GAS AND HEAT FLUXES DURING MULTIPLE EFFUSIVE ERUPTIONS OF PITON DE LA FOURNAISE (RÉUNION) AND THEIR IMPLICATIONS FOR MAGMATIC PROCESSES

This study investigates five eruptions with different temporal trends of erupted mass and sulfur dioxide (SO2) at Piton de la Fournaise (La Réunion). We acquired the daily SO2 emissions from three ultraviolet (UV) satellite instruments (the Ozone Monitoring Instrument [OMI], the Ozone Mapping and Profiler Suite [OMPS] and the Tropospheric Monitoring Instrument [TROPOMI]). The Time-Averaged-Lava-Discharge Rate (TADR) has been obtained from two automatic systems: MIROVA and MODVOLC. Assuming that the lava volumes measured in the field are the most accurate, MIROVA gives the best estimation among all the methods investigated. It has also been demonstrated that a petrological approach might be a viable alternative, especially during cloudy weather that compromises the hot spot detection. Finally, in several eruptions we observe a terminal increase in TADR and SO2 emissions. We suggest that a deeper input may be at the origin of this eruptive process causing a pressurization of the magmatic system

Lava Volume from Remote Sensing Data: Comparisons with Reverse Petrological Approaches for Two Types of Effusive Eruption

Five effusive eruptions of Piton de la Fournaise (La Réunion) are analyzed to investigate temporal trends of erupted mass and sulfur dioxide (SO2) emissions. Daily SO2 emissions are acquired from three ultraviolet (UV) satellite instruments (the Ozone Monitoring Instrument (OMI), the Ozone Mapping and Profiler Suite (OMPS), and the Tropospheric Monitoring Instrument (TROPOMI)) and an array of ground-based UV spectrometers (Network for Observation of Volcanic and Atmospheric Change (NOVAC)). Time-averaged lava discharge rates (TADRs) are obtained from two automatic satellite-based hot spot detection systems: MIROVA and MODVOLC. Assuming that the lava volumes measured in the field are accurate, the MIROVA system gave the best estimation of erupted volume among the methods investigated. We use a reverse petrological method to constrain pre-eruptive magmatic sulfur contents based on observed SO2 emissions and lava volumes. We also show that a direct petrological approach using SO2 data might be a viable alternative for TADR estimation during cloudy weather that compromises hot spot detection. In several eruptions we observed a terminal increase in TADR and SO2 emissions after initial emission of evolved degassed magma. We ascribe this to input of deeper, volatile-rich magma into the plumbing system towards the end of these eruptions. Furthermore, we find no evidence of volatile excess in the five eruptions studied, which were thus mostly fed by shallow degassed magma

Lava Volume from Remote Sensing Data: Comparisons with Reverse Petrological Approaches for Two Types of Effusive Eruption

co-auteur étrangerInternational audienceFive effusive eruptions of Piton de la Fournaise (La Réunion) are analyzed to investigate temporal trends of erupted mass and sulfur dioxide (SO2) emissions. Daily SO2 emissions are acquired from three ultraviolet (UV) satellite instruments (the Ozone Monitoring Instrument (OMI), the Ozone Mapping and Profiler Suite (OMPS), and the Tropospheric Monitoring Instrument (TROPOMI)) and an array of ground-based UV spectrometers (Network for Observation of Volcanic and Atmospheric Change (NOVAC)). Time-averaged lava discharge rates (TADRs) are obtained from two automatic satellite-based hot spot detection systems: MIROVA and MODVOLC. Assuming that the lava volumes measured in the field are accurate, the MIROVA system gave the best estimation of erupted volume among the methods investigated. We use a reverse petrological method to constrain pre-eruptive magmatic sulfur contents based on observed SO2 emissions and lava volumes. We also show that a direct petrological approach using SO2 data might be a viable alternative for TADR estimation during cloudy weather that compromises hot spot detection. In several eruptions we observed a terminal increase in TADR and SO2 emissions after initial emission of evolved degassed magma. We ascribe this to input of deeper, volatile-rich magma into the plumbing system towards the end of these eruptions. Furthermore, we find no evidence of volatile excess in the five eruptions studied, which were thus mostly fed by shallow degassed magma

