GMPV8.2 | Hydrothermal alteration in volcanic settings
Hydrothermal alteration in volcanic settings
Convener: Alexandra Kushnir | Co-conveners: Claire HarnettECSECS, Marlene Villeneuve, Thomas R. Walter
| Fri, 19 Apr, 08:30–10:15 (CEST)
Room -2.33
Posters on site
| Attendance Fri, 19 Apr, 16:15–18:00 (CEST) | Display Fri, 19 Apr, 14:00–18:00
Hall X1
Posters virtual
| Attendance Fri, 19 Apr, 14:00–15:45 (CEST) | Display Fri, 19 Apr, 08:30–18:00
vHall X1
Orals |
Fri, 08:30
Fri, 16:15
Fri, 14:00
Hydrothermal systems exert crucial influence on volcanic hazards. For example, hydrothermal alteration can reduce the strength of edifice- and dome-forming rocks, increasing the likelihood of volcano spreading and flank collapse, and high pore pressures that develop within hydrothermal systems can promote phreatic/phreatomagmatic explosions and further increase volcano instability. On the other hand, hydrothermal systems also offer the opportunity to exploit minerals of economic interest, and their heat can be harnessed to produce energy. A detailed understanding of hydrothermal systems, fluid-rock interactions in hydrothermal systems, and the resulting effects of alteration, using multidisciplinary studies, is required to better anticipate the hazards posed, to exploit the economic opportunities they provide, and to execute engineering design. We invite diverse contributions dedicated to the characterisation, imaging, monitoring, and hazard/economic assessment of volcanic hydrothermal systems and associated fluid-rock interactions. Contributions can be based on fieldwork, laboratory work, modelling, or a combination of these approaches. Because understanding hydrothermal systems requires multidisciplinary, collaborative teamwork, we welcome contributions based on any subdiscipline (e.g., geology, geophysics, geochemistry, engineering) and using any technique or method (e.g., geological mapping, magnetic, gravity, and spectroscopic methods, laboratory experiments, gas monitoring, numerical modelling). It goes without saying that we hope to have a diverse session in terms of both speakers and audience.

Orals: Fri, 19 Apr | Room -2.33

Chairpersons: Claire Harnett, Alexandra Kushnir, Marlene Villeneuve
On-site presentation
Maya Kopylova

Rodingite is a calc-silicate rock containing garnet, diopside, and chlorite that develop via metasomatic replacement of ultramafic to felsic rocks. Rodingites are found in ophiolites subjected to low-grade metamorphism or hydrothermal serpentinization of the surrounding ultramafic rocks. Here we report a previously unrecognized volcanic environment for rodingite formation. Rodingites can result from skarn-like reactions between kimberlite and xenoliths of silicate country rocks.

We studied hypabyssal and pyroclastic kimberlites from Renard and Gahcho Kue clusters (Canada) and Orapa (RSA). The kimberlites entrain 20-90 vol.% of granitoid and gneiss (Renard 65, Gahcho Kue) and basalt (Orapa) xenoliths collectively called silicate country rocks. Skarn-like reactions triggered by gradients in the chemical potentials of Si, Al, Ca, and Mg across the xenolith–kimberlite contacts produce concentric reaction zones within the xenoliths and a reaction halo in the surrounding contaminated kimberlite. The original mineralogy of the unreacted xenoliths is replaced by prehnite, diopside, pectolite, wollastonite, serpentine, garnet, calcic hydrosilicates (hydrogarnet, xonotlite, amphiboles). In the kimberlite halo, diopside and phlogopite form, carbonate is leached out, olivine is completely serpentinized. Rodingites develop by moderate degrees of reaction, and are replaced with monomineral serpentine and chlorite if metasomatism advances further.

Petrographic evidence for post-emplacement, metasomatic development of rodingite assemblages in kimberlite matches the Perple_X phase equilibria calculations that model the formation of the reactive mineralogy in the subsolidus, at T<600oC. Mass transfer of major elements is demonstrated by bulk compositions, thermodynamic modelling, and conserved element ratio analysis. The xenoliths experience Si loss and gain of Ca and Mg; the opposite trends are observed in the adjacent kimberlite. A metasomatic rather than hydrothermal origin of the reactive mineralogy is suggested by the inability to model hydrogarnet and pectolite under fluid saturated conditions. The observed mineralogy is accurately reproduced only when H2O and CO2 are modelled as components controlled by the kimberlite composition.

Rodingite patches in kimberlites have many common characteristics with rodingites in ophiolites and serpentinites, i.e. 1) a wide range of protoliths; 2) a juxtaposition of different lithologies; 3) synchronicity with the proximal serpentinization; 4) local sources of elemental mass transfer; 5) a combination of desilication and Ca gain; 6) a low-T character; 7) a common replacement of garnet by hydrous sheet silicates in subsequent de-rodingization.

Rodingite formation in kimberlites is distinct from the classic rodingization in several aspects. In kimberlites, rocks of contrasting compositions are juxtaposed on the smaller scale creating centimeter-sized domains of rodingite mineralogy. An influx of H2O is not required, as the deuteric hydrous fluid is already in excess in the cooling kimberlite. In this environment, serpentinization is driven by the Si flux. Rodingites in kimberlites form at surface pressure. De-rodingization is not a result of gradually decreasing P-Ts, but is due to the continued mass transfer and cooling. Our observations suggest that hydration, serpentinization of clinopyroxene, decarbonation of marine sediments and moderate P= 3-6 kb are not pre-requisites for rodingite formation. A new geological environment of rodingite formation in kimberlites expands the range of parameters for rodingization.

How to cite: Kopylova, M.: A new geological environment for rodingite formation: Metasomatism in kimberlite volcanoes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2974,, 2024.

Virtual presentation
Germano Solomita, Francecsa Corrado, Giuseppina Balassone, Nicola Mondillo, Angela Mormone, Harald Strauss, and Monica Piochi

The Tolfa volcanic district (TVD, Latium, Central Italy) is an extinct hydrothermal setting where the effect of fluid-rock interactions can be studied also as a key to unravel the processes taking place in active volcanic systems. TVD is characterized by a sulfate alteration associated with a pervasive deposition of opaline and/or microcrystalline silica, consisting of mineral replacements, veins and agate druses, and argillic facies in the pre-existing volcanic host rocks (Conte et al., 2022; Lombardi and Sheppard 1977; Marchesini et al., 2023).

The TVD hydrothermal alteration products have received attention in the previous literature with some petrographic and isotopic studies of sulfates and clay minerals, as well as scattered and incomplete rock and fluid geochemistry (Cinti et al., 2011; Lombardi and Sheppard 1977). These early studies, however, suggest that TVD is of particular interest in terms of both alteration and mineralization features.

This study is part of a bigger project on the Allumiere-Tolfa district and focuses on the aluminium sulfate and kaolinite-rich deposits of the Tolfa hydrothermal alteration area, deepening the investigation of the processes that generated the alteration materials. We conducted a field campaign at Tolfa and Allumiere and in several surrounding localities up to the Santa Severa mountains. Various collected samples have been analyzed by integrating different techniques: X-ray diffraction (XRD), scanning electron microscopy with microanalysis by energy-dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FT-IR), whole-rock geochemistry and stable O- and S- isotope geochemistry.

The mineralogical analyses show a wide morphological and compositional variability of the sulfates mineralization. An alunite group mineral occurs as solid solutions. We identified natroalunite as well as walthierite, usually characterized by lamellar/tabular and pseudocubic habits; instead, alluminium-phosphate–alunite (APA) is present as pseudocubic crystals. Kaolinite group minerals have been also detected and, in particular, kaolinite is widespread, occurring in well-shaped grains, often in vermicular packages. Quartz is abundant in most of the samples, associating to alunites. Fe-Ti oxides contain Cr, V, Ni. Alunite can be characterized for high Ba or Sr content and a distinct P-rich core. The ẟ34S varies between 1.2 and 10.3‰ and the ẟ18O between -5.7 and 11.7‰, depending on mineral type. The preliminary data collected are useful (i) to improve the knowledge of mineral chemistry of these deposits and to characterize in detail alunite and kaolinite minerals; (ii) to constrain the sulfate mineralizations by the hydrothermal fluids, (iii) to have a background for the investigation of mineralization zones and processes, and (iv) to compare this case study with hydrothermal alteration and processes from extinct and quiescent volcanic districts, such as Campi Flegrei and Ischia in Campania.

Cinti D et al. (2011).  Chemical Geology 284, no. 1-2: 160-181.

Conte A. et al. (2022). Physics and Chemistry of Minerals, 49(10), p.39.

Lombardi G., Sheppard S.M.F. (1977) Petrographic and isotopic studies of the altered acid volcanics of the Tolfa-Cerite area, Italy: the genesis of the clays. Clay Miner 12(2):147–162.

Marchesini B. et al. (2023) Structural control on the alteration and fluid flow in the lithocap of the Allumiere-Tolfa epithermal system. Journal of Structural Geology, 105035.


How to cite: Solomita, G., Corrado, F., Balassone, G., Mondillo, N., Mormone, A., Strauss, H., and Piochi, M.: Hydrothermal alteration at the Tolfa volcanic district (Latium, Italy): texture, mineral and isotope chemistry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16850,, 2024.

On-site presentation
Roberto Davoli, Katharina Engels, Cristian Montanaro, Tullio Ricci, Alessandra Sciarra, and Bettina Scheu

Alteration by rock-fluid interaction can significantly impact rock and soil properties influencing fluid flow within hydrothermal aquifers, and shaping surface features and degassing in geothermal settings over time. Hence, the understanding of the spatial distribution of thermal manifestations in correlation with the geological setting and alteration type remains essential for unravelling the processes controlling the transport of fluids from within the hydrothermal aquifers to the surface.

Previous studies focused on the effect of alteration within hydrothermal aquifers on a scale of hundreds of meters. In contrast, limited research has explored the influence of subsurface lithologies – including their intrinsic and altered permeability – and their spatial distribution on degassing activity. This study examines subsoil portions across the active geothermal fields of Krafla caldera in Iceland, to explore the relationship between various soils/lithologies and primary degassing areas. In particular, we investigate how hydrothermal alteration influences the petrophysical characteristics of these lithologies, thereby modulating surface-level fluid circulation.

In two field campaigns carried out in 2022 and 2023, we assessed the in situ petrophysical properties of over 200 samples across 22 sites in the Víti and Hveragil regions. Moreover, we conducted subsoil diffuse CO2 flux measurements for specific profiles. Field permeability ranged from 10-11 to 10-16 m2, with CO2 fluxes varying between 1.25 to 2628.33 g/m2 day. Additionally, we examined the grain size distribution and the componentry of selected subsoil layers.

Our findings delineate the presence of both active (thermal) and inactive (non-thermal) regions, depicting the variable impact of hydrothermal alteration on the subsoil properties. In active geothermal sites, the dominance of mineral dissolution/replacement facilitates the formation of preferential pathways for fluid flow. Within inactive areas, mineral cementation of pores and fractures appears to act as barriers to fluid movement. These outcomes offer crucial insights for comprehending and quantifying the effect of hydrothermal alteration on fluid dynamics, shedding light on the progression of surficial manifestations and degassing patterns within active geothermal areas.

How to cite: Davoli, R., Engels, K., Montanaro, C., Ricci, T., Sciarra, A., and Scheu, B.: Subsurface soil and lithology alteration shaping degassing at Krafla caldera, Iceland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16537,, 2024.

On-site presentation
Michael Heap, Geoffroy Avard, and Patrick Baud

Reductions to the permeability of a volcanic system can increase pore fluid pressure and, in turn, promote volcanic hazards such as erratic explosive behaviour and slope failure. Hydrothermal alteration, ubiquitous at volcanoes worldwide, is one mechanism thought to reduce permeability and therefore increase volcanic hazard potential. Turrialba, Poás, and Rincón de Vieja, active stratovolcanoes in Costa Rica, are characterised by erratic explosive behaviour thought to be the result of the formation, maturation, and rupture of hydrothermal seals that clog the conduit. To better understand this process, we present here a systematic study in which we assessed the textural, mineralogical, and physical properties of hydrothermal seals from Turrialba, Poás, and Rincon de Vieja volcanoes, ejected as ballistics following recent explosive activity. We first document the type and intensity of the hydrothermal alteration preserved in the collected ballistics using X-ray powder diffraction and scanning electron microscopy. All samples are characterised by pervasive acid-sulphate alteration. Prior to sample preparation, the permeability of these ballistics was first estimated using a TinyPerm III, a portable handheld air permeameter. We find that the hydrothermal seal is characterised by low values of permeability, between 10−15 and 10−13 m2. Cylindrical samples were then prepared for a systematic physical property (porosity, permeability, P-wave velocity, thermal properties, and mechanical strength) characterisation in the laboratory. These laboratory data, together with fluid flow modelling, highlight how alteration can create a low-permeability hydrothermal seal that promotes cyclic, but erratic, explosive volcanic behaviour.

