Volcano-hydrothermal System Research Articles

A Late-Pleistocene confined volcanic debris avalanche promoted by hydrothermal alteration at the Tinguiririca volcano (Andes of Central Chile)

Distinguishing volcanic debris avalanche deposits from other epiclastic breccia could be complex. For more than 60 years, the Tinguiririca deposit (sourced from the homonymous volcano) in the Andes of Central Chile has been described by different authors as glacial moraines, a lahar, a volcanic debris avalanche, and even a debris flow deposit. To decipher its obscured origin and emplacement dynamics, we have carried out a detailed investigation of its distribution, contact relationships, sedimentology, and facies. Our findings unravel that the 57 km-long deposit is 5 to 300 m thick, totalling a reconstructed volume of 3.64 ± 0.05 km3. It is composed of unsorted heterometric breccias formed by clasts and blocks arranged in mixed and matrix facies characterised by distinctive lithological domains. In general, three clasts lithologies are dominant, consisting of black and grey andesites and hydrothermally altered clasts with jigsaw cracks and fractures. The deposit overlies terraced colluvium along the valleys and forms hummocks and ridges. Emplacement velocities estimates range from 39.6 m/s to 108.4 m/s. Therefore, the Tinguiririca deposit should represent a massive volcanic debris avalanche that formed after a lateral collapse that affected the ancient Tinguiririca Volcanic Complex, during the Late Pleistocene (between 45 ± 18 and c. 19.2 ± 1.2 ka). The abundance of hydrothermal minerals within the deposit's matrix and clasts (i.e., illite, phengite, epidote, tridymite, chlorite, hematite, jarosite, and alunite) all represent the volcano's hydrothermal system that likely favoured rock weakness and edifice collapse. Finally, the new interpretation is valuable for evaluating volcanic hazards and requires further mapping and research efforts.

Fluid-mineral dynamics at the Rincón de la Vieja volcano—hydrothermal system (Costa Rica) inferred by the study of major, minor and rare earth elements in the hyperacid crater lake

Volcanic lakes are complex natural systems and their chemical composition is related to a myriad of processes. The chemical composition of major, minor, Rare Earth Elements (REE) and physico-chemical parameters at the hyperacid crater lake of Rincón de la Vieja volcano (Costa Rica) are here investigated during February 2013–August 2014. The study of the lake chemical composition allows to identify the main geochemical processes occurring in the lake and to track the changes in the volcanic activity, both important for active volcanoes monitoring. The total REE concentration (∑REE) dissolved in the crater lake varies from 2.7 to 3.6 mg kg−1 during the period of observation. REE in the water lake samples normalized to the average volcanic local rock (REEN-local rock) are depleted in light REE (LREE). On the contrary REEN-local rock in the solids precipitated (mainly gypsum/anhydrite), from lake water samples in laboratory at 22°C, are enriched in LREE. The low variability of (La/Pr)N-local rock and (LREE/HREE)N-local rock ratios (0.92–1.07 and 0.66–0.81, respectively) in crater lake waters is consistent with the low phreatic activity (less than 10 phreatic eruptions in 2 years) observed during the period of observation. This period of low activity precedes the unrest started in 2015, thus, it could be considered as a pre-unrest, characterized by infrequent phreatic eruptions. No clear changes in the REE chemistry are associated with the phreatic eruption occurred at mid-2013. The results obtained investigating water-rock interaction processes at the Rincón de la Vieja crater lake show that rock dissolution and mineral precipitation/dissolution are the main processes that control the variability of cations composition over time. In particular, precipitation and dissolution of gypsum and alunite are responsible for the variations of REE in the waters. Despite the low variations of (La/Pr)N-local rock and (LREE/HREE)N-local rock ratios, this study allows to suggest that REE can be used, together with major elements, as practical tracers of water-rock interaction processes and mineral precipitation/dissolution at active hyperacid crater lakes over time, also during periods of quiescence and low phreatic activity.

