Caldera System Research Articles

Hydrothermal calcite formation in Campi Flegrei caldera, Italy: unraveling carbon sink processes in alkaline volcanic systems

Understanding carbon dioxide emissions variability in volcanic regions is vital for detecting instabilities in the subvolcanic plumbing system, crucial for managing both volcanic and environmental risks. While changes in magmatic sources drive these variations, non-magmatic processes can complicate signal interpretation, especially in caldera environments. Here, geothermal systems can sequester CO2 within the bedrock through hydrothermal calcite precipitation, significantly impacting surface-level CO2 emissions. Unfortunately, few studies have explored this phenomenon, examining hydrothermal calcite origins and their effects on carbon balances and temporal gaseous patterns in active volcanic settings. Our study developed a specialized methodology for quantifying CO2 sequestered in hydrothermal calcites within alkaline caldera systems. We focused on analyzing hydrothermal calcite in lithics from volcanic deposits of eruptions of varying ages, Volcanic Explosivity Index (VEI), and eruptive vent locations to enhance the representativeness of the entire caldera bedrock. Unlike core samples from geothermal wells, which are infrequent and limited to specific depths, lithics can be easily collected, offering a comprehensive understanding of CO2 sequestration. Through extensive 3D textural characterization and isotopic investigations on hydrothermal calcite within lithic fragments from selected alkaline volcanic deposits in the Campi Flegrei caldera, our findings emphasized the significant influence of calcite sinks on the overall CO2 budget released by volcanoes throughout their evolution.

Topographic Controls on Pyroclastic Density Current Hazard at Aluto Volcano (Ethiopia) Identified Using a Novel Zero‐Censored Gaussian Process Emulator

AbstractAluto volcano (Central Ethiopia) displays a complex, hybrid topography, combining elements typical of caldera systems (e.g., a central, flat caldera floor) and stratovolcanoes (e.g., relatively high and steep, radial flanks, related to eruptions occurring clustered in space). The most recent known eruptions at Aluto have commonly generated column‐collapse pyroclastic density currents (PDCs), a hazardous phenomenon that can pose a significant risk to inhabited areas on and around the volcano. In order to analyze and quantify the role that Aluto's complex topography has on PDC hazard, we apply a versatile probabilistic strategy, which merges the TITAN2D model for PDCs with a novel zero‐censored Gaussian Process (zGP) emulator, enabling robust uncertainty quantification at tractable computational costs. Results from our analyses indicate a critical role of the eruptive vent location, but also highlight a complex interplay between the topography and PDC volume and mobility. The relative importance of each factor reciprocally depends on the other factors. Thus, large PDCs (≥0.1–0.5 km3) can diminish the influence of topography over proximal regions of flow propagation, but PDCs respond to large‐ and small‐scale topographic features over medial to distal areas, and the zGP captures processes like PDC channelization and overbanking. The novel zGP can be applied to other PDC models and can enable specific investigations of PDC dynamics, topographic interactions, and PDC hazard at many volcanic systems worldwide. Potentially, it could also be used during volcanic crises, when time constraints usually only permit computation of scenario‐based hazard assessments.

Magma mixing between rhyolite and pseudotachylite as the origin for the Glencoe ‘flinty crush rock’

The Glencoe caldera is a well-studied example of a caldera system exposed to intermediate depths along the glacially excavated glen. We present a first quantitative assessment of clast-size population and matrix chemistry from the flinty crush rock that occurs on the main ring faults. Size–shape metrics of clasts differ from those of a ‘normal’ pseudotachylite from the Outer Hebrides. Both samples display good power-law clast-size populations, once allowance is made for dissolution of a portion of clasts into the melt that contained them, with fractal dimensions of 2.7 and 4.0 for the Outer Hebrides and Glencoe samples respectively. Mass-balance calculations of flinty crush rock matrix chemistry imply an origin by mixing between rhyolite and pseudotachylite that was derived from semipelitic host rock. This means that the flinty crush rock was transported some distance from the point of frictional heating, as previously proposed, as semipelitic rocks are not present at the surface at Stob Mhic Mhartuin but are likely to be present at depth. This transport, and mixing with rhyolite magma, would have provided the time and thermal energy for clast dissolution beyond that possible during normal pseudotachylite generation and quenching on fault wall rocks. Supplementary material: Results of simulations, major element compositions of the matrix in the flinty crush rock sample and a supplementary figure are available at https://doi.org/10.6084/m9.figshare.c.7084992

