input of ocean water into the hydrothermal system to aid in developing an exploitable geochemical database for prospective geochemical modelling analysis, leading to possible inferences on the influence of salinity on; 1) the gas-water-rock interactions in the shallow hydrothermal reservoir 2) scrubbing effects and 3) forcing conditions responsible for the measured surface gas emissions. We investigate this by numerically modelling flow transport of NaCl brines (wt.% 5, 25 and 35) using TOUGH2 software. Brines were modelled to represent volcanic- or marine- sourced brines at high temperature and pressure conditions (300°C - 350°C and 195Pa respectively). Steady state solutions of varying mass injections of orders of 1.0E^-4 kg/s/m² and 1.0E^-5 kg/s/m² resulted with the brines concentrating at heights of ~1100m a.s.l. at the summit and exiting the system, depending on the adopted permeability values. Thus implying that strong permeability contrasts due to sealing effects promoted by argillic alterations influences the trapping of brines in the upper edifice. 

How to cite: Glynn, C., Moretti, R., and Rosas-Carbajal, M.: Brine Vs Marine Water as Sources of Halogen-rich Hydrothermal Fluids at La Soufrière de Guadeloupe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2557, https://doi.org/10.5194/egusphere-egu22-2557, 2022.

Vulcano is an active volcanic island located in the south-central sector of the Aeolian Archipelago (Tyrrhenian Sea, Italy). The most recent active edifice is the La Fossa crater located in the center of the island, neighboring the main settlement Vulcano Porto. Its eruptive history is characterized by frequent transitions from phreatomagmatic to minor magmatic activity. The last eruption occurred in 1888–90 with strong phreatic (“Vulcanian”) eruption pulses. During the last decades, it has undergone several periods of volcanic unrest, accompanied by increasing degassing, rising fumarole temperatures, changing gas compositions, or increasing groundwater- and soil temperatures. Major unrest periods were reported in the 1920s, 1940s, and 1990s. Here we report on the ongoing crisis that initiated in September 2021. Rapidly increasing degassing levels and fumarole temperatures, accompanied by seismic activity and surface deformation were detected and monitored by the monitoring network (INGV bulletin reports). The fast evolution and dynamics of the crisis caused authorities to raise the alert level to orange and led to temporary evacuations in Vulcano Porto. We monitored this crisis from the beginning by monthly drone-based optical and thermal infrared overflights. The drone data was processed by using the Structure-from-Motion approach, allowing to generate spatially dense optical and thermal infrared maps. This way we captured the response of the hydrothermal system at the surface in great detail, were able to monitor the spatio-temporal evolution of the high-temperature fumarole field but also associated mean and low-temperature anomalies of diffuse degassing areas. We compared observations to a previous study considering in detail the structure and thermal expression of the La Fossa fumarole field, and the hydrothermal alteration associated (Müller et al., 2021, JVGR). Major aspects of changes observed at the surface during the crisis that could be constrained are (i) an increase of fumarole temperatures, (ii) the development of new fumarole vents, (iii) the evolution of a thermal aureole surrounding the major fumarole field at a distance, and (iv) the formation of a net-shaped thermal anomaly network. Changes are presented on a spatial and temporal scale and highlight the dynamics of degassing systems at the surface with implications for volcanic monitoring and hydrothermal alteration research and suggest that unrest is detectable at fumaroles but also at diffuse degassing zones elsewhere affecting a larger region of the La Fossa cone. 

How to cite: Müller, D., Pisciotta, A., and Walter, T. R.: Structural and geothermal changes mapped by drone-based photogrammetry and thermal infrared during the 2021 volcanic crisis of Vulcano Island, Sicily, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4966, https://doi.org/10.5194/egusphere-egu22-4966, 2022.

In Lesser Antilles arc several volcanic islands present high potential for the high enthalpy geothermal production. Saint-Kitts Island is one of them, where numerous surface evidences of active hydrothermal system such as fumaroles, boiling water are present. All of these activities are structurally controlled. Field analysis highlights four main families of faults or fractures with NE-SW, NW-SE, N-S, and E-W trends respectively.

The interaction of fluids (rain and sea waters) with host rock leads to meteoric and hydrothermal alterations of rocks according to depth of infiltration, usually consisting in their argillization. The characterization of these alterations allows a better understanding of the hydrothermal system of St Kitts and to estimate the geothermal potential of the resource in this island.

In the Frigate Bay area, in the southern part of the island, the rocks present an intensive structural and petrological transformations related to hydrothermal fluid flows. From study of mineralogical transformations with microscopic, SEM, EPMA, Raman and XRD analysis, we identify clays minerals (kaolinite, smectite), sulphates (jarosite, alunite, gypsum), quartz, opal, calcite and chlorite. These paragenesis are consistent with fluids above 100°C and allow to constrain the spatio-temporal activity of the hydrothermal system and the geothermal reservoir. Petrophysical properties, on a selected set of representative petro-structural facies, show large ranges of variation, porosity from 2 to 40%, permeability from 10^-3 to 3 D; grain density between 2.84 and 2.31 g.cm^-3, thermal conductivity is relatively low, 0.5 to 2 W.m^-1.K^-1. Samples alteration results in increasing of porosity and decrease in density. In turn the porosity increasing causes a decrease in the thermal conductivity.

These investigations allows us to interpret this site as part of a hydrothermal paleosystem and consider as an analogue of the deep northern part of the island under a current hydrothermal activity.

How to cite: Favier, A., Chibati, N., Diraison, M., Corsini, M., Géraud, Y., Lardeaux, J.-M., and Navelot, V.: Structural, mineralogical and petrophysical characterization of the hydrothermal system in Saint-Kitts Island (Lesser Antilles), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11655, https://doi.org/10.5194/egusphere-egu22-11655, 2022.