How to cite: Heap, M., Avard, G., and Baud, P.: Hydrothermal sealing and erratic explosive behaviour at Turrialba, Poás, and Rincón de Vieja volcanoes (Costa Rica), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4420,, 2024.

On-site presentation
Antonio Troiano, Anna Mocerino, Claudio De Paola, Fabio Pagliara, Maria Giulia Di Giuseppe, Roberto Isaia, and Rosa Di Maio

The island of Vulcano is situated in the southern part of the Salina-Lipari-Vulcano ridge and, together with these two islands, constitutes the central sector of the Aeolian archipelago. It is a complex active volcanic system whose last eruption occurred between 1888 and 1890. Since then, intense fumarolic activity accompanied the various phases of unrest of the associated hydrothermal system, which made Vulcano the object of various geophysical, geochemical, and hydrogeological studies.

The aim of the study here presented was the investigation of the superficial hydrothermal system of Vulcano, specifically the northern sector of the island, where the ‘La Fossa’ caldera is located, which has been the main degassing area since the beginning of the last period of unrest. The survey has been conducted using geoelectrical methods because of the efficiency that these methods revealed in the previous studies on active volcanic systems, especially on Vulcano. The distribution of resistivity in the subsoil, indeed, is influenced by parameters such as temperature, permeability, porosity, presence of fluids and their salinity, which allow the reconstruction of the changes in the lithological and structural features with depth and the evaluation of the presence of discontinuity, such as faults, that have a fundamental role in the circulation of fluids.

During the survey, electric resistivity, chargeability, and self-potential data were jointly acquired along five profiles with lengths between 700 m and 2500 m and an investigation depth of approximately 200 m. The tomographies resulting from the data processing provide information on the evolution of the hydrothermal system, allowing us to identify the prominent resistivity anomalies and the pattern of hydrothermal fluid circulation. By integrating the results obtained with the ones emerging from previous studies conducted on the island, it is possible to implement a more exhaustive evaluation of the hydrothermal mechanism in the areas adjacent to the La Fossa caldera and how the flow is facilitated or hindered by the main fault system of Vulcano, that is necessary to analyze in order to previously verify the actualization of conditions favorable to the overpressure or decompression of the fluids, which could trigger an explosive eruptive event.

How to cite: Troiano, A., Mocerino, A., De Paola, C., Pagliara, F., Di Giuseppe, M. G., Isaia, R., and Di Maio, R.: Characterization of the shallow hydrothermal system of Vulcano Island (Eolie islands, Italy)  through Electrical Resistivity Tomography (ERT) and Induces Polarization (IP) tomography., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18301,, 2024.

On-site presentation
Maia Kidd, Gabor Kereszturi, Michael Heap, Ben Kennedy, and Jonathan Procter

Volcanoes are dynamic and complex natural systems, constantly changing through eruptions, alteration, and erosion. Hydrothermal systems are ubiquitous on volcanoes, causing physical and mechanical change to rock properties via hydrothermal alteration. The most commonly measured rock physical properties are porosity and uniaxial compressive strength (UCS) as they provide insight as to their history and potential mechanical behavior. Porosity and UCS are affected by the primary properties and emplacement history (e.g., volatile content, crystallinity, cooling rate, composition, fragmentation type) and the post-emplacement conditions (e.g., surface weathering and hydrothermal alteration). This creates highly heterogenous rock masses, with variation occurring across mm to m scales. Currently, destructive testing is required to measure UCS and porosity (destructive of the wider sample) accurately and needs a large volume of samples to capture the heterogeneity of volcanic rock masses. This testing is cost and time prohibitive, requiring large sample volume, in terms of sample size and number, which often require shipping to specialist labs. Schmidt hammers can be used to estimate UCS non-destructively, however, they produce inaccurate results on soft rocks such as hydrothermally altered rocks. Here, we present a new non-destructive method for predicting porosity and UCS across a range of volcanic rocks, from non-altered to highly altered.

This study uses visible-near infrared (VNIR) to shortwave infrared (SWIR) wavelengths (350-2500 nm) reflectance spectroscopy to predict porosity and UCS via Partial Least Squares Regression (PLSR). Reflectance spectroscopy is a non-destructive method that is sensitive to both physical (surface roughness and crystal/particle size) and chemical (mineral species and abundance) properties of volcanic rocks. Because these rock attributes also influence the physical and mechanical properties of rock, reflectance spectroscopy could be used to quantitatively predict porosity and UCS. This study used experimentally deformed volcanic rocks from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria), La Soufrière de Guadeloupe (Eastern Caribbean), Volvic (France), and Cracked Mountain (Canada) to evaluate the accuracy of PLSR-based predictions for porosity and UCS. The training samples encompass a wide range of volcanoes, alteration degree (non-altered, silicic, argillic, and phyllic alteration), mineralogical differences (initial composition from basalt to rhyolite and alteration products), and textural differences (original textures such as lava and pyroclastic, and alteration textures including veins). Model sensitivity is evaluated by adding randomly individual samples to the training database or performing leave one group out cross validation based on characteristics (e.g., alteration mineral types, textural features, or volcano location). From this analysis, specific alteration mineralogy can be evaluated for its effect on porosity and UCS predictions such as the role of phyllosilicate formation causing a reduction in UCS. The proposed non-destructive method via VNIR-SWIR spectroscopy can complement existing rock mechanical testing methods to better quantify highly heterogenous volcanic and hydrothermal systems and their rock successions.

How to cite: Kidd, M., Kereszturi, G., Heap, M., Kennedy, B., and Procter, J.: Linking hydrothermal alteration and volcanic rock mechanics through VNIR-SWIR spectroscopy  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6858,, 2024.

On-site presentation
Gabor Kereszturi, Antonio M. Álvarez-Valero, Nessa D’Mello, Mercedes Suárez Barrios, Rachelle Sanchez, Craig Miller, and Daniel A. Coulthard

Andesitic composite volcanoes located at convergent plate margins can host extensive zones of hydrothermally altered rocks, which can influence their rock mechanics and potentially modulate eruptive activity. Hydrothermal minerals typically form along fractures and permeable zones above cooling and degassing magma bodies. A robust understanding of the vertical and lateral distribution of hydrothermal minerals can therefore reveal the location of (former) magma bodies/fluid source and can indicate areas susceptible for future flank collapse processes.

This study investigates the type and extent of hydrothermal alteration within the Wahianoa Formation (160-80 ky) of Ruapehu volcano in New Zealand by integrating field observations, Scanning Electron Microscopy (SEM-EDS), Shortwave Infrared (SWIR) reflectance spectroscopy, X-Ray Diffraction (XRD), sulfur isotope systematics and Inductively coupled plasma mass spectrometry (ICP-MS), thermodynamic modelling and airborne geophysics. Ruapehu shows a diverse suite of weathering and hydrothermal alteration minerals formed in relation to the present and fossil hydrothermal systems. Wahianoa Formation is one of the oldest formations, showing remarkable diversity of hydrothermal alteration that has never been studied before. The distal rock has only supergene alteration with abundant goethite, hematite and phyllosilicate mineral associations, while the hydrothermally altered rock are rich in phyllosilicates, Fe-oxides, pyrite, jarosite, alunite, gypsum anhydrite, and native sulphur. The latter is interpreted to be formed under intermediate and advanced argillic alteration conditions (>150 °C and low pH). In contrast, the some of the exposed outcrops within the upper Wahianoa valley show distinct mineralogy, that is rich in quartz, pyrite, illite(-chlorite) and tourmaline, indicating a transition from the advanced argillic conditions toward more phyllic alteration type (>220 °C and more neutral pH). Our results indicate a complex hydrothermal system developed within the Wahianoa Formation between 150-80 ky, providing a great example to study vertical and lateral mineralogical changes. A new model has been proposed to integrate hydrothermal alteration history into the Mt Ruapehu’s evolution that can better depict ongoing alteration processes and triggers for flank instability and volcanic hazards associated hydrothermal systems.

How to cite: Kereszturi, G., Álvarez-Valero, A. M., D’Mello, N., Suárez Barrios, M., Sanchez, R., Miller, C., and Coulthard, D. A.: Vertical and lateral changes of hydrothermal alteration in andesitic composite volcanoes – Linking flank collapse with hydrothermal activity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7009,, 2024.

On-site presentation
Benjamin De Jarnatt, Thomas R. Walter, Michael J. Heap, Daniel Mueller, Julia Nikutta, and Antonino Fabio Pisciotta

Hydrothermal alteration is well recognized to change the physical and mechanical properties of volcanic rocks and promote instability and flank collapse. Here, we investigate La Fossa of Vulcano Island (Italy), the southernmost exposure of the Aeolian volcanic archipelago, and its associated regions of hydrothermal alteration. La Fossa’s accessibility, altered flanks, history of mass wasting events, and periods of escalating fumarole activity set an ideal environment for a natural laboratory. We used drone remote sensing methods coupled with field and ongoing laboratory rock property measurements to classify regions of hydrothermal alteration and assess their associated rock properties. Our results have (1) identified a heterogenous distribution of alteration intensity and alteration types, (2) distinguished a relationship between decreasing rock strength and increasing alteration, and (3) correlated regions with the weakest rock strength with hydrothermally altered flanks. This combined approach allows us to explore the relationships between hydrothermal alteration, rock strength, and flank instability of La Fossa. 

How to cite: De Jarnatt, B., Walter, T. R., Heap, M. J., Mueller, D., Nikutta, J., and Pisciotta, A. F.: Hydrothermally altered rocks of La Fossa, Vulcano island (Italy): Implications for flank instability from coupled drone photogrammetry and rock mechanical measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4726,, 2024.

On-site presentation
Agata Poganj, Michael J. Heap, and Patrick Baud

Hydrothermal alteration, which alters the chemical composition and physical state of volcanic rocks, can weaken a volcanic edifice, potentially leading to instability and collapse, thereby endangering the livelihoods of neighbouring residents. Instability scenarios can be modelled using large-scale numerical models, the accuracy of which depends on acquiring the physical and mechanical properties of volcanic rocks from laboratory measurements. Routinely, laboratory studies provide measurements for small sample suites (< 10) and, as a result, we do not fully grasp the range of rock properties that describe a particular unit or volume of the volcano, nor is it clear whether the few samples collected are representative. Here, we introduce a method that can help us select the most appropriate value, or range of values, for the physical and mechanical properties of volcanic rocks for large-scale numerical models.

We collected nearly 550 rocks from seven different sampling sites at La Soufrière de Guadeloupe, an active andesitic stratovolcano in the Eastern Caribbean. The rocks were assigned an alteration grade index, from 1 (least altered) to 5 (most altered), based on a visual assessment of their alteration. Their bulk densities were then measured in the field using the Archimedes method and, lastly, their strengths were measured using a point load tester, a field strength measuring apparatus. Alteration grade index ranges from 1 to 5 , bulk densities range from less than 1000 to 2700 kg/m3, and point load strength values span from 0.012 to 8.53 MPa . Point load strength and alteration distribution maps for the seven sampling locations show not only that rock physical properties vary greatly at an individual location, but also that the distributions of alteration and strength vary from place to place, highlighting the large heterogeneity of the dome at La Soufrière de Guadeloupe.

To calibrate these field data, we took small samples of the tested rocks back to the laboratory for porosity and uniaxial compressive strength measurements. Porosity measurements, ranging from 0.02 to 0.8, exhibited a strong correlation with field density values. The extremely altered rocks, deemed alteration grade index 5, despite having porosities ranging from 0.11 to 0.8, did not exceed point load strength of 2 MPa, suggesting that strength was not solely dependent on porosity. We also found that strength decreases as a function of increasing porosity and alteration grade index. Although uniaxial compressive strength tests revealed synchronicity with point load strength data, our current goal is to perform further tests to better constrain this relationship.

The point load test is a field method that can simplify the logistic problems behind strength assessments of volcanic domes, and likewise enlarge the sampling suite of individual studies. Calibrating field results to uniaxial compressive strength laboratory data will allow our data to be used in volcano stability models and accommodate direct conversion, from point load values to uniaxial compressive strength values, on-site. The potential abundance of available data, and spatial strength distributions, could make the large-scale models more realistic, and consequently more accurate and reliable.

How to cite: Poganj, A., Heap, M. J., and Baud, P.: Alteration and strength heterogeneity of the volcanic dome at La Soufrière de Guadeloupe (Eastern Caribbean), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-174,, 2024.

On-site presentation
Caroline Martel, François Décossin, Laurent Arbaret, Rémi Champallier, Michael Heap, Marina Rosas-Carbajal, and Jean-Christophe Komorowski

Flank collapse represents a significant hazard at explosive volcanoes. Hydrothermal alteration is considered to promote volcano instability, but there are few data and models to robustly assess this hypothesis. In the framework of the Mygale ANR project, a multidisciplinary approach that combines geological data, geophysical and geochemical techniques, laboratory measurements, and modelling is proposed to assess the role of alteration on the stability of La Soufrière de Guadeloupe (France). We focus here on the benefit of combining time-series and flow-through experiments with thermodynamic and kinetic modelling for constraining the alteration process below the lava dome of La Soufrière. Flow-through experiments under pressure and temperature provide in-situ permeability data during ongoing alteration of pristine andesite. Thermodynamic modelling is essential for evaluating the fluid phase composition driving rock alteration at depth. Kinetic modelling is precious for predicting the alteration sequence on timescales longer than experimentally achievable. Eventually combining such geophysical and geochemical data on natural and experimental samples will allow to propose a large-scale 4D numerical model to assess the influence of alteration on volcano stability and improve monitoring of volcano instability.

How to cite: Martel, C., Décossin, F., Arbaret, L., Champallier, R., Heap, M., Rosas-Carbajal, M., and Komorowski, J.-C.: Hydrothermal alteration of lava domes: coupled contribution of experimentation and thermodynamic modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5948,, 2024.

Posters on site: Fri, 19 Apr, 16:15–18:00 | Hall X1

Display time: Fri, 19 Apr 14:00–Fri, 19 Apr 18:00
Chairpersons: Alexandra Kushnir, Claire Harnett, Thomas R. Walter
Alexandra Kushnir, Cindy Mikaelian, and Andrea Mazzeo

Reactions between aqueous fluids and rocks result in secondary mineral formation – alteration – that can block permeable pathways in the Earth’s crust and change the hydraulic properties of geological systems. The propensity, extent, and timescales of rock alteration are therefore important controls on crustal permeability. However, understanding the intricate relationships that result in rock alteration and changes in permeability - including dissolution, transport, and redistribution of chemical compounds - represents a technical challenge and remains a key unsolved problem in the Earth sciences. As a result, we do not fully understand how these processes modify the structure of permeable channels and over what timescales they may hamper crustal fluid flow, limiting, for example, our ability to effectively model the efficiency of geothermal reservoirs or forecast volcanic eruptions. The SNSF-funded ReSiDue project – Redistribution of Silica and Deposition under a volcanic edifice - addresses how the redistribution of the most common compound in the Earth’s crust - silica (SiO2) - changes the permeability of rocks under volcanically relevant conditions. Silica alteration is ubiquitous in volcanic systems and results in changes to rock structure and thus hydraulic properties. However, the processes that lead to silica alteration remain understudied, especially at high temperatures, because they involve complex feedbacks between fluid-rock interactions and petrophysics. Using a mix of detailed petrophysical, microstructural and geochemical characterization, and water-rock interaction experiments, we are quantifying 1) the physical and chemical conditions promoting silica alteration in volcanic edifices, 2) how fluid-flow pathways in rocks change over time as a result of silica alteration, 3) how these changes modify permeable flow, and 4) on what timescales these processes are active. The newly operational High temperature Reactive flOw permeabiLity Device – HAROLD – allows for in situ monitoring of permeability evolution over time, during reactive flow. This apparatus can operate at temperatures up to 500°C and confining and pore fluid pressures of 100 MPa and 20 MPa, respectively, allowing us to target conditions relevant to volcanic edifices and shallow geothermal reservoirs. This systematic understanding of how silica alteration changes the permeability of volcanic rocks will help refine volcanic outgassing models, with potential application to risk assessment and hazard mitigation efforts. However, beyond volcanology, this project sets the infrastructural, experimental and analytical foundation needed to more broadly study the relationships between rock-fluid interactions and crustal fluid flow, including in geothermal systems.

How to cite: Kushnir, A., Mikaelian, C., and Mazzeo, A.: ReSiDue: A coupled rock physics and geochemical approach to understanding how rock-fluid interactions change permeability in volcanic systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4729,, 2024.

Claire Harnett, Michael Heap, Valentin Troll, and Thomas Walter

Hydrothermal alteration gradually and imperceptibly changes the chemical and physical state of the rocks inside a volcano, creating a soft and unstable (or 'rotten') interior. However, the link between 'soft' volcanoes and unpredictable volcanic events remains poorly resolved. ROTTnROCK is a 6-year ERC project, running from 2024-2030, funded through the European Research Council's Synergy Call. The growing ROTTnROCK team will investigate the role of hydrothermal alteration in unpredictable volcanic hazards, such as volcanic instability, alteration-induced eruption triggering, and caldera fault (re-)activation. Specifically, we will use remote sensing and geophysics to identify where and at what scales alteration is occurring. Laboratory investigations will study the chemical fingerprint of alteration and its effects on rock mechanical properties and strength. These approaches will be coupled with 4D volcano stability simulations to produce an innovative and optimised hazard assessment workflow. We will work at selected target sites that show evidence of strong hydrothermal alteration, either associated with flank collapse, collapsing lava domes, crater lakes, or active collapse calderas. This project will transform our understanding of hydrothermal alteration and its impact on volcanic hazards, and will pave the way for strategies to predict and mitigate unexpected volcanic events caused by hydrothermal alteration.

How to cite: Harnett, C., Heap, M., Troll, V., and Walter, T.: ROTTnROCK: a new ERC project to investigate the influence of hydrothermal alteration on volcano instability, eruption triggers and fault reactivation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6347,, 2024.

