The eruptive history of the Trous Blancs pit craters, La Réunion Island: The origin of a 24 km long lava flow

International audienceThe assessment of volcanic hazards is strongly based on the past eruptive behaviour of volcanoes and its morphological parameters. Since past eruption characteristics and their frequency provide the best probabilities of such eruptions for the future, understanding the complete eruptive history of a volcano is one of the most powerful tools in assessing the potential hazards or eruptions. At Piton de la Fournaise (PdF) volcano (La Réunion, Indian Ocean), the most frequent style of activity is the effusion of lava flows, which pose the greatest hazard by invasion of inhabited areas and destruction of human property. Here we examined the eruptive history of a previously uninvestigated area, believed to be the origin of a 24 km long lava flow.The eruptions recurrence time of PdF is about one eruption every 9 months in the central caldera. Besides this central activity, eruptive vents have been built along three main rift zones cutting the edifice during the last 50 kyrs. In this study we focused on the largest rift zone of about 15 km width and 20 km length, which extends in a north westerly direction between PdF and the nearby Piton des Neiges volcanic complex. This rift zone is typified by deep seismicity (up to 30 km), emitting mostly primitive magmas, indicative of high fluid pressures (up to 5 kbar) and large volume eruptions. Our area of investigation focused on four consecutively aligned pit craters called the Trous Blancs. These have been identified [1] as the source area of one of the youngest (ca. 6 kyrs) and largest lava field, which extends for 24 km from a height of 1800m asl, passing Le Tampon and Saint Pierre city, until it reaches the coast. To gain insight into the development of this eruption and possible future similar activity, we collected new field data (including stratigraphic logs, a geological map of the area, C-14 dating and geochemical analyses of the eruptive products).Fieldwork revealed that the eruption initiated with intense fountaining activity, producing a m-thick bed of loose black scoria, which becomes densely welded in its upper part. It was followed by an alternation of volume rich lava effusions and strombolian activity and deposition of meter-thick massive units of olivine basalt, alternating with coarse scoria beds in the proximal area. Activity ended with the emplacement of a dm sized bed of glassy, dense scoria and a stratified lithic breccia, marking the pit crater formation.Preliminary dating suggested that this type of eruption could have a millennial recurrence time at PdF. Reoccurring similar activity on the NW rift represents a major source of risk for this now densely populated region (more than 150,000 people living in the affected area)

Influence of glass polymerisation and oxidation on micro-Raman water analysis in alumino-silicate glasses

International audienceThe development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: not, vert, similar490 cm−1; HF: not, vert, similar960 cm−1) and total water stretching (H2OT: not, vert, similar3550 cm−1) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe3+/FeT from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses (CH2OT from 0.4 to 7.0 wt%) with NBO/T varying from 0.01 to 0.76. In the explored domain of water concentration, external calibration procedure based on the H2OT band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H2OT height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO2, Na2O + K2O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO2, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses: View the MathML source where CH2OT is the total water content (in wt%) dissolved in glass; TOTN represents the computed ILF/IHF variation as a function of the calculated NBO/T and SM parameters; IH2ON is the H2O band height scaled to ratio of the reference bands; k is the linearity spectrometer response on the H2OT band in function of water content. The water concentrations of the reference glasses are reproduced using this equation with a standard deviation of 0.06 wt%. The adopted parameterisation provides a useful tool towards the characterisation of composition dependence of micro-Raman procedures for silicate glasses. We show, based on the widest range of glass compositions so far investigated, that accurate evaluation of dissolved water content is achieved by micro-Raman spectroscopy

Les variations chimiques des clinopyroxènes calciques, témoins de la complexité des processus pétrogénétiques à l'intérieur d'un réservoir magmatique (Exemple des ponces des Champs Phlégréens Campanie, Italie)

