Étude des variations en éléments traces dans un carbonate archéologique de la ville antique d'Ostie : conséquences environnementales et archéologiques

International audienceHigh resolution laser ablation ICP-MS analyses are performed along the growth axis of an archaeological carbonate deposit from a water tower in the antique city of Ostia (Italy). Results show a composite structure with high frequency cycles, thought to be seasonal, and longer-term variations in Mg/Ca, Sr/Ca, Ba/Ca, Pb/Ca and U/Ca ratios. Changes in the broad trace element composition at the beginning of the deposit are interpreted as the hallmark of important modifications in the sources supplying the water to the city. Our study suggests that trace elements analysis in archaeological carbonates have the potential to provide high-resolution seasonal records, which could give insights into historic environment variability and past policy of natural resource administration.Les étapes de croissance d'un dépôt de carbonate provenant d'un château d'eau de la ville antique d'Ostie sont étudiées à l'aide d'un ICP-MS à ablation laser. Les résultats montrent une structure composite, avec des cycles hautes fréquences, probablement saisonniers, se superposant à un signal à plus long terme pour les rapports élémentaires suivants : Mg/Ca, Sr/Ca, Ba/Ca, Pb/Ca et U/Ca. Un changement de composition au début du dépôt est interprété comme la mémoire de modifications importantes dans la localisation de la source d'eau qui approvisionnait la ville. À l'issue de cette étude, on peut entrevoir les potentialités de l'analyse des éléments traces dans les carbonates archéologiques, afin d'aborder avec une haute résolution la question environnementale durant la période historique et, ainsi de mieux cerner les politiques passées de gestion des ressources naturelles

Géochimie de l'arc du Vanuatu (évolution spatio-temporelle des édifices volcaniques et des sources mantelliques.)

L archipel volcanique actif du Vanuatu s édifie au coeur du pacifique sud-ouest au niveau de la frontière convergente des plaques australienne et pacifique. Je présente ici une nouvelle étude géochimique des laves du Vanuatu à partir de la détermination des compositions en éléments majeurs et traces, et des compositions isotopiques (Sr-Nd-Hf-Pb) d une centaine d échantillons de laves (< 2 Ma).L étude des magmas les plus primitifs a permis de mettre en évidence la variation de composition des sources mantelliques le long de l arc et d individualiser 3 segments : "central", dans la zone de collision de la ride D Entrecasteaux, "sud" en face du bassin Nord Fidjien, et "extrême sud" en face du bassin Sud Fidjien. La composition des roches des différentes structures subductées influence celles des laves des volcans adjacents via le composant de subduction, sous forme de fluides et de produits de fusion. Les laves des îles situées en face de la ride D Entrecasteaux sont issues d un manteau enrichi ("type-MORB indien"), différent de celui échantillonné par les autres laves ("type-MORB pacifique"). Cette ride apporte probablement en subduction un composant ancien, pouvant être assimilé à un fragment de croûte inférieure.L étude locale de certaines îles a permis de caractériser la différenciation des laves par cristallisation fractionnée, d identifier des processus d assimilation crustale, et de révéler la présence de magma provenant de portions de manteau distinctes, ayant subi un métasomatisme différent.Ces travaux révèlent une extrême hétérogénéité du manteau sous l arc du Vanuatu, témoignant de la complexité des processus géologiques impliqués au niveau de cette zone de subduction.The Vanuatu island arc is located in the SW Pacific at the convergent boundary between the Australian and Pacific plates. I present here a new geochemical study of the Vanuatu lavas based on major and trace element as well as isotopic (Sr-Nd-Hf-Pb) analyses of approximately one hundred lava samples (< 2 My).Examination of the most primitive magmas reveals the variation of the mantle source composition along the arc and demarcates three segments: a "central" segment in front the d Entrecasteaux ridge, a "south" segment in front of the North Fiji Basin, and a "far south" segment in front of the South Fiji Basin. The composition of the different subducted basins highly influenced those of the adjacent volcanoes via mass transfer of aqueous fluid and melt subduction components. Lavas from islands facing the D'Entrecasteaux Ridge come from an enriched mantle (Indian MORB-like), distinct from that sampled at other islands (Pacific MORB-like). Additionally, this ridge likely carries with it an ancient component that may reflect entrainment of lower crust material.Localized study of some volcanic islands reveals processes related to the differentiation of lavas by fractional crystallization, processes related to crustal assimilation, and indicates the presence of magma coming from different portions of mantle with slightly different metasomatic histories.This body of work highlights extreme mantle heterogeneity under Vanuatu islands arc and testifies to the complex nature of its associated geological processes.BREST-SCD-Bib. electronique (290199901) / SudocMARSEILLE-MAP (130552318) / SudocSudocFranceF

