Copahue volcano, located at the border between Chile and Argentina, has been among the most active volcanoes in Patagonia for the last 30 years. Copahue hosts a hyperacidic lake in its active crater, and many manifestations of intense hydrothermal activity, motivating studies of the volcano’s internal structure and how alteration may affect edifice stability. In this work, we study the structure, composition, and rock strength of the upper eastern flank of Copahue volcano, which is the most common path used to access the crater by tourists. It is also where the main water springs originating from the crater lake are found, and where most of the ejected material is deposited due to topography and the prevailing wind direction. To study the field-scale edifice structures, we installed a muon detector and computed a muon density image. In addition, we analyzed rock samples from the eastern flank in the laboratory. Using a Scanning Electron Microscope combined with Energy Dispersive X-ray analysis and X-ray Powder Diffraction, we visualized the effects of alteration and identified the minerals present. Finally, we measured the bulk density, dry uniaxial compressive strength (UCS), and static Young’s modulus of the samples. The muon imaging suggests mainly low-density values in the eastern side of the volcano, that we interpret as glacial and/or ice deposits, interleaved with tephras and tuffs, and presence of large edifice fractures. Well-defined high-density anomalies correspond to lava flows that outcrop in some parts of the edifice. Most of the samples analyzed have suffered hydrothermal alteration through rock dissolution, and precipitation of silica and secondary minerals, in accordance with the hyperacidic hydrothermal environment. The laboratory measurements suggest variable rock strength values, which appear to be controlled by rock type and primary porosity. Our multidisciplinary approach indicates that the eastern flank of Copahue volcano is characterized by mainly low-cohesion, low-density and variable-strength materials. The structures mapped, together with the rock properties measured, are key for a future flank stability assessment.

Grande Comore, the westernmost island within the Madagascar Comoros Volcanic (MCV) chain, hosts two juxtaposed basaltic volcanoes, Karthala and La Grille, with contrasting lava geochemical signatures and eruption frequencies. Their formation and dynamics have been explained either by a mantle plume or, more recently, as part of a transtensional volcano-tectonic system. Understanding their individual development is key to resolving persistent geodynamic contradictions within the Comoros and improving volcanic hazard assessment. We integrate the morphology and spatial distribution of scoria cones with a new database of lava, and, for the first time, tephra compositions to constrain magma pathways. We classify cone-dense regions into four main clusters (SGC, KAR, CGC, and LAG) and four sub-clusters (KARa, KARb, KARc, and LAGa). Within Karthala, cone size weakly correlates with primitive mafic magma compositions, whereas no such relationship occurs at La Grille clusters. This indicates a developed, shallow plumbing system beneath Karthala and its absence beneath La Grille. Cone base orientation reveals that Karthala and La Grille volcanism is influenced by regional rifting, but only Karthala is influenced by edifice loading. Edifice-induced compositional filtering promotes lateral migration of primitive magmas and vertical ascent of evolved magmas, while regionally controlled rift zones channel evolved magmas to the center of the island. Recent eruption convergence/resurgence cycles suggest that future eruptions at Karthala will likely be constrained to the edifice of Karthala while La Grille's rare eruptions have only tectonic triggers. Active areas on Grande Comore are thus well constrained and more widespread than previously thought.

