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

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

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

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