Matias Tramontini, Marina Rosas-Carbajal, Fabio I. Zyserman, Pascale Besson, Jacques Marteau, and Michael Heap

Muography is a non-invasive geophysical method that relies on the detection of muons, which are subatomic particles generated by the interaction of cosmic rays with the Earth's atmosphere. The physical quantity estimated by this method is the opacity, which represents the amount of matter traversed by muons along their trajectories, resulting in energy loss and scattering for the particles. Thus, absorption muography consists of deploying a muon detector targeting the volcano and registering the muons traversing it per unit of time and trajectory. From these data, radiographs of average density of extensive rock volumes can be obtained using a single measuring instrument and from a singular measurement position.

Copahue volcano is located in the Andes mountain range and is considered the highest-risk volcano in Argentina due to its proximity to two towns situated within an 8 km radius of the volcano's crater. Additionally, the region attracts a significant number of tourists, leading to a substantial increase in the population of both localities. The latest eruptive cycle, initiated in 2012, has maintained a near-continuous state of activity, marked by ash emissions, crater explosions, and seismic activity. In this work, we study the hydrothermal alteration at Copahue volcano through a combination of muography and laboratory measurements of the chemical and physical properties of rock samples.

The muography dataset was acquired by installing a muon detector on the eastern flank of Copahue volcano, situated at an altitude of approximately 2500 meters above sea level. For the laboratory analyses, we collected rock blocks with the objective of representing a diverse spectrum of alteration stages within Copahue volcano. Through this selection process, we captured variations in mineralogical composition, geochemical signatures, and physical properties that correspond to different stages of hydrothermal alteration. We carried out a series of examinations on the rock samples extracted from the targeted flank, such as X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) analyses that identified mineralogical compositions and geochemical signatures associated with hydrothermal processes. We also carried out additional measurements, including density, porosity, permeability, thermal properties, and uniaxial compressive strength, contributing to a comprehensive understanding of the physical properties of the samples. In addition, we performed microscopic examinations using a scanning electron microscope (SEM) to study the microstructural changes induced by hydrothermal alteration. This integrative approach, between muography and detailed laboratory measurements on rock samples, aims to reveal correlations between subsurface density variations and hydrothermal alteration observed at the microscopic and macroscopic scales.

How to cite: Tramontini, M., Rosas-Carbajal, M., Zyserman, F. I., Besson, P., Marteau, J., and Heap, M.: Investigating hydrothermal alteration in Copahue volcano (Argentina/Chile) using muography and laboratory measurements on rock samples, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6565,, 2024.

Claudio De Paola, Maria Giulia Di Giuseppe, Roberto Isaia, Fabio Pagliara, and Antonio Troiano

The island of Vulcano is one of the most exposed volcanic edifices of the Aeolian islands. Vulcano has been responsible for many eruptions, including at least three phreatic eruptions. The most recent eruption on the island occurred between 1888 and 1890 AD, and since then, Vulcano’s hydrothermal system has been the site of different volcanic unrest. In 2021, geochemical and geophysical parameters showed changes, suggesting a potential increase in magmatic fluid within the shallow geothermal system. GPS data revealed an expansion of the crater zone, consequently increasing the Vulcano alert level.

The main volcano-tectonic structures and geothermal fluid pathways have been resolved down to a depth of 2.5 km through a magnetotelluric (MT) survey conducted in 2022 (Di Giuseppe et al., 2023). A more detailed definition and mapping of the significant volcano-tectonic structures were subsequently acquired through a high-resolution Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) investigation campaign conducted in 2023. These surveys provided original information regarding the main structure of the caldera and the related subsurface fluid circulation. The overall characterization of the link between the shallow geothermal system and the volcano activity furnished by the electromagnetic and electrical investigations represented the basis for developing thermo-fluid dynamic modeling of Vulcano island using the TOUGH2 numerical code. Adopting a conceptual model of the island mainly derived from the results of the MT and ERT investigations and considering literature and monitoring data collected immediately before and after the 2021 Vulcano unrest phase, a thermodynamic stationary state has been reconstructed, which likely represents a reliable description of the actual condition of the volcano. Subsequently, perturbation analysis of such a thermodynamical state has been performed to characterize the recent unrest phase involving Vulcano during the past two years and to reconstruct its primary mechanisms.

How to cite: De Paola, C., Di Giuseppe, M. G., Isaia, R., Pagliara, F., and Troiano, A.: Characterization of the 2021 unrest phase of Vulcano Island (Eolie islands, Italy) through numerical modelling: an integrated approach based on Electrical Resistivity Tomography and Magnetotelluric data., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18827,, 2024.

Aida Mendieta, Marina Rosas-Carbajal, Raphaël Bajou, Patrick Baud, Sébastien Deroussi, Tomaso Esposti Ongaro, Jean-Christophe Komorowski, Alexandra R.L. Kushnir, Marlène Villeneuve, and Michael J. Heap

Hydrothermal alteration plays a major role in volcanic instability. Improving techniques that allow us to better understand the timescales of alteration in active volcanoes is thus paramount. Since the reactivation of the fumarolic field at the top of the dome of La Soufrière de Guadeloupe (Guadeloupe, France) in 1992 and the expansion of its surface area in recent years, the area of study is subjected to prolonged variable alteration that has promoted past flank collapses and can also influence permeability and thus shallow depth overpressurization. During a field campaign in May 2022 we performed 25 electrical resistivity tomography (ERT) profiles in the summit of La Soufrière, next to active fumaroles and acid boiling ponds. These ERT profiles were inverted using the open-source code pyGIMLi. Thanks to the ERT profiles we are able to roughly map the altered areas to a depth of about 20 m in this zone of La Soufrière. Some of the byproducts of alteration that have been identified in La Soufrière are clays, sulfates and pyrite. Thus, we infer that the low electrical resistivity zones (<20 Ωm) correspond to alteration and high electrical resistivity (>1500 Ωm) corresponds to unaltered rock. Low electrical resistivity anomalies are observed north of the Breislack fault, near the fumaroles. The explored north-most region is characterized by higher values of electrical resistivity. We take into account ground temperature and spatial variability to interpret the electrical conductivity anomalies and we use this first high-resolution resistivity model to plan a repetition of our experiment. This time-lapse experiment will allow us to estimate the evolution of hydrothermal alteration in the volcano’s summit over a 2-year period in the context of the current ongoing multiparameter unrest.

How to cite: Mendieta, A., Rosas-Carbajal, M., Bajou, R., Baud, P., Deroussi, S., Esposti Ongaro, T., Komorowski, J.-C., Kushnir, A. R. L., Villeneuve, M., and Heap, M. J.: Imaging hydrothermal alteration with electrical resistivity tomography in La Soufrière de Guadeloupe volcano, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9520,, 2024.

Amelie Klein, David E. Jessop, Franck Donnadieu, Joanny Pierre, and Roberto Moretti

Quantifying subsurface fluid circulation and the associated heat and gas fluxes provides crucial clues for interpreting the evolution of volcanic unrest in volcanoes with active hydrothermal systems. To better constrain the distribution of current hydrothermal activity on La Soufrière de Guadeloupe, we conducted repeated mapping of diffuse ground CO2 degassing, ground temperature, and self-potential on the dome summit in 2022-2023.

We produced maps by interpolating these data, which allowed us to identify areas of fluid recharge into the hydrothermal system and the areas of ascending hydrothermal flows. We were also able to quantify the convective and conductive ground heat fluxes and the area affected by ground heating.

Our measurements allowed us, for the first time, to quantify the ground CO2 flux of the dome, estimate the condensation depth of ascending fluids, and relate it to soil permeability. We found diffuse ground CO2 degassing corresponds to about half of the CO2 emissions from the summit fumaroles.

Further, we performed continuous self-potential measurements at the summit to analyse the temporal variability of underground fluid fluxes. The combination of these measurements and comparison with high-resolution thermal images taken in recent years allows us to track the spatial and temporal evolution of the shallow hydrothermal fluid circulation. Moreover, we were able to constrain the controlling factors of the observed changes in the surface manifestations in the NE sector of the summit, where a fumarole field has been developing since 2012. Our results imply that monitoring soil degassing and ground temperature gradients can enhance understanding of the sealing state of the dome, which is crucial for assessing potential hazards linked to fluid pressurisation.

How to cite: Klein, A., Jessop, D. E., Donnadieu, F., Pierre, J., and Moretti, R.: Hydrothermal fluid circulation at La Soufrière de Guadeloupe inferred from soil CO2 degassing, thermal flux, and self-potential, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9869,, 2024.

Geological & Structural properties of the Öksüt High Sulfidation Epithermal Gold Deposit South-Central Anatolia (Turkey)
Duygu İşbil, David A. Bickford, Yücel Öztaş, Göktuğ Söğütçü, and Hasan Yaradılmış
Federica Salomone and David Dolejš

Ionization of water and speciation of aqueous electrolytes exerts important control on chemical composition, acid-base interaction, and fluid-mineral equilibria in hydrothermal systems. Several competing approaches are available to describe H2O, HCl and NaCl speciation under hydrothermal conditions: (i) electrostatic models using the Born theory (Tanger and Helgeson 1988; Shock et al. 1992, Djamali and Cobble 2009), (ii) semi-empirical correlations with H2O density (Marshall and Franck 1981, Mesmer et al. 1988, Holland and Powell 1998); (iii) hydration models (Pitzer 1982; Tanger and Pitzer 1989) and (iv) diverse theoretical models (e.g., Akinfiev and Diamond 2003, Lvov et al. 2018). Available models were compared to experimental data (n = 606) covering temperature from 0 to 800 °C and pressure of 1 to 8000 bar, with several measurements up to 1000 °C and 133 kbar. Various approaches consistently and accurately reproduce the experimental H2O ionization up to 400 °C and above 300 bar. By contrast, at higher temperatures or lower pressures the models substantially diverge and reveal conceptual deficiencies. For HCl dissociation, the electrostatic, density and virial models are accurate to 400 °C and 1500 bar, at higher temperatures remain consistent but rapidly inaccurate and at higher pressures they diverge. This critical evaluation identifies density models as most promising for formulating new equation of state for ionic species. Here we combine individual thermodynamic contributions representing intrinsic species properties, mechanical and electrostatic solute-solvent interaction during hydration, and standard-state conversion term. We use the experimental datasets for H2O, HCl and NaCl ionization to evaluate the performance and accuracy of optional functional forms for heat capacity, hydration compression and its coupling to the standard-state conversion term. Our preliminary results indicate that intrinsic enthalpy, entropy and volume coupled with heat capacity linearly dependent on temperature, compression and standard-state conversion terms, without involvement of electrostatic contribution or chemical hydration afford the most accurate description of thermodynamic properties of ionic species. This model provides basis for accurate prediction and modeling of mineral-fluid and melt-fluid equilibria in upper-crustal magmatic and hydrothermal systems.


Akinfiev N.N., Diamond L.W. (2003) Geochim. Cosmochim. Acta 67, 613-627.

Djamali E., Cobble J.W. (2009) J. Phys. Chem. 113, 2398-2403.

Lvov S.N. et al. (2018) J. Molecul. Liq. 270, 62-73.

Marshall W.L., Franck E.U. (1981) J. Phys. Chem. Ref. Data 10, 295–304.

Pitzer K.S. (1982) J. Phys. Chem. 86, 4704-4708.

Shock E.L. et al. (1992) J.  Chem. Soc. Faraday Trans. 88, 803-826.

Tanger IV J.C., Helgeson H.C. (1988) Am. J. Sci. 288, 19-98.

Tanger IV J.C., Pitzer K.S. (1989) J. Phys. Chem. 93, 4941-4951.

How to cite: Salomone, F. and Dolejš, D.: Critical evaluation and development of thermodynamic models for ionization of H2O, HCl and NaCl at high temperatures and pressures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15937,, 2024.

François Décossin, Caroline Martel, Laurent Arbaret, Rémi Champallier, Philippe Penhoud, Mohamed Azaroual, Fabrice Muller, Jean-Christophe Komorowski, and Michael Heap

Volcano unrest associated to ascent of magmatic fluids (magma, brines, gases) at shallow depths in the presence of a very active hydrothermal system, can promote or enhance extensive hydrothermal rock alteration and form fragile discontinuities within the edifice. A process that can favour flank instability and culminate in partial flank collapse, engendering significant risks to the surrounding population. In the MYGALE ANR project, we focus on hydrothermal alteration timescales of andesitic rocks to better assess the hazard of volcano flank instability at La Soufrière de Guadeloupe (Eastern Caribbean, France). The conditions and kinetics of hydrothermal alteration reactions of the volcanic rocks of La Soufrière lava dome are determined by three approaches: mineralogical, experimental, and by modelling. Firstly, we characterized the natural alteration sequence of 20 samples from the lava dome and lava flows showing different degrees of alteration and porosity. SEM and XRD analyses of the samples show that the plagioclases are replaced by secondary minerals such as kaolinite, natroalunite, and amorphous silica. Secondly, we performed time-series fluid flow-through experiments, in which fluids are circulated through a pristine and porous andesite core (representative of the unaltered state of the present-day lava dome at La Soufrière). We varied temperature (200-250 °C), pressure (100-150 bar), duration (from days to months), and fluid composition (H2O-HCl mixtures). Rock permeability is measured in-situ and the mineralogical changes are characterized by post-experiment using various methods (SEM, EDS, EMPA, X-ray microtomography, and XRD). A sharp decrease in permeability of four orders of magnitude (from 10-14 to 10-18 m2) during the first 3-6 days was observed when using pure water as the percolating pore fluid. The cause of this large and fast decrease in permeability is currently being investigated. Experiments using H2O-HCl fluids are in progress attempting to reproduce the alteration sequence of the natural samples. Thirdly, we performed thermodynamic and kinetic numerical modelling using Perple_X, Phreeqc, and GEM-Selektor codes, in order to better constrain the conditions (especially the fluid phase composition) for hydrothermal alteration and to explore the alteration kinetics for timescales longer than those attained experimentally.

How to cite: Décossin, F., Martel, C., Arbaret, L., Champallier, R., Penhoud, P., Azaroual, M., Muller, F., Komorowski, J.-C., and Heap, M.: Experimental and numerical thermos-kinetic modelling of hydrothermal alteration of volcanic rocks - Example of La Soufrière de Guadeloupe (Eastern Caribbean , France), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17627,, 2024.

Valentin Troll, Thomas R. Walter, Herlan Darmawan, Michael J. Heap, Claire E. Harnett, Frances M. Deegan, Nadhirah Seraphine, Harri Geiger, and Daniel Müller

Catastrophic lava dome collapse is considered an unpredictable volcanic hazard because the physical properties, stress conditions, and internal structure of lava domes are not well understood. To better explain the locations of recent dome instability events at Merapi volcano, Indonesia (1), we combined geochemical and mineralogical analyses, rock physical property measurements, drone-based photogrammetry, and numerical modelling. We show that a linear fissure and a horseshoe-shaped alteration zone that formed in 2014 was buried by lava extrusion in 2018. The linear fissure controlled the location of the new lava dome, while the horseshoe shaped zone influenced subsequent instability. Geomechanical, mineralogical, and geochemical data suggest that such alteration zones are characterised by mechanically weak, hydrothermally altered materials, and we show that the new lava dome is collapsing along this now-hidden horseshoe shaped and comparatively weak alteration zone (2). To derive an improved general understanding of this phenomenon, we then combined recent laboratory data for the mechanical behaviour of dome rocks with discrete element method models to show that the presence of weak zones within lava domes increases instability, which is exacerbated when the size of the zone increases or when the zone is positioned off-centre (3). Our results highlight that improved understanding of dome architecture and compositional variations due to hydrothermal alteration within domes is essential for assessing hazards associated with dome and edifice failure at volcanoes worldwide.

How to cite: Troll, V., Walter, T. R., Darmawan, H., Heap, M. J., Harnett, C. E., Deegan, F. M., Seraphine, N., Geiger, H., and Müller, D.: Concealed hydrothermal alteration zones provide mechanical weaknesses within lava domes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5415,, 2024.

Posters virtual: Fri, 19 Apr, 14:00–15:45 | vHall X1

Display time: Fri, 19 Apr 08:30–Fri, 19 Apr 18:00
Chairpersons: Claire Harnett, Alexandra Kushnir, Marlene Villeneuve
Elmar Albers, Felix Genske, Lilian Böhringer, Jessica N. Fitzsimmons, Shelby Gunnells, Tea Isler, Jonathan Mette, Jeffrey S. Seewald, Maren Walter, Gunter Wegener, Christopher R. German, and Vera Schlindwein and the R/V Polarstern PS137 Science Team

Deep-sea hydrothermal systems are particularly difficult to locate and investigate in the ice-covered Arctic Ocean. Here we report findings of a system in the ultraslow-spreading Lena Trough at 81°22’N, from Expedition PS137 of the icebreaker R/V Polarstern in the summer of 2023. The site, named ‘Lucky B’, was first discovered after the recovery of massive sulfides in a dredge haul in 1999 (Snow et al., 2001) on the western flank of the Lucky Ridge, a 130 km-long ultramafic topographic high.

We investigated the buoyant and non-buoyant parts of Lucky B’s hydrothermal plume making use of physical sensors and geochemical hydrothermal tracers. The non-buoyant plume was found at approx. 300–400 m above the seafloor and was characterized by high turbidity and pronounced anomalies in oxidation–reduction potential and temperature (up to ~80 mV and 0.02°C, respectively). In its buoyant part, the plume contained high dissolved H2 (~450 nmol/L) and CH4 (~250 nmol/L).

Our seafloor observations, using deep-sea robotics, revealed widespread traces of hydrothermal activity at approx. 3,000–3,300 m water depth. These included hydrothermally discolored rocks and sediment and local outflow of clear hydrothermal fluids with abundant macrofauna. Guided by our discoveries, follow-on operations with the Norwegian icebreaker R/V Kronprins Haakon resulted in ROV dives to a large black smoker vent field with a number of several m-high chimneys. The visual appearance of the suggests vent fluid temperatures similar to those of other high-temperature ultramafic-influenced systems (e.g., 365°C at Rainbow; Charlou et al., 2002).

Confirmation awaits comparison with ongoing analyses of hydrothermal plume samples (He isotope signatures, Fe and Mn concentrations, CH4:Mn ratios) and of sulfide chimneys dredged in 1999 (mineral assemblages).

In the Arctic, Lucky B is the first hydrothermal system that has been traced to its seafloor source that is associated with ultramafic rocks outcropping at the seafloor. Additional work will be required to conceive its role on biogeochemical cycles in the ice-covered ocean.

How to cite: Albers, E., Genske, F., Böhringer, L., Fitzsimmons, J. N., Gunnells, S., Isler, T., Mette, J., Seewald, J. S., Walter, M., Wegener, G., German, C. R., and Schlindwein, V. and the R/V Polarstern PS137 Science Team: Discovery of the “Lucky B” ultramafic-hosted hydrothermal field in the ultraslow-spreading Lena Trough, Arctic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12269,, 2024.