Viscosity of crystal-free silicate melts from the active submarine volcanic chain of Mayotte

Following an unprecedented seismic activity that started in May 2018, a new volcanic edifice, now called Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Fani Maoré, one tephriphonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature in the air. At super-liquidus temperatures, basanites have the lowest viscosity (0.11–0.34 to 0.99–1.16 log10 Pa⸱s), phonolites the highest (1.75–1.91 to 3.10–3.89 log10 Pa⸱s), while the viscosity of the tephriphonolite falls in between (0.89–1.97 log10 Pa⸱s). Near the glass transition, viscosity measurements have only been performed for one phonolite melt because obtaining pure glass samples for the basanite and tephriphonolite compositions was unsuccessful. This was due to the formation of nanolites upon quench as evidenced by Raman spectroscopy. The phonolite viscosity ranges from of 10.19 log10 Pa⸱s at 1058 K to 12.30 log10 Pa⸱s at 986 K. Comparison with existing empirical models revealed an underestimation of 1.2 to 2.0 log units at super-liquidus and undercooled temperatures, respectively, for the phonolite. This emphasizes (i) the lack of data falling along the alkaline basanite-to-phonolite magmatic differentiation trend to calibrate empirical models, and (ii) the complexity of modeling viscosity variations as a function of temperature and chemical composition for alkaline compositions. The new measurements indicate that, at eruptive temperatures between 1050 °C and 1150 °C (1323–1423 K), the oxidized, anhydrous, crystal-free and bubble-free basanite melt is very fluid, presenting a viscosity around 2.6 log10 Pa⸱s. In contrast, the anhydrous phonolite crystal- and bubble-free melt would have a viscosity around 6–10 log10 Pa⸱s at expected eruptive temperatures, which range from 800 to 1000 °C (1073–1273 K). Considering that both basanite and phonolite lavas from the Mayotte submarine volcanic chain contain <6% crystals and a significant amount of water, such viscosity values are probably upper limits. The new viscosity measurements are essential to define eruptive models and to better understand the storage and transport dynamics of Comoros Archipelago magmas, and of alkaline magmas in general, from the source to the surface

Metastable liquid immiscibility in the 2018–2021 Fani Maoré lavas as a mechanism for volcanic nanolite formation

International audienceNanoscale liquid immiscibility is observed in the 2018-2021 Fani Maoré submarine lavas (Comoros archipelago). Heat transfer calculations, Raman spectroscopy, scanning and transmission electron microscopy reveal that in contrast to thin (500 µm) outer rims of homogeneous glassy lava (rapidly quenched upon eruption, >1000 °C s-1), widespread liquid immiscibility is observed in thick (1 cm) inner lava rims (moderately quenched, 1-1000 °C s-1), which exhibit a nanoscale coexistence of Si-and Al-rich vs. CaFe Fe-, and Ti-rich melt phases. In this zone, rapid nanolite crystallization contrasts with the classical crystallization process inferred for the slower cooled (< 1 °C s-1) lava interiors. The occurrence of such metastable liquid immiscibility at eruptive conditions controls physicochemical characteristics of nanolites and residual melt compositions. This mechanism represents a common yet frequently unobserved feature in volcanic products, with the potential for major impacts on syn-eruptive magma degassing and rheology, and thus on eruptive dynamics

Viscosity of silicate melts from the active submarine volcanic chain of Mayotte

International audienceFollowing an unprecedented seismic activity that started in May 2018, a new volcanic edifice, now called Mont Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Mont Fani Maoré, one tephri-phonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature. At super-liquidus temperatures, basanites have the lowest viscosity (0.11-0.99 log10 Pa⸱s), phonolites the highest (0.91-3.89 log10 Pa⸱s), while the viscosity of the tephr