Hidden pressurized fluids prior to the 2014 phreatic eruption at Mt Ontake

A large fraction of volcanic eruptions does not expel magma at the surface. Such an eruption occurred at Mt Ontake in 2014, claiming the life of at least 58 hikers in what became the worst volcanic disaster in Japan in almost a century. Tens of scientific studies attempted to identify a precursor and to unravel the processes at work but overall remain inconclusive. By taking advantage of continuous seismic recordings, we uncover an intriguing sequence of correlated seismic velocity and volumetric strain changes starting 5 months before the eruption; a period previously considered as completely quiescent. We use various novel approaches such as covariance matrix eigenvalues distribution, cutting-edge deep-learning models, and ascribe such velocity pattern as reflecting critically stressed conditions in the upper portions of the volcano. These, in turn, later triggered detectable deformation and earthquakes. Our results shed light onto previously undetected pressurized fluids using stations located above the volcano-hydrothermal system and hold great potential for monitoring.

Water-rock exchange of Sr isotopes evaluated through a reactive transport model: Application to the El Chichón hydrothermal system

The strontium isotope ratio (87Sr/86Sr) is a natural tracer commonly used to determine water sources, water flow trajectories, and mixing relationships. In high-temperature hydrothermal systems, the low solubility of retrograde minerals such as calcite and anhydrite may significantly reduce the dissolved Sr concentration. Further contribution of Sr along the flow path from isotopically distinct sources will alter and potentially mask the original Sr isotopic signature of the fluid, limiting the potential of 87Sr/86Sr data for reconstructing fluid flow patterns. This study investigates the tracing potential of 87Sr/86Sr in terms of fluid circulation within the El Chichón volcano hydrothermal system. Although limestone aquifers are evidenced in the region, the 87Sr/86Sr signature of most thermal fluids from the volcano does not indicate any major Sr contribution from such lithologies, but rather reveals that Sr is inherited from volcanic rocks. An isotope-enabled reactive transport model is performed to investigate under which conditions a connection between the limestone and volcanic aquifers is compatible with chemical and Sr isotopic compositions of the thermal springs. Results indicate that the 87Sr/86Sr signature of a sedimentary fluid with moderate Sr concentration can be completely modified into a magmatic signature after interacting with Sr-rich primary minerals, such as plagioclases, within short period of times (<1 year). Primary minerals release magmatic Sr to the fluid while secondary minerals (such as calcite) progressively precipitate and remove the initial sedimentary Sr. The El Chichón hydrothermal system hence represents a case where the 87Sr/86Sr of most springs does not keep any memory of the circulation through the sedimentary basement, as magmatic Sr dominates the isotopic signature. A certain analogy exists with hydrothermal fluids vented at oceanic spreading centers, whose 87Sr/86Sr signature departs from seawater and reflects Sr inherited from water-basalt interaction.

ГЕОХИМИЧЕСКАЯ ХАРАКТЕРИСТИКА ТЕРМАЛЬНЫХ ИСТОЧНИКОВ ПРИВЕРШИННОЙ ЧАСТИ ВУЛКАНА ЭБЕКО (о. ПАРАМУШИР, КУРИЛЬСКИЕ ОСТРОВА)

The chemical and isotopic composition of hot springs at the apical part of the active Ebeko volcano (Paramushir Island, Kuril Islands) is characterized based on the data obtained from fieldwork in 2020–2021. Thermal waters discharging from one of the sources of the Kuzminka River are ultra-acid (рН&lt;2) Al-Ca- SO -Cl type with mineralization up to 5 g/l and temperature up to 70°С. The anionic composition of waters is formed due to the dissolution in groundwater of acidic volcanic gases, partially «purified» in the main reservoir of the volcano's hydrothermal system. The cationic composition of waters, including rare-earth elements, is formed by isochemical dissolution of host rocks in the equivalent of 5 g per 1 liter of water. Differences in the isotopic composition and ratios of macrocomponents (SO /Cl, Al+Fe/Ca+Mg/Na+K) of waters of the near the summit prings and the northwestern slope suggest the presence of different-level aquifers in the hydrothermal system confined to the edifice of the Ebeko volcano.