Gathering insights into volcanic risk from Auckland to the Andes, Antilles, and Arabia: an unexpected journey to professorship in volcanology

In 2020 I was promoted to Professor at Waipapa Taumata Rau the University of Auckland (UoA), joining the small circle of women (now three, the others being Kathleen Campbell and Philippa Black, also from UoA) who had achieved promotion to Professor in Geology in Aotearoa New Zealand, and the first ever in the field of volcanology. This promotion was a gratifying and somewhat unexpected achievement for me considering that I had started out studying languages and linguistics! In this contribution, which is based on my “inaugural” lecture as Professor in 2021, I provide an overview of my journey, framed primarily through the work of my doctoral students. I pay particular focus on the Andes where I worked on large silicic caldera systems, the Lesser Antilles, characterised by andesite and dacitic dome complexes and stratovolcanoes, and the distributed basaltic volcanism in Auckland and Saudia Arabia. I share some insights gained along the way and describe how these have influenced a shift in my research from more fundamental petrology to more applied volcanology over the course of my career.

The Colíder Paleoproterozoic felsic volcanism: New insights into stratigraphy and petrogenesis in the southern Amazonian Craton

The eastern side of Alta Floresta Mineral Province (AFMP) (União do Norte region) comprises an effusive/subvolcanic phase, explosive phase, and epiclastic rocks. The effusive/subvolcanic phase comprises massive porphyritic rhyolite with different degrees of crystallinity, porphyritic rhyolite with flow structure, and microcrystalline rhyolite. The explosive phase is characterized by welded lapilli tuff, lapilli tuff with flow structure, stratified lapilli tuff, and tuff with accretionary lapilli. Sandstones and siltstones characterize epiclastic rocks. There are porphyritic rhyolite dikes that cut the volcanic sequence. The rocks of the Colíder Group are rhyolites (>70 % SiO2) with calcic-alkaline to alkaline-calcic affinity. A rhyolitic rock is predominantly ferroan and metaluminous, while a pyroclastic rock is predominantly magnesian to ferroan and meta- to peraluminous in composition. The rocks have a high LREE content, and slight fractionation of HREE suggests they have hybrid geochemical characteristics of high potassium calc-alkaline, shoshonitic, and suggestive affinities of A-type magmas, probably produced in a post-collisional orogenic environment. These rocks have a relatively high level of Ba and Rb and a negative anomaly for Sr, P, Nb, Ta, and Ti, suggesting an intra-plate mantle source with contributions from a metasomatized mantle edge. The rocks do not have adakitic characteristics nor fertility for Cu-Au (Sr/Y < 1.9 and LaN/YbN < 38.0) as in other regions of the AFMP. The welded lapilli tuff rocks show U-Pb age of 1838 ± 17 Ma and 1817 ± 2 Ma and are the older explosive phases. The effusive/subvolcanic phase has U-Pb ages of 1800 ± 3 Ma and 1792 ± 3 Ma obtained in the massive porphyritic rhyolite. The age interval of approximately 46 Ma (1838–1792 Ma) suggests the presence of two or more volcanic cycles or distinct volcanic events. Based on the interpretation of volcanic and epiclastic deposits and remote sensing products, the paleoenvironmental reconstruction includes subaerial with effusive/subvolcanic and explosive stages in a probable eroded volcanic caldera system with epiclastic rocks from a fluvial depositional system. This volcanic scenario provides more information about the Colíder–Teles Pires volcano-plutonic event, uniquely mapped in AFMP, and offers new insights into the tectonic frame of the Amazonian craton.

Insights into the explosive eruption history of the Campanian volcanoes prior to the Campanian Ignimbrite eruption