Viscosity of silicate melts from the active submarine volcanic chain of Mayotte

International audienceFollowing an unprecedented seismic activity that started in May 2018, a new volcanic edifice, now called Mont Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Mont Fani Maoré, one tephri-phonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature. At super-liquidus temperatures, basanites have the lowest viscosity (0.11-0.99 log10 Pa⸱s), phonolites the highest (0.91-3.89 log10 Pa⸱s), while the viscosity of the tephr

Magma ascent and lava flow field emplacement during the 2018–2021 Fani Maoré deep-submarine eruption insights from lava vesicle textures

International audienceThe 2018–2021 Fani Maor´e submarine eruption (offshore of Mayotte, Mozambique Channel) extruded a bulk volume of ~6.5 km3 of basanite magma onto the seafloor at a depth of 3300 m, with effusion rates ranging from 150 to 200 m3/s in the first year of the eruption, to less than 11 m3/s in the final months. Six oceanographic campaigns provided a large sample set covering the entire flow field at high spatial and temporal resolution.These samples allow us to precisely track syn-eruptive degassing processes through quantification of textural parameters including porosity, pore connectivity, vesicle number density (NV) and vesicle size distributions (VSD). Three different textural facies have been distinguished. (1) Vesicular lavas (average porosity of 35%) display unimodal VSDs, high NV (14–214 mm 3), and small and spherical vesicles. (2) Lavas with intermediate porosities (25%) have scarce small vesicles, VSDs shifted towards larger vesicles, and low NV (0.2–39 mm 3). (3) Dense lavas with low porosities (14%) display bimodal VSDs distribution, a dominant mode of small vesicles, and low NV (0–87 mm 3). The early phase of activity (Phase 1, June 2018 – May 2019) built the main edifice and was fed by rapid ascent and closed-system degassing of volatile-rich magma ascending from a deep reservoir to the seafloor (Facies 1). Distal samples collected from lava flows emitted during Phase 2, between June and July 2019, show large and irregular shape vesicles mostly related to bubble growth and coalescence, and outgassing during emplacement (Facies 2). These lavas are interpreted to be emplaced during extension of a lava tube system which began to develop during Phase 1. The final phase (Phase 3, August 2019 – January 2021) was associated with lava effusion located at the northwest lava flow front, 6 km from the summit. Phase 3 involved a more degassed magma due to the increase in the length of the magma pathway (Facies 3). Phase 3 lavas were also extremely outgassed and associated with construction of a new complex lava flow field with tumuli and multiple ephemeral vents (lava breakouts). The heterogeneous textures within the studied samples reflect changing ascent and effusion rates with time, leading to emplacement of lava flows which varied depending on the degree of degassing and effusion rate. We conclude that emplacement of the Fani Maor´e large submarine lava flow fields developed through extensive and prolonged tube systems this being supported by the high effusionrates

Évolution magmatique temporelle de l’éruption sous-marine de Fani Maoré, située à 50 km à l’est de Mayotte, révélée par un échantillonnage in situ et un suivi pétrologique

International audienceThe “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.L’éruption au large de Mayotte a été intensément surveillée en appliquant des méthodes similaires aux éruptions sub-aériennes. Une étude pétrologique et géochimique des échantillons dragués couplée à de nombreux relevés bathymétriques, nous a permis de suivre l’évolution du magma au cours de l’éruption. Le trajet du magma change après un an de remontée directe (Phase 1), un réservoir magmatique sous-crustal et plus différencié est alors échantillonné (Phase 2). Un mois plus tard, le trajet change à nouveau et engendre une migration du site éruptif à 6 km au nord-ouest de l’édifice principal (Phase 3). La signature pétrologique des coulées de lave révèle à la fois une évolution par cristallisation fractionnée et un mélange syn-eruptif avec un magma téphri-phonolitique. Nous démontrons qu’une éruption à haut débit impliquant de grands volumes de magma basanitique et provenant d’un réservoir profond peut interagir avec des réservoirs plus superficiels et remobiliser le magma éruptible. Ceci a des implications significatives en termes de risques quant à la capacité de ces grandes éruptions à réactiver des réservoirs inactifs peu profonds provenant d’un système magmatique transcrustal et potentiellement situé à distance du site éruptif