How to remove the seasonal atmospheric effect: An application on a surface volcanic temperature at Nisyros active volcano, Hellenic Volcanic Arc

Surface temperature at a volcanic system is the result of a mixture of two heat sources originating one from above and another from below the ground surface. The atmosphere significantly affects the surface's temperature of the volcano, and removal of the atmospheric effect is required for meaningful interpretations prior to any characterization of anomalous volcanic activity. Time series of meteorological data were acquired from the closest weather station to Nisyros volcano, and surface volcanic temperature from the southern part of Nisyros caldera were analyzed. Time series analysis took place at different time scales through a decomposition method. The decomposition of the time series into long-, seasonal-, and short-term components was performed by the application of the Kolmogorov-Zurbenko (KZ) filter to separate the noise that appears on the obtained data. The Time Series Regression (TSR) model was applied in all time scales. The long- and the short-term components of the surface volcanic temperature show no significant correlation with the atmospheric variables, while the seasonal term indicates a very strong association with an R2 equal with 99%. After the removal of the atmospheric effect, our model indicates that there is a normal activity of the seasonal heat emission in Nisyros with no anomalous signals in the temperature residuals during our survey. This methodology may facilitate the process of filtering a seasonal atmospheric effect and better comprehend the heat emission of a volcano-hydrothermal system from temperature surface readings.

Subsurface structure identification of ‘X’ geothermal prospect area based on gravity and magnetotelluric data

Exploration stage has the highest risk, due to discover the interest reservoir zone include high temperature and pressure, middle to high permeability, and non-acidic fluid. Permeability is one of the most difficult parameters to be determined in this stage. Fault zone could be able to control and provide fluid flow within reservoir. It is usually related to high permeability zone. Fault identification become an important and interesting parameter in to be explore. The aim of this research is identified subsurface structure to build up a conceptual model of geothermal system. The methods include magnetotelluric data analysis and 3D MT structure interpretation together with first horizontal derivative and second vertical derivative analysis of gravity data. Geochemistry and geology data are used as supporting data. There are structure types of normal and reverse fault identified in the research area. MT and gravity interpretation show that structures are direct to northeast-southwest and northwest-southeast. Up-flow zone is predicted between TPP-TPB manifestations. Structures supposed to highly contribute in providing permeability zone in the research area. Manifestations fluid have the characteristics of meteoric and magmatic fluid (volcano-hydrothermal system). The clay cap thickness average is predicted between 500–700 m. The reservoir thickness average is predicted between 1500–2000 m. Temperature estimation of the reservoir is 240–280 °C. Heat source might be located between TPB and TPP manifestations. Subsurface structure analysis might be useful to increase success ratio in drilling.

Feasibility to Use Continuous Magnetotelluric Observations for Monitoring Hydrothermal Activity. Numerical Modeling Applied to Campi Flegrei Volcanic System (Southern Italy)

The magnetotelluric (MT) method is useful for monitoring geophysical processes because of a large dynamic depth range. In this paper, a feasibility study of employing continuous MT observations to monitor hydrothermal systems for both volcanic hazard assessment and geothermal energy exploitation is presented. Sensitivity of the MT method has been studied by simulating spatial and temporal evolution of temperature and gas saturation distributions in a hydrothermal system, and by calculating the MT response at different time steps. Two possible scenarios have been considered: the first is related to an increase in the fluid flow rate at the system source, the second is associated to an increase in the permeability of the rocks hosting the hydrothermal system. Numerical simulations have been performed for each scenario, and the sensitivity of the MT monitoring has been analyzed by evaluating the time interval needed to observe significant variations in the MT response. This study has been applied to the hydrothermal system of the Campi Flegrei (southern Italy) and it has shown that continuous MT measurements are not sensitive enough to detect a significant increase in the source fluid flow rate over time intervals less than ten years. On the contrary, if the permeability of the upwelling zone increases, a measurable change in the MT response occurs over a time interval ranging from six months to three years, depending on the extent of the permeability increase. Such findings are promising and suggest that continuous MT observations in active volcanic areas can be useful for imaging volcano-hydrothermal system activity.

Hydrothermal fluid migration due to interaction with shallow magma: Insights from gravity changes before and after the 2015 eruption of Cotopaxi volcano, Ecuador

On August 14, 2015 Cotopaxi Volcano (Ecuador) erupted with several phreatomagmatic explosions after nearly 135 years of quiescence. Unrest began in April 2015 with an increase in the number of daily seismic events and inflation of the flanks of the volcano. Time-lapse gravity measurements started at Cotopaxi volcano in June 2015. Although minor gravity changes were detected prior to eruptive activity, the largest gravity variations at Cotopaxi were measured between October 2015 and March 2016, when other geophysical parameters had reached background levels. Inverse modelling of GPS data suggests a deep intrusion prior to the eruptive activity, while inverse modelling of post-eruptive gravity changes suggests variations in the volcano hydrothermal system. Deformation, seismicity, and gravity changes are consistent with the intrusion of a deep magmatic source between April and August 2015. Part of the magma rose from depth and interacted with the hydrothermal system, causing the phreatomagmatic activity and pushing hydrothermal fluids from a deep aquifer into a shallow perched aquifer.

Temporal variations of repeating low frequency volcanic earthquakes at Ngauruhoe Volcano, New Zealand

A persistent swarm of micro-earthquakes was observed at Ngauruhoe volcano, New Zealand from 2005 to 2010, marking the first unrest episode at the volcano since 1975. Detailed waveform and spectral analysis of the continuous seismic record revealed eight earthquake families (F1–F8) within a sequence of 8561 low-frequency earthquakes. An analysis of stacked waveforms of each family allowed the families to be linked systematically into four super-families that may indicate four different mechanisms, and/or source locations or geometries.The first super-family mostly occurred in 2005 and showed stable waveforms and no significant evolution. The second super-family was dominant over a two-year period starting in mid-2006 and displayed a slow evolution with progressively higher frequency. The third super-family emerged from early 2008 with intermittent occurrence and high seismicity, and progressed to higher peak frequency in 2010. The fourth super-family was mainly observed from late 2008 to mid-2009 and had the highest peak frequency in 2009.The observed evolutionary pattern may be driven by a modest multi-year input of gas and heat from a deeper relict magmatic zone into the shallow volcano hydrothermal system. The interplay of gas from below and seasonal meltwater from above may have modulated the seismic activity and may relate to repeated over-pressurization and failure of constricted evolving pathways within a pervasive crack system.

Seawater-rock interaction at Ushishir volcano-hydrothermal system, Kuril Islands

Ushishir volcano is located in the middle of the Kuril Arc. The Ushishir crater, a closed bay connected with the ocean by a narrow and shallow strait is characterized by a strong hydrothermal activity. Boiling springs, hot pools, fumaroles and shallow submarine vents are manifestations of a magmatic-seawater hydrothermal system with the discharging solution similar in chemical and isotopic composition to the seafloor hydrothermal fluids. The main features of the Ushishir fluids are: (1) water has close to zero δD and a large oxygen isotopic shift (6 7‰); (2) high boron concentration (~70 ppm); (3) a significant uptake of Ca and Sr from the rock and Ca/Sr higher than that for seawater with 87Sr/86Sr ~0.7037, a bit higher than the rock value (0.7032). The measured onshore discharge of boiling water is ~ 5 kg/s; however, a large plume of the discoloured seawater releasing from the outer submarine slope of the volcano indicates a much higher total mass and heat output.

Volcano-hydrothermal system and activity of Sirung volcano (Pantar Island, Indonesia)

Sirung is a frequently active volcano located in the remote parts of Western Timor (Indonesia). Sirung has a crater with several hydrothermal features including a crater lake. We present a timeseries of satellite images of the lake and chemical and isotope data from the hyperacid hydrothermal system. The fluids sampled in the crater present the typical features of hyperacidic systems with high TDS, low pH and δ34SHSO4–δ34SS0 among the highest for such lakes. The cations concentrations are predominantly controlled by the precipitation of alunite, jarosite, silica phases, native sulfur and pyrite which dominate the shallow portions of the hydrothermal system. These minerals may control shallow sealing processes thought to trigger phreatic eruptions elsewhere. Sparse Mg/Cl and SO4/Cl ratios and lake parameters derived from satellite images suggest gradual increase in heat and gas flux, most likely SO2-rich, prior to the 2012 phreatic eruption. An acidic river was sampled 8 km far from the crater and is genetically linked with the fluids rising toward the active crater. This river would therefore be a relevant target for future remote monitoring purposes. Finally, several wells and springs largely exceeded the World Health Organization toxicity limits in total arsenic and fluoride.

Geochemical characterisation of Taal volcano-hydrothermal system and temporal evolution during continued phases of unrest (1991–2017)

Taal volcano (Luzon Island, Philippines) has last erupted in 1977 but has known some periods of increased activity, characterised by seismic swarms, ground deformation, increased carbon dioxide flux and in some cases temperature anomalies and the opening of fissures. We studied major, trace element and sulphur and strontium isotopic composition of Taal lake waters and hot springs over a period of 25 years to investigate the geochemical evolution of Taal volcano's hydrothermal system and its response to volcanic unrest.Long-term evolution of Main Crater Lake (MCL) composition shows a slow but consistent decrease of acidity, SO4, Mg, Fe and Al concentrations and a trend from light to heavy sulphate, consistent with a general decrease of volcanic gases dissolving in the hydrothermal system. Na, K and Cl concentrations remain constant indicating a non-volcanic origin for these elements. Sulphate and strontium isotopic data suggest this neutral chloride-rich component represents input of geothermal water into Taal hydrothermal system. A significant deviation from the long-term baseline can be seen in two samples from 1995. That year, pH dropped from 2.6 to 2.2, F, Si and Fe concentrations increased and Na, K and Cl concentrations decreased. Sulphate was depleted in 34S and temperature was 4 °C above baseline level at the time of sampling. We attribute these changes to the shallow intrusion of a degassing magma body during the unrest in 1991–1994.More recent unrest periods have not caused significant changes in the geochemistry of Taal hydrothermal waters and are therefore unlikely to have been triggered by shallow magma intrusion. A more likely cause for these events is thus pressurisation of the hydrothermal reservoir by increasing degassing from a stagnant magma reservoir. Our study indicates that new magmatic intrusions that might lead to the next eruption of Taal volcano are expected to change the geochemistry of MCL in the same way as in 1994–1995, with the most notable effects being changes in temperature, pH, F and Si concentrations.

Hydrovolcanic ash emission between August 14 and 24, 2015 at Cotopaxi volcano (Ecuador): Characterization and eruption mechanisms

Cotopaxi is an active, hazardous and ice-covered stratovolcano 60km southeast of Quito, (Ecuador) whose last major eruption occurred in 1877. During 2001–2002, volcanic unrest characterized by volcanic seismicity and deformation ended without eruptive activity. On April 2015, a new increase of seismicity, SO2 emissions, thermal anomalies and edifice deformation, evolved into the onset of a new eruptive cycle, beginning August 14. We sampled and measured the ash fall deposits to the west of Cotopaxi between August 14 and 24, 2015. The ash collected was analyzed using grain size, X-Ray fluorescence, X-Ray diffraction and scanning electron microscope (SEM-EDS), revealing the eruptive products to be compound of dense fragments (mostly lithics), diverse types of scoria, pumice, free fractured crystals, glassy particles and aggregates. Most of hydrothermal alteration is observed during the initial stage of the eruption (14–15 August; including Cu oxides and Fe minerals in the lithics). The glassy particles were blocky morphology, and textural changes were recognized over 10days of eruption, varying from null or low vesicularity to low-to-moderate vesicularity, occasionally exhibiting molten or subrounded textures. The bulk ash has a basaltic-andesitic composition (~55.67wt% of SiO2), while clusters of selected particles (likely juvenile) analyzed through SEM+EDS reveal dacitic composition (65.67 and 65.8wt% SiO2). Plagioclase, clinopyroxene and orthopyroxene are the main minerals present, with accessory anhydrite, melanterite and pyrite (these typically observed during the initial stage of eruption). These variations in addition to the geophysical background, led us to interpret this eruption as the result of the volcano's hydrothermal system disruption due to a shallow, low-volume magma input, which initially evolved into phreatic activity at surface level. Further activity up to 24 August was triggered by the indirect interaction between magma and the depleted hydrothermal system, generating a magmatic-hydrothermal eruption. The issue is important for evaluating unrest periods at active stratovolcanoes, and the impact of their initial, low-volume ash falls in neighboring communities.

