Abstract. The 2014 eruption at Piton de la Fournaise (PdF), La Réunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4  ×  106 m3). The pyroclastic material was composed of golden basaltic pumice along with fluidal, spiny iridescent and spiny opaque basaltic scoria. Density analyses performed on 200 lapilli reveal that while the spiny opaque clasts are the densest (1600 kg m−3) and most crystalline (55 vol. %), the golden pumices are the least dense (400 kg m−3) and crystalline (8 vol. %). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 vol. %) than the spiny (Strombolian-like) clasts (0–5 vol. %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments and lava that mirror the cooler portion of the shallow reservoir. Exponential decay of the magma ascent and output rates through time revealed depressurization of the source during which a stratified storage system was progressively tapped. Increasing syn-eruptive degassing and melt–gas decoupling led to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new input of hot magma into the 2014 reservoir and confirm the emission of a single shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. Fast volatile exsolution and crystal–melt separation (second boiling) were triggered by deep pre-eruptive magma transfer and stress field change. Our study highlights the possibility that shallow magma pockets can be quickly reactivated by deep processes without mass or energy (heat) transfer and produce hazardous eruptions with only short-term elusive precursors.

In spite of its major role on the atmospheric volatile budget, climate, and tracking magmatic transfers, mantle (CO 2 ) degassing below volcanoes is still poorly understood. Most of the studies on this scientific topic lack constraint on the CO 2 concentration of primary melts, the depth at which it starts degassing, and the extent of this process in the mantle. In this study of Piton de la Fournaise (PdF) volcano, we couple geochemistry of low solubility gases (He, Ar, CO 2 , d 13 C) in fluid inclusions (FIs) and petro-chemistry of magmatic inclusions on a set of olivine and clinopyroxene crystals from basalts and ultramafic enclaves.We constrain basaltic melt degassing at PdF over a large pressure range (from 4 GPa up to the surface). Based on CO 2 -He-Ar systematics, we infer that extensive degassing occurs already in the upper mantle (4-1 GPa) and it is favored by multiple steps of magma ponding and differentiation up to the mantle-crust underplating depth (0.4 GPa). Thus, we calculate that basaltic melts injected at crustal depth (<0.4 GPa) have already exsolved $94 ± 5 wt% of their primary CO 2 content in accordance with (1) the evolved and degassed signature of erupted lavas and (2) the weakness of inter-eruptive gas emissions in the active area bearing low-temperature vapor-dominated fumaroles. Our results at PdF strongly contrast with previous findings on other ocean island volcanoes having a higher magma production rate and faster magma ascent, like Kilauea (Hawaii), whose basalts experience only limited extent of differentiation and degassing. We propose that extensive degassing already in the upper mantle can be a common process for many volcanoes of the Earth and is tightly dependent on the dynamics of magma ascent and differentiation across multiple ponding zones.Based on the modeling developed in this study, we propose a new estimation of the CO 2 content (up to 3.5 ± 1.4 wt%) in primary basaltic melts at PdF leading to a carbon content in the mantle source of 716 ± 525 ppm. This new estimation is considerably higher than the few previous calculations performed for Ocean Island Basalts (OIB) systems. Another implication of this work involves the possible bias between the d 13 C measured in volcanic gas emissions (<À6‰) and that of primary vapour phase (À0.5 ± 0.5‰) constrained in this work. This bias would confirm the early step of extensive CO 2 degassing within the upper mantle and could represent an alternative for the hypotheses of carbon recycling or mantle heterogeneity in support of the low d 13 C signature of some mantle reservoirs. This study bears significant implications on the global budget of volcanic ⇑ Corresponding author at: Observatoire Volcanologique du Piton de la Fournaise ((G. Boudoire). volatile emissions, chiefly regarding the contribution of past and future emissions of volcanic CO 2 to climate dynamics, and on volcanic gas monitoring.

Nyiragongo volcano is known for its active lava lake and for socioeconomic issues arising from future possible eruptive events having major impacts on the community living in the Virunga region. The 2020 field expedition inside the summit crater has allowed the collection of unprecedented field observations to state on the current activity. Since the February 2016 intracrater event, the crater floor level has been rising much faster than during the 2010–2016 period. The current activity is reminiscent of the 1970–1972 and 1994–1995 periods preceding the lava lake drainage events in 1977 and 2002. Numerical simulations, successfully validated with data over the past 30 years, show that the rising of the crater floor could slow down in the next months/years and reach a critical equilibrium. Based on the past eruptive history and on the current activity, a flank eruption in the March 2024 to November 2027 interval could be a possible scenario.

Abstract. The 2014 eruption at Piton de La Fournaise (PdF), la Reunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity, and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4 Mm3). The pyroclastic material was composed of spiny-opaque, spiny-iridescent, and fluidal scoria along with golden pumice. Density analyses performed on 200 lapilli reveal that the spiny-opaque clasts are the densest (1600 kg/m3) and richest in crystals (54 vol%), and the golden pumices are the lightest (400 kg/m3) and poorest in crystals (14 vol%). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 %) than the spiny (Strombolian-like) clasts (0–5 %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments which mirror the cooler portion of the shallow reservoir. Progressive tapping of these distinct portions leads to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new hot input of magma and confirm the involvement of a single, shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. We found that the eruption was triggered by water exsolution, favoured by the shallow depth of the reservoir, rather than cooling and chemical evolution of the stored magma.

