Lead isotopes behavior in the fumarolic environment of the Piton de la Fournaise volcano (Reunion Island)

International audienceThe recent activity of the Piton de la Fournaise volcano offers a rare opportunity to address the issue of Pb isotope behavior in volcanic fumaroles, as the composition of the degassing source is accurately and precisely known. Gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca. 100 degrees C include Na-K sulfate (aphthitalite), Ca-Cu sulfate (e.g., gypsum), Na sulfate (thenardite), Ca-Mg-Al-Fe fluoride (e.g., ralstonite) and native sulfur. The high-temperature deposits show trace element patterns typical of volcanic gas (with Pb concentration up to 836 ppm) while the low-temperature deposits are depleted in most volatile elements (Pb <1 ppm) with the exception of Pd and Tl (in fluorides) and Se (in native sulfur). Only for low-temperature fluoride samples do Pb isotope compositions plot significantly outside the field of lavas. The isotopic shift is ascribed to leaching ubiquitous unradiogenic phases (e.g., sulfides) by acidic gas condensates. The similarity in Pb isotope signature between lavas and sublimate samples more representative of the gas phase (sulfates) indicates that the net fractionation of Pb isotopes resulting from volatilization and condensation processes is smaller than the precision of Pb isotope measurements (better than 60 ppm/a.m.u.). The absence of net fractionation could result from negligible isotope fractionation during Pb volatilization followed by extensive condensation of gaseous Pb, with possibly significant isotopic fractionation at this stage. Although this scenario has to be refined by more direct measurement of the gas phase, and its general applicability tested, it suggests that a small fraction (<10\%) of initially volatilized Pb ultimately escapes to the atmosphere, while the remaining dominant fraction is trapped in sublimates. As sublimates are rapidly dissolved and entrained by runoff, the fumarolic environment appears as a factory efficiently transferring isotopically unfractionated Pb from magmas towards the hydrological system and seawater. Resolving very small isotopic differences between magmas and their gaseous products remains an analytical challenge. High-precision Pb isotope measurements rest not only on instrumental performance but also on high-yield chemistry, as Pb isotopes drastically fractionate (800 ppm/a.m.u.) upon elution on anionic resin. For 50\% Pb recovery, the estimated isotopic bias is plus or minus 60-80 ppm/a.m.u., depending on which of the early (isotopically light) or late (isotopically heavy) Pb fraction is lost. (c) 2012 Elsevier Ltd. All rights reserved

Textural and geochemical analysis of a pumice polisher with grooves from the Magdalenian site of Duruthy (Sorde, Landes, France)

Magdalenian archaeological sites often contain grooved polishers used to polish other implements (for example, Capitan and Peyrony 1928). These objects are generally made from sandstone but a few are made from pumice. Bibliographic research revealed that only four sites have yielded pumice stone polishers (fig. 1, fig. 2, tab. 1). These are the sites of Gourdan (Piette 1873), Isturitz (Passemard 1944), Aitzbitarte IV (Barandiaran 1965) and Arancou (Chauchat et al. 1999). Several polishers were found in the Duruthy rock-shelter, Sorde-L'abbaye, Landes. Most of these polishers are in sandstone (fig. 3) but one of them is different and has the appearance of pumice (fig. 4). The textural study and geochemical analysis (fig. 5, fig. 6, tab. 2) of this polisher indicate that it was made from rhyolitic pumice. The geochemical characteristics of this material indicate that it is not a product of French volcanism. The fact that all the sites where pumice polishers were found are located near the Atlantic Ocean suggests that floated pumices may have been collected along the ocean shores. Such rhyolitic pumices could have been produced during the eruption of a volcano on an island in the Atlantic Ocean. Although we have not yet identified the volcano responsible for this eruption, this remains the most likely hypothesis. In this respect, these artefacts offer a new approach to the study of the exploitation of the littoral zone by the last Palaeolithic hunters in southwest France and northern Spain

Heterogenous initial Sr isotope compositions of highly envolved volcanic rocks from the Main Ethiopian Rift, Ethiopia

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The Cenozoic volcanic province of Tibesti (Sahara of Chad): major units, chronology, and structural features