Geochemical processes assessed by Rare Earth Elements fractionation at “Laguna Verde” acidic-sulphate crater lake (Azufral volcano, Colombia)

The geochemical behaviour of major elements, Fe, Al, Mn, and Rare Earth Elements (REE) was investigated in the “Laguna Verde” acidic crater lake of Azufral volcano (Colombia). The cold lake water (T close to 10 °C) is sulphate-dominated, due to absorption and oxidation of H2S (pH 2.1–2.7, Eh 196–260 mV), and Na-enriched (Total Dissolved Solids 0.79 g L−1). The total amount of REE dissolved in the lake ranges from 3.3 to 9.1 ppb. The REE patterns normalized to the local rocks show a Light Rare Earth Elements (LREE) depletion quite constant in the 15 samples. Similar patterns were already found in the acidic sulphate springs of Nevado del Ruiz volcano-hydrothermal system, caused by the precipitation of alunite and jarosite, absorbing LREE and hence removing them from solution. Alunite and jarosite minerals are not oversaturated at chemical-physical conditions within the lake itself, but alunite becomes oversaturated for temperatures above ≈100 °C, reigning in the underlying hydrothermal system. Water temperatures close to 75 °C were found in the northern part of the lake. Coupling the distribution of REE in lake water (LREE depleted) and the saturation indexes, we suggest that the distribution of REE in the lake water is the result of the alunite precipitation in the northern part of the lake and/or in the deeper hydrothermal system. The acidic hydrothermal fluids mobilize the REE with contents up to ≈5 orders of magnitude higher than seawater; acidic-hydrothermal systems, such as acidic crater lakes, can hence be considered potential REE “reservoirs”.

Bayesian estimation of magma supply, storage, and eruption rates using a multiphysical volcano model: Kīlauea Volcano, 2000–2012

Estimating rates of magma supply to the world's volcanoes remains one of the most fundamental aims of volcanology. Yet, supply rates can be difficult to estimate even at well-monitored volcanoes, in part because observations are noisy and are usually considered independently rather than as part of a holistic system. In this work we demonstrate a technique for probabilistically estimating time-variable rates of magma supply to a volcano through probabilistic constraint on storage and eruption rates. This approach utilizes Bayesian joint inversion of diverse datasets using predictions from a multiphysical volcano model, and independent prior information derived from previous geophysical, geochemical, and geological studies. The solution to the inverse problem takes the form of a probability density function which takes into account uncertainties in observations and prior information, and which we sample using a Markov chain Monte Carlo algorithm. Applying the technique to Kīlauea Volcano, we develop a model which relates magma flow rates with deformation of the volcano's surface, sulfur dioxide emission rates, lava flow field volumes, and composition of the volcano's basaltic magma. This model accounts for effects and processes mostly neglected in previous supply rate estimates at Kīlauea, including magma compressibility, loss of sulfur to the hydrothermal system, and potential magma storage in the volcano's deep rift zones. We jointly invert data and prior information to estimate rates of supply, storage, and eruption during three recent quasi-steady-state periods at the volcano. Results shed new light on the time-variability of magma supply to Kīlauea, which we find to have increased by 35–100% between 2001 and 2006 (from 0.11–0.17 to 0.18–0.28 km 3 /yr), before subsequently decreasing to 0.08–0.12 km 3 /yr by 2012. Changes in supply rate directly impact hazard at the volcano, and were largely responsible for an increase in eruption rate of 60–150% between 2001 and 2006, and subsequent decline by as much as 60% by 2012. We also demonstrate the occurrence of temporal changes in the proportion of Kīlauea's magma supply that is stored versus erupted, with the supply “surge” in 2006 associated with increased accumulation of magma at the summit. Finally, we are able to place some constraints on sulfur concentrations in Kīlauea magma and the scrubbing of sulfur by the volcano's hydrothermal system. Multiphysical, Bayesian constraint on magma flow rates may be used to monitor evolving volcanic hazard not just at Kīlauea but at other volcanoes around the world. • A model of Kilauea Volcano is developed which predicts diverse observations. • Bayes' Theorem relates model, data, and independent prior information. • Probabilistic rates of magma supply, storage, and eruption are resolved. • Kilauea's magma supply has waxed and waned from 2000 to 2012. • Changes in supply control, in part, eruptive hazard at the volcano.