Muography is a non-invasive geophysical method that relies on the detection of muons, which are subatomic particles generated by the interaction of cosmic rays with the Earth's atmosphere. The physical quantity estimated by this method is the opacity, which represents the amount of matter traversed by muons along their trajectories, resulting in energy loss and scattering for the particles. Thus, absorption muography consists of deploying a muon detector targeting the volcano and registering the muons traversing it per unit of time and trajectory. From these data, radiographs of average density of extensive rock volumes can be obtained using a single measuring instrument and from a singular measurement position. Copahue volcano is located in the Andes mountain range and is considered the highest-risk volcano in Argentina due to its proximity to two towns situated within an 8 km radius of the volcano's crater. Additionally, the region attracts a significant number of tourists, leading to a substantial increase in the population of both localities. The latest eruptive cycle, initiated in 2012, has maintained a near-continuous state of activity, marked by ash emissions, crater explosions, and seismic activity. In this work, we study the hydrothermal alteration at Copahue volcano through a combination of muography and laboratory measurements of the chemical and physical properties of rock samples. The muography dataset was acquired by installing a muon detector on the eastern flank of Copahue volcano, situated at an altitude of approximately 2500 meters above sea level. For the laboratory analyses, we collected rock blocks with the objective of representing a diverse spectrum of alteration stages within Copahue volcano. Through this selection process, we captured variations in mineralogical composition, geochemical signatures, and physical properties that correspond to different stages of hydrothermal alteration. We carried out a series of examinations on the rock samples extracted from the targeted flank, such as X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) analyses that identified mineralogical compositions and geochemical signatures associated with hydrothermal processes. We also carried out additional measurements, including density, porosity, permeability, thermal properties, and uniaxial compressive strength, contributing to a comprehensive understanding of the physical properties of the samples. In addition, we performed microscopic examinations using a scanning electron microscope (SEM) to study the microstructural changes induced by hydrothermal alteration. This integrative approach, between muography and detailed laboratory measurements on rock samples, aims to reveal correlations between subsurface density variations and hydrothermal alteration observed at the microscopic and macroscopic scales.

Intraplate volcanism provides remarkable insight into diverse sources in the mantle because its source can be quite shallow, or as deep as the core-mantle boundary, and its origin can be as diverse as recycled crustal material or undifferentiated mantle. While geophysical approaches can in some cases locate the source of magmas, a geochemical approach is necessary to characterize both the nature of the source and the way it melts to produce the erupted lavas.Here we present geochemical and isotopic data obtained on a new submarine volcano (Fani Maore ') that was discovered in 2019 next to Mayotte Island in the Comoros. The radiogenic isotope data are remarkably uniform at subdued values intermediate between HIMU and EM1 compositions but trace element contents are unusual with a marked enrichment in Ba (Ba/Th approximate to 370 compared with the ocean island basalt (OIB) average of 100) and depletion in Pb (Ce/Pb approximate to 70 versus 25 for average OIB). This unique data set suggests that the basanites formed by melting of a carbonated mantle source that was highly enriched in Ba and volatiles. A similar source is also present under the East African Rift where contemporaneous basanite and carbonatite eruptions are known. This establishes a possible link between the volcanic activity of the Comoros and the East African Rift zone. More generally, it demonstrates that carbonated sources are more common in the mantle than previously thought and can be traced using trace element geochemistry. Other volcanoes in the world carry similar characteristics and we suggest that carbonated mantle sources explain the geochemical peculiarities of not only Fani Maore ' in the Comoros but also those of Cape Verde volcanics and more generally those of many HIMU-like OIBs, in particular the so-called 'young HIMU' OIBs.

Although the input of desert dust as a key source of trace metals in the Southern Ocean (SO) has been previously studied, the dissolution process of metals in surface waters, particularly iron (Fe), remain poorly understood. Given the crucial role of Fe in primary production and the biological carbon pump in the SO, we focused on experimental estimations of Fe dissolution from Patagonian dust, the primary natural dust source in the SO. Our study considered both current and projected future conditions, encompassing sea-surface warming, acidification, increased photosynthetically active radiation, and doubled dust inputs. Through controlled laboratory experiments using filtered SO seawater, conducted over 7 days, we assessed changes in particulate Fe (pFe) concentrations, Fe redox speciation (Fe(II)/Fe(III)), and in the mineralogy of Fe-bearing dust in abiotic condition. The predominant minerals in the dust included quartz and aluminosilicates, with silicon (Si), aluminum (Al), and Fe as the major elements. No significant alterations in the mineralogy and the elemental composition of the dust were recorded during the dissolution experiments, neither under present nor under projected future conditions. The particulate Fe(II)/Fe(III) ratio remained consistently at 0.25 during the experiments, unaffected by changed conditions. Consequently, changes in environmental conditions in the SO would therefore not significantly alter the mineralogy and redox speciation of pFe in the Patagonian dust. On the contrary, pFe exhibited a dissolution rate of 3.8% and 1.6% per day under present and future conditions, respectively. The environmental changes anticipated for 2100 in the SO will likely to result in a decrease in the dissolution rate of pFe. Thus, even though a doubling of dust input by 2100 is anticipated, it will unlikely provide significantly more dissolved Fe (dFe) in seawater in the SO. Consequently, the future intensification of Patagonian dust inputs may not alleviate the Fe limitation in the SO.

Abstract We describe four Quaternary volcanic phonolitic explosive edifices containing mantle xenoliths on Petite-Terre Island (Mayotte, Comoros Archipelago, Western Indian Ocean) to quantifying magma fragmentation processes and eruptive dynamics. Petite-Terre explosive volcanism is the westernmost subaerial expression of a 60 km volcanic chain, whose eastern submarine tip has been the site of the 2018–2021 sub-marine eruption which saw the birth of a new volcano, Fani Maoré. The scattered recent volcanic activity and the persistence of deep seismic activity along the volcanic chain requires to constrain the origin of past activity as a proxy of possible future volcanic activity on land. Through geomorphology, stratigraphy, grain size and componentry data we show that Petite-Terre tuff rings and tuff cones are likely formed by several closely spaced eruptions forming a monogenetic volcanic complex. The eruptive sequences are composed of few, relatively thin (cm-dm) coarse and lithic rich pumice fallout layers containing abundant ballistic clasts, and fine-ash rich deposits mostly emplaced by dilute pyroclastic density current (PDCs). All deposits are dominated by vesiculated, juvenile (pumice clasts, dense clasts, and obsidian) and non-juvenile clasts from older mafic scoria cones, coral reef and the volcanic shield of Mayotte as well as mantle xenoliths. We conclude that phonolitic magma ascended directly and rapidly from the mantle and first experienced a purely magmatic fragmentation at depth (≈ 1 km deep). The fragmented pyroclasts underwent a second shallower hydromagmatic, fragmentation where they interacted with liquid water, producing fine ash and building the tuff ring and tuff cone morphologies.

Piton de la Fournaise is an active shield volcano located in the eastern area of the Réunion Island (Indian Ocean) whose activity is characterized by effusive and explosive episodes with the emission of scarcely differentiated magmas with mostly tholeiitic affinity. The presently active edifice has grown within the Enclos Fouqué caldera, a polylobate plain bounded on its western side by the 80–200 m high Bellecombe vertical cliffs. This escarpment exposes a vertical sequence of 12 lava flows cut by a dike with an age > 5.5 kyrs. In this work, the Bellecombe products were investigated by X-ray fluorescence, Inductively Coupled Plasma Mass Spectroscopy, a Scanning Electron Microscope and X-ray computed microtomography in order to characterize the evolution over time of the magmatic system feeding the eruptive activity prior to the Enclos Fouqué caldera collapse. The results indicate that lava flows share a geochemical affinity with the two main series documented at Piton de la Fournaise, namely, Steady State Basalts (SSB) at the bottom and top of the sequence and Abnormal basalt Group (AbG) with different degrees of differentiation in the central part. The emission of these two different products in both a restricted area and timespan testifies to the dynamic activity of the plumbing system, capable of shifting rapidly from central to eccentric activity in the recent past.

The “Fani Maoré” eruption off the coasts of Mayotte has been intensively monitored by applying methods similar to those used for subaerial eruptions. Repeated high-resolution bathymetric surveys and dredging, coupled with petrological analyses of time-constrained samples, allowed tracking the evolution of magma over the whole submarine eruptive sequence. Indeed, after one year of direct ascent (Phase 1), basanitic magma switched to a different pathway that sampled a tephri-phonolitic subcrustal reservoir (Phase 2). Later, the magma pathway shifted again in the crust resulting in a new eruption site located 6 km northwest of the main edifice (Phase 3). The petrological signature of lava flows reveals both an evolution by fractional crystallization and syn-eruptive mixing with a tephri-phonolitic magma.We demonstrate that high-flux eruption of large volumes of basanitic magma from a deep-seated reservoir can interact with shallower reservoirs and remobilize eruptible magma. This has significant hazards implications with respect to the capacity of such large eruptions to reactivate shallow-seated inactive reservoirs from a transcrustal magmatic system that could be located potentially at a distance from the high-flux eruptive site.

Basaltic eruptions are commonly associated with lava emissions and relatively weak explosive activities, but they can sometimes produce strong explosive eruptive phases. In April and November 2005, two paroxysmal eruptive events occurred within the summit crater of Karthala basaltic shield volcano (Grande Comore Island, Comoros), which hosted a water lake before each of these events. Both 2005 ash plumes spread across the Comoros Archipelago and heavily impacted the whole Grande Comore Island. Associated deposits on the volcano summit are extremely fine-grained (up to 50 wt% of fine ash < 63 mu m for some analyzed layers) and rich in millimeter-sized rounded accretionary lapilli aggregates. Field observations, as well as textural and chemical analyses performed on both coarse-and fine-grained pyroclasts permit to identify juvenile and non-juvenile components and quantify their peculiar characteristics. Coarse ash (710-1000 mu m) mainly consists of juvenile pumice particles (vesicle number density N-V = 4.5 10(4) mm(-3) and gas to melt ratio V-G/V-L = 1.5, on average), characterized by glassy groundmasses and representative of magma portions ascending quickly within the eruptive conduits (up to 10 m s(-1)). A relatively low amount of juvenile scoria particles are also observed in the coarse ash fractions, which are characterized by magma degassing (N-V = 4.7 10(4) mm(-3) and V-G/V-L = 0.5 on average) and associated crystallization (occurrence of dendritic microlites). Non-juvenile fragments (from blocks to coarse ash) are dense lava or intrusive fragments. Their amount decreases exponentially towards the fine ash fractions, which are mainly composed of juvenile, blocky, dense and glassy particles that are characterized by unambiguous textural signs of brittle fragmentation (hackle lines, stepped features and cracks). We support that Molten Fuel-Coolant Interactions between highly porous fast ascending basaltic magmas and external waters occurred during the paroxysmal phases of the studied eruptions, leading to a brittle-dominant and efficient regime of magma fragmentation. Variable but large amount of fine ash grains through the stratigraphic depth of the deposits can be ascribed to the brittle failure of the vesicle walls of the initial porous magma. Concurrently, thermohydraulic explosions caused the host rock fragmentation at shallow level, generating the relatively coarse non-juvenile particles. A short-lived episode of intense lava fountaining associated with steam explosions eventually occurred at the end of the November 2005 paroxysm, forming the last and relatively coarse tephra layer at the top of the studied eruptive sequence. Each paroxysmal phase lasted about a day as each associated water lake and shallow water table progressively vaporized and dried away. Both eruptions ended with lava pond and weak lava fountaining activities confined within the summit crater. We conclude that the contributions of both magmatic processes and phreatomagmatic interaction mechanisms ultimately generated the grain size, grain component and grain texture variabilities observed within the paroxysmal deposits. This work contributes to a better understanding of the generation of unusual fine ash from basaltic explosions as well as their eruptive dynamics and associated mechanisms, from magma ascent in the conduit to the fragmentation level and the interaction with intra-crateric lake waters.