Detecting renewal of volcanic activity is a challenging task and even more difficult in tropical settings. Continuous measurements of soil CO2 flux were carried out at the Piton de la Fournaise volcano during 2013–2016. Since this site is in the tropics, periods of heavy rainfall are in the norm. Measurements covered volcanic unrest after a hiatus of 3.5 years. We find that while temperature has the strongest effect, extreme rainfall causes short‐term noise. When corrected and filtered from the environmental influence, soil CO2 time series permit to detect a major deep magmatic event during March–April 2014, 3 months before the first eruption of the new activity phase. Correlation with geophysical data sets allows timing of further stages of upward fluid ascent. Our study validates soil CO2 flux monitoring in tropical environments as a valuable tool to monitor magma transfer and to enhance understanding of volcano unrest down to the lithospheric mantle.

Peripheral diffuse degassing of CO 2 from the soil occurs across the western flank of Piton de la Fournaise volcano (La Réunion Island, Indian Ocean) along a narrow zone. In this area, carbon isotopic analysis on soil gas samples highlights significant mixing between magmatic and organic end-members. The zones with the strongest magmatic signature (highest δ 13 C) overlap spatial distribution of hypocenters recorded shortly before and during volcano reactivation and allow discriminating a N135° degassing lineament, with a minimum length of 11 km and 140 ± 20 m-width. Such orientation is in accordance with that of an old dyke network along the rift zone and with N120-130° and N140-155° lineaments related to the inheritance of oceanic lithosphere structures. Our findings show that this N135° lineament represents a preferential magmatic pathway for deep magma transfer below the volcano flank. Moreover, spatial distributions of recent eccentric cones indicate a well-founded possibility that future eruptions may by-pass the shallow plumbing system of the central area of the volcano, taking a lateral pathway along this structure. Our results also confirm that Piton de la Fournaise activity is linked to a laterally shifted plumbing system and represent a major improvement in identifying the main high-risk area on the densely populated western flank of the volcano.

Abstract. The STRAP (Synergie Transdisciplinaire pour Répondre aux Aléas liés aux Panaches volcaniques) campaign was conducted over the entire year of 2015 to investigate the volcanic plumes of Piton de La Fournaise (La Réu-nion, France). For the first time, measurements at the local (near the vent) and at the regional scales were conducted around the island. The STRAP 2015 campaign has become possible thanks to strong cross-disciplinary collaboration between volcanologists and meteorologists. The main observations during four eruptive periods (85 days) are summarised. They include the estimates of SO 2 , CO 2 and H 2 O emissions, the altitude of the plume at the vent and over different areas of La Réunion Island, the evolution of the SO 2 concentration, the aerosol size distribution and the aerosol extinction profile. A climatology of the volcanic plume dispersion is also reported. Simulations and measurements show that the plumes formed by weak eruptions have a stronger interaction with the surface of the island. Strong SO 2 mixing ratio and particle concentrations above 1000 ppb and 50 000 cm −3 respectively are frequently measured over a distance of 20 km from Piton de la Fournaise. The measured aerosol size distribution shows the predominance of small particles in the volcanic plume. Several cases of strong nucleation of sulfuric acid have been observed within the plume and at the distal site of the Maïdo observatory. The STRAP 2015 campaign provides a unique set of multi-disciplinary data that can now be used by modellers to improve the numerical parameterisations of the physical and chemical evolution of the volcanic plumes.

Petrological and geochemical (major, trace, Sr-Nd isotope) data for recent (<5 kyr old) basalts that sporadically erupt on the western flank of Piton de la Fournaise (PdF), one of the most active volcanoes on Earth, allow the tracking of magma transfer and evolution from mantle to crustal depths. In the western peripheral area of PdF we document the broadly synchronous eruptions of : (1) primitive olivine- and olivine-clinopyroxene transitional basalts with tholeiitic affinity that are closely associated in space with (2) transitional olivine basalts with alkaline affinity, and (3) hybrid lavas, intermediate between the “alkaline” and the “tholeiitic” end-members. The composition of the latter overlaps with that of the lavas frequently erupted from the conduit system feeding the main summit cone. AlphaMELTS modelling, and fluid inclusion and clinopyroxene barometry, constrain the conditions of magma storage at 10-30 km, and the ascent of magma from the upper mantle to the shallow crustal plumbing system. Variable degrees of mantle melting, together with minor source heterogeneity and contamination with cumulate-derived partial melts, contribute to the diversity of PdF magmas. However, all these processes do not represent the dominant factors that produce the large variability we found in major element composition. Indeed, the composition of basalts erupted from PdF peripheral centers is strongly controlled by polybaric olivine-clinopyroxene fractionation at pressures higher than 3 kbar. Crystal textures and geochemical modelling suggest that fast magma ascent is critical to prevent clinopyroxene dissolution. Conversely, long-lasting magma stagnation promotes pyroxene resorption and magma differentiation. “Central” eruptions occurring close to the PdF summit cone emit variably more evolved melts, which result from olivine-clinopyroxene-plagioclase differentiation at intermediate-shallow pressure (<3 kbar and in most cases <1 kbar). Deep and extensive magma mixing before injection into the crustal magma conduit system, located below the summit region, results in the apparent homogeneity of basalts erupted from the central area. As regard peripheral eruptions, deep-seated stagnation of basaltic melts and differentiation at the mantle-crust transition zone (ca. 4 kbar) produces a range of magma compositions. We demonstrate that rapid magma ascent from deep-seated reservoirs can bypass the central plumbing system. The eruptions of these magmas both in the central area and on the densely populated flanks have major consequences in terms of volcanic hazard at PdF.