Reconstruction of an Ordovician seafloor volcanohydrothermal system: a case study from the Copper Coast, southeastern Ireland using field, geochemical and fluid inclusion data

AbstractVolcanic rocks in south County Waterford include flow-top hyaloclastite, pillow lavas and peperite, which are formed typically by sub-aqueous eruption or intrusion into unconsolidated sediment. Element mobility in wet sediment during emplacement of volcanic intrusions was reconstructed on a variety of spatial scales using bulk-rock and mineral analysis. Magma-sediment and magma-water interactions enhanced hydrothermal alteration. The chemistry of chlorite was a function of mixing between an Fe-rich magmatic fluid and a Mg-rich meteoric fluid. Chlorite geothermometry yields temperatures of formation between 230 and 388°C compatible with other metamorphic indicators. Fluid inclusion microthermometric data from genetically-related mineralized quartz veins reveal a hydrothermal vein mineralization event that occurred at lower temperatures during the end stage of volcanic activity. A convection driven mixing trend reflects the trapping of co-existing brine with entrained seawater concomitant with, the late stages of emplacement of the Bunmahon Volcano intrusions.

Volcano–hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur

Ebeko volcano at the northern part of Paramushir Island in the Kuril island arc produces frequent phreatic eruptions and relatively strong fumarolic activity at the summit area ~1000m above sea level (asl). The fumaroles are characterized by low-temperature, HCl- and S-rich gas and numerous hyper-acid pools (pH<1) without drains. At ~550m asl, in the Yurieva stream canyon, many hot (up to 87°C) springs discharge ultra-acidic (pH1–2) SO4–Cl water into the stream and finally into the Sea of Okhotsk. During quiescent stages of degassing, these fumaroles emit 1000–2000t/d of water vapor, <20t/d of SO2 and <5t/d of HCl. The measurement of acidic hot Yurieva springs shows that the flux of Cl and S, 60–80t/d each, is independent on the volcanic activity in the last two decades. Such high flux of Cl is among the highest ever measured in a volcano–hydrothermal system. Oxygen and hydrogen isotopic composition of water and Cl concentration for Yurieva springs show an excellent positive correlation, indicating a mixing between meteoric water and magmatic vapor. In contrast, volcanic gas condensates of Ebeko fumaroles do not show a simple mixing trend but rather a complicated data suggesting evaporation of the acidic brine. Temperatures calculated from gas compositions and isotope data are similar, ranging from 150 to 250°C, which is consistent with the presence of a liquid aquifer below the Ebeko fumarolic fields. Saturation indices of non-silicate minerals suggest temperatures ranging from 150 to 200°C for Yurieva springs. Trace elements (including REE) and Sr isotope composition suggest congruent dissolution of the Ebeko volcanic rocks by acidic waters. Waters of Yurieva springs and waters of the summit thermal fields (including volcanic gas condensates) are different in Cl/SO4 ratios and isotopic compositions, suggesting complicated boiling–condensation–mixing processes.