The Campanian Volcanic Zone (CVZ) comprises multiple active volcanoes and includes the highly productive Campi Flegrei and Ischia caldera systems. These caldera volcanoes have produced probably the largest eruptions in Europe in the past 200 ka, such as the Monte Epomeo Green Tuff (MEGT; Ischia) at ca. 56 ka and the Campanian Ignimbrite (CI; Campi Flegrei) at ca. 40 ka, which form widespread isochrons across the Mediterranean region. These closely-spaced volcanic centres erupt phonolitic to trachytic glass compositions that are similar, and thus it can be challenging to correlate tephra deposits to specific volcanic sources. Here we present a detailed tephrostratigraphy for pre-CI eruption activity using the units preserved within a sequence at the coastal Acquamorta outcrop, on the western side of the CI caldera rim. Both the MEGT and CI units are present in the section, and they bracket twelve eruption units that were logged and sampled. New major and trace element glass chemistry data have been acquired for these Acquamorta tephra deposits. Three eruption deposits from Ischia and nine from Campi Flegrei are identified, which helps constrain the tempo of volcanic activity of these centres between the large caldera-forming eruptions. The three Ischia tephra deposits between the MEGT and the CI are indistinguishable based on both major and trace element glass chemistry and cannot be correlated to a specific or known eruption in this interval, such as the Schiappone tephra. The compositional variations between the Campi Flegrei eruptions reveal temporal shifts in the composition of the tephra deposits that reflect changes in the magmatic system prior to the CI eruption. These deposits indicate that there were at least nine eruptions at Campi Flegrei within 16 ka of the enormous CI eruption, and suggest that there was no significant period of repose before the caldera generating eruption.

Subsurface structural control of geothermal resources in a magmatic rift: gravity and magnetic study of the Tulu Moye geothermal prospect, Main Ethiopian Rift

Since the Quaternary, extension and magmatism in the Main Ethiopian Rift (MER) have been mainly focused into narrow magmatic segments that have numerous volcanic centers and caldera collapses that offer favorable conditions for the occurrence of geothermal resources. However, the subsurface structure of the volcanic systems (0–10 km) and their link to the distribution of shallow geothermal resources remain unclear. To investigate the role of subsurface structures on the occurrence of these resources, we conducted gravity and magnetic studies combined with geological constraints within the Tulu Moye Geothermal Prospect (TMGP), one of the current geothermal prospects in the central MER associated with caldera collapses. Gravity data from the Global Gravity Model plus (GGMplus 2013) and ground magnetic data transformed into residual and derivative maps reveal that shallow magmatic intrusions occur under the volcanic centers (Tulu Moye, Bora, and Bericha). Our interpretation along with recent magnetotelluric model suggests that only the intrusion beneath Tulu Moye is currently magmatically active and includes partial melt, consistent with it being a primary heat source for the geothermal system. A new caldera formation model is proposed where the TMGP hosts an older large caldera (about 25 km diameter) within which there are several smaller nested caldera systems associated with the Bora, Bericha, and Tulu Moye volcanoes. Along with existing geologic, seismic, and magnetotelluric studies, our gravity and magnetic analysis indicate the interaction between NNE-SSW (rift-parallel) and NW-SE (cross-rift) trending faults, along with shallow magmatic intrusions and caldera systems, suggesting that such a large geothermal system is possible under these conditions.

Vents opening susceptibility map of the Colli Albani long dormant volcano

Despite their protracted periods of inactivity, long-dormant volcanoes may be highly hazardous, as their reactivation can be characterized by violent explosive eruptions. An example of such volcanoes is the Colli Albani caldera, onto which deposits Rome Capital City is built, Italy. Its last volcanic activity was characterized by voluminous maar-forming phreatomagmatic eruptions dated between 36 and 25ka, but the volcano has produced several maar lake overflows during the Holocene till historical times. Presently, Colli Albani is affected by recurrent seismic events, anomalous heat flow, ground uplifts, hydrothermal circulation and gas emissions. For these reasons, the Italian Civil Protection has recently listed Colli Albani among the ten active volcanoes of Italy, but products for the evaluation of its volcanic hazards lacking. This work presents the first study on vent opening susceptibility mapping at Colli Albani. We explore the potential of an available geological dataset for building, through geographic information system analysis, an index that classifies areas at different vents opening susceptibility (low, moderate and high). Such result highlights as a solid geological mapping is a prerequisite for the volcanic hazard assessment, especially in remote or poorly studied long-dormant volcanoes such as caldera systems where the location of new vents could occur in different volcano sectors.

High resolution magnetic anomalies, volcanism and tectonics of the active “La Fossa” vulcanic system (Vulcano island) and Lipari island (South Italy)

In this paper we present the new high-resolution magnetic anomaly map of the La Fossa Caldera system and Lipari island (Southern Italy), obtained by merging two low-altitude aeromagnetic surveys. In these islands a variegated ensemble of magnetic anomalies develops in the north-south direction. The La Fossa Caldera is characterized by very high-intensity and short-wavelength magnetic anomalies, related to mafic intrusive/effusive sources, mainly aligned along the NNW-SSE and NS faults. Instead, the Lipari island is characterized by lower intensity anomalies related to sources with more evolved chemistry, elongated in the NE-SW, NNE-SSW and EW of the subordinate faults. Both the two sets of structures belong to the Tindari-Letojanni strike slip fault-system, a regional lineament along which the southern side of the Aeolian Archipelago is emplaced. The study identifies three distinctive magnetic zones on the Lipari island: the southern, the central, and the north-western ones. The southern zone is characterized by negative magnetic monopoles in correspondence with the younger rhyolitic domes and a Curie Isotherm upwelling. The central part of the island has an average lower intensity of positive anomalies, corresponding to the less evolved products of the intermediate Lipari volcanic epochs. The north-western side is characterized by higher intensity anomalies related to older volcanic epochs. The interplay among strike-slip tectonics, chemistry of the uprising magmas, and the thermal setting has contributed to the overall anomaly pattern in this sector of the Aeolian Archipelago. This interpretation is supported by the application of digital enhancement to the total intensity magnetic anomaly field and by the spatial correlation analysis of the magnetic and volcano-tectonic features. The obtained insights are useful to better understand the relationship between arc volcanism and tectonics. Moreover, they can also outline new inferences to forecast future eruptions of the active La Fossa volcanic system. In fact, recent unrest signals were registered at La Fossa Cone, which is one of the present-day active volcanoes of the Aeolian Archipelago, along with Stromboli island. Key pointsLa Fossa volcanic system of Vulcano and Lipari islands high resolution aeromagnetic surveys; Strike slip tectonics magnetic anomaly signature; Spatial correlation analysis of volcano-tectonic and magnetic features.

Remote Characterization of the 12 January 2020 Eruption of Taal Volcano, Philippines, Using Seismo-Acoustic, Volcanic Lightning, and Satellite Observations

ABSTRACT On 12 January 2020, an eruption began on the shores of the Main Crater Lake (MCL) of Taal Volcano—a caldera system on the southern end of Luzon Island in the Philippines. Taal, one of the most active volcanoes in the Philippines, is located 30 km south of Manila—a major metropolitan area with a population of 13.5 million people. Eruptive activity intensified throughout the day on 12 January, producing prolific volcanic lightning, ashfall, and a sustained plume that reached 16–17 km altitude. The chronology of events was well documented by the Philippine Institute of Volcanology and Seismology and the Tokyo Volcanic Ash Advisory Center. The wealth of data collected during the eruption provides a unique opportunity to investigate how the combination of different remote sensing methods may complement local observations and monitoring. Remote systems tend to provide lower resolution data but are also less likely to be compromised by the eruptive activity, thus providing continuous records of eruptive processes. Here, we present a postevent analysis of the 12 January activity, including data from long-range lightning, infrasound, and seismic arrays located at distances up to several thousands of kilometers from the volcano. By combining these datasets, we distinguish five phases of activity and infer a major shift in eruption behavior around 12:00 on 12 January (UTC). The remote observations suggest that the most of the water within the MCL (∼42 million m3) was vaporized and incorporated into the volcanic plume within the first 12 hr of the eruption.

Quaternary caldera-forming eruptions at the Sanzugawa caldera, NE Japan, revealed by zircon U-Pb geochronology

Recent application of zircon U-Pb geochronology has contributed to deciphering the evolution of caldera systems worldwide. However, in the Tohoku region, NE Japan, this is not the case, although it is well studied as a typical island arc subduction system. Here, U-Pb dating of zircon suggests that the Sanzugawa caldera, the largest caldera in Tohoku since late Miocene, initiated its caldera-forming volcanic activity at ∼7 Ma and culminated its activity in late Pliocene to Quaternary (3.0–1.5 Ma). This is contrary to a previous notion that the caldera developed in late Miocene to Pliocene (6–3 Ma) based on mainly whole-rock K-Ar dating results. This finding may also question a current hypothesis of relatively subdued volcanism at 3.5–1.5 Ma in NE Japan and thus shows the need to date other calderas in the Tohoku region with zircon U-Pb and/or Ar-Ar methods to better understand the magmatic history of this region.

The geology and geophysics of Lake Tarawera, New Zealand: Implications for sublacustrine geothermal activity

The Okataina Volcanic Centre is a large caldera system in the Taupo Volcanic Zone, New Zealand, which includes the Haroharo caldera, where the geothermally active Lake Tarawera is located at its southwestern boundary. Here, we show the results of high-resolution bathymetric mapping of the lake floor combined with gravity, magnetic and heat-flow surveys carried out over the lake, constrained by geochemical sampling. Our combined data elucidate the detailed geometry of the SW structural margin of the Haroharo caldera bisecting Lake Tarawera, and the relationship with the regional faults related with the Taupo Volcanic Zone rift.In this context, the spatial distribution of heat-flow in Lake Tarawera appears controlled by fluid-focusing permeable zones associated with the structural lineaments and by peripheral, gravity-driven circulation of meteoric water associated with cooling of highly permeable rhyolite domes around the lakeshore. The structural boundary of the Haroharo caldera provides first-order control of heat-flow in the lake, with WNW striking normal faults to a lesser degree. An estimated total conductive output of ∼5.3 MW escaping through the floor of Lake Tarawera is refracted by the thick layer of sediments, providing an impermeable layer to the convection of fluids, which in turn diminishes heat output compared to other lakes in this region.Our results contribute to modelling of hydrothermal circulation at the Okataina Volcanic Centre, and when integrated with data from the other lakes and the terrestrial geothermal output, it will contribute to determine the global geothermal output at this caldera system. The relationships to eruptive history and structural setting are the key to modelling similar geothermal system of other silicic calderas worldwide.

Storage, evolution, and mixing in basaltic eruptions from around the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand

The Okataina Volcanic Centre (OVC) is the most recently active caldera system in the Taupō Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are several examples of basaltic to basaltic-andesite contributions to OVC eruptions. These range from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions from within and around the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that eruptions sample multiple distinct reservoirs during ascent to the surface. The most abundant basaltic component is generated by cooling-induced crystallisation of a common, oxidised, volatile-rich basaltic melt at various depths within the crust that mixes upon ascent. Despite similar bulk compositions, these four eruptions are texturally distinct from each other as a result of their wide variation in eruption style.

Development of a Bayesian event tree for short-term eruption onset forecasting at Taupō volcano

Taupō volcano, located within the Taupō Volcanic Zone (TVZ) in the central North Island of Aotearoa-New Zealand, is one of the world’s most active silicic caldera systems. Silicic calderas such as Taupō are capable of a broad and complex range of volcanological activity, ranging from minor unrest episodes to large destructive supereruptions. A critical tool for volcanic risk management is eruption forecasting. The Bayesian Event Tree for Eruption Forecasting (BET_EF) is one probabilistic eruption forecasting tool that can be used to produce short-term eruption forecasts for any volcano worldwide. A BET_EF model is developed for Taupō volcano, informed by geologic and historic data. Monitoring parameters for the model were obtained through a structured expert elicitation workshop with 30 of Aotearoa-New Zealand’s volcanologists and volcano monitoring scientists. The eruption probabilities output by the BET_EF model for Taupō volcano’s 17 recorded unrest episodes (between 1877 and 2019) were examined. We found time-inhomogeneity in the probabilities stemming from both the changes over time in the monitoring network around Taupō volcano and increasing level of past data (number of non-eruptive unrest episodes). We examine the former issue through the lens of the latest episodes, and the latter by re-running the episodes assuming knowledge of all 16 other episodes (calibration to 2021 data). The time variable monitoring network around Taupō volcano and parameter weights had a substantial impact on the estimated probabilities of magmatic unrest and eruption. We also note the need for improved monitoring and data processing at Taupō volcano, the existence of which would prompt updates and therefore refinements in the BET_EF model.

Pumice Raft Detection Using Machine-Learning on Multispectral Satellite Imagery

Most of Earth’s volcanic eruptions occur underwater, and these submarine eruptions can significantly impact large-scale Earth systems (e.g., enhancing local primary production by phytoplankton). However, detecting submarine eruptions is challenging due to their remote locations, short eruption durations, lack of sea surface signature (if eruptions do not breach the surface), and the transient nature of the surface manifestations of an eruption (e.g., floating pumice clasts, hydrothermal fluids). We can utilize global satellite imagery of 10–30 m resolution (e.g., Landsat 8, Sentinel-2) to detect new eruptions; however, the large data volumes make it challenging to systematically analyze satellite imagery globally. In this study, we address these challenges by developing a new semi-automated analysis framework to detect submarine eruptions through supervised classification of satellite images on Google Earth Engine. We train our algorithm using images from rafts produced by the August 2019 eruption of Volcano F in the Tofua Arc and present a case study using our methodology on satellite imagery from the Rabaul caldera region in Papua New Guinea. We potentially find a large number of new unreported pumice rafts (in ∼16% of images from 2017–present). After analysis of the spatial pattern of raft sightings and ancillary geophysical and visual observations, we interpret that these rafts are not the result of a new eruption. Instead, we posit that the observed rafts represent remobilization of pumice clasts from previous historical eruptions. This novel process of raft remobilization may be common at near-shore/partially submarine caldera systems (e.g., Rabaul, Krakatau) and may have significant implications for new submarine eruption detection and volcanic stratigraphy.

ISOTOPE 18O OF ALTERATION ROCKS AT WELL C-1 KAMOJANG GEOTHERMAL FIELD, WEST JAVA, INDONESIA

The purpose of this study is to determine the reservoir condition system of Well C-1. Kamojang geothermal field located in the Quaternary volcanic complex caldera system (0.452 to 1.2 Ma). The volcanic complex contributes to the formation of vapor-dominated high-temperature geothermal systems (250oC). The potential of the electricity produced from this field was 260 MWe. This study describes a new method, namely the 18O isotope of alteration rock, to determine reservoir conditions. Another approach is to use a petrographic analytical form, X-ray diffraction, and isotope 18O and D on some core and cutting samples. Isotope abundances are measured using an isotope ratio mass spectrometer (IRMS). Zoning mineral alteration in Well C-1 comprises the cristobalite-montmorillonite zone, illite-montmorillonite zone, and chlorite-epidote zone. Hydrothermal fluids that generate steam in geothermal systems were derived from meteoric water. These fluids interact with rocks based on the ratio of reservoir water-rock interaction, Well C-1. The research shows that the value of the fraction of oxygen atoms of rocks in the system (1-x) was 0.16. That indicated that the reservoir containing fluid 16% or reservoir is rock-dominated. This research on 18O isotope is a novelty because it differs from what has been done by previous researchers who studied reservoir characteristics using water 18O isotope. Meanwhile, this study uses the 18O isotope of alteration rock to identify reservoir conditions. It is hoped that this method can be used in other geothermal fields.

The Quaternary Kurobegawa Granite: an example of a deeply dissected resurgent pluton

The Quaternary Kurobegawa Granite, central Japan, is not only the youngest known granitic pluton exposed on the Earth’s surface, it is one of few localities where both Quaternary volcanics and related plutons are well exposed. Here, we present new zircon U–Pb ages together with whole rock and mineral geochemical data, revealing that the Kurobegawa Granite is a resurgent pluton that was emplaced following the caldera-forming eruption of the Jiigatake Volcanics at 1.55 ± 0.09 Ma. Following the eruption, the remnant magma chamber progressively cooled forming the voluminous Kurobegawa pluton in the upper crust (~ 6 km depth) until ~ 0.7 Ma when resurgence caused rapid uplift and erosion in the region. This is the first study to document the detailed spatiotemporal evolution of resurgent pluton for a Quaternary caldera system. Our new findings may contribute significantly to understanding the fate of active caldera systems that can produce supereruptions.

Textural analysis and emplacement conditions of well-preserved Orosirian felsic volcanic rocks of northern Amazon Craton, Brazil

Located in the Amazon Craton, the Uatumã magmatism (1.89–1.87 Ga) consists in one of the oldest Silicic Large Igneous Provinces (SLIPs) on Earth. For a long time, the access to these deposits in the northern Amazon Craton (Erepecuru–Trombetas Domain) has been set back for volcanological studies due to dense vegetation cover and the absence of roads. Recent studies identify two Orosirian volcanic units in the region: the Iricoumé Group (1.89–1.87 Ga) related to the Uatumã magmatism, and the Igarapé Paboca Formation (1.99–1.94 Ga), associated to an older magmatism. Both units are widespread in the Erepecuru–Trombetas Domain and include effusive and explosive deposits. In this paper, we apply textural analyses and rheological estimations to determine the eruption and emplacement conditions of these two volcanic sequences. Textural analyses were carried out through fieldwork and petrography, including a systematic classification of lavas and volcaniclastic rocks. Rheological parameters were determined using geochemistry data to obtain melt viscosity (η) and temperature, zircon saturation (TZr), liquidus (TL), and glass transition temperatures (TG), for anhydrous and hydrous compositions. Textural analyses indicate the predominance of volcaniclastic facies with abundant eutaxitic and parataxitic textures. Rheological estimations reveal TL of 1020 °C, TZr 650–905 °C, and TG 640–753 °C for anhydrous Iricoumé Group melts. Eruptive viscosity estimations range from 8.4 to 11.7 log η (Pa.s). Igarapé Paboca melts present higher temperatures, with TL of 1050 °C, TZr 710–880 °C, and TG 670–740 °C. Modeling using hydrous compositions indicate that minute amounts of water can strongly affect the rheology of the studied melts, reducing η, TL, TZr, and TG. The petrographic features indicative of hydrous magma reinforces the role of H2O as a controlling agent in the fragmentation of Iricoumé and Igarapé Paboca melts. The pyroclastic samples are marked by elevated ΔTZr - TG relationships indicative of high emplacement temperatures above the TG. Our results indicate that the high temperatures and the presence of network-modifier cations in the studied melts favored the development of extensive welded ignimbrites associated with low-eruption columns, likely developed in fissural and/or caldera systems.

