Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania

International audienceOldoinyo Lengai is the only volcano on Earth currently erupting natrocarbonatites, of which the source and genesis remain controversial. Cognate xenoliths and fumaroles were sampled at the summit of Oldoinyo Lengai, and deep crustal xenoliths from Oltatwa maar, in 2010 and 2014, after the 2007-2008 sub-Plinian eruption. The summit cognate xenoliths provide direct information on the isotopic composition of the mid-crustal magma chamber that was active during the 2007-2008 explosive eruption. Cognate xenolith-hosted pyroxenes from Oldoinyo Lengai have an average 3 He/ 4 He = 6.58 ± 0.46 R A , similar to values from nearby silicate volcanoes (4.95-7.30 R A), and reflecting a sub-continental lithospheric mantle (SCLM) signature. This similarity implies that Oldoinyo Lengai carbonatites form from a similar mantle reservoir as the nearby silicate volcanoes. We identify SCLM, metasomatized by fluids/melts derived from the depleted convective mantle, as the common source of magmas in the Arusha volcanic province. Fumarole measurements highlight that fumarolic 3 He/ 4 He values have been relatively constant since at least 1988, indicating that dramatic changes to the crater region morphology during the 2007-2008 eruption did not affect the architecture of the hydrothermal system, which is probably connected to the crustal magma chamber(s). Moreover, the similarity between 3 He/ 4 He values from the mid-crustal magma chamber (6.58 ± 0.46 R A) and fumaroles (7.31 ± 0.24 R A) of Oldoinyo Lengai attests that helium is not subjected to atmospheric contamination or crustal assimilation during transport to the surface

Architecture de la plomberie du volcan carbonatitique Oldoinyo Lengai : nouvelles contraintes sur la source, les transferts hydrothermaux, et la différenciation magmatique dans la chambre active: nouvelles contraintes sur la source, les transferts hydrothermaux, et la différenciation magmatique dans la chambre active