Geochemistry of REE, Zr and Hf in a wide range of pH and water composition: The Nevado del Ruiz volcano-hydrothermal system (Colombia)

The geochemical behaviour of Rare Earth Elements, Zr and Hf was investigated in the thermal waters of Nevado del Ruiz volcano system. A wide range of pH, between 1.0 and 8.8, characterizes these fluids. The acidic waters are sulphate dominated with different Cl/SO4 ratios. The important role of the pH and the ionic complexes for the distribution of REE, Zr and Hf in the aqueous phase was evidenced. The pH rules the precipitation of authigenic Fe and Al oxyhydroxides producing changes in REE, Zr, Hf amounts and strong anomalies of Cerium. The precipitation of alunite and jarosite removes LREE from the solution, changing the REE distribution in acidic waters.Y–Ho and Zr–Hf (twin pairs) have a different behaviour in strong acidic waters with respect to the water with pH near-neutral. Yttrium and Ho behave as Zr and Hf in waters with pH near neutral-to-neutral, showing super-chondritic ratios. The twin pairs showed to be sensitive to the co-precipitation and/or adsorption onto the surface of authigenic particulate (Fe-, Al-oxyhydroxides), suggesting an enhanced scavenging of Ho and Hf with respect to Y and Zr, leading to superchondritic values. In acidic waters, a different behaviour of twin pairs occurs with chondritic Y/Ho ratios and sub-chondritic Zr/Hf ratios. For the first time, Zr and Hf were investigated in natural acidic fluids to understand the behaviour of these elements in extreme acidic conditions and different major anion chemistry. Zr/Hf molar ratio changes from 4.75 to 49.29 in water with pH<3.6. In strong acidic waters the fractionation of Zr and Hf was recognized as function of major anion contents (Cl and SO4), suggesting the formation of complexes leading to sub-chondritic Zr/Hf molar ratios.

Gas geochemistry of hydrothermal fluids of the S. Miguel and Terceira Islands, Azores

We present here for the first time a complete dataset of the chemical and isotopic compositions of fumarolic fluids collected on the S. Miguel (Fogo and Furnas volcanoes) and Terceira (Pico Alto volcano) Islands. The data are analyzed and discussed, to provide both a comprehensive picture of the thermodynamic conditions of the hydrothermal systems on these two islands, and to give new insight into the origins of these fluids, for a better understanding of the geodynamic context of the Azores archipelago. For S. Miguel Island, the gas equilibria in the H2–CO2–CO–CH4–H2O system suggest temperatures of the hydrothermal system reservoirs from 190°C to 280°C for the Fogo volcano fumaroles, which are 30–35°C higher than those measured for geothermal wells. The equilibrium temperatures estimated for the Furnas volcano hydrothermal system are from 200°C to 275°C. Equilibrium vapor at a temperature of ∼190°C is inferred for the fumarolic fluids discharged on Terceira Island.The He isotopic composition of ∼9.6R/Ra measured in the fumaroles of Terceira suggests that a plume-like source, presumably from the lower mantle, feeds this hydrothermal system. The relatively low 3He/4He ratios (from 5.21 to 5.35R/Ra) and higher CO2/3He ratios of S. Miguel fluids suggest an addition of ∼45% of radiogenic He and ∼30% of crustal CO2 to a plume-like composition as at Terceira.A mainly meteoric origin is inferred for the fumarolic water, whereas the unreactive gas species (He, Ar, N2) arise from mixing processes between an atmospheric-like component and a magmatic component. On S. Miguel Island, the estimated magmatic fluid composition in terms of the ratios of N2/40Ar (62±6) suggests a plume-like mantle source. Deep derived N2 isotope compositions characterized by very negative δ15N values (δ15N⩽−14‰) are inferred for the fluids of the Terceira Island. Also S. Miguel fluids are compatible with the same source with a possible involvement of crustal-derived Nitrogen. These values are more negative than the typical compositions of both the upper mantle and plume-like mantle, which thus supports the possible presence of a 15N-depleted source, presumably from the lower mantle, in the region.