Origin of volatile species and aqueous fluids in the Los Humeros Geothermal Field, Mexico

The Los Humeros Geothermal Field, discovered in the late 1970s, is the last geothermal field in Mexico to be significantly developed in recent years. Located in a nested caldera system of the Central Mexican Volcanic Belt, the field is within an andesitic reservoir, with low porosity and permeability, limiting the recharge of the system. Here, a new dataset of noble gas and stable isotopes (δ2H and δ 18O), tritium (3H), CO2 contents, halogens (Cl content and δ37Cl, δ81Br), and isotopes of Sr (87Sr/86Sr) were analyzed and interpreted along with previously published noble gas data. The goal was to identify the fluid sources present in the Los Humeros reservoir. This study highlights a complex hydrogeological scenario, with multiple generations of meteoric waters. Furthermore, some volatile species of mantle origin (He, Ar, C) appear to be partially decoupled from the aqueous phase, providing only partial information on the origin and evolution of the waters present in the system. The northern sector of the field contains meteoric waters having interacted with the basalt-andesitic reservoir or deeper melts (87Sr/86Sr = 0.7040) and containing mantle He and Ar. Together with this fluid, a meteoric component containing tritium confirms the presence of local and rapid meteoric recharge in this sector. The central and southern sectors of the field contain meteoric waters having interacted with carbonates from the basement. However, the relationship between 87Sr/86Sr and CO2/3He suggests a possible decoupling between mantle C and He and the aqueous phase, transporting Sr. Of particular interest, limestone C seems to be sourced from subducting slab components from the northern sector of the field, while the often-cited C contribution from local basement metacarbonates could be negligible. Finally, chlorine is sourced by magmatic HCl and partitioned in the steam phase, causing acidic waters.

Study of Surface Emissions of 220Rn (Thoron) at Two Sites in the Campi Flegrei Caldera (Italy) during Volcanic Unrest in the Period 2011–2017

The study concerns the analysis of 220Rn (thoron) recorded in the surface soil in two sites of the Campi Flegrei caldera (Naples, Southern Italy) characterized by phases of volcanic unrest in the seven-year period 1 July 2011–31 December 2017. Thoron comes only from the most surface layer, so the characteristics of its time series are strictly connected to the shallow phenomena, which can also act at a distance from the measuring point in these particular areas. Since we measured 220Rn in parallel with 222Rn (radon), we found that by using the same analysis applied to radon, we obtained interesting information. While knowing the limits of this radioisotope well, we highlight only the particular characteristics of the emissions of thoron in the surface soil. Here, we show that it also shows some clear features found in the radon signal, such as anomalies and signal trends. Consequently, we provide good evidence that, in spite of the very short life of 220Rn compared to 222Rn, both are related to the carrier effect of CO2, which has significantly increased in the last few years within the caldera. The hydrothermal alterations, induced by the increase in temperature and pressure of the caldera system, occur in the surface soils and significantly influence thoron’s power of exhalation from the surface layer. The effects on the surface thoron are reflected in both sites, but with less intensity, the same behavior of 222Rn following the increasing movements and fluctuations of the geophysical and geochemical parameters (CO2 flux, fumarolic tremor, background seismicity, soil deformation). An overall linear correlation was found between the 222−220Rn signals, indicating the effect of the CO2 vector. The overall results represent a significant step forward in the use and interpretation of the thoron signal.