Diopside and salite occur together in latitic and trachytic pumices in Phlegrean Fields. Optical and chemical studies by electron microprobe (Camebax) on clinopyroxenes, and optical thermometry on the two phases inclusions (glass + gaz) (which can be used to get a minimal crystallization of the host mineral) present in these crystals show that : — diopside crystallize from a trachybasaltic melt 1,155 °C ± 10 °C, — cognate diopsidic pyroxene exist either as phenocrysts in a latitic and trachytic pumices, or as core of salite in latitic pumice, — inversed zoning and oscillatory zoning (with diopsidic and salitic compositions) occur in these salites, — minimal crystallization temperatures measured on the melt inclusions are : 1,020 °C ± 10 °C, 1,040 °C ± 10 °C for more ferriferous salite in trachyte. These variations can be correlated with compositional variations of the phenocrysts. — Melt inclusions of some apatite included in the pyroxene of trachyte point out the same temperature 1,020 °C ± 10 °C. The results suggest the coexistence of different cogenetic magmas at different differentiation degrees. The observed compositional change from trachybasaltic and trachytic melts is consistent with a fractional crystallization process with removal of diopside and olivine in the first step then salite, plagioclase, magnetite and a more and more important role of the sanidine in the most evolved liquids. These hypotheses are in agreement with the magma chamber model proposed by Armienti et al. (in press).La coexistence de deux pyroxènes (diopside, salite) a été observée dans les ponces latitiques et trachytiques des Champs Phlégréens (Italie). L'étude optique, l'analyse par microsonde électronique de ces pyroxènes, ainsi que la thermométrie optique des inclusions vitreuses présentes dans ces phases minérales ont permis de mettre en évidence : — La cristallisation du diopside à partir d'un liquide trachybasaltique à 1 155 ± 10 °C. — La présence de diopsides hérités : soit sous forme de phénocristaux dans les ponces latitiques et trachytiques, soit au coeur des salites de la latite. — L'existence de zonations inverses et oscillatoires de ces dernières. — Une température minimale de cristallisation de 1 125 ± 10 °C pour les salites de la latite, de 1 020 ± 10 °C-1 040 ± 10 °C pour les salites plus ferrifères des trachytes. L'ensemble de ces résultats suggère la coexistence de magmas cogénétiques à différents degrés de différenciation. L'évolution du liquide trachybasaltique vers des liquides latitiques et trachybasaltiques, par séparation de diopside et d'olivine puis de salite, de plagioclase, de magnétite avec une importance croissante du rôle de la sanidine, témoigne d'un processus de cristallisation fractionnée.Métrich Nicole. Les variations chimiques des clinopyroxènes calciques, témoins de la complexité des processus pétrogénétiques à l'intérieur d'un réservoir magmatique (Exemple des ponces des Champs Phlégréens Campanie, Italie). In: Bulletin de Minéralogie, volume 106, 3, 1983. pp. 353-364

Redox state of volatiles and their relationships with Iron in silicate melts. Implications for magma degassing

International audienceThe question of how degassing of H-C-O-S-Cl fluids may change Fe3+/ΣFe ratio and redox conditions in silicatemelts has been widely addressed via the experiments and studies of natural systems. After a brief summary of thesolubility data and the possible role of the volatile species (CO2, H2O, S, Cl) on the redox of magmas during theirdecompression, the review focuses on the behavior of water and sulfur redox during magma degassing, the relationshipsbetween these two species with iron in silicate melts through experimental petrology, geochemical investigations,and modeling of natural systems using cutting-edge methods to determine the redox of S and Fe

Détermination des teneurs en carbone de quelques verres basaltiques ; analyses par réactions nucléaires