Deep-sea submarine eruptions are the least known type of volcanic activity, due to the difficulty of detecting, monitoring, and sampling them. Following an intense seismic crisis in May 2018, a large submarine effusive eruption offshore the island of Mayotte (Indian Ocean) has extruded at least 6.5 km(3) of magma to date, making it the largest monitored submarine eruption as well as the largest effusive eruption on Earth since Iceland's 1783 Laki eruption. This volcano is located along a WNW-ESE volcanic ridge, extending from the island of Petite Terre (east side of Mayotte) to about 3,500 m of water depth. We present a detailed petrological and geochemical description of the erupted lavas sampled by the MAYOBS 1, 2, and 4 cruises between May and July 2019 and use these to infer characteristics and changes through time for the whole magmatic system and its dynamics from the source to the surface. These cruises provide an exceptional time-series of bathymetric, textural, petrological, and geochemical data for the 2018-2019 eruptive period, and hence bring an invaluable opportunity to better constrain the evolution of magma storage and transfer processes during a long-lived submarine eruption. Integrating the petrological signatures of dredged lavas with geophysical data, we show that the crystal-poor and gas-rich evolved basanitic magma was stored at mantle depth (>37 km) in a large (>= 10 km(3)) reservoir and that the eruption was tectonically triggered. As the eruption proceeded, a decrease in ascent rate and/or a pathway change resulted in the incorporation of preexisting differentiated magma stored at a shallower level. Magma transfer from the deep mantle reservoir is syn-eruptive, as indicated by transfer times estimated from diffusion in zoned olivine crystals that are much shorter than the total eruption duration. Our petrological model has important hazard implications concerning the rapid and stealthy awakening of a deep gas-rich magma reservoirs that can produce unusually high output rates and long-lived eruption. Sudden tapping of large crystal poor reservoirs may be the trigger mechanism for other rarely witnessed high-volume (>1 km(3)) effusive events. (C) 2021 Elsevier B.V. All rights reserved.

Since 2018, the submarine east flank of Mayotte Island (Comoros archipelago) is the site of a major eruption located at 3.5 km depth bsl on a WNW-ESE volcanic ridge. Samples brought by oceanographic cruises carried out to monitor this seismo-volcanic crisis indicate that this volcanic ridge is built by a bimodal sodic alkaline magmatic series that includes basanites and phonolites. A petrological study of dredged samples allowed us to image the magmatic system feeding the volcanic ridge and to determine the link between basanitic and phonolitic magmas. The magmatic system feeding the volcanic ridge comprises multiple levels of magma storage. Basanitic magmas generated at 80–100 km mantle depth are stored in two or more deep reservoirs (≥ 37 km) and then in shallower basanitic and phonolitic lenses located close to the Moho interface before rising the surface. This study identifies three possible scenarios: (1) the deep basanitic magma rises directly and quickly to the surface from the deep mantle reservoir (as is currently happening 60 km offshore), (2) the basanitic magma stalls in a shallower reservoir near the Moho before resuming its ascent toward the surface and erupting as porphyritic basanite, (3) the basanitic magma stops and evolves to phonolite in these sub-crustal reservoirs. The phonolitic lavas are produced by approximately 80% fractional crystallization (34% clinopyroxene, 30% anorthoclase feldspar, 15.5% magnetite, 12.5% olivine, 5% apatite and 4% ilmenite) of a hydrous basanitic magma at mantle depths (P > 0.6 GPa) under reduced oxygen fugacity (~ FMQ-1). In this third scenario, the phonolitic magma might be reactivated by the arrival of a new batch of deeper basanitic magma.

It is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic shield volcanoes. At Piton de La Fournaise (La Réunion Island, France), eruptive activity occurs mostly within the non-populated Enclos Fouqué caldera and generally does not pose any risk to the population. However, historical observations, recent monitoring data, and field work on tephra deposits suggest that some eruptions have produced unusual and unexpected explosive phases. A comprehensive sampling of tephra from major historical and recent eruptions on this volcano allowed us to perform systematic componentry, grain size, and chemical and morphological analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions. This integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation efficiencies and multiple associated mechanisms. Primary ductile and partially brittle fragmentation of various intensity of juvenile magma emitted during Hawaiian fountaining or mild Strombolian explosions were identified as the most common fragmentation mechanisms, in particular during the 2014–2018 dominantly effusive eruptive sequence. In parallel, we distinguished more efficient short-lived fragmentation events related to (i) plug pressurization and brittle fragmentation enhanced by syn-eruptive crystallization, (ii) magma/water interactions, (iii) rare phreatic explosions, and (iv) secondary fragmentation producing fine ash during caldera collapse. We conclude that textural, geochemical, and morphological analyses make it possible to identify and characterize the variability in eruptive processes, with the grain size, and the componentry of the ash particles being probably the most important parameters to quantify both the efficiency and the nature of the fragmentation.