International audienceUsing both field relationships and some absolute ages, the sequence of volcanic units in the Cenozoic Tibesti Volcanic Province (TVP) (Chad) is established as follows: (1) plateau volcanism, between at least 17 and 8 Ma, consisting of flood basalts and silicic lava plugs, with intercalated ignimbritic sheets in the upper basalt succession increasing in amount upwards. Ages decrease from NE to SW, following the migration of the small NW-SE flexures concentrating the feeding dike swarms; (2) Late Miocene large central composite volcanoes exhibiting diverse and original structures. Some of them (Tarso Toon, Ehi Oyé, and Tarso Yéga) are located along a major NNE fault, representing the main tectonic direction in Tibesti since Precambrian times; (3) construction of three large ignimbritic volcanoes, associated with significant updoming of the basement, ending with the collapse of large calderas: Voon (about 5–7 Ma), Emi Koussi (2.4–1.33 Ma), and Yirrigué (0.43 Ma); (4) basaltic activity, starting at about 5–7 Ma, and essentially consisting of cinder cones and associated lava flows (Tarso Tôh, Tarso Ahon, and Tarso Emi Chi); and (5) final volcanic activity represented by post-Yirrigué caldera activity in the Tarso Toussidé Volcanic Complex, and especially Ehi Toussidé (the only active volcano in Tibesti), plus Ehi Timi and Ehi Mousgou volcanoes, similar to Ehi Toussidé. The two tectonic directions controlling some volcanic features of the province correspond to the major old lithospheric structures delimiting the volcanic province, namely, the great NW-SE Tassilian flexure to the SW and a major NE-NNE fault zone to the E. Unusual conditions of uplift and erosion in the TVP enable exceptional exposure of the internal structure of its volcanoes

Structure of Puy de Dôme volcano (Chaîne des Puys, France): towards a revised model

The project TOMUVOL, aimed at developing a method for muonic tomography of volcanic edifices, has chosen the Puy de Dôme as an experimental test site. In fact, this volcano with a simple external shape has a complex internal structure that may allow testing the performances of this new method. The Puy de Dôme is a composite lava dome, about 11 ka old. It is composed of two distinct morphological units. The first one would consist of a classic bristled lava-dome whereas the second one would be a dome erected on the eastern flank of the first one, after a large flank collapse. However, between 2004 and 2011, fresh outcrops revealed by the building of touristic facilities on the summit and at the basis of the mountain, afforded the collection of new data inducing new questions. The field was then more systematically explored and the results of a LiDAR survey centred on the Puy de Dôme and recorded in March 2011 were examined

Multi-stage growth of the trachytic lava dome of the Puy de Dôme (Chaîne des Puys, France). Field, geomorphological and petro-geochemical evidence

International audienceUnderstanding lava dome eruptions is a major concern in volcanology regarding the assessment of associated hazards. This question is usually addressed through the study of active calc-alkaline domes, however, alkaline domes remaining poorly known. The Puy de Dôme is the most prominent volcano of the Chaîne des Puys Quaternary intraplate volcanic field in the French Massif Central. It is a complex trachytic lava dome ca. 11 ka old. Field investigations, geomorphological and petro-geochemical studies were performed to understand the magmatic system of this dome and reconstruct its volcanic evolution. The different lava facies were fully characterised and their relationships with the volcano building stages have been established. A new four-step evolution is proposed for the growth of this edifice. Mineralogical and geochemical variations document magmatic differentiation with self-mixing and minor crustal contamination in a zoned reservoir within the upper crust. Magma mixing and crustal contamination through fluids interactions during magma ascent and the involvement of fumarolic activity throughout the edifice building are also documented. Further, this work highlights the specific features of the Puy de Dôme compared to calc-alkaline domes: lack of lava flows and almost complete magma degassing during ascent, resulting in the lack of significant explosive activity until the last eruption step

Environmental variability in Lake Kivu and cholera risk in Bukavu from 2008 to 2012

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Environmental variability in Lake Kivu and cholera risk in Bukavu from 2008 to 2012

International audienc

Environmental variability in Lake Kivu and cholera risk in Bukavu from 2008 to 2012

International audienc

The ultimate summit eruption of Puy de Dôme volcano (Chaîne des Puys, French Massif Central) about 10,700 years ago

The Puy de Dôme volcano is a trachytic lava dome, about 11,000 y old. New pyroclastic layers originating from the volcano itself were discovered covering the summit and the flanks of the volcano. These pyroclastic layers do not fit with the previous interpretation, assuming a non-explosive dome-forming eruption. The tephra display pyroclastic surge features and exhibit fresh trachytic lapilli, basement lithics, allogeneous basaltic lava and clinker fragments requiring an open vent eruption. This ultimate eruption occurred after a period of rest, long enough for vegetation to develop on the volcano, as evidenced by carbonized plant fragments. Radiocarbon dating of some of these fragments gave an age of c.10,700 y also suggesting a significant rest duration