Carbon concentrations have been determined in basaltic glasses using the 12C(d,p)13C nuclear reaction technique. The selected energy of the incident charged particles (1.4 MeV) allows an accurate determination of the deep carbon. In oceanic tholeiitic glasses (glassy rims of pillow-lava) from E.P.R. (20°49') carbon ranges in concentration from 30 ppm (close to the detection limit) to 130 ppm (PCO2) = 25-30 MPa) after the nucleation of the CO2 vesicles. The dissolved carbon contents of the alkali basalt glasses, trapped in the olivine phenocrysts (Fo81.7) from Etna volcano, vary from 450 to 700 ppm. But carbonates (daughter minerals) occur on the walls of the shrinkage bubbles of the melt inclusions as the result of post-entrapment reactions between a CO2 fluid phase and the glass. Carbon is much lower (60 ppm) in the melt inclusions of olivine (Fo82.96) from Piton de la Fournaise volcano (Reunion Island). Our results exhibit : — a heterogeneous distribution of carbon in the submarine glasses, probably related with the CO2 vesicles nucleation process, — a relation between the carbon content of the melt inclusions and the pressure (PCO2) of crystallization of the host mineral.La réaction 12C(d,p)13C a été utilisée, avec une énergie des deutérons incidents de 1,4 MeV, pour mesurer la concentration en carbone dissous dans différents verres. Les teneurs en carbone déterminées pour les verres tholéiitiques océaniques varient de 30 ppm à 130 ppm (PCO2 = 25-30 MPa), après nucléation des vésicules de CO2. Les verres (basalte alcalin) piégés par les olivines (Fo81.7) de l'Etna (éruption de 1669, 1763 et 1892), contiennent entre 450 et 700 ppm de carbone à l'état dissous, le rapport C/S variant de 0,16 à 0,35. Le carbone est aussi exprimé sous la forme de carbonates cristallisés sur les parois des bulles de retrait de ces inclusions vitreuses. Des concentrations en carbone de 60 ppm (C/S<0,1) ont été mesurées dans les inclusions vitreuses des olivines du Piton de la Fournaise (Ile de la Réunion). Nos résultats ont mis en évidence : — une distribution hétérogène du carbone dans les verres océaniques, en relation avec le processus de nucléation des vésicules de CO2. — une relation entre la teneur en carbone des inclusions vitreuses et la pression (PCO2) lors de la cristallisation des minéraux hôtes.Métrich Nicole, Mosbah Michelle. Détermination des teneurs en carbone de quelques verres basaltiques ; analyses par réactions nucléaires. In: Bulletin de Minéralogie, volume 111, 5, 1988. pp. 511-522

Water content, δD and δ11B tracking in the Vanuatu arc magmas (Aoba Island): Insights from olivine-hosted melt inclusions

International audienceIon microprobe measurements of H and B isotopic ratios and H2O, B and trace element contents are reported here for a series of melt inclusions typical of alkaline basalts of Aoba Island in the central part of Vanuatu arc (Southwestern Pacific). The melt inclusions, hosted in olivine Fo86–90, display large ranges in trace element concentrations and hydrogen (δD from − 48.2 to + 61.7‰) and boron (δ11B from − 11.9 to + 6.4‰) isotopic compositions. The high deuterium enrichment (δD ≥ 0‰) observed in a small subset of melt inclusions requires a proton diffusion loss through the olivine network, in addition to late-stage magma interactions with aqueous saline fluids. These melt inclusions are therefore not considered as representative of the magma from which the olivine grew. In most melt inclusions, positive correlations between H2O, K2O, Ba and Sr lead us to determine the K2O/H2O (1.5 ± 0.2), H2O/Ba (46 ± 3 × 10− 4) and H2O/Sr (29 ± 2 × 10− 4) ratios of Aoba basalts.Overall correlations between δ11B, B/Nb, and B/Nd testify to the mixing between slab-derived fluids, preferentially enriched in δ11B and fluid mobile elements and a relatively depleted MORB-type mantle wedge beneath Aoba Island. Heavy δ11B (on average 5.4 ± 0.7‰) indicate slab-derived fluids, possibly involving serpentine, which would have a mean δD value of − 28.4 ± 7‰. The chemical and isotopic variability recorded by Aoba magmas (melt inclusions) is consistent with the geodynamic context of ridge-arc collision in the central segment of Vanuatu arc

Constraints on the Origin of Nepheline-Normative Primitive Magmas in Island Arcs Inferred from Olivine-hosted Melt Inclusion Compositions

International audienc

Paroxysms at Stromboli volcano (Italy): source, genesis and dynamics

International audienceFor nearly 1300 years Stromboli has been renowned not only for its continuous degassing activity and mild explosions at the summit craters, but also for short-lived, violent explosive events of variable scale known as major explosions and paroxysms. It is important to understand the triggering mechanisms of paroxysms, as this violent explosive activity can impact social and economic life on the island. To this end, we focus on the 1456 and 1930 paroxysms and on the most recent events, in July and August 2019. We present new data on the geochemistry of the 2019 bulk pumice, along with a compilation of data from the literature, chemical profiles in olivine crystals, and the physical parameters of explosive eruptions of wide ranging magnitude and intensity. Trace element concentrations (Nb, La and Ba) and ratios (Rb/Th) indicate that the 2019 pumice samples plot in the domain of magma batches erupted within the last 20 years at Stromboli. As a whole, there is no correlation between magma geochemistry and magnitude or intensity of explosive eruptions, which span a range of ~3 orders of magnitude (from major explosions to large paroxysms) based on estimates of erupted tephra volumes. In contrast, olivine compositions are a good proxy for total tephra volumes, suggesting that the magma source can affect the magnitude and intensity of the final eruption. They also indicate that in July 2019 the plumbing system reached steady-state conditions prior to paroxysm, whereas olivine and clinopyroxene zoning in products from the August 2019 paroxysm testifies to magma recharge. For small and large paroxysms, timescales were derived from Fe-Mg diffusion profiles in olivine. In both types of explosion, the last phases of crystallization indicate rapid magma ascent rates two to ten days prior to eruption. Lastly, we discuss the possible relative influence on eruption dynamics of flank collapse, lava outpouring through fractures opening, and partial emptying of the shallow conduits. These phenomena may enhance volatile-rich magma ascent by increasing the decompression rate, although pressurization of the crustal system and the deep refilling by magma and its CO2-rich gas phase play a major role in triggering paroxysms

Sulfur and boron isotope study of high-Ca impact glasses from the K/T boundary: Constraints on source rocks

A boron and sulfur isotope study has been conducted by ion microprobe on 20 high-Ca yellow impact glass particles from the K/T boundary deposit in Haiti. The yellow glasses are S-rich with up to 1.24 wt% SO3 and show a large range of δ34S values (+1.5 ± 2‰ to +13.2 ± 4‰) and δ11B values (-9.8 ± 2‰ to +11.0 ±2‰). In addition to the previously recognized andesite (or continental crust), anhydrite, and carbonate components, continental sedimentary rocks containing clays and diagenetic minerals (e.g., loess or sandstones) are indicated by the high B contents and low δ11B values of some yellow glasses. This component could correspond to the Todos Santos red-bed formation, recognized by drilling in the Yucatan and likely to be present at Chicxulub. Mass balance mixing calculations (based on SiO 2, CaO, B contents, and δ11B values) show that all the isotopic characteristics (δ11B, δ34S, and δ18O) of the yellow glasses can be explained by mixing during impact of very variable proportions of andesite (31 to 75%, mean 49%), sulfate (5 to 47%, mean 30%), red sandstone (3 to 26%, mean 14%), and carbonate (0 to 26%, mean 7%). The major element compositions modeled for the yellow glasses, taking into account the large volatilization of SO2 and CO 2, match closely the natural composition of the yellow glasses. The present observations suggest that the amount of SO2 released in the atmosphere represents ε 18.0 wt% of the impacted terranes, whereas only ε 2.6 wt% of the impacted terranes is released as CO2 to the atmosphere