The uniqueness of Oldoinyo Lengai to emit natrocarbonatite lavas makes this volcano a natural laboratory to study the genesis of these magmas. New helium isotopic data permit to assert that the signature of the fumaroles has been constant since 1988 despite the radical morphological change of the summit crater after the last sub-Plinian eruption in 2007-2008. The alternation of the effusive and explosive eruptions does not cause major modifications in the hydrothermal system architecture, which is inferred to be deeply rooted. Cognate xenoliths that were emitted during the eruption in 2007-2008 represent a unique opportunity to document the igneous processes occurring within the active magma chamber. The comparison between the noble gas (helium) isotopic compositions of the active magma chamber and those of the other silicate volcanoes of the Arusha region indicates that both types of magmatism have similar sources, identified as being a typical sub-continental lithospheric mantle, which was previously metasomatized by asthenospheric fluids. Moreover, these isotopic signatures confirm that no crustal contamination has occurred during the magma ascent from the mantle to the surface. Detailed petrographic descriptions coupled to a thermo-barometric approach, and to the determination of volatile solubility models for a phonolite composition, allow us to identify the melt evolution at magma chamber conditions and the storage parameters. These results indicate that the magma injected in 2007 has a phonolitic composition and contains a high amount of volatiles (3.2 wt.% H2O and 1.4 wt.% CO2) as well as a temperature around 1060° C. This magma subsequently evolved in the crustal magma chamber located at 11.5 ± 3.5 km depth until reaching a nephelinite composition and a temperature of 880°C. During the differentiation in the magma chamber, the silicate magma is enriched in calcium, sodium, magnesium and iron, whereas the content of silicate, potassium and aluminum decreases. Our results support previous studies related to this eruption, and are similar to the historical products emitted during the whole volcano history, permitting the suggestion that no major modification in the plumbing system has occurred during the Oldoinyo Lengai evolution. The trace elements (REE, LILE and HFSE) measured in the minerals and melt inclusions reveal a concentration reaching 100 to 1000 times the primitive mantle composition. A preliminary experimental study based on the recharge melt composition (phonolite) and identified magma chamber conditions (P, T) permits to reproduce the immiscibility between silicate and carbonatite liquids, key processes at the origin of the Oldoinyo Lengai carbonatites. The continuation of this experimental study will lead to a better comprehension of the carbonatite genesis, thus improving our understanding of the processes that are responsible for the enrichment in trace elementsLa particularité de l’Oldoinyo Lengai à émettre des laves natrocarbonatitiques fait de ce volcan un laboratoire naturel pour l’étude de la genèse de ces magmas. De nouvelles mesures isotopiques en hélium nous ont permis de constater que la signature des fumerolles est constante depuis 1988 malgré le changement morphologique considérable du cratère sommital lors de la dernière éruption subplinienne de 2007-2008. L’alternance des éruptions explosives et effusives n’engendre donc aucune modification majeure dans l’organisation du système hydrothermal qui est par conséquent profondément enraciné. Les xénolites cogénétiques qui ont été émis lors de l’éruption de 2007-2008 permettent d’étudier directement les processus magmatiques qui se déroulent dans la chambre magmatique active. La comparaison des signatures isotopiques des gaz rares (hélium) de la chambre magmatique et des volcans silicatés de la région d’Arusha montre que les deux types de magmatisme ont une source analogue identifiée comme un manteau lithosphérique subcontinental préalablement métasomatisé par des fluides asthénosphériques. De plus, ces signatures isotopiques confirment l’absence de contaminations crustale lors de la remontée du magma entre le manteau source et la surface. Une description pétrographique de détail couplée à une approche thermobarométrique, ainsi qu’à la détermination des modèles de solubilité des volatils dans les liquides phonolitiques, nous a permis d’identifier l’évolution du liquide dans la chambre magmatique et ses paramètres de stockage. Les résultats nous révèlent que le magma injecté en 2007 a une composition phonolitique et des teneurs élevées en volatils (3.2 wt.% de H2O et 1.4 wt.% de CO2) ainsi qu’une température d'environ 1060° C. Ce magma évolue ensuite dans la chambre magmatique crustale se trouvant à 11.5±3.5 km de profondeur jusqu’à atteindre une composition de néphélinite et une température de 880°C. Pendant sa différenciation, le magma silicaté s’enrichit en calcium, sodium, magnésium et fer alors que sa concentration en silice, potassium et aluminium décroit. Ces résultats concordent avec les précédents relatifs à cette éruption, ou aux produits volcaniques plus anciens émis tout au long de la vie du volcan. Cette similarité suggère qu’aucun changement majeur n’ait eu lieu dans l’organisation de la plomberie du volcan Oldoinyo Lengai au cours de son évolution. Les mesures en éléments traces (REE, HFSE et LILE) dans les minéraux cristallisés lors de cette séquence de différenciation, et les inclusions magmatiques associées montrent un enrichissement pouvant atteindre de 100 à 1000 fois la composition du manteau primitif. Une étude expérimentale préliminaire s’appuyant sur la composition du liquide de recharge (phonolite) et les conditions (P, T) identifiées pour la chambre magmatique nous a permis de reproduire l'immiscibilité entre un liquide silicaté et carbonatitique, processus à l’origine de la formation des carbonatites de l’Oldoinyo Lengai. La poursuite de ces travaux expérimentaux permettra de mieux contraindre la genèse des magmas carbonatitiques et ainsi comprendre les processus en jeux dans l’enrichissement en éléments traces des magmas carbonatitique

Architecture of the plumbing of the Oldoinyo Lengai carbonatitic volcano : New constraints on the source, hydrothermal transfer, and magmatic differentiation in the active chamber

La particularité de l’Oldoinyo Lengai à émettre des laves natrocarbonatitiques fait de ce volcan un laboratoire naturel pour l’étude de la genèse de ces magmas. De nouvelles mesures isotopiques en hélium nous ont permis de constater que la signature des fumerolles est constante depuis 1988 malgré le changement morphologique considérable du cratère sommital lors de la dernière éruption subplinienne de 2007-2008. L’alternance des éruptions explosives et effusives n’engendre donc aucune modification majeure dans l’organisation du système hydrothermal qui est par conséquent profondément enraciné. Les xénolites cogénétiques qui ont été émis lors de l’éruption de 2007-2008 permettent d’étudier directement les processus magmatiques qui se déroulent dans la chambre magmatique active. La comparaison des signatures isotopiques des gaz rares (hélium) de la chambre magmatique et des volcans silicatés de la région d’Arusha montre que les deux types de magmatisme ont une source analogue identifiée comme un manteau lithosphérique subcontinental préalablement métasomatisé par des fluides asthénosphériques. De plus, ces signatures isotopiques confirment l’absence de contaminations crustale lors de la remontée du magma entre le manteau source et la surface. Une description pétrographique de détail couplée à une approche thermobarométrique, ainsi qu’à la détermination des modèles de solubilité des volatils dans les liquides phonolitiques, nous a permis d’identifier l’évolution du liquide dans la chambre magmatique et ses paramètres de stockage. Les résultats nous révèlent que le magma injecté en 2007 a une composition phonolitique et des teneurs élevées en volatils (3.2 wt.% de H2O et 1.4 wt.% de CO2) ainsi qu’une température d'environ 1060° C. Ce magma évolue ensuite dans la chambre magmatique crustale se trouvant à 11.5±3.5 km de profondeur jusqu’à atteindre une composition de néphélinite et une température de 880°C. Pendant sa différenciation, le magma silicaté s’enrichit en calcium, sodium, magnésium et fer alors que sa concentration en silice, potassium et aluminium décroit. Ces résultats concordent avec les précédents relatifs à cette éruption, ou aux produits volcaniques plus anciens émis tout au long de la vie du volcan. Cette similarité suggère qu’aucun changement majeur n’ait eu lieu dans l’organisation de la plomberie du volcan Oldoinyo Lengai au cours de son évolution. Les mesures en éléments traces (REE, HFSE et LILE) dans les minéraux cristallisés lors de cette séquence de différenciation, et les inclusions magmatiques associées montrent un enrichissement pouvant atteindre de 100 à 1000 fois la composition du manteau primitif. Une étude expérimentale préliminaire s’appuyant sur la composition du liquide de recharge (phonolite) et les conditions (P, T) identifiées pour la chambre magmatique nous a permis de reproduire l'immiscibilité entre un liquide silicaté et carbonatitique, processus à l’origine de la formation des carbonatites de l’Oldoinyo Lengai. La poursuite de ces travaux expérimentaux permettra de mieux contraindre la genèse des magmas carbonatitiques et ainsi comprendre les processus en jeux dans l’enrichissement en éléments traces des magmas carbonatitiquesThe uniqueness of Oldoinyo Lengai to emit natrocarbonatite lavas makes this volcano a natural laboratory to study the genesis of these magmas. New helium isotopic data permit to assert that the signature of the fumaroles has been constant since 1988 despite the radical morphological change of the summit crater after the last sub-Plinian eruption in 2007-2008. The alternation of the effusive and explosive eruptions does not cause major modifications in the hydrothermal system architecture, which is inferred to be deeply rooted. Cognate xenoliths that were emitted during the eruption in 2007-2008 represent a unique opportunity to document the igneous processes occurring within the active magma chamber. The comparison between the noble gas (helium) isotopic compositions of the active magma chamber and those of the other silicate volcanoes of the Arusha region indicates that both types of magmatism have similar sources, identified as being a typical sub-continental lithospheric mantle, which was previously metasomatized by asthenospheric fluids. Moreover, these isotopic signatures confirm that no crustal contamination has occurred during the magma ascent from the mantle to the surface. Detailed petrographic descriptions coupled to a thermo-barometric approach, and to the determination of volatile solubility models for a phonolite composition, allow us to identify the melt evolution at magma chamber conditions and the storage parameters. These results indicate that the magma injected in 2007 has a phonolitic composition and contains a high amount of volatiles (3.2 wt.% H2O and 1.4 wt.% CO2) as well as a temperature around 1060° C. This magma subsequently evolved in the crustal magma chamber located at 11.5 ± 3.5 km depth until reaching a nephelinite composition and a temperature of 880°C. During the differentiation in the magma chamber, the silicate magma is enriched in calcium, sodium, magnesium and iron, whereas the content of silicate, potassium and aluminum decreases. Our results support previous studies related to this eruption, and are similar to the historical products emitted during the whole volcano history, permitting the suggestion that no major modification in the plumbing system has occurred during the Oldoinyo Lengai evolution. The trace elements (REE, LILE and HFSE) measured in the minerals and melt inclusions reveal a concentration reaching 100 to 1000 times the primitive mantle composition. A preliminary experimental study based on the recharge melt composition (phonolite) and identified magma chamber conditions (P, T) permits to reproduce the immiscibility between silicate and carbonatite liquids, key processes at the origin of the Oldoinyo Lengai carbonatites. The continuation of this experimental study will lead to a better comprehension of the carbonatite genesis, thus improving our understanding of the processes that are responsible for the enrichment in trace element

Mono- and Dialkyl Glycerol Ether Lipids in Anaerobic Bacteria: Biosynthetic Insights from the Mesophilic Sulfate Reducer Desulfatibacillum alkenivorans PF2803(T) (vol 81, pg 3157, 2015)

International audienc

Desulfatiferula berrensis sp. nov., a n-alkene-degrading sulfate-reducing bacterium isolated from estuarine sediments.

A novel sulfate-reducing bacterium designated strain BE2801(T) was isolated from oil-polluted estuarine sediments (Berre Lagoon, France). Cells were Gram-stain-negative, motile, slightly curved or vibrioid rods. Optimal growth of strain BE2801(T) occurred at 30-32 °C, 0.5-1.5% NaCl (w/v) and pH 7.2-7.4. Strain BE2801(T) grew with C4 to C20 fatty acids or C12 to C20 n-alkenes as electron donors. Acetate and carbon dioxide were the oxidation products. The major cellular fatty acids were C16 : 0, C(16 : 1)ω7c and C(18 : 1)ω7. The DNA G+C content was 50.2 mol%. 16S rRNA and dsrAB gene sequence analysis indicated that strain BE2801(T) was a member of the family Desulfobacteraceae within the class Deltaproteobacteria. DNA-DNA hybridization with the most closely related taxon demonstrated 14.8 % relatedness. Based on phenotypic and phylogenetic evidence, strain BE2801(T) ( = DSM 25524(T) = JCM 18157(T)) is proposed to be a representative of a novel species of the genus Desulfatiferula, for which the name Desulfatiferula berrensis sp. nov. is suggested

Variscan lamprophyres in the Lower Penninic domain (Central Alps): age and tectonic significance

Lamprophyre dykes have been recently discovered in blocks of gneiss embedded in a calcschist formation of wildflysch type that forms the top of the Mesozoic-Tertiary metasedimentary cover of the Antigorio nappe (the Teggiolo zone) in the Val Bavona (Lower Penninic, NW Ticino, Switzerland). The presence of the lamprophyres gives a clue to the possible source of these blocks. Similar dykes occur in the N part of the Maggia nappe where they are intruded into the Matorello granite and the surrounding gneisses. We studied these lamprophyres at two localities in the Teggiolo zone (Tamierpass and Lago del Zott) and at one locality in the Maggia nappe (Laghetti). Detailed mineralogical and geochemical investigations confirm their great similarity, particularly between the Tamier and Laghetti dykes. They all recrystallized during Alpine metamorphism under amphibolite facies conditions and lost their primary mineral assemblages and textures. The chemistry reveals a calc-alkaline affinity, a limited differentiation range, features of mineral accumulation and intense remobilization in some cases. The lamprophyres are characterized by a high mg# and relatively low contents in REE and other incompatible elements. In situ SHRIMP and LA-ICPMS U-Pb zircon dating yielded ages of 284.8 +/- 1.7 Ma (Tamier), 290.0 +/- 1.3 Ma (Zott) and 290.5 +/- 3.7 Ma (Laghetti). These ages are compatible with the general late- to post-Variscan magmatic evolution of the Helvetic and Lower Penninic domains. The lamprophyres are considered as melts derived from the lithospheric Variscan mantle, variously hybridized and differentiated at the contact with crustal material during late- to post-orogenic extension. These lamprophyres are chemically distinct from earlier lamprophyres of Visean age, emplaced together with their associated granites in transcurrent fault zones during the Variscan orogenic compression. The similarity of these different dykes suggests that the front of the Maggia nappe is a likely source of the gneissic blocks embedded in the calcschists at the top of the Teggiolo zone. They would have been provided by the advancing Maggia nappe during its thrusting over the Antigorio nappe and simultaneous closure of the Teggiolo sedimentary basin