A Typical Sublithospheric Mantle beneath the Tello Region, SE-Ngaoundéré (Cameroon Line)

Peridotite xenoliths, raised to the surface by alkaline basalts or kimberlites,provide us direct information on the processes and composition of the upper mantle. They are the major source of information on the state of stress,pressure and temperature in the deep mantle. They are thus a source of petrological and geochemical information that is generally not available on the Earth’s surface. Fresh spinel-lherzolite xenoliths exhibit a protogranular components of the Tello volcano. The Tello is the continental sector of the Cameroon Line, located in the South East of the town of Ngaoundéré at 75 km approximately between (N7° 13’, N7° 14’) and (E13° 40’ and E13° 60’). Minerals’ composition of the xenoliths is ~64% olivine, ~24% orthopyroxene, ~11% clinopyroxene and ~1% spinel. Significant variation in (Cr/Cr + Al) of the system shows the reciprocal nature of the spinel solution.The Tello spinel lherzolites show internal chemical homogeneity and represent a normal upper mantle. Their mineral chemistries suggest equilibrium condition of 830° - 925° and 1.4 GPa-2.3 GPa. These data suggest that there is good correlation bracket between increasing activity of Al2O3 and decreasing of practionning of TiO2 into spinel. The AlIV and AlVI contents vary by 0.05-0.2 and 0.03-0.2 respectively. The majority of samples caracterise the lithospheric mantle

Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)

International audienceWe report the mineralogy, geochemistry and geochronology of the mafic rocks from the Bamenda Mountains, part of the Cameroon Volcanic Line (CVL), in order to discuss the origin and evolution of the magmas in this part of the CVL. Mafic rocks in the Bamenda Mountains are basanites, basalts, hawaiites and mugearites with an alkaline affinity. K-Ar ages have been obtained on 10 samples and range from 17.6 Myr to present. Trace element and isotopic compositions (Sr-Nd-Pb) show that some samples among the oldest are slightly contaminated by a crustal component with high La/Nb and 87Sr/86Sr ratios and low Pb isotopic ratios. The mafic rocks strongly resemble OIB in their trace element compositions. Some samples possess a positive Sr and Eu anomaly which cannot be explained by a process of plagioclase accumulation. These anomalies are also observed in some pyroxenites found as xenoliths in the Adamawa volcanic province further north. Furthermore, non-contaminated samples have high Pb isotopic ratios and point towards an HIMU component similar to the St. Helena mantle plume. We propose that the Bamenda mafic magmas with positive Sr and Eu anomalies were formed by hybridization of asthenospheric melts with melts formed by the partial melting of pyroxenites. Samples without these anomalies result from the hybridization of the same asthenopheric melts with melts coming from the metasomatized, amphibole-bearing, lithospheric mantle

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

De la pluie à l'eau souterraine (apport du traçage naturel (ions majeurs, isotopes) à l'étude du fonctionnement des aquifères volcaniques (bassin d'Argnat, Auvergne, France))

Les propriétés hydrogéologiques de la Chaîne des Puys sont liées à ses particularités climatiques, géologiques, morphologiques et environnementales qui vont contraindre l'eau souterraine en terme d'origines quantité qualité et parcours dans les aquifères volcaniques. Lacomposition chimique et isotopique de l'eau reflétant l'ensemble des processus intervenant lors du cycle de l'eau, ses variations au sein et entre les différents compartiments permettent donc de préciser le rôle de ces différents facteurs . La caractérisation chimique (ions majeurs) et isotopiques (d18O, d2H, d13C) des différentes étapes du cycle de l'eau peut donc permettre de préciser le fonctionnement des aquifères de la Chaîne des Puys. Dans cette perspective, notre travail s'est articulé autour de deuxthèmes: 1) la définition du signal d'entrée des aquifères régionaux, préalable indispensable à toute hydrogéologique, 2) l'étude du fonctionnement du bassin d'Argnat localisé au Nord de la Chaîne des Puys.CLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF