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<600^oC. 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, https://doi.org/10.5194/egusphere-egu24-2974, 2024.

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, https://doi.org/10.5194/egusphere-egu24-16850, 2024.

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, https://doi.org/10.5194/egusphere-egu24-16537, 2024.

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, https://doi.org/10.5194/egusphere-egu24-4420, 2024.

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, https://doi.org/10.5194/egusphere-egu24-18301, 2024.

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, https://doi.org/10.5194/egusphere-egu24-6858, 2024.

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, https://doi.org/10.5194/egusphere-egu24-7009, 2024.

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, https://doi.org/10.5194/egusphere-egu24-4726, 2024.

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, https://doi.org/10.5194/egusphere-egu24-174, 2024.

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, https://doi.org/10.5194/egusphere-